American Journal of Medical Genetics Part C (Seminars in Medical Genetics) 166C:156–172 (2014)

A R T I C L E

Megalencephaly and Hemimegalencephaly: Breakthroughs in Molecular Etiology GHAYDA M. MIRZAA*

AND

ANNAPURNA PODURI**

Megalencephaly (MEG) is a developmental disorder characterized by brain overgrowth that occurs due to either increased number or size of neurons and glial cells. The former may be due to either increased neuronal proliferation or decreased apoptosis. The degree of brain overgrowth may be extensive, ranging from generalized MEG affecting the entire cortex–as with mutations in PTEN (phosphatase and tensin homolog on chromosome ten)–to unilateral hemispheric malformations–as in classic hemimegalencephaly (HME). On the other hand, some lesions are more focal or segmental. These developmental brain abnormalities may occur in isolation in some individuals, whereas others occur in the context of a syndrome involving dysmorphic features, skin findings, or other organ system involvement. Brain overgrowth disorders are often associated with malformations of cortical development, resulting in increased risk of epilepsy, intellectual disability, and autistic features, and some are associated with hydrocephalus. The past few years have witnessed a dramatic leap in our understanding of the molecular basis of brain overgrowth, particularly the identification of mosaic (or post-zygotic) mutations in core components of key cellular pathways such as the phosphatidylinositol 3-kinase (PI3K)-vakt murine thymoma viral oncogene homolog (AKT)-mTOR pathway. These molecular insights have broadened our view of brain overgrowth disorders that now appear to span a wide spectrum of overlapping phenotypic, neuroimaging, and neuropathologic features and molecular pathogenesis. These molecular advances also bring to light the possibility of pathway-based therapies for these often medically devastating developmental disorders. © 2014 Wiley Periodicals, Inc. KEY WORDS: megalencephaly; hemimegalencephaly; polymicrogyria; somatic mosaicism; overgrowth; PI3K-AKT-mTOR pathway; Ras/MAPK pathway

How to cite this article: Mirzaa GM, Poduri A. 2014. Megalencephaly and hemimegalencephaly: Breakthroughs in molecular etiology. Am J Med Genet Part C 166C:156–172.

INTRODUCTION The etiologies of focal malformations of cortical development have long been a puzzle. Unlike most forms of lissencephaly and microcephaly where brain morphology appears affected globally in most forms, suggesting a genetic disorder affecting the entire cortex, focal malformations such as focal cortical dysplasia (FCD), hemimegalencephaly

(HME), and focal megalencephalies suggest another pattern in which only part of the developing brain appears to have experienced a genetic aberration. Brain overgrowth phenotypes range from very localized lesions to more diffuse multifocal forms. These are conditions were asymmetry is the rule, and where etiology had long been elusive. Non-genetic etiologies were previously sought to explain the pres-

ence of a severely overgrown and dysplastic portion of the brain with the neighboring cortex seemingly normal. However, one of the earliest and first recognized neurogenetic syndromes, tuberous sclerosis complex (TSC), featured just this type of patchy malformation, the cortical tuber. For over a decade, it has been known that germline loss of function mutations in TSC1 and TSC2, causing hyperactivation of

Grant sponsor: NINDS; Grant number: NS069784. The authors have no conflicts of interest to disclose. Dr. Ghayda M. Mirzaa is clinical and molecular geneticist in the Department of Human Genetics at Seattle Children's Hospital and Seattle Children's Research Institute. Her research interests focus on the clinical and molecular spectrum of developmental brain disorders and overgrowth genetic syndromes. Dr. Annapurna Poduri is a clinician-scientist at Boston Children's Hospital in the Division of Epilepsy and Clinical Electrophysiology, Department of Neurology, where she is the director of the Hospital's Epilepsy Genetics Program and a member of the Translational Research Program Investigator Service. She is a Co-Investigator of the NINDS-supported Center without Walls Epi4K. Her research interests focus on the discovery and modeling of familial and de novo causes of early onset epilepsy and brain malformations. *Correspondence to: Ghayda M. Mirzaa, M.D., Department of Pediatrics, Division of Genetic Medicine, University of Washington, Center for Integrative Brain Research, Seattle Children's Research Institute, 1900 9th Avenue, Seattle, WA 98101. E-mail: [email protected] **Correspondence to: Annapurna Poduri, M.D., M.P.H., Division of Epilepsy and Clinical Electrophysiology, Department of Neurology, Fegan 9, Boston Children's Hospital, 300 Longwood Avenue, Boston, MA 02115. E-mail: [email protected] DOI 10.1002/ajmg.c.31401 Article first published online in Wiley Online Library (wileyonlinelibrary.com): 28 May 2014

ß 2014 Wiley Periodicals, Inc.

ARTICLE

the mammalian target of rapamycin (mTOR) signaling cascade, are associated with cytomegaly and disorganized lamination within the cerebral cortex, the hallmark features of TSC. Furthermore, loss of function mutations of PTEN, an upstream phosphatase that inhibits the PI3K-AKT-mTOR pathway, are well known causes of generalized megalencephaly (MEG) in syndromic (Cowden and Bannayan– Riley–Ruvalcaba syndromes) and nonsyndromic phenotypes (MEG with autism). In this context, the identification of somatic mosaicism in HME and MEG phenotypes makes sense but has only recently come to attention. Mosaicism, as exemplified by these disorders, provides a new set of mechanisms previously under-appreciated as causes of neurological disease [Poduri et al., 2013]. Single cell sequencing of cortical tubers and the detection of somatic TSC1/2 mutations was a foreshadowing of our very rapidly improved understanding of the genetics of these disorders [Crino et al., 2010]. In this review, we will highlight genetic etiologies of brain overgrowth disorders broadly and then review the clinical spectrum and molecular pathogenesis of a recently emerging class of brain overgrowth phenotypes associated with mutations in the PI3K-AKTmTOR pathway. Identification of new genes is ongoing. However, the biologic effects of these mutations on brain and body overgrowth and genotype–phenotype correlations are still under investigation, and we anticipate that these will be areas of continued discovery in the coming years.

SYNDROMES AND GENES ASSOCIATED WITH BRAIN OVERGROWTH: A GENERAL OVERVIEW Megalencephaly (MEG) is classically defined as an oversized and overweight brain that exceeds the age-related mean by 2 or more standard deviations. Clinically, the distinction between megalencephaly (enlarged brain) and macrocephaly (enlarged head overall) relies on neuroimaging examination of the brain

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

Megalencephaly (MEG) is classically defined as an oversized and overweight brain that exceeds the age-related mean by 2 or more standard deviations. Clinically, the distinction between megalencephaly (enlarged brain) and macrocephaly (enlarged head overall) relies on neuroimaging examination of the brain and recognition of enlarged cerebral structures.

and recognition of enlarged cerebral structures. Whereas MEG is associated with specific syndromes, macrocephaly can be caused by a myriad of causes such as hydrocephalus or ventriculomegaly, enlarged extra-axial spaces, and thickened skull bones. Therefore, the distinction between MEG and macrocephaly is clinically helpful towards accurate diagnosis. MEG has long been classified based on pathogenesis into metabolic and non-metabolic (or anatomic) subtypes [DeMyer, 1972, 1986]. Cellular hypertrophy due to cellular edema or accumulation of metabolic substrates can cause MEG in a wide range of neurometabolic syndromes, such as Canavan disease, glutaric aciduria type I, lysosomal storage disorders, among others (Table I). A growing number of developmental (or non-metabolic) genetic syndromes are known to be associated with generalized or focal MEG, including HME. Table II represents a broad overview of genetic disorders where MEG is a defining/diagnostic or common feature. Brain overgrowth in these syndromes varies widely in severity, distribution and co-occurrence of other malformations of cortical development from a mild (and often relatively) enlarged brain with a normal cortex to

157

bilateral MEG with diffuse cortical dysplasia, as discussed below. Finally, reciprocal copy number changes are known to be associated with brain growth dysregulation (i.e., MEG and microcephaly) (Table III). Other important overgrowth conditions include Sotos syndrome, Weaver syndrome, Simpson Golabi Behmel syndrome, and nevoid basal cell carcinoma syndrome. While they are beyond the scope of this review, many of the same principles apply to these disorders as to the conditions that affect the brain predominantly.

MOLECULAR PATHWAYS OF MEGALENCEPHALY AND HEMIMEGALENCEPHALY Cellular growth of neuronal elements is an intricately orchestrated process, as discussed by Drs. Alcantara and O’Driscoll in this series. Dysregulation of a number of critical pathways is known to be associated with human brain overgrowth phenotypes, as highlighted in Table II. Dysregulation of two particular critical cellular pathways, the Ras/ mitogen-activated protein kinase (MAPK) pathway and the PI3K-AKTmTOR pathway, appear to account for the largest number of known MEG/ HME syndromes. Both pathways are associated with multiple diverse cellular functions including cellular proliferation, differentiation, cell cycle regulation, survival, and metabolism. Not surprisingly, these two pathways are functionally related. Given their critical developmental roles, germline mutations of genes in both pathways are believed to be embryonic lethal. When dysregulated, regardless of the specific gene or protein alteration, the ensuing syndromes exhibit numerous overlapping phenotypic features spanning many organ systems. Furthermore, both pathways have been extensively studied in the cancer field and constitute very attractive targets for pathway (small molecule) inhibitor therapy to treat various malignancies. Whereas most mutations in the RASopathies are germline, the emerging

158

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

ARTICLE

TABLE I. Genes, Clinical Features, and Metabolic Abnormalities of Neurometabolic Syndromes Associated with MEG Syndrome

Gene

Clinical features

Cerebral organic acid disorders and disorders of lysine metabolism ASPA Progressive severe ID, SZ, N-Acetylaspartic OA, spasticity, aciduria opisthotonus (Canavan disease)a Glutaric aciduria (GA) GCDH Neonatal MAC, ID, type Ia dyskinesia, choreoathetosis, dystonia

Neuroimaging findings Diffuse symmetric WM abnormalities

L-2-Hydroxyglutaric aciduria

L2HGDH

Progressive MAC (50%), ID, SZ, extrapyramidal signs

D-2-Hydroxygylatric aciduria

D2HGDH

Neonatal epileptic encephalopathy with severe ID, hypotonia, CM to mild DD/no symptoms

Frontotemporal atrophy (95%), delayed myelination, high signal intensity in the dentate nucleus, subdural effusion/hemorrhage Swollen subcortical WM, progressive loss of arcuate fibers, severe cerebellar atrophy, signal intensities in the dentate nuclei and globi pallidi, low signal intensities in the thalami Delayed and abnormal gyration, myelination and opercularization, VMEG, cysts over head of the caudate nucleus

ID, HSM, SZ, tone abnormalities, DYSM, HSM, macular cherry red spot

Diffuse hypomyelination, mild T2 hyperintensities of the caudate nucleus and putamen

Hypotonia, motor weakness, SZ, hyperacusis, macular cherry red spot, blindness, spasticity, MAC by 18 months of age Organomegaly and bony abnormalities less common

Similar to GM1

PN, opisthotonus, SZ, hyperpyrexia, blindness, loss of bulbar functions, hypotonia

Diffuse WM abnormalities, diffuse cerebral atrophy, calcifications (thalamus, BG, periventricular WM)

Lysosomal storage diseases a Disorders of Sphingolipid Metabolism GLB1 Generalized gangliosidosis GM1 (early infantile)a

GM2 gangliosidosis Tay-Sachs disease (infantile)a

HEXA

Sandhoff diseasea

HEXB

Krabbe disease (globoid cell leukodystrophy) (early infantile)a

GALC

Similar to GM1

Metabolic abnormalities #Aspartoacylase (ASPA), "N-acetyl-aspartic acid (NAA) #Glutaryl-CoA dehydrogenase, "Glutaryl-CoA, "Acylcarnitines:free carnitine, "Urinary dicarboxylic acids #L-2-hydroxyglutarate dehydrogenase, "L-2hydroxyglutaric acid (CSF> plasma), "hydroxydicarboxylic acids (CSF), "Lysine (CSF, blood)

#D-2-hydroxyglutaric acid dehydrogenase, "D-2hydroxyglutaric acid

#b-galactosidase, "GM1 ganglioside, asialo-GA1 (neurons), "oligosaccharide, minor glycolipids, glycopeptides (visceral organs) #Hexosaminidase A, "GM2ganglioside (neurons)

#Hexosaminidase A and B, "GM2-ganglioside, asialoGM2 (neurons), "Globosides, oligosaccharides (viscera) #Galactosylceramidase, "Galactosylceramide (globoid cells), "Galactosylphingosine (oligodendrocytes, Schwann cells)

ARTICLE

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

159

TABLE I. (Continued) Syndrome

Gene

Mucopolysaccharidoses (MPS) Hurler syndrome IDUA (type IH)

Clinical features HSM, CNS, DM, DYS, OPH, CAR

Neuroimaging findings WM abnormalities, cerebral atrophy, cervical myelopathy WM abnormalities, cerebral atrophy, cervical myelopathy WM abnormalities, cerebral atrophy, cervical myelopathy

Hunter syndrome (type II)

IDS

HSM, CNS, DM, DYS, OPH, CAR, SK

Sanfilippo syndrome (type III)

SGSH(IIIA), NAGLU (IIIB), HGSNAT(IIIC), GNS (IIID)

CNS, DM (þ/), DYS (þ/)

Morquio syndrome (type IV]

GALNS(IVA), GLB1(IVB)

DM, CAR, OPH (þ/)

WM abnormalities, cerebral atrophy, cervical myelopathy

ARSB

HSM, DM, DYS, OPH, CAR

WM abnormalities, cerebral atrophy, cervical myelopathy

GNPTAB

HSM, CNS, DM, DYS, OPH, CAR

Mucolipidosis type III

GNPTAB (a/b), GNPTG (g)

HSM (þ/), CNS (þ/), DM, DYS (þ/), CAR

Mannosidosis

MAN2B1 (a), MANBA (b)

HSM, DM, DYS, CAR, CNS (þ/)

Cerebral atrophy, WM abnormalities (occasionally) Cerebral atrophy, WM abnormalities (occasionally) Partially empty sella turcica, cerebellar atrophy, WM abnormalities (a)

GFAP

ID, SZ, paraparesis, feeding problems

MLC1, HEPACAM

Progressive spasticity, ataxia

Maroteaux-Lamy syndrome (type VI) Mucolipidosesb Mucolipidosis type II (I-cell disease)

Leukoencephalopathiesa a Alexander disease (infantile and juvenile forms)

Megalencephalic leukoencephalopathy with subcortical cysts

WM abnormalities (frontally-predominant), calcification of the BG, cerebellar changes, HYD Extensive symmetric, WM changes with subcortical cyst

Metabolic abnormalities #Iduronidase, "Heparan sulfate, "Dermatan sulfate #Iduronate-2-sulfatase, "Heparin sulfate, "Dermatan sulfate #Heparan N- sulfatase (IIIA), #N-acetyl-glucosaminidase (IIIB), #Acetyl CoA glucosamine N-acetyl transferase (IIIC), #N-acetyl-glucosamine-6sulfatase (IIID), "Heparan sulfate #N-acetylgalactosamine-6sulfatase (IVA), #b-galactosidase (IVB), "Keratan sulfate #N-acetyl-galactosamine-4sulfatase, "Dermatan sulfate

#Transferasec

#Transferasec

#a-mannosidase (a), "a-mannosides (a), #b-mannosidase (b), "b-mannosides (b) —

—

BG, basal ganglia; CAR, cardiovascular involvement; CC, corpus callosum; CM, cardiomyopathy; CNS, central nervous system regression; ID, developmental delay; DM, dysostosis multiplex; DYS, dysmorphic features; HL, hearing loss; HSM, hepatosplenomegaly; MAC, macrocephaly; OA, optic atrophy; OPH, ocular anomalies [corneal clouding, ophthalmoplegia]; PN, peripheral neuropathy; SK, dermatological findings; SZ, seizures; VMEG, ventriculomegaly; WM, white matter. Inheritance: Most of these disorders are AR in inheritance with the exception of Hunter syndrome (XL), Alexander disease (AD), and megalencephalic leukoencephalopathy with subcortical cysts due to HEPACAM mutations. a Other leukoencephalopathies associated with megalencephaly (as indicated in the table): Canavan disease, glutaric aciduria type I, infantile generalized, or GM1, gangliosidosis, infantile GM2 gangliosidosis (Tay-Sachs; Sandhoff diseases), infantile Krabbe disease. b May not be true MAC. c Lysosomal UDP-N-acetylglucosamine-I-phosphotransferase. Reference: Mirzaa et al. [2012].

Protein function

Kinase

Kinase

PIK3CA

PIK3R2

PI3K-AKT-MTOR PTEN Phosphatase, tumor suppressor

Gene

De novo/dominant

Post zygotic/mosaic (rare germ line)

De novo/dominant

Inheritance

MPPH syndrome

Klippel–Trenaunay syndrome (KTS)

HME Somatic overgrowth: CLOVES, Fibroadipose hypoplasia, Isolated macrodactyly

MCAP syndrome

HME

MEG, capillary malformations, digit anomalies (polydactyly, syndactyly), segmental somatic overgrowth, connective tissue/ skin laxity As above Variable segmental somatic overgrowth, digital anomalies, spinal anomalies, cutaneous vascular malformations; also isolated macrodactyly in some cases Cutaneous VM (capillary, venous, lymphatic), varicose veins, unilateral hypertrophy of bones and soft tissues Postaxial polydactyly, MEG

Overgrowth, hamartomatous intestinal polyposis, lipomas, penile pigmented macules, malignancy risk similar to CS Macrocephaly (asymmetric; maybe seen in HME)

Bannayan–Riley– Ruvalcaba syndrome

Cowden syndrome

Mild DYSM (frontal bossing, midface hypoplasia, biparietal narrowing) Mucocutaneous lesions, malignancy risk (breast, thyroid, endometrium)

Clinical features

Megalencephalyautism syndrome

Syndrome

MEG, perisylvian PMG, HYD, mega CC

HYD, calcifications, HME reported in some individuals

ID/SZ (rare)

ID, epilepsy, tone abnormalities

As above HME and Chiari malformation reported in some individuals

As above Some have ID

HME: VMEG, MCD, WM abnormalities (ipsilateral) HYD, VMEG, CBTE, PMG, thick CC

Cerebellar dysplastic gangliocytoma (Lhermitte-Duclos disease) —

ID (10%)

Autistic features, ID (70%), SZ (25%), proximal myopathy (60%) ID (severe), SZ (intractable), hemiparesis ID, SZ, hypotonia (variable)

MEG

MRI findings

ASD, ID

Neurologic findings

TABLE II. Genes, Syndromes, and Pathways Associated with Megalencephaly and Hemimegalencephaly

160 AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) ARTICLE

Kinase

STE20-related kinase adaptor (“pseudokinase”)

Tumor suppressor

GTPase-RHEB

Kinase

AKT3

STRADA/ LYK5

TSC1, TSC2

TBC1D7

MTOR Post-zygotic/ mosaic De novo/dominant

Recessive

De novo/dominant (somatic mosaicism described)

Recessive

De novo/dominant

Post-zygotic/ mosaic

Inheritance

ID, macrocephaly, patellar dislocation, celiac disease HME

MPPH HME Polyhydramnios, MEG, symptomatic epilepsy (PMSE) syndrome Tuberous sclerosis complex (TSC) (associated with HME, FCD)

Proteus syndrome (associated with HME)

Syndrome

As above

Skin (hypomelanotic macules, facial angiofibromas, shagreen patches, fibrous facial plaques, ungal fibromas), angiomyolipomas, rhabdomyomas Osteoarticular problems, celiac disease, myopia, astigmatism

Asymmetric and disproportionate hamartomatous overgrowth of multiple tissues, connective tissue and epidermal nevi, dysregulated adipose tissue, VM, hyperostosis As above As above DYSM, strabismus, skeletal muscle hypoplasia, nephrocalcinosis

Clinical features

Sprouty-related

GTPase

SPRED1

HRAS De novo/dominant

De novo/dominant

Costello syndrome

Legius syndrome

CALs, freckling, lipomas, macrocephaly, no tumor manifestations FTT, short stature, coarse facial features, fine, curly or sparse hair, papillomata, HCM, CHD, malignancy risk (15%)

Cyclin; cell cycle MPPH As above control Ras/mitogen-activated protein kinase (MAPK) pathway “the RASopathies” (associated with absolute or relative macrocephaly) NF1 RasGAP De novo/dominant Neurofibromatosis 1 CALs, axillary freckling, cutaneous neurofibromas, short stature

CCND2

Kinase

Protein function

AKT1

Gene

TABLE II. (Continued)

ID (100%), hypotonia (most), SZ (20-50%)

Optic glioma (15%), UBOs, CC abnormalities, HYD LD (50-75%), (severe ID 3%), ADHD, headaches (20%), SZ (10%) ID/LD, ADHD, headaches, SZ

CBTE, VMEG/HYD

—

As above

As above

Cerebral calcifications

Calcifications, abnormalities of the CC, HYD, HME reported in some individuals As above As above VEMG (mild), subependymal dysplasia, WM abnormalities SEN, cortical tubers, SEGAs, WM abnormalities, HME/FCD

MRI findings

As above

As above

ID (mild), behavioral abnormalities, LD

SZ (80%), ID (50%), ASD/PDD (40– 50%), ADHD

As above As above ID, hypotonia, SZ, ASD

ID (20%), SZ (13%)

Neurologic findings

ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) 161

Kinase

Kinase GTPase

Phosphatase GTPase Kinase RasGEF GTPase Scaffolding

MAP2K1

MAP2K2 KRAS

PTPN11 NRAS RAF1 SOS1 RIT1 SHOC2

Histone methyltransferase

Mediator complex

EZH2

MED12

Transcriptional regulation NSD1 Histone methyltransferase

Kinase

Protein function

BRAF

Gene

novo/dominant novo/dominant novo/dominant novo/dominant novo/dominant novo/dominant

X-linked

De novo/dominant

De novo/dominant

De De De De De De

De novo/dominant De novo/dominant

De novo/dominant

De novo/dominant

Inheritance

Opitz–Kaveggia (FG) syndrome Lujan (Lujan–Fryns) syndrome

Weaver syndrome

Sotos syndrome

CFC syndrome Noonan syndrome CFC syndrome CFC syndrome Noonan syndrome Noonan syndrome Noonan syndrome Noonan syndrome Noonan syndrome Noonan syndrome Noonan syndrome with loose anagen hair

Noonan syndrome (NS)

Cardiofaciocutaneous (CFC) syndrome

Syndrome

Characteristic facies (prominent hypertelorism, micrognathia, deep horizontal skin crease), camptodactyly Imperforate anus, characteristic facial features, broad thumbs Marfanoid habitus, maxillary hypoplasia, palate and dental problems, long hands,

Prenatal and postnatal overgrowth, characteristic facial gestalt, advanced bone age

Cardiac abnormalities (VHD, HCM, dysrhythmias), DYSM, multiple cutaneous abnormalities Short stature, CHD (PVS, HCM), characteristic facies, webbed neck, coagulation defects, lymphatic dysplasias As above As above As above As above As above As above As above As above As above As above Skin and hair abnormalities (sparse, thin, slow growing hair with pigment abnormalities), features of NS

Clinical features

TABLE II. (Continued)

ID (97%), hypotonia (90%), SZ (70%) ID (mild-mod), behavioral abnormalities

Hypotonia, ID/ behavioral problems (very common), SZ (25%) ID (81%)

Abnormalities of CC, VMEG, HET Dysgenesis of the CC

Pachygyria, VMEG, cysts of the septum pellucidum (rare)

Prominent trigone, VMEG/HYD, XAX, CC abnormalities, CSP

As above As in the above As above As above As above As above As above As above As above As above As above

VMEG, CBTE

ID (variable), language delay

As above As in the above As above As above As above As above As above As above As above As above As above

HYD/VMEG, cortical atrophy, ACC, NMD

MRI findings

ID (80%), SZ (50%), hypotonia

Neurologic findings

162 AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) ARTICLE

Glypican; cell surface heparan sulfate proteoglycans

Protein function X-linked

Inheritance

Exoribonuclease

DIS3L2

Zinc finger

Other less common genes and disorders RIN2 Ras effector protein

GLI3

Sonic Hedgehog (Shh) signaling PTCH1 Patched; receptor for sonic hedgehog

Centrioleassociated

OFD1

Recessive

Recessive De novo/dominant

De novo/dominant

Recessive

X-linked

Mitotic regulation, centrosome and microtubule assembly KIF7 Kinesin De novo/dominant

Glypicans GPC3

Gene

MACS syndrome (macrocephaly, alopecia, cutis laxa, and scoliosis)

Acrocallosal syndrome Greig cephalosyndactyly

Nevoid basal cell carcinoma (Gorlin) syndrome

Simpson-GolabiBehmel syndrome (type 2) Perlman syndrome

Acrocallosal syndrome

Simpson-GolabiBehmel syndrome

Syndrome

Coarse facial features, gingival hyperplasia, severe joint hypermobility, soft redundant skin, sparse hair, short stature, severe scoliosis

Jaw keratocysts, basal cell carcinomas (BCCs), coarse facial features, facial milia, skeletal anomalies (bifid ribs, wedgeshaped vertebrae) As above Polydactyly (pre-, post-axial, mixed), ocular hypertelorism, craniosynostosis

Fetal gigantism, renal hamartomas, nephroblastomatosis, risk for Wilms tumor

Ciliary dyskinesia, respiratory problems, DYSM, short fingers

Polysyndactyly, hypertelorism

hyperextensible digits Prenatal overgrowth, characteristic facies (macroglossia, macrostomia, central groove of lower lip, ocular hypertelorism), supernumerary nipples

Clinical features

TABLE II. (Continued)

—

As above ID, SZ (90%), medulloblastoma (PNET) (5%)

Abnormalities of the CC, HET, WM abnormalities, cerebral atrophy

VMEG

ACC

HYD, CBTE, ACC (all rare)

MRI findings

ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) 163

Rab GTPase; cellular endocytosis

Protein function X-linked

Inheritance XLID, autism, epilepsy and macrocephaly

Syndrome Macrocephaly

Clinical features ID/MR, ASD, SZ

Neurologic findings —

MRI findings

Notes 1. This table includes only gene-known MEG/HME disorders and does not include others where underlying genetic etiology is unknown as of writing this manuscript (such as Macrocephaly, megalocornea, motor and mental retardation (MMMM) syndrome, Macrosomia, obesity, macrocephaly, and ocular abnormalities (MOMO), for example). This list also does not include skeletal dysplasias known to be associated with MEG (such as achondroplasia and thanatophoric dysplasia). 2. HME has also been reported with other somatic manifestations such as hypomelanosis of Ito and linear nevus sebaceous syndrome. Reference: Mirzaa et al. [2012]. ACC, agenesis of the corpus callosum; ADHD, attention-deficit-hyperactivity disorder; ASD, autism spectrum disorder; CAL, café au lait macules; CBTE, cerebellar tonsillar ectopia; CC, corpus callosum; CHD, congenital heart disease; CLOVES, congenital lipomatous asymmetric overgrowth of the trunk, lymphatic, capillary, venous, and combined-type vascular malformations, epidermal nevi, skeletal and spinal anomalies; CSP, cavum septum pellucidum; DYSM, dysmorphic features; FCD, focal cortical dysplasia; FTT, failure to thrive; HCM, hypertrophic cardiomyopathy; HET, heterotopias; HME, hemimegalencephaly; HYD, hydrocephalus; ID, intellectual disability; LD, learning disability; MEG, megalencephaly; MPPH, megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome; NMD, neuronal migration disorder; PDD, pervasive developmental disorder; PMG, polymicrogyria; PNET, primitive neuroectodermal tumor; PVS, pulmonary valve stenosis; SZ, seizures; UBO, unidentified bright object; VHD, valvular heart disease; VM, vascular malformation; VMEG, ventriculomegaly; WM, white matter; XAX, enlarged extra-axial space; XLID, X-linked intellectual disability.

RAB39

Gene

TABLE II. (Continued)

164 AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) ARTICLE

spectrum of PI3K-AKT-mTOR pathway phenotypes (particularly in upstream components such as PIK3CA) includes predominantly post-zygotic mutations present in a mosaic pattern. The related MEG syndromes involve almost every organ system in the body including the brain (intellectual disability, autism, epilepsy, hydrocephalus, Chiari malformation), heart and vascular system (conduction defects, heart-great vessel anomalies), skin (capillary malformations, epidermal nevi), connective tissue (skin laxity, joint hypermobility), skeleton (polydactyly, syndactyly), and others. Mutations within the PI3K-AKTmTOR pathway are associated with the most severe brain overgrowth phenotypes including marked brain overgrowth (occipito-frontal circumference, OFC, more than 4 standard deviations above the mean) and HME, a serious medical condition typically associated with severe early onset intractable epilepsy and poor developmental outcome. While some syndromes in both pathways are considered cancer syndromes, as identified mutations are “activating” causing enhanced pathway activation, some of the novel germline and mosaic mutations are not as robustly activating as those associated with oncogenesis and require further study. We will specifically focus on an area of rapidly increasing knowledge related to PI3K-AKTmTOR-related brain overgrowth phenotypes, their clinical and neuroimaging features, and molecular pathogenesis.

PI3K-AKT RELEATED MEG AND HME: THE CLINICAL AND NEUROIMAGING SPECTRUM

In addition to PTEN-related disorders, upstream mutations in core components of the PI3K-AKT-mTOR pathway have now been identified in classic HME and a variety of MEG syndromes including the MEG-capillary malformation syndrome (MCAP) and the MEG-polymicrogyria-polydactyly-hydrocephalus syndrome (MPPH). Collectively, these disorders not only overlap molecularly but also share clinical, neuroimaging, and neuropathologic features.

ARTICLE

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

165

TABLE III. Reciprocal Copy Number Changes Associated with Brain Growth Dysregulation (Megalencephaly and Microcephaly) Locus

CNV

Gene

Size

Syndrome/pathway

Refs.

1q21.1

Duplication Deletion Duplication

— — AKT3

MAC MIC MEG

— — PI3K-AKT; MEG and HME

Deletion Duplication Deletion Duplication Deletion Duplication Deletion Duplication Deletion

— MYCN — — NSD1 — — — —

MIC MEG MIC MIC MEG MIC MAC MIC MAC

PI3K-AKT; postnatal MIC Regulates growth an apoptosis Feingold syndrome — Sotos syndrome, transcriptional regulator — — — —

Brunetti-Pierri et al. [2008] Brunetti-Pierri et al. [2008] Poduri et al. [2012], Wang et al. [2013], Chung et al. [2014] Ballif et al. [2012] Malan et al. [2010] Van Bokhoven et al. [2005] Rosenfeld et al. [2013] Tatton-Brown et al. [2005] Aalfs et al. [1995] Van Bon et al. [2011] Shinawi et al. [2009] Shinawi et al. [2009]

1q43.44

2q24.3a 5q35.5 10q22–23 16p11.2

HME, hemimegalencephaly; MAC, macrocephaly; MEG, megalencephaly; MIC, microcephaly. a This disorder is associated with triphalangeal thumb, and dysmorphic facial features similar to Weaver syndrome.

PTEN-Related Disorders Loss of function mutations of PTEN have been identified in a wide range of MEG phenotypes including two well-known cancer predisposition syndromes: Cowden and Bannayan–Riley– Ruvalcaba syndrome (BRRS) syndromes [Liaw et al., 1997; Marsh et al., 1997]. These disorders constitute a clinical spectrum associated with prenatal onset MEG, hamartomas, lipomas, intestinal polyps, and various types of cutaneous vascular malformations [Gorlin et al., 1992; Tan et al., 2011]. Neurologically, affected individuals have hypotonia, delayed gross motor skills and, in some, proximal myopathy [Marsh et al., 1999]. Prenatal onset

Pten loss of function mutant mice develop macrocephaly and behavioral abnormalities such as reduced social activity, increased anxiety and sporadic seizures, closely resembling the human phenotype of PTEN-related disorders.

progressive MEG is typical and OFCs are usually 4–5 (and up to 8) standard deviations above the mean, whereas body overgrowth is typically mild (þ1–3 SD). PTEN mutation carriers are at increased risk for various tumors, most notably of the breast, thyroid, and endometrium. Finally, PTEN mutations constitute the largest single gene defects in autistic children with MEG, with estimates of mutations between 1% and 17% in this cohort [Butler et al., 2005; Buxbaum et al., 2007]. While OFCs in this cohort vary, one of the earliest reports showed OFCs of þ7–8 SD above the mean [Butler et al., 2005]. An average OFC of þ4.35 SD was recently reported in 6 PTEN mutation-positive individuals with autism [Hobert et al., 2014]. Pten loss of function mutant mice develop macrocephaly and behavioral abnormalities such as reduced social activity, increased anxiety and sporadic seizures, closely resembling the human phenotype of PTEN-related disorders [Kwon et al., 2001; Ogawa et al., 2007]. While most children with PTEN mutations have uniform bilateral MEG with grossly normal cortical cytoarchitecture (Fig. 1A–C), there are several published cases of asymmetric or focal

brain phenotypes in individuals with germline PTEN mutations. These include HME and linear epidermal nevi in a child whose family has features of BRRS, and FCD with focal intractable epilepsy in a child with features of Cowden syndrome [Merks et al., 2003; Elia et al., 2012] (Fig. 1D–F). PIK3CA-Related Disorders Post-zygotic gain-of-function mutations in PI3KCA have been recently identified in a growing number of segmental brain and body overgrowth disorders (Table IV). These phenotypes include somatic overgrowth disorders such as CLOVES syndrome (congenital lipomatous asymmetric overgrowth of the trunk, lymphatic, capillary, venous, and combined-type vascular malformations, epidermal nevi, skeletal and spinal anomalies), fibroadipose hyperplasia, isolated macrodactyly and Klippel–Trenaunay syndrome, and predominantly brain overgrowth phenotypes such as HME and MCAP syndrome [Kurek et al., 2012; Lindhurst et al., 2012; Poduri et al., 2012; Rivière et al., 2012; Lee et al., 2012; Rios et al., 2013]. The severity and extent of brain and body overgrowth in these phenotypes is

166

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

variable. This is particularly true with brain involvement that ranges from bilateral generalized MEG with PMG (as with classic MCAP), to HME to dysplastic MEG (Fig. 1G–L). The tissue distribution and types of post-zygotic mutations are expected to be among the primary determinants of the developmental phenotypes that are often severe, as we discuss below. The classic neuroimaging features of MCAP are marked diffuse MEG with

The classic neuroimaging features of MCAP are marked diffuse MEG with bilateral perisylvian polymicrogyria (PMG), although this latter feature may not be present in a large number of children. Some individuals also have marked cerebellar enlargement, with a large and crowded posterior fossa and ensuing cerebellar tonsillar ectopia that may necessitate surgical decompression. bilateral perisylvian polymicrogyria (PMG), although this latter feature may not be present in a large number of children. Some individuals also have marked cerebellar enlargement, with a large and crowded posterior fossa and ensuing cerebellar tonsillar ectopia that may necessitate surgical decompression [Conway et al., 2007a,b; Mirzaa et al., 2012a]. Somatic manifestations of MCAP include cutaneous vascular malformations (typically capillary malformations), polydactyly, syndactyly, and focal somatic overgrowth—overlapping with but typically milder than other somatic PIK3CA-related disorders [ClaytonSmith et al., 1997; Moore et al., 1997; Conway et al., 2007b; Mirzaa et al., 2012a].

The MegalencephalyPolymicrogyria- PolydactylyHydrocephalus (MPPH) Syndrome This rare developmental syndrome is characterized predominantly by prenatal onset MEG and bilateral perisylvian PMG. Hydrocephalus and postaxial polydactyly are more variable features seen in nearly half of reported individuals [Kariminejad et al., 2012; Mirzaa et al., 2013]. A subset of children also has a distinctly thick corpus callosum (megacorpus callosum). De novo germline mutations in three core PI3K-AKTmTOR pathway genes are now known to be associated with MPPH including, in order of frequency, PIK3R2, CCND2, and AKT3 [Rivière et al., 2012; Nakamura et al., 2014; Mirzaa et al., 2014] (Fig. 2A–C, G–L). These mutations are mostly germline mutations with a narrow mutational spectrum. For example, a single recurrent PIK3R2 (p.Gly373Arg) mutation has been reported in most MPPH children. Postaxial polydactyly is a more frequent feature in PIK3R2 versus CCND2-positive children.

Hemimegalencephaly HME is a severe brain malformation characterized by overgrowth of all or part of a cerebral hemisphere, often with ipsilateral severe cortical dysplasia or dysgenesis, white matter hypertrophy and a dilated and dysmorphic lateral ventricle. It is often an isolated congenital abnormality, but there are sporadic associations with neurocutaneous and overgrowth syndromes in the literature including with Proteus syndrome, Klippel–Trenaunay syndrome, linear nevus sebaceous (LNS) syndrome, TSC, neurofibromatosis type 1, and hypomelanosis of Ito [Cristaldi et al., 1995; Sharma et al., 2009; Pavlidis et al., 2012]. HME constitutes the most severe brain overgrowth phenotype not only morphologically but also because most children with HME experience early onset intractable epilepsy, typically within the first few months of life. Children with HME can present with focal seizures or epilepsy syndromes such as

ARTICLE

infantile spasms. Developmental delay is often early and severe. Within the affected hemisphere, neuroimaging reveals regions of apparent PMG, pachygyria, subcortical, and periventricular gray matter heterotopia. However, various morphological abnormalities outside the involved cerebral hemisphere have been reported such as ipsilateral cerebral vascular dilatation, ipsilateral and bilateral cerebellar enlargement with dysplastic folia, and ipsilateral olfactory nerve enlargement [Sato et al., 2007]. Moreover, contralateral volume loss (or hemimicrencephaly) with white matter abnormalities have been reported [Shiroishi et al., 2010]. Although it is not determined whether these abnormalities are developmental or acquired, these and other more widespread or asymmetric malformations are believed to partially account for poor seizure control and poor post-hemispherectomy outcome in some individuals. Activating mosaic mutations in three PI3K-AKT-mTOR pathway genes have now been reported in isolated HME including PIK3CA (four patients), AKT3 (two patients), and MTOR (one patient) [Lee et al., 2012; Poduri et al., 2012]. Further, duplications of 1q encompassing AKT3 have been identified in two HME patients with presumed activation of the gene [Poduri et al., 2012]. Duplications of AKT3 have also been reported in children with macrocephaly, focal PMG, and intellectual disability [Wang et al., 2013; Chung et al., 2014]. Interestingly, other less common patterns of focal MEG with cortical dysplasia have been described in the literature such as total or diffuse HME, localized MEG (hemi-hemimegalencephaly), and multilobar cortical dysplasia that share similar neuropathological findings to HME including large neurons, cortical dyslamination, with or without dysmorphic and ectopic neurons, heterotopia, balloon cells, and abnormal white matter [Barkovich and Chuang, 1991; Nakahashi et al., 2009; Blümcke and Mühlebner, 2011]. It is therefore expected that these more focal manifestations may share the same molecular pathogenesis.

Figure 1. The PI3K-AKT-mTOR associated neuroimaging spectrum (part I). A–C: T1-weighted mid-sagittal, T2-weighted axial, and T1-weighted coronal images of a child with PTEN-related hamartoma tumor syndrome due to a germline PTEN mutation (p. Gln17X). Note megalencephaly, large cerebellum, crowded posterior fossa and mild cerebellar tonsillar ectopia. This patient had a choroid plexus carcinoma and underwent placement of a left frontal approach ventricular catheter. D–F: Autopsy (D) and CT scan images (E,F) of a child with a maternally inherited PTEN mutation (IVS5 þ 1delG) showing segmental dysplastic megalencephaly with a markedly enlarged and cerebral hemisphere, periventricular cysts, and focal cortical dysplasia (adapted with permission from Merks et al., J Med Genet, 2003). This child also had facial linear epidermal naevi ipsilateral to the severely affected cerebral hemisphere. G–I: T1-weighted mid-sagittal, and T2-weighted axial and coronal MRI images of a child with the megalencephaly-capillary malformation syndrome (MCAP) due to a mosaic PIK3CA mutation (p.Glu726Lys). Note generalized megalencephaly, enlarged cerebellum, crowded posterior fossa, moderate cerebellar tonsillar ectopia, bilateral perisylvian polymicrogyria (arrowheads, H,I), and moderate to severe ventriculomegaly with stretching of the corpus callosum. J–L: T1-weighted, and T2-weighted axial and coronal images of a child with a mosaic PIK3CA mutation (p.Glu545Lys) with bilateral asymmetric megalencephaly, severe bilateral cortical dysplasia, dysplastic, and enlarged ventricles. This child also had severe segmental somatic overgrowth (also published in Riviere et al., Nature Genetics, 2013).

168

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

ARTICLE

TABLE IV. Summary of the PI3K-AKT Associated Developmental Brain Phenotypes Gene

Type of mutation

Inheritance

PTEN AKT3

Loss of function Gain of function

PIK3CA

Gain of function

De novo/dominant Post-zygotic/mosaic, De novo/dominant Post-zygotic/mosaic

PIK3R2 CCND2 AKT1

Gain of function Gain of function Gain of function

De novo/dominant De novo/dominant Post-zygotic/mosaic

CNS phenotype

Non-CNS phenotype

MEG-autism, HME, FCD HME, MPPH

Cowden, BRRS —

Megalencephaly (MCAP), HME MPPH MPPH HMEa

Somatic overgrowth (CLOVES/ FH, macrodactyly, MCAP) Polydactyly Polydactyly Proteus syndrome

BRRS, Bannayan–Riley–Ruvalcaba syndrome; FH, fibro-adipose hyperplasia; HME, hemimegalencephaly; MCAP, megalencephalycapillary malformation syndrome. a HME reported in Proteus syndrome, but no AKT1 mutations have been identified in affected brain tissues to our knowledge.

PI3K-AKT-MTOR RELEATED MEG AND HME: MOLECULAR SPECTRUM AND INSIGHTS INTO MOLECULAR PATHOGENESIS Mutations of the above mentioned PI3K-AKT-mTOR pathway genes, whether loss of function mutations of

Mutations of the above mentioned PI3K-AKTmTOR pathway genes, whether loss of function mutations of PTEN or gain of function mutations of PIK3CA, AKT3, and PIK3R2, all share a common functional endpoint, namely activation of the pathway. PTEN or gain of function mutations of PIK3CA, AKT3, and PIK3R2, all share a common functional endpoint, namely activation of the pathway (Table IV). Mutations of PTEN, PIK3R2, and AKT3 have been predominantly germline. Whereas mutations of PIK3CA and the Proteus syndrome gene, AKT1, have been mosaic, providing a molecular explanation for the wide phenotypic

variability in their attendant overgrowth phenotypes [Lindhurst et al., 2011]. The phenotypic spectrum of PIK3CA-related disorders is particularly wide and, while the exact mechanisms by which mutations result in these manifestations are currently under study, some preliminary genotype–phenotype correlations can be suggested. For example, the same PIK3CA mutation (p.Glu545Lys) has been identified in the four children with HME so far [Lee et al., 2012]. The mutational spectrum of MCAP syndrome, on the other hand, is wide and has not included any of the so-called mutation “hotspots” seen in cancer (p.Glu542Lys, p.Glu545Lys, p.His1047Arg, p.His1047Leu) [Samuels and Ericson, 2006; Samuels and Waldman, 2010; Rivière et al., 2012; Mirzaa et al., 2013]. While these mutations are all activating, they may not as robustly activating as those seen in cancer, given that the cancer risk in these phenotypes does not appear to be much increased; although natural history, and particularly cancer risk, data are lacking. For more than a decade now, hyperactivation of the mTOR signaling downstream of PI3K-AKT (due to loss of function mutations of TSC1 and TSC2, for example) have been considered to provide a pathological link between TSC, HME, and FCD by extensive studies [Crino, 2007; Lim and Crino, 2013]. Further, mTOR inhibition reversed neuronal hypertrophy in Pten-deficient mice and amelio-

rated a subset of Pten-associated abnormal behaviors, thereby substantiating evidence that the mTOR pathway downstream of PTEN is critical for its complex phenotype [Kwon et al., 2003; Zhou et al., 2009]. However, the recent identification of mutations in CCND2 in MPPH syndrome sheds a novel insight into the molecular pathology of these phenotypes [Mirzaa et al., 2014]. CCND2 is a member of the D-type

Recent data demonstrate accumulation of degradation resistant CCND2 in individuals with MPPH and also, interestingly, in lymphoblastoid cell lines of individuals with upstream mutations in PIK3CA, PIK3R2, and AKT3. cyclin family critically required for G1/S transition during the cell cycle [Matsushime et al., 1991; Inaba et al., 1992; Ross et al., 1996; Glickstein et al., 2006, 2009]. Identified mutations within CCND2 affect highly conserved terminal residues that include targets for glycogen synthase kinase 3b (GSK3b)-phosphorylation and, ultimately, its’ ubuiquitin mediated degradation [Kida et al., 2007]. Recent data

ARTICLE

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

169

Figure 2. The PI3K-AKT-mTOR associated neuroimaging spectrum (part II). A–C: T1-weighted mid-sagittal, and T2-weighted axial and coronal images of a child with MPPH syndrome due to a de novo germline PIK3R2 mutation (p.Gly373Arg) showing generalized megalencephaly and bilateral perisylvian polymicrogyria. D–F: T1-weighted and T2-weighted axial and coronal images of a child with MPPH syndrome due to a germline AKT3 mutation (p.Arg465Trp) showing bilateral but asymmetric megalencephaly, bilateral perisylvian PMG, and mild enlargement of the lateral ventricles. G–I, T1-weighted mid-sagittal and T2-weighted axial and coronal images of a child with isolated hemimegalencephaly due to a mosaic AKT3 (p.Glu17Lys) mutation. J–L: T2-weighted mid-sagittal and axial and T1-weighted coronal images of a child with MPPH syndrome due to a de novo germline mutation in CCND2 (p.Thr289Ala) showing generalized megalencephaly, bilateral extensive perisylvian polymicrogyria, marked ventriculomegaly status post shunt placement.

170

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

ARTICLE

syndromes is treated with anti-epileptic drugs that do not specifically address the molecular defects that we now know to be the basis of their disease. The use of numerous small molecule PI3K-AKTmTOR pathway inhibitors to alleviate some of these developmental defects is currently under study. While specific therapy is already available for the downstream TSC-mTOR pathway by everolimus [Kingwell, 2013; Krueger et al., 2013], it is clear that the brain and somatic overgrowth phenotypes associated with upstream PI3K-AKT-mTOR pathway mutations result from increased activation of multiple pathways downstream of AKT, leading us to predict that successful treatment strategies will need to downregulate more than one of these pathways. Figure 3. Schematic of PI3K-AKT-mTOR associated Meg Phenotypes. A simplified schematic diagram showing key PI3K-AKT-mTOR genes (PIK3CA, PIK3R2, PTEN, AKT3, and CCND2) with their associated phenotypes.

demonstrate accumulation of degradation resistant CCND2 in individuals with MPPH and also, interestingly, in lymphoblastoid cell lines of individuals with upstream mutations in PIK3CA, PIK3R2, and AKT3 [Mirzaa et al., 2014]. These data implicate for the first time the involvement of another critical effector pathway downstream of PI3K-AKT: the cell cycle pathway downstream of GSK3b (Fig. 3). Clearly, further investigation is necessary to determine the detailed molecular mechanisms of brain overgrowth in this pathway and how these various critical downstream pathways interact in these phenotypes.

THERAPIES AND FUTURE DIRECTIONS The past few years have witnessed exciting advances in our understanding of the molecular pathogenesis of brain overgrowth. It follows that individuals

Some of the key functional deficits of these disorders, such as intellectual disability,

autism, epilepsy, hydrocephalus, are naturally attractive targets for treatment that today are still treated empirically. For example, the epilepsy associated with some MEG syndromes is treated with anti-epileptic drugs that do not specifically address the molecular defects that we now know to be the basis of their disease. with disorders of the PI3K-AKTmTOR and the interacting Ras/ MAPK pathways may have the option in the future of pathway-based rational treatment. Some of the key functional deficits of these disorders, such as intellectual disability, autism, epilepsy, hydrocephalus, are naturally attractive targets for treatment that today are still treated empirically. For example, the epilepsy associated with some MEG

ACKNOWLEDGMENTS The authors thank our patients and their families for their valuable and ongoing contributions and support of our research. A.P. was supported by the NINDS (NS069784).

REFERENCES Aalfs CM, Hoovers JM, Nieste-Otter MA, Mannens MM, Hennekam RC, Leschot NJ. 1995. Further delineation of the partial proximal trisomy 10q syndrome. J Med Genet 32:968–971. Ballif BC, Rosenfeld JA, Traylor R, Theisen A, Bader PI, Ladda RL, Sell SL, Steinraths M, Surti U, McGuire M, Williams S, Farrell SA, Filiano J, Schnur RE, Coffey LB, Tervo RC, Stroud T, Marble M, Netzloff M, Hanson K, Aylsworth AS, Bamforth JS, Babu D, Niyazov DM, Ravnan JB, Schultz RA, Lamb AN, Torchia BS, Bejjani BA, Shaffer LG. 2012. High-resolution array CGH defines critical regions and candidate genes for microcephaly, abnormalities of the corpus callosum, and seizure phenotypes in patients with microdeletions of 1q43q44. Hum Genet 131:145–156. Barkovich AJ, Chuang SH. 1991. Unilateral megalencephaly: Correlation of MR imaging and pathologic characteristics. Ann Neurol 30:139–146. Blümcke I, Mühlebner A. 2011. Neuropathological work-up of focal cortical dysplasias using the new ILAE consensus classification system - practical guideline article invited by the Euro-CNS Research Committee. Clin Neuropathol 30:164–177. Brunetti-Pierri N, Berg JS, Scaglia F, Belmont J, Bacino CA, Sahoo T, Lalani SR, Graham B, Lee B, Shinawi M, Shen J, Kang SH, Pursley A, Lotze T, Kennedy G, Lansky-Shafer S,

ARTICLE Weaver C, Roeder ER, Grebe TA, Arnold GL, Hutchison T, Reimschisel T, Amato S, Geragthy MT, Innis JW, Obersztyn E, Nowakowska B, Rosengren SS, Bader PI, Grange DK, Naqvi S, Garnica AD, Bernes SM, Fong CT, Summers A, Walters WD, Lupski JR, Stankiewicz P, Cheung SW, Patel A. 2008. Recurrent reciprocal 1q21.1 deletions and duplications associated with microcephaly or macrocephaly and developmental and behavioral abnormalities. Nat Genet 40:1466–1471. Butler MG, Dasouki MJ, Zhou X-P, Talebizadeh Z, Brown M, Takahashi TN, Miles JH, Wang CH, Stratton R, Pilarski R, Eng C. 2005. Subset of individuals with autism spectrum disorders and extreme macrocephaly associated with germline PTEN tumour suppressor gene mutations. J Med Genet 42: 318–321. Buxbaum JD, Cai G, Chaste P, Nygren G, Goldsmith J, Reichert J, Anckarsäter H, Rastam M, Smith CJ, Silverman JM, Hollander E, Leboyer M, Gillberg C, Verloes A, Betancur C. 2007. Mutation screening of the PTEN gene in patients with autism spectrum disorders and macrocephaly. Am J Med Genet B Neuropsychiatr Genet 144B:484– 491. Chung BK, Eydoux P, An Karnebeek CD, Gibson WT. 2014. Duplication of AKT3 is associated with macrocephaly and speech delay. Am J Med Genet Part A 9999:1–2. Clayton-Smith J, Kerr B, Brunner H, Tranebjaerg L, Magee A, Hennekam RC, Mueller RF, Brueton L, Super M, Steen-Johnsen J, Donnai D. 1997. Macrocephaly with cutis marmorata, haemangioma and syndactyly–a distinctive overgrowth syndrome. Clin Dysmorphol 6:291–302. Conway RL, Danielpour M, Graham JM, Jr. 2007a. Surgical management of cerebellar tonsillar herniation in three patients with macrocephaly-cutis marmorata telangiectatica congenita. Report of three cases. J Neurosurg 106:296–301. Conway RL, Pressman BD, Dobyns WB, Danielpour M, Lee J, Sanchez-Lara PA, Butler MG, Zackai E, Campbell L, Saitta SC, Clericuzio CL, Milunsky JM, Hoyme HE, Shieh J, Moeschler JB, Crandall B, Lauzon JL, Viskochil DH, Harding B, Graham JM. 2007b. Neuroimaging findings in macrocephaly-capillary malformation: A longitudinal study of 17 patients. Am J Med Genet Part A 143A:2981–3008. Crino PB. 2007. Focal brain malformations: A spectrum of disorders along the mTOR cascade. Novartis Found Symp 288:260– 272;discussion 272–281. Crino PB, Aronica E, Baltuch G, Nathanson KL. 2010. Biallelic TSC gene inactivation in tuberous sclerosis complex. Neurology 74: 1716–1723. Cristaldi A, Vigevano F, Antoniazzi G, Di Capua M, Andreuzzi A, Morselli G, Iorio F, Fariello G, Trasimeni G, Gualdi GF. 1995. Hemimegalencephaly, hemihypertrophy and vascular lesions. Eur J Pediatr 154:134– 137. DeMyer W. 1972. Megalencephaly in children. Clinical syndromes, genetic patterns, and differential diagnosis from other causes of megalocephaly. Neurology 22:634–643.

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS) DeMyer W. 1986. Megalencephaly: Types, clinical syndromes, and management. Pediatr Neurol 2:321–328. Elia M, Amato C, Bottitta M, Grillo L, Calabrese G, Esposito M, Carotenuto M. 2012. An atypical patient with Cowden syndrome and PTEN gene mutation presenting with cortical malformation and focal epilepsy. Brain Dev 34:873–876. Glickstein SB, Alexander S, Ross ME. 2006. Differences in cyclin D2 and D1 protein expression distinguish forebrain progenitor subsets. Cereb Cortex 17:632–642. Glickstein SB, Monaghan JA, Koeller HB, Jones TK, Ross ME. 2009. Cyclin D2 is critical for intermediate progenitor cell proliferation in the embryonic cortex. J Neurosci 29:9614– 9624. Gorlin RJ, Cohen MM, Condon LM, Burke BA. 1992. Bannayan-Riley-Ruvalcaba syndrome. Am J Med Genet 44:307–314. Hobert JA, Embacher R, Mester JL, Frazier TW, II. Eng C. 2014. Biochemical screening and PTEN mutation analysis in individuals with autism spectrum disorders and macrocephaly. Eur J Hum Genet 22:273–276. Inaba T, Matsushime H, Valentine M, Roussel MF, Sherr CJ, Look AT. 1992. Genomic organization, chromosomal localization, and independent expression of human cyclin D genes. Genomics 13:565–574. Kariminejad A, Radmanesh F, Rezayi A-R, Tonekaboni S-H, Gleeson JG. 2012. Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome: A case report. J Child Neurol 28:651–657. Kida A, Kakihana K, Kotani S, Kurosu T, Miura O. 2007. Glycogen synthase kinase-3beta and p38 phosphorylate cyclin D2 on Thr280 to trigger its ubiquitin/proteasome-dependent degradation in hematopoietic cells. Oncogene 26:6630–6640. Kingwell K. 2013. Neuro-oncology: Everolimus for astrocytoma in tuberous sclerosis complex. Nat Rev Neurol 9:6. Krueger DA, Wilfong AA, Holland-Bouley K, Anderson AE, Agricola K, Tudor C, Mays M, Lopez CM, Kim M-O, Franz DN. 2013. Everolimus treatment of refractory epilepsy in tuberous sclerosis complex. Ann Neurol 74:679–687. Kurek KC, Luks VL, Ayturk UM, Alomari AI, Fishman SJ, Spencer SA, Mulliken JB, Bowen ME, Yamamoto GL, Kozakewich HPW, Warman ML. 2012. Somatic mosaic activating mutations in PIK3CA cause CLOVES syndrome. Am J Hum Genet 90:1108–1115. Kwon CH, Zhu X, Zhang J, Knoop LL, Tharp R, Smeyne RJ, Eberhart CG, Burger PC, Baker SJ. 2001. Pten regulates neuronal soma size: A mouse model of Lhermitte-Duclos disease. Nat Genet 29:404–411. Kwon C-H, Zhu X, Zhang J, Baker SJ. 2003. mTor is required for hypertrophy of Pten-deficient neuronal soma in vivo. Proc Natl Acad Sci USA 100:12923–12928. Lee JH, Huynh M, Silhavy JL, Kim S, DixonSalazar T, Heiberg A, Scott E, Bafna V, Hill KJ, Collazo A, Funari V, Russ C, Gabriel SB, Mathern GW, Gleeson JG. 2012. De novo somatic mutations in components of the PI3K-AKT3-mTOR pathway cause hemimegalencephaly. Nat Genet 44:941–945.

171

Liaw D, Marsh DJ, Li J, Dahia PL, Wang SI, Zheng Z, Bose S, Call KM, Tsou HC, Peacocke M, Eng C, Parsons R. 1997. Germline mutations of the PTEN gene in Cowden disease, an inherited breast and thyroid cancer syndrome. Nat Genet 16:64–67. Lim K-C, Crino PB. 2013. Focal malformations of cortical development: New vistas for molecular pathogenesis. Neuroscience 252: 262–276. Lindhurst MJ, Sapp JC, Teer JK, Johnston JJ, Finn EM, Peters K, Turner J, Cannons JL, Bick D, Blakemore L, Blumhorst C, Brockmann K, Calder P, Cherman N, Deardorff MA, Everman DB, Golas G, Greenstein RM, Kato BM, Keppler-Noreuil KM, Kuznetsov SA, Miyamoto RT, Newman K, Ng D, O’Brien K, Rothenberg S, Schwartzentruber DJ, Singhal V, Tirabosco R, Upton J, Wientroub S, Zackai EH, Hoag K, Whitewood-Neal T, Robey PG, Schwartzberg PL, Darling TN, Tosi LL, Mullikin JC, Biesecker LG. 2011. A mosaic activating mutation in AKT1 associated with the Proteus syndrome. N Engl J Med 365:611– 619. Lindhurst MJ, Parker VER, Payne F, Sapp JC, Rudge S, Harris J, Witkowski AM, Zhang Q, Groeneveld MP, Scott CE, Daly A, Huson SM, Tosi LL, Cunningham ML, Darling TN, Geer J, Gucev Z, Sutton VR, Tziotzios C, Dixon AK, Helliwell T, O’Rahilly S, Savage DB, Wakelam MJO, Barroso I, Biesecker LG, Semple RK. 2012. Mosaic overgrowth with fibroadipose hyperplasia is caused by somatic activating mutations in PIK3CA. Nat Genet 44:928–933. Malan V, Chevallier S, Soler G, Coubes C, Lacombe D, Pasquier L, Soulier J, Morichon-Delvallez N, Turleau C, Munnich A, Romana S, Vekemans M, Cormier-Daire V, Colleaux L. 2010. Array-based comparative genomic hybridization identifies a high frequency of copy number variations in patients with syndromic overgrowth. Eur J Hum Genet 18:227–232. Marsh DJ, Dahia PL, Zheng Z, Liaw D, Parsons R, Gorlin RJ, Eng C. 1997. Germline mutations in PTEN are present in Bannayan-Zonana syndrome. Nat Genet 16:333–334. Marsh DJ, Kum JB, Lunetta KL, Bennett MJ, Gorlin RJ, Ahmed SF, Bodurtha J, Crowe C, Curtis MA, Dasouki M, Dunn T, Feit H, Geraghty MT, Graham JM, Hodgson SV, Hunter A, Korf BR, Manchester D, Miesfeldt S, Murday VA, Nathanson KL, Parisi M, Pober B, Romano C, Eng C, et al. 1999. PTEN mutation spectrum and genotypephenotype correlations in Bannayan-RileyRuvalcaba syndrome suggest a single entity with Cowden syndrome. Hum Mol Genet 8:1461–1472. Matsushime H, Roussel MF, Ashmun RA, Sherr CJ. 1991. Colony-stimulating factor 1 regulates novel cyclins during the G1 phase of the cell cycle. Cell 65:701–713. Merks JHM, De Vries LS, Zhou X-P, Nikkels P, Barth PG, Eng C, Hennekam RCM. 2003. PTEN hamartoma tumour syndrome: Variability of an entity. J Med Genet 40:e111. Mirzaa GM, Conway RL, Gripp KW, LermanSagie T, Siegel DH, deVries LS, Lev D, Kramer N, Hopkins E, Graham JM, Jr. Dobyns WB. 2012a. Megalencephaly-

172

AMERICAN JOURNAL OF MEDICAL GENETICS PART C (SEMINARS IN MEDICAL GENETICS)

capillary malformation (MCAP) and megalencephaly-polydactyly-polymicrogyriahydrocephalus (MPPH) syndromes: Two closely related disorders of brain overgrowth and abnormal brain and body morphogenesis. Am J Med Genet Part A 158A:269– 291. Mirzaa GM, Ashwal S, Dobyns WB. 2012b. In: Ashwal S, Swaiman K, editors. Disorders of brain size. In: Swaiman’s Pediatric Neurology: Principles and Practice, (volume 2). Elsevier Saunders. pp 173–201. Mirzaa GM, Rivière J-B, Dobyns WB. 2013. Megalencephaly syndromes and activating mutations in the PI3K-AKT pathway: MPPH and MCAP. Am J Med Genet C Semin Med Genet 163C:122–130. Mirzaa GM, Parry DA, Fry AE, Giamanco KA, Schwartzentruber J, Vanstone M, Logan CV, Roberts N, Johnson CA, Singh S, Kholmanskikh SS, Adams C, Hodge RD, Hevner RF, Bonthron DT, Braun KPJ, Faivre L, Rivière J-B, St-Onge J, Gripp KW, Mancini GMS, Pang K, Sweeney E, Van Esch H, Verbeek N, Wieczorek D, Steinraths M, Majewski J. FORGE Canada Consortium. Boycott KM, Pilz DT, Ross ME, Dobyns WB, Sheridan EG, 2014. De novo CCND2 mutations leading to stabilization of cyclin D2 cause megalencephaly-polymicrogyriapolydactyly-hydrocephalus syndrome. Nat Genet. 46:510–515. Moore CA, Toriello HV, Abuelo DN, Bull MJ, Curyr CJR, Hall BD, Higgins JV, Stevens CA, Twersky S, Weksberg R, Dobyns WB. 1997. Macrocephaly-cutis marmorata telangiectatica congenita syndrome: A distinct disorder with developmental delay and connective tissue abnormality. Am J Med Genet 70:67–73. Nakahashi M, Sato N, Yagishita A, Ota M, Saito Y, Sugai K, Sasaki M, Natsume J, Tsushima Y, Amanuma M, Endo K. 2009. Clinical and imaging characteristics of localized megalencephaly: A retrospective comparison of diffuse hemimegalencephaly and multilobar cortical dysplasia. Neuroradiology 51:821– 830. Nakamura K, Kato M, Tohyama J, Shiohama T, Hayasaka K, Nishiyama K, Kodera H, Nakashima M, Tsurusaki Y, Miyake N, Matsumoto N, Saitsu H. 2014. AKT3 and PIK3R2 mutations in two patients with megalencephaly-related syndromes: MCAP and MPPH. Clin Genet 85:396–398. Ogawa S, Kwon C-H, Zhou J, Koovakkattu D, Parada LF, Sinton CM. 2007. A seizureprone phenotype is associated with altered free-running rhythm in Pten mutant mice. Brain Res 1168:112–123. Pavlidis E, Cantalupo G, Boria S, Cossu G, Pisani F. 2012. Hemimegalencephalic variant of epidermal nevus syndrome: Case report and

literature review. Eur J Paediatr Neurol 16:332–342. Poduri A, Evrony GD, Cai X, Elhosary PC, Beroukhim R, Lehtinen MK, Hills LB, Heinzen EL, Hill A, Hill RS, Barry BJ, Bourgeois BFD, Riviello JJ, Barkovich AJ, Black PM, Ligon KL, Walsh CA. 2012. Somatic activation of AKT3 causes hemispheric developmental brain malformations. Neuron 74:41–48. Poduri A, Evrony GD, Cai X, Walsh CA. 2013. Somatic mutation, genomic variation, and neurological disease. Science 341: 1237758. Rios JJ, Paria N, Burns DK, Israel BA, Cornelia R, Wise CA, Ezaki M. 2013. Somatic gain-offunction mutations in PIK3CA in patients with macrodactyly. Hum Mol Genet 22: 444–451. Rivière J-B, Mirzaa GM, O’Roak BJ, Beddaoui M, Alcantara D, Conway RL, St-Onge J, Schwartzentruber JA, Gripp KW, Nikkel SM, Worthylake T, Sullivan CT, Ward TR, Butler HE, Kramer NA, Albrecht B, Armour CM, Armstrong L, Caluseriu O, Cytrynbaum C, Drolet BA, Innes AM, Lauzon JL, Lin AE, Mancini GMS, Meschino WS, Reggin JD, Saggar AK, Lerman-Sagie T, Uyanik G, Weksberg R, Zirn B, Beaulieu CL, Majewski J, Bulman DE, O’Driscoll M, Shendure J, Graham JM, Jr. Boycott KM, Dobyns WB. 2012. De novo germline and Post-zygotic mutations in AKT3, PIK3R2 and PIK3CA cause a spectrum of related megalencephaly syndromes. Nat Genet 44: 934–940. Rosenfeld JA, Kim KH, Angle B, Troxell R, Gorski JL, Westemeyer M, Frydman M, Senturias Y, Earl D, Torchia B, Schultz RA, Ellison JW, Tsuchiya K, Zimmerman S, Smolarek TA, Ballif BC, Shaffer LG. 2013. Further evidence of contrasting phenotypes caused by reciprocal deletions and duplications: Duplication of NSD1 causes growth retardation and microcephaly. Mol Syndromol 3:247–254. Ross ME, Carter ML, Lee JH. 1996. MN20, a D2 cyclin, is transiently expressed in selected neural populations during embryogenesis. J Neurosci 16:210–219. Samuels Y, Ericson K. 2006. Oncogenic PI3K and its role in cancer. Curr Opin Oncol 18:77–82. Samuels Y, Waldman T. 2010. Oncogenic mutations of PIK3CA in human cancers. Curr Top Microbiol Immunol 347: 21–41. Sato N, Yagishita A, Oba H, Miki Y, Nakata Y, Yamashita F, Nemoto K, Sugai K, Sasaki M. 2007. Hemimegalencephaly: A study of abnormalities occurring outside the involved hemisphere. AJNR Am J Neuroradiol 28: 678–682.

ARTICLE Sharma S, Sankhyan N, Kabra M, Kumar A. 2009. Hypomelanosis of Ito with hemimegalencephaly Dermatol Online J 15:12. Shinawi M, Li P, Kang SH, Shen J, Belmont JW, Scott DA, Probst FJ, Craigen WJ, Graham B, Pursley A, Clark G, Lee J, Proud M, Stocco A, Rodriguez D, Kozel B, Sparagana S, Roeder E, McGrew S, Kurczynski T, Allison L, Amato S, Savage S, Patel A, Stankiewicz P, Beaudet A, Cheung SW, Lupski JR. 2009. Recurrent reciprocal 16p11.2 rearrangements associated with global developmental delay, behavioral problems, dysmorphism, epilepsy, and abnormal head size. J Med Genet 47:332–341. Shiroishi MS, Jackson HA, Nelson MD, Jr. Bluml S, Panigrahy A. 2010. Contralateral hemimicrencephaly in neonatal hemimegalencephaly. Pediatr Radiol 40:1826–1830. Tan M-H, Mester J, Peterson C, Yang Y, Chen J-L, Rybicki LA, Milas K, Pederson H, Remzi B, Orloff MS, Eng C. 2011. A clinical scoring system for selection of patients for PTEN mutation testing is proposed on the basis of a prospective study of 3042 probands. Am J Hum Genet 88:42–56. Tatton-Brown K, Douglas J, Coleman K, Baujat G, Cole TR, Das S, Horn D, Hughes HE, Temple IK, Faravelli F, Waggoner D, Turkmen S, Cormier-Daire V, Irrthum A, Rahman N. 2005. Genotype-phenotype associations in Sotos syndrome: An analysis of 266 individuals with NSD1 aberrations. Am J Hum Genet 77:193–204. Van Bokhoven H, Celli J, Van Reeuwijk J, Rinne T, Glaudemans B, Van Beusekom E, Rieu P, Newbury-Ecob RA, Chiang C, Brunner HG. 2005. MYCN haploinsufficiency is associated with reduced brain size and intestinal atresias in Feingold syndrome. Nat Genet 37:465–467. Van Bon BWM, Balciuniene J, Fruhman G, Nagamani SCS, Broome DL, Cameron E, Martinet D, Roulet E, Jacquemont S, Beckmann JS, Irons M, Potocki L, Lee B, Cheung SW, Patel A, Bellini M, Selicorni A, Ciccone R, Silengo M, Vetro A, Knoers NV, De Leeuw N, Pfundt R, Wolf B, Jira P, Aradhya S, Stankiewicz P, Brunner HG, Zuffardi O, Selleck SB, Lupski JR, De Vries BBA. 2011. The phenotype of recurrent 10q22q23 deletions and duplications. Eur J Hum Genet 19:400–408. Wang D, Zeesman S, Tarnopolsky MA, Nowaczyk MJM. 2013. Duplication of AKT3 as a cause of macrocephaly in duplication 1q43q44. Am J Med Genet Part A 161A:2016–2019. Zhou J, Blundell J, Ogawa S, Kwon C-H, Zhang W, Sinton C, Powell CM, Parada LF. 2009. Pharmacological inhibition of mTORC1 suppresses anatomical, cellular, and behavioral abnormalities in neural-specific Pten knock-out mice. J Neurosci 29:1773–1783.