3

Cancer Biomarkers 14 (2014) 3–16 DOI 10.3233/CBM-130374 IOS Press

Human glycosylation disorders Donna Krasnewich National Institute of General Medical Sciences, National Institutes of Health, Building 45 Room 2As25h, Bethesda, MD 20892, USA Tel.: +1 301 594 0943; Fax: +1 301 480 2228; E-mail: [email protected]

Abstract. Over the past 20 years, clinical disorders of glycosylation have expanded to include over 50 recognized defects in the network of glycobiologic pathways. In parallel, more cases have been recognized by astute clinicians increasing both the number of known affected individuals as well as the breadth of clinical features attributed to these disorders. The descriptions of affected individuals may include a functional adult with cognitive impairments, a developmentally normal child with significant gastrointestinal symptoms, a severely ill infant or a fetus with hydrops fetalis. These clinical cases have led to the recognition of gene mutations affecting different enzymes and transporters active in the interconnected synthetic pathways of the myriad of oligosaccharides with essential roles in human development and biology. Keywords: Glycosylation, glycobiology, congenital disorders of glycosylation

1. Introduction Since about 2% of the human genome is known to code for proteins involved in glycosylation pathways much of the field of clinical glycobiology has yet to be explored [1–3]. The history of the discovery of clinical disorders of glycosylation actually started with the testing, in children with a metabolic disorder of unknown etiology, for known biomarkers of alcohol abuse. Helena Stibler had noted that isoelectric focusing of the serum glycoprotein, transferrin, revealed unusual patterns of N-linked oligosaccharides in the serum of patients with chronic alcohol abuse [4]. The technique separated glycosylated transferrin that typically had two attached N-linked oligosaccharides, each carrying two negatively charged sialic acids. Any differences in these four negative charges changed the isoelectric focusing pattern and suggested an abnormal N-linked glycan structure. In 1984, using the transferrin isoelectric focusing test pioneered by Helena Stibler, Jaak Jaeken diagnosed the first children with a glycosylation disorder, termed “carbohydrate deficient glycoprotein syndrome” [4,5]. These children were shown to have an abnormal serum transferrin isoelectric focusing pattern suggesting that they had a defective N-linked oligosaccharide synthetic pathway. While this test could detect an abnormal product of the

pathway, glycobiologists and eventually molecular diagnostics were needed to determine which individual step, in this complex synthetic pathway, was defective in each affected individual. This began an era of discovery [6]. Over the next 20 years, assays were developed, genes were cloned and individuals were diagnosed. It became clear that there were individuals with abnormal synthesis of glycans other than N-linked glycans, opening up a larger more inclusive arena in medicine and research, the human glycosylation disorders. In the past 10 years, rare disorders resulting from the abnormal synthesis of O-linked glycans, glycolipids, glycosaminoglycans (GAGs) and glycosylphosphatidylinositol (GPI) anchors have also been described [2,6]. It is more difficult for the clinical community to screen for individuals with these disorders because there are no serum based tests available in diagnostic laboratories. The clinical manifestations of these disorders involve many organ systems and may range from mild to quite severe. In contrast with the blood test for N-linked glycan disorders, the current diagnostic testing for these groups is often invasive and the certainty of the diagnosis is frequently delayed. This is a harder path for both the affected individual and their family; better diagnostic tools would not only

c 2014 – IOS Press and the authors. All rights reserved ISSN 1574-0153/14/$27.50 

4

D. Krasnewich / Human glycosylation disorders

change their early medical course but would also improve strategies for testing new therapeutic agents.

2. The metabolic pathways affected in clinical glycosylation disorders 2.1. Introduction to glycosylation Medical teams are reporting increasing numbers of rare, inherited glycosylation disorders which involve defective steps in many different oligosaccharide synthetic pathways, including N-linked, O-linked, lipidlinked oligosaccharides GAGs and GPI anchors [7,8]. This implies that these protein and lipid-linked sugar chains play specific and high impact roles in human development and physiology. From years of basic scientific research it is known that N-linked glycosylation plays an important role in protein-protein complex formation, protease resistance, protein folding and stability, and cell-cell recognition and communication. O-linked glycosylation has been shown to determine ABO blood type, and play roles in lymphocyte targeting in inflammatory responses and antibacterial immune defense. Both N- and O-linked glycans have also been shown to be involved in embryonic development. Lipid-linked oligosaccharides (LLOs) are important to the function and integrity of membranes. GAGs or proteoglycans, long repeating disaccharide chains extending from a small core glycan include heparin sulphate, heparin, chondroitin and dermatan sulphate. They are involved in joint cushioning and binding of growth factors, cytokines and morphogens during development [2]. GPI anchor glycans are linked to proteins which anchor the glycan into membranes and affect membrane diffusion, signaling pathways and intracellular protein sorting [7,8]. 2.2. N-linked glycosylation synthetic pathway N-linked glycosylation is the addition of a complex oligosaccharide to a site specific asparagine in a class of proteins called N-linked glycoproteins. The synthesis of the oligosaccharide begins on the lipid structure, dolichol, where a 14 sugar precursor is made starting with the addition of two N-acetyl glucosamine (GlcNAc) units, nine mannose (Man) and three glucose (Glc) units, which are assembled by a series of monosaccharide and structure specific glycosyltransferases. This assembled glycan is then transferred, en bloc, to an appropriate asparagine in a newly synthe-

sized protein, now a glycoprotein. This transferred glycan is then trimmed of three Glc and up to six Man. GlcNac, galactose (Gal), sialic acid (SA) and even fucose (Fuc) are then added to create a branched structure that defines important functional characteristics of the attached glycoprotein. The branching patterns and monosaccharide additions to N-linked oligosaccharides are highly regulated and may be quite dynamic with the cell type and environment playing a role in determining the final structure. Defects in N-linked glycosylation manifest on a cellular level in differences in protein folding and turnover, peptide agonist-receptor interaction and cell to cell recognition. These differences in turn lead to changes in human embryology and physiology seen in individuals with Congenital Disorders of Glycosylation (CDGs) [1,9]. 2.3. O-linked glycosylation and GPI anchor synthetic pathways O-linked glycosylation starts with the initial addition of a monosaccharide to serine or threonine in a newly synthesized protein destined to be an O-linked glycoprotein [10]. This monosaccharide may be mannose, in the case of O-mannosylated proteins, N-acetyl galactosamine (GalNAc), in O-GalNAc proteins, or xylose or serine in GAGs. The synthesis of O-linked oligosaccharides is even more complex and diverse than that of N-linked glycans and their initial linkage predicts the rest of the structure. For example, after mannose is added in O-mannose glycans, GlcNAc, Gal and SA are added. However, in O-linked GalNAc oligosaccharides, the addition of GalNAc to serine or threonine, the basis of the mucin structure, is extended by Gal, GlcNAc, Fuc and SA forming branched or linear structures. The most studied role of O-mannosylated glycans in humans is the stabilization and function of alpha dystroglycan, a glycoprotein that is involved in connecting the extracellular to the intracellular space in skeletal muscle cells. In humans O-GalNAc oligosaccharides, or mucins, are part of the lubricating mucous membranes that create a barrier in the mouth, nose and gastrointestinal tract against pathogens. O-GalNAc glycans are also on the surface of white blood cells, guiding their trafficking into blood vessels. Oligosaccharides linked to lipids, LLOs, have important roles in lipid rafts in cell membranes as well as in cell-cell recognition and are especially abundant in the central and peripheral nervous system. These special biologic compounds are synthesized starting with

D. Krasnewich / Human glycosylation disorders

an O-linkage between Glc or Gal and a ceramide which is then extended. GAGs, on the other hand, are typically found in the extracellular space, and are synthesized initially with the linkage of xylose to serine. This is followed by the addition of two Gal molecules and one glucuronic acid unit followed by the linking of repeating disaccharides. GPI anchors are formed by the stepwise addition of monosaccharides, de-N-acetylation and the addition of ethanolamine phosphate onto phosphatidylinositol starting with GlcNAc in the endoplasmic reticulum (ER). This complex synthetic pathway starts on the cytoplasmic face of the ER and is completed on the ER luminal side with attachment of the protein by a multisubunit transamidase. The enzymes involved in this pathway include PIG-A, PIG-L, PIG-W, PIG-B, PIGM, PIG-V, PIG-N, PIG-O, and PIG-F.

3. Diagnosis and clinical features of N-linked CDGs 3.1. Diagnosis of N-linked synthetic defects As a group of clinical disorders, the CDGs affect about 800 individuals worldwide, with the most common CDG involving an N-linked synthetic defect in the PMM2 gene coding for phosphomannomutase. This type of CDG is termed PMM2-CDG (CDG-Ia), with 500 cases reported and an incidence estimated at 1/20,000. Almost all types of CDGs are recessive in inheritance with a few found to be X-linked. The clinical findings reflect the role of N-linked glycosylation in the embryology and biology of many organ systems including the brain and peripheral nervous system, eyes, liver, kidneys, bone marrow, bone, endocrine organs and growth. Clinical findings may include developmental delay, hypotonia, and peripheral neuropathy with or without abnormalities in brain MRI reflecting brain development. Children may have failure to thrive or growth impairment, hepatopathy, coagulopathy, immunodeficiency, protein losing enteropathy, osteopenia, congenital renal abnormalities and renal dysfunction, functional and/or anatomic eye involvement, and endocrine abnormalities. The clinical manifestations of the N-linked CDGs are widely variable both between and within types (Table 1). The diagnosis should be considered in any child or adult with multi-organ system involvement, and different combinations of affected organ systems lead to a wide range of phenotypes [11–14].

5

The glycosylation status of serum transferrin is the most popular indicator of congenital disorders of glycosylation. While the glycosylation of many circulating glycoproteins can be affected by this disorder, transferrin, which typically has two bientennary Nlinked glycans attached, has been used both because it is a sensitive marker and diagnostic laboratories have experience with the nuances of the testing results. The glycosylation of this protein can be analyzed by several methods including isoelectric focusing, ion exchange high performance liquid chromatography and capillary electrophoresis. Currently most diagnostics are being performed on GC/Mass spectrometry. Electrospray-ionization mass spectrometry (ESIMS) is also a highly informative method as it detects both differences in monosaccharide content as well as the loss of an entire glycan from the serum transferrin glycoprotein. N-linked CDGs are divided into Type I and Type II, Type I CDGs involve defects in the LLO synthesis and transfer to the protein while Type II CDGs involve defects in the processing of protein-bound sugar chains. Transferrin glycosylation analysis will not detect isolated O-linked glycosylation synthesis disorders because there is no O-linked glycosylation on serum transferrin. Most recently, spectrometry based analytic tools used to profile both N-linked and O-linked glycan chain structures from whole serum or plasma glycoproteins are being utilized to supplement diagnostic transferrin glycoform analysis. This type of profiling can detect glycans that are not attached to transferrin and may include high mannose chains, hybrid, fucosylated or glucosylated oligosaccharides or mucin core 2 O-glycans [15]. Mass spectrometry profiling facilitates the diagnosis of Type II CDGs, conserved oligomeric golgi (COG) defects and combined Type I and Type II CDG defects. Urine free oligosaccharides when performed with mass spectrometry of glycans may identify O-fucosylation defects such as B3GALTLCDG (Peter’s Plus syndrome) [16] and free glycans in urine are currently the only way to detect GCS1-CDG (CDG-IIb) [17]. The detection of false positive transferrin glycosylation abnormalities makes diagnosis more complicated. False positive results, or abnormal transferrin glycosylation, can be found in transferrin of individuals affected with alcoholism, uncontrolled galactosemia, and fructose intolerance. In addition, transferrin glycosylation may appear normal in individuals with specific CDG types including GCS1-CDG (CDG-

6

D. Krasnewich / Human glycosylation disorders Table 1 Clinical features of CDGs Presentation in infancy Clinical features in adulthood Normal pregnancy, delivery, birth weight Variable intellectual impairment Failure to thrive and hypotonia Ataxia, dysarthria, dysmetria Ascites, multi-organ system failure Kyphoscoliosis and skeletal involvement Hydrops fetalis Range of functional life skills Risk of thrombosis Neurology Developmental delay Ataxia, dysarthria, dysmetria Hypotonia Hyporeflexia Seizures Stroke-like episodes Peripheral neuropathy with muscle wasting Congenital muscular dystrophy Brain anomalies on MRI

Gastroenterology Failure to thrive Gastroesophogeal reflux Liver dysfunction/elevated liver function tests

Endocrinology Hypergonadotropic hypogonadism Hyperinsulinemic hypoglycemia Growth dysregulation

Cardiology Pericardial effusion Hypertrophic cardiomyopathy

Dermatology Atypical fat distribution Inverted nipples Ichthyosis Cutis laxa

Nephrology Microcystic kidneys Proteinuria Nephrotic syndrome

Hematology Risk of thrombosis Decreased levels of pro-coagulation cascade Decreased levels of Protein C, Protein S, Antithrombin III Immunodeficiency

Orthopedics Osteopenia Kyphosis/Kyphoscoliosis Chest deformaties Contractures Skeletal dysplasia

Ophthalmology Esotropia Retinitis pigmentosa Myopia Congenital eye anomalies Cataracts

IIb), SLC35C1-CDG (CDG-IIc), and SLC35A1-CDG (CDG-IIf). False negatives have been reported in molecular diagnostics proven cases of CDG. Highlighting the complexity of diagnostics in N-linked CDGs there was a recent report of several individuals with abnormal glycosylation in infancy which appeared normal later in childhood, despite persistence of the clinical issues in these children [18]. Alpha-1antitrypsin, another serum N-linked glycoprotein, has also been used for supportive evidence of abnormal Nlinked glycosylation in an individual. While the analysis of serum transferrin may detect altered glycosylation, the exact defective step of the pathway cannot be determined by these analytic methods. To determine the molecular diagnosis several approaches can be taken. Molecular testing for candidate genes can be done either on an individual basis or in a panel to search for functional variants, or known mutations, in genes already reported to be altered in cases of CDG. The validation of a variant in a specific gene defines a CDG type, with 50 types now identified. Historically, the types were denoted as types Ia-In and IIaetc. in the order that they were identified. A revised

nomenclature is now used with gene name-CDG and the number if a numbered type has already been defined, i.e., PMM2-CDG (CDG-Ia) [19]. The definition of type is useful for medical teams and families to learn from reported cases or if a couple with an affected child would like prenatal diagnostics for another pregnancy to help them with reproductive choices.

4. Clinical features of individuals with N-linked CDGs 4.1. PMM2-CDG (CDG-Ia)- the most common N-linked CDG Individuals with N-linked CDGs show a tremendous range of symptoms, from death in infancy to mildly involved adults. The most common type of N-linked CDG, PMM2-CDG (CDG-Ia) results from deleterious mutations in PMM2, one of two genes that encode phosphomannomutases, the enzymes that converts mannose-6-phosphate to mannose-1-phosphate. Mannose-1-phosphate is the precursor of GDP-man-

D. Krasnewich / Human glycosylation disorders

7

nose, with mutations in this gene leading to limitation of the availability of this charged sugar in the synthetic pathway. About 60% of individuals with N-linked CDGs carry the diagnosis of PMM2-CDG (CDG-Ia) [20–23]. PMM2-CDG (CDG-Ia) should be considered in a child with developmental delay and hypotonia (low muscle tone) in combination with any of the following findings: failure to thrive, hepatic dysfunction including elevated transaminases, coagulopathy with low serum concentration of factors IX and XI, protein C, protein S and/or antithrombin III, pericardial effusion, hypothyroidism or hypogonadism, strabismus or esotropia, abnormal fat distribution, osteopenia, seizures, stroke-like episodes, scoliosis and cerebellar atrophy with a small brain stem. In adolescents and adults, the diagnosis of PMM2-CDG(CDG-Ia) should be considered in individuals with medical histories suggestive of the following clinical features: non-progressive cerebellar dysfunction with ataxia (poor balance), dysarthria (slurred speech) and dysmetria (poor fine motor function), cognitive impairment, unexplained deep venous thrombosis or strokelike episodes, peripheral neuropathy with or without muscle wasting, absent puberty in females, small testes in males, retinitis pigmentosa, progressive scoliosis with truncal shortening and joint contractures. The diagnosis of PMM2-CDG (CDG-Ia) should also be considered in a fetus with non-immune hydrops fetalis [11, 24]. The clinical manifestations of N-linked CDGs are evidence of the importance of these glycans in the embryologic development of the brain, eyes, kidneys, liver, bone and other organ systems. The symptoms in each of these organ systems, in individuals affected with CDG, form a compendium of the functional roles of N-linked glycans in humans.

are independent with apparently normal intelligence. Seizures are common, typically first occurring in childhood, and progressive peripheral neuropathy with muscle wasting is seen variably in teens and adults. Strokelike episodes, sudden hemiplegia or loss of function of an extremity, are another complication that have been reported in early childhood through the teen years. The etiology of these typically reversible episodes is not well understood [22].

4.1.1. Central and peripheral nervous system Hypotonia and developmental delay are typically the first recognized neurologic signs in infants with PMM2-CDG (CDG-Ia). Affected children continue to make slow developmental progress without regression throughout their lives. Ataxia, dysarthria and dysmetria signs of cerebellar dysfunction, are seen in early childhood. The cerebellum may appear normal size in infancy, however, often there is less cerebellar growth than growth in other parts of the brain resulting in the common finding of a small cerebellum. Cognition varies greatly, most children have intellectual impairment while a few individuals, diagnosed in adulthood,

4.1.4. Cardiac The most frequent cardiac finding in infants with PMM2-CDG (CDG-Ia) is a small pericardial effusion that disappears over time. There are also reports of hypertrophic cardiomyopathy, however, congenital heart disease, reflecting abnormal development of the heart in the affected fetus has only been reported once suggesting less involvement of N-linked glycan structure in early cardiac development.

4.1.2. Gastrointestinal tract and liver Children with PMM2-CDG (CDG-Ia) commonly present with poor growth or failure to thrive. Oral motor dysfunction and reflux, make feeding difficult for infants, although these symptoms disappear as the children develop and adults eat a normal diet. Elevated transaminases, a reflection of liver dysfunction, and decreased serum levels of both pro- and anti- coagulation factors, made by the liver, are signs of the common episodic hepatopathy which does not progress to liver failure. Liver biopsies have been performed on children prior to diagnosis and show steatosis, fibrosis and involvement of the portal tracts [25]. The elevated liver function tests typically decrease by age five, however, the coagulopathy continues to be worrisome through adulthood [26]. 4.1.3. Renal Renal involvement is typically limited in individuals affected with PMM2-CDG (CDG-Ia), with echogenic, cystic kidneys, reflecting abnormal embryologic development, being the most common finding [27]. There are reports of affected children with nephrotic syndrome, and even rarer reports of progressive renal failure. Typically, renal function is preserved throughout the life of the affected individuals.

4.1.5. Ophthalmologic Infants with PMM2-CDG (CDG-Ia) have nearsightedness with decreased eye muscle strength leading

8

D. Krasnewich / Human glycosylation disorders

to esotropia but functional eyesight is maintained throughout life. Progressive retinitis pigmentosa, has been reported in affected children with variable age of onset and becomes more symptomatic in affected adults. 4.1.6. Orthopedic Both children and adults with PMM2-CDG (CDGIa) show radiologic evidence of osteopenia (decreased bone mineralization) without significantly increased risk of fracture. As children grow they may develop contractures of the extremities especially if hypotonia is profound. The onset of scoliosis and progressive kyphosis (spinal curvature) in childhood with truncal shortening and chest deformity may lead to functional limitation in adulthood. 4.1.7. Dermatologic One of the cardinal features of PMM2-CDG (CDGIa) are the unusual fat pads seen in early childhood including suprapubic fat pads with a symmetric fat accumulation and distinctive lipodystrophy of the buttocks. These disappear with age thus are an age dependent diagnostic finding. 4.1.8. Endocrinologic Endocrine issues in individuals with PMM2-CDG (CDG-Ia) are present in many of the hormone pathways. Affected individuals may present with hypergonadotropic hypogonadism. While there is minimal pubertal development in females, males undergo normal pubertal changes followed by hypogonadism in adulthood with low testosterone levels and small testes. The adrenal glands are also impacted with some individuals having low cortisol or transcortin, and most children having failure to thrive suggesting growth dysregulation. Each of these hormonal cascades include glycosylated receptors and agonist peptides, yet the exact mechanism of how defective N-linked oligosaccharide structures change endocrine function continues to be explored [28]. 4.1.9. Adulthood Adults with PMM2-CDG (CDG-Ia) continue to gain skills and thrive into adulthood with the oldest known patient in her sixties. Their cheerful, interactive personalities highlight their function despite, in most cases, both cognitive and physical disabilities. Affected adults live in assisted living settings with cognitive impairment and ataxia causing limitations in their functional life skills and independence. Otherwise affected adults are relatively healthy and social members of the community [29].

4.2. Clinical features of N-linked glycan defects- Type I defects and defects in LLO synthesis Many other more rare types of N-linked CDGs have been diagnosed in fewer patients, sometimes only one or two families, making it difficult to fully define the clinical phenotype (Table 1). There are currently more than 50 recognized types of CDG, including both Nlinked, O-linked and others, with a type defined when causative mutations in a novel gene have been identified. Clinical features now reported in affected individuals with less common CDG types include: normal development with severe protein losing enteropathy and hepatic involvement in MPI-CDG (CDG-Ib) and ALG8-CDG (CDG-Ih). Icthyosis (skin abnormalities) in MPDU1-CDG (CDG-If) and DK1-CDG (CDG-Im) and cutis laxa (loose skin) is a feature of ATP6VOA2CDG [30,31]. Developmental eye anomalies including coloboma and optic atrophy are seen in ALG6CDG (CDG-Id), ALG2-CDG (CDG-Ii) and SRD5A3CDG [32–34]. Other hematologic anomalies including increased risk of infections in ALG12-CDG (CDG-Ig) and CDG-COG4, increased peripheral lymphocytes can be seen SLC35C1-CDG (CDG-IIc) and thrombocytopenia in SLC35A1-CDG (CDG-IIf) [35–38]. One of the only treatable CDG types is MPI-CDG (CDG-Ib), mainly a hepatic-intestinal disorder. More than 20 patients have been reported. Symptoms begin in the first year of life with vomiting, abdominal pain, protein-losing enteropathy, recurrent thrombosis (blood clots), gastrointestinal bleeding, liver disease and hypoglycemia (low blood sugar). This is the only CDG that is currently treatable, with improvement in responding to oral D-mannose supplementation. Mannose supplementation is hypothesized to bypass the defective step in the pathway, the conversion of fructose6-P to mannose-6-P, to support the synthesis of GDPmannose, an essential precursor in the early stages of N-linked oligosaccharide synthesis [39]. ALG6-CDG (CDG-Ic) is the second most common CDG type, a disorder in protein N-glycosylation, with at least 30 patients identified [12]. The clinical features of ALG6-CDG (CDG-Ic) are similar to PMM2CDG (CDG-Ia), including axial hypotonia, strabismus, seizures, areflexia, ataxia, moderate to severe developmental delay, feeding difficulties and coagulopathy ALG1-CDG (CDG-Ik) has at least 20 patients identified to date [40], Severe neurological complications such as acquired microcephaly, seizures, and optic atrophy are common. There are a group of CDG types with N-linked glycan synthesis defects where between 2 and 10 patients

D. Krasnewich / Human glycosylation disorders

9

Table 2 Genes and enzymes involved in N-glycosylation defects OMIM gene # phenotype # 601785 212065 154550 602579 605907 608540 607905 607906 608750 601110 604566 603147 608103 608104 606941 608776 613666 613661 607144 607143 300776 300884 603564 615042 614202 604346 602202 614507 611908 612015 191350 608093 160995 602616 212066 601336 606056 601385 611093 604346 614202

N-Linked glycosylation defects Enzyme deficiency Phosphomannomutase-2 deficiency

CDG type designation PMM2-CDG (CDG-Ia)

Mannosephosphate Isomerase deficiency

MPI-CDG (CDG-Ib)

Mannosyltransferase 1 deficiency

ALG1-CDG (CDG-Ik)

Mannosyltransferase 2 deficiency

ALG2-CDG (CDG-Ii)

Mannosyltransferase 6 deficiency

ALG3-CDG (CDG-Id)

Glucosyltransferase 1 deficiency

ALG6-CDG (CDG-Ic)

Glucosyltransferase 3 deficiency

ALG8-CDG (CDG-Ih)

Mannosyltransferases 7-9 defiency

ALG9-CDG (CDG-Il)

Mannosyltransferase 11 deficiency

ALG11-CDG (CDG-Ip)

Mannosyltransferase 8 deficiency

ALG12-CDG (CDG-Ig)

UDP-GlcNAc transferase

ALG13-CDG (CDG-Is)

Dol-P-Man Synthase Complex

DPM2-CDG (CDG-Iu)

Alpha-1,2-mannosidase

MAN1B1-CDG

Subunit of the OST complex

DDOST-CDG (CDG-Ir)

Flippase of Man5GlcNA2-PP-Dol

RFT1-CDG (CDG-In)

UDP-GlcNAc:Dol-P-GlcNAc-P Transferase deficiency

DPAGT1-CDG (CDG-Ij)

Oligosaccharyl transferase subunit 3B deficiency N-Acetylglucosaminyl transferase 2 deficiency

MGAT1-CDG MGAT2-CDG (CDG-IIa)

Glucosidase 1 deficiency

GCS1-CDG (CDG-IIb)

Oligosaccharyltransferase subunit 3A deficiency aka Tumor Suppressor 3

TUSC3-CDG

Alpha 1,2 Mannosidase deficiency

MAN1B1-CDG

have been reported including: ALG12-CDG(CDG-Ig), RFT1-CDG (CDG-In), ALG3-CDG(CDG-Id), ALG8CDG (CDG-Ih), ALG9-CDG (CDG-IL)and ALG11CDG (CDG-Ip). Certainly, the spectrum of the phenotype of these disorders will evolve as more affected individuals are identified. The features of each of these types include developmental delay, hypotonia, seizures and failure to thrive. ALG12-CDG (CDGIg) is unique because affected individuals have immunodeficiency [41]. RFT1-CDG (CDG-In) was first described in 2008 with the hallmark feature being sensori-neuronal deafness along with the other features mentioned above. Some of these individuals also have microcephaly, inverted nipples, coagulopathy and kyphoscoliosis [42]. Descriptions of children

with ALG11-CDG (CDG-Ip) also include the finding of deafness in these medically fragile children [40,43]. Individuals with ALG3-CDG (CDG-Id) have significant visual impairment along with the more common clinical features [44]. Some of them may have liver disease and protein-wasting enteropathy. Hyperinsulinemic hypoglycemia and coloboma were reported in a single ALG3-CDG(CDG-Id) patient. ALG8-CDG (CDG-Ih), ALG9-CDG (CDG-IL) and ALG11-CDG (CDG-Ip) all have clinical presentations of multi-organ system involvement [11]. In the group of CDG with glycosyltransferase defects affecting dolichol-linked oligosaccharides in the N-linked oligosaccharide synthetic pathway there are two types where only a single affected individual has

10

D. Krasnewich / Human glycosylation disorders Table 3 Genes and enzymes involved in O-linked, multiple, GPI Anchor and dolichol synthesis disorders

OMIM gene # phenotype # Enzyme deficiency Congenital disorders of glycosylation o-linked, multiple and GPI anchor glycosylation defects 211900 Polypeptide N-acetylgalactosaminyl transferase 3 deficiency 601756 236670 Protein-O-Mannosyltransferase 1 deficiency 607423 613150 Protein-O-Mannosyltransferase 1 deficiency 607439 606822 Protein-O-Mannose B-1,2-N-Acetylglucosaminyl transferase deficiency 253280 613154 N-Acetylglucosaminyl transferase-like Protein 608840 610308 O-Fucose-specific B1,3-N-Glucosyltransferase deficiency 261540 604327 B-1,4 Galactosyl transferase 7 deficiency 130070 606374 Glucuronosyl transferase-I deficiency 245600 608183 Chondroitin Sulfate Synthase 1 deficiency 605282 614828 GTDC2 614830 614631 Isoprenoid synthetase domain containing protein 614643 Multiple glycosylation defects 607091 B-1,4-Galactosyl transferase 1 deficiency 605881 GDP-Fucose transporter deficiency 266265 171900 Phosphoglucomannose deficiency 612934 606494 Sialyltransferase for sLe-a 615006 611090 Conserved oligomeric golgi (COG) defects 606973 COG 1 deficiency 611209 613489 COG 4 deficiency 613612 COG 5 deficiency 606977 COG 6 deficiency 608779 COG 7 deficiency 611182 COG 8 deficiency Glycosphingolipid and glycosylphosphatidylinositol (GPI) anchor defects 604402 GM3 Synthase deficiency 609056 606097 Phosphatidylinositolglycan, Class N deficiency 614080 610274 Phosphatidylinositolglycan, Class V deficiency 239303 300868 Phosphatidylinositolglycan, Class A deficiency 300818 605947 Phosphatidylinositolglycan, Class L deficiency 280000 614730 Phosphatidylinositolglycan, Class O deficiency 614749

CDG designation GALNT3-CDG POMT1-CDG POMT2-CDG POMGNT1-CDG LARGE-CDG B3GALTL-CDG B4GALT7-CDG B3GAT3-CDG CHSY1-CDG Muscular dystrophy-dystroglycanopathy Muscular dystrophy-dystroglycanopathy

B4GALT1-CDG (CDG-IId) SLC35C1-CDG (CDG-IIc) PGM1-CDG, GSD-XIV ST3GAL3-CDG

COG1-CDG (CDG-IIg) COG4-CDG (CDG-IIj) COG5-CDG (CDG-Iii) COG6-CDG (CDG-IIl) COG7-CDG (CDG-IIe) COG8-CDG (CDG-IIh) SIAT9-CDG PIGN-CDG PIGV-CDG PIGA-CDG PIGL-CDG PIGO-CDG

D. Krasnewich / Human glycosylation disorders

been described, DPAGT1-CDG (CDG-Ij) and ALG2CDG (CDG-Ii). The child with DPAGT1-CDG had congenital microcephaly and intractable seizures in addition the more typical CDG clinical features [45]. The child diagnosed with ALG2-CDG (CDG-Ii) had bilateral colobomas and a unilateral cataract at 2 months of age along with more typical features of CDG [34]. 4.3. Clinical Features of N-linked glycan defectsType II synthetic defectsOligosaccharyltransferase deficiency and defects in processing the protein-linked oligosaccharide The Type II CDG disorders are considered an emerging group of CDGs, their recognition has lagged behind Type I CDGs. The mechanisms of Type II CDG disorders involved the transfer of the oligosaccharide from dolichol to the nascent protein and the processing afterwards to their final, complex form. These disorders include defects in glycosyltransferases as well as other protein complexes integral this latter part of the N-linked synthetic pathway. These disorders illustrate the importance of the correct processing of the proteinlinked oligosaccharide in human biology and development. Examples of type II CDG disorders include: TUSC3-CDG and MAGT1-CDG result from defects in oligosaccharyltransferase complex subunits. These disorders present with non-syndromic intellectual disability [46,47]. The current gold standard of transferrin glycoform analysis fails to detect either of these disorders. The mild clinical and biochemical findings in TUSC3-CDG and MAGT1-CDG may be related to functional compensation between these two subunits. MGAT2 –CDG (CDG-IIa) was the first disorder identified in N-glycan processing and involves a defect in a glycosyltransferase involved in the transfer of a monosaccharide to a more mature protein linked oligosaccharide [48]. GCS1-CDG (CDG-IIb), with the metabolic defect involving a glucosidase, was first reported a decade ago in a female newborn of consanguineous parents with multiple congenital anomalies, hypotonia with hypermobile joints, hepatomegaly and low IgA [49]. MAN1B1-CDG, a deficiency of mannosidase, has recently been described in 12 patients from 5 families with autosomal recessive nonsyndromic intellectual disability with some dysmorphic features and seizures but no other organ involvement [50]. 4.4. Clinical features of dolichol synthesis defects Incorrectly synthesized dolichol, the carrier for the early part of the N-linked glycan synthetic path-

11

way, underlie another group of CDGs which include: SRD5A3-CDG (CDG-Iq), DK1-CDG (CDG-Im), DHDDS-CDG, DPM1-CDG (CDG-Ie), DPM3-CDG (CDG-Io), and MPDU1-CDG (CDG-If). Of clinical interest, they often present with eye, skeletal and skin problems [51] (Table 3). Among the dolichol synthesis defects, SRD5A3CDG (CDG-Iq) has been reported in approximately 20 patients. Their presentations include significant eye findings such as coloboma, hypoplasia of the optic disc and/or cataracts. Their skin involvement is also significant with ichthyosis typically present. In addition, the more typical features of other CDG types were seen. Some individuals with SRD5A3-CDG may have a normal transferrin profile, therefore molecular sequencing of the SRD5A3 gene is required for diagnosis. The metabolic defect in DK1-CDG (CDG-Im) is in dolichol kinase, the final step in the synthesis of dolichol phosphate. Four DK1-CDG (CDG-Im) patients were reported in 2007 with ichthyosis and sparse hair at birth or progressive hair loss during the neonatal period, all had neurologic involvement died in infancy. 4.5. N-linked glycan synthetic defects – Unique mechanisms SLC35C1-CDG (CDG-IIc) is most commonly known as Leukocyte Adhesion deficiency type II syndrome (LAD II). Less than 10 individuals have been reported since this disorder was first identified in 1992 and the metabolic defect is in the GDP fucose transporter. The clinical presentation of individuals with SLC35C1-CDG (CDG-IIc) includes the typical CDG findings, however, individuals have marked neutrophilia even when healthy, they have an absence of pus formation at the site of infection [37]. SLC35A1-CDG (CDG-IIf) is caused by a defect in the CMP-sialic acid transporter. Only one patient has been reported, presenting with spontaneous, recurrent episodes of bleeding, thrombocytopenia and neutropenia [52]. ATP6V0A2-CDG is autosomal recessive cutis laxa type 2A [30] caused by a proton pump deficiency. These patients skin problems, congenital brain and palate anomolies. SEC23B-CDG also known as Congenital Dyserythropoietic Anemia type II (CDAII) has recently found to be due to mutations in the gene coding for SEC23B, a COPII component, coat protein complex for vesicle budding from the endoplasmic reticulum. More than 300 patients have been reported. CDAII is characterized by ineffective erythropoiesis, hemolysis, erythroblast morphological abnormalities [53].

12

D. Krasnewich / Human glycosylation disorders

Defects in the Conserved Oligomeric Golgi (COG) Complex are considered mixed O-linked and N-linked glycan synthetic defects. COG-CDGs are due to defects in one of the eight subunits of conserved oligomeric golgi (COG) complexes. These complexes are important parts of determining the structure and function of the Golgi apparatus. Defects have been reported in COG1-CDG (CDG-IIg), COG4-CDG (CDGIIj), COG5-CDG (CDG-IIi), COG6-CDG (CDG-IIL), COG7-CDG (CDG-IIe), and COG8-CDG (CDG-IIh). Patients with COG1-CDG (CDG-IIg), COG6-CDG (CDG-IIL), COG7-CDG (CDG-IIe), and COG8-CDG (CDG-IIh) deficiency have severe phenotypes typical of individuals with other CDG types. COG4-CDG (CDG-IIj) or COG5-CDG (CDG-Iii) deficient patients have a relatively mild phenotype. COG1-CDG (CDGIIg) patients may present with skeletal abnormalities that appear as a cerebro-costo-mandibular-like syndrome. Eight patients have been reported with COG7CDG (CDG-IIe), all with fevers of unknown origin, and some with wrinkled skin, cutis laxa, and adducted thumbs. Only 3 patients have been identified with COG8-CDG. Neuromuscular features in COG8-CDG (CDG-IIh) may resemble mitochondrial disorders [54]. 4.6. O-linked glycan, GAG, glycolipid and glycosylphosphatidylinositol (GPI) anchor synthetic defects O-linked glycosylation are glycans attached to the threonine or serine of proteins. They are not processed but built sequentially by glycosyltransferases, however, the simple appearing sequential nature of the synthetic pathway disguises the incredible diversity of the resulting O-linked structures. As research continues, their biologic functional roles continue to evolve with known roles including blood group antigens, saliva mucous, extracellular matrix, and immune response. O-linked glycosylation synthesis defects include defective synthesis of protein O-linked oligosaccharides or alpha-dystroglycan disorders as well as proteoglycan synthesis defects. The muscle-eye-brain disease group or alpha-dystroglycan disorders includes individuals with POMT1/POMT2-CDG, POMGNT1CDG, FKTN-CDG, and LARGE-CDG all have significant developmental delay. Individuals with proteoglycan synthesis defects typically have bone and/or cartilage involvement but normal cognitive function and include EXT1/EXT2CDG, SLC35D1-CDG, LFNG-CDG, CHST6-CDG, CHST14-CDG. Some other proteoglycan synthesis

disorders lead to bone and cartilage involvement with developmental and intellectual disabilities. These include CHSY1-CDG, B3GAT3-CDG, B4GALT7-CDG, GALNT3-CDG, and B3GALTL-CDG. The diagnosis of the O-linked disorders is difficult because, at present, there are no blood screening tests, like transferrin glycosylation analysis in N-linked CDGs. This work has been hindered by the complexity of the Olined glycans and the organ system specificity of the initial O-linked linkage. For example there are few easily accessed circulating proteins with the O-mannose linkage, mannose to serine or threonine, so diagnosis of metabolic disorders of this type of glycan currently requires muscle biopsy, making diagnostic screening higher risk and technically more difficult. Research in this arena is ongoing. 4.7. O-linked glycan synthetic defects. Alpha-dystroglycan glycosylation disorders O-linked glycan synthetic defects lead to a group of congenital muscular dystrophies having clinical involvement of the muscle, eye and brain. This group includes POMT1/POMT2-CDG, also known as WalkerWarburg syndrome, which is a rare, often fatal, neuronal migration disorder characterized by brain and eye abnormalities with congenital muscular dystrophy. These individuals can show congenital brain anomalies including “cobblestone” lissencephaly, agenesis of the corpus callosum, cerebellar hypoplasia, hydrocephaly, and sometimes encephalocoele [55]. FKTNCDG, another O-linked glycan defect, is also known as Fukuyama congenital muscular dystrophy (FCMD). FCMD presents with variable clinical features including 1) severe muscle, eye, and brain anomalies, 2) a congenital muscular dystrophy without intellectual disability or, 3) a mild limb-girdle form of muscular dystrophy. POMGNT1-CDG, also known as, muscleeye-brain disease (MEB), is similar, but less severe than Walker-Warburg syndrome. The diagnosis of each of these disorders is made by the lack of immunostaining of correctly glycosylated alpha-dystroglycan in a muscle biopsy from an individual where clinical suspicion has been raised [56]. The complexity of this diagnostic assay in the lab and recognized risk of a muscle biopsy make this test unlikely to be used to screen individuals for this group of disorders, leading to underdiagnosis of these disorders in the community.

D. Krasnewich / Human glycosylation disorders

5. GAG synthetic defects Deficiencies in proteoglycan biosynthesis are often detected with reduced immunostaining by specific antibodies towards different glycosaminoglycan chains in cells or tissues. Confirmation is by molecular analysis of genes in the synthetic pathway [2,11]. Skeletal and skin findings are often features in these disorders. Skeletal and skin findings with normal cognitive ability is seen in individuals affected by EXT1/EXT2-CDG, SLC35D1-CDG, LFNG-CDG, CHST3-CDG and CHST14-CDG. Skeletal and skin features with intellectual impairment is seen in individuals diagnosed with B4GALT7-CDG, B3GAT3-CDG, CHSY1-CDG, and GALNT3-CDG . Over 70% of the cases with autosomal dominant hereditary multiple exostoses carry the diagnosis of EXT1/EXT2-CDG. Their disease is characterized by osteochondromas at the end of the long bones and affected individuals are at increased risk of developing chondrosarcomas and osteosarcomas [57]. The exostosin (EXT) family of genes encodes glycosyltransferases involved in heparan sulfate synthesis. SLC35D1-CDG presents as Schneckenbecken dysplasia, a rare lethal skeletal dysplasia and LFNG-CDG is an autosomal recessive spondylocostal dysotosis type 3 with extensive vertebral anomalies, and camptodactyly of fingers. CHST3-CDG, reported in approximately 20 individuals, causes spondyloepiphyseal dysplasia with congenital joint dislocation. CHST14-CDG is the cause of a musculo-contractural type of Ehlers-Danlos, more commonly known as Adducted Thumb-Clubfoot syndrome. CHSY1-CDG causes autosomal recessive Temtamy preaxial brachydactyly syndrome, characterized by limb malformations, digital anomalies, short stature, and hearing loss [58,59]. B3GAT3-CDG is a newly described CDG caused by deficiency in a glucuronyltransferase involved in the synthesis of dermatan sulfate, chondroitin sulfate and heparan sulfate. Clinical features include congenital heart defects such bicuspid aortic valve and aortic root dilatation, multiple joint dislocations and short stature Three patients with B4GALT7-CDG presented with Ehlers-Danlos syndrome progeroid form, with premature aging, macrocephaly, loose, elastic skin, thin, atrophic scars, joint hyperlaxity, hypotonia and developmental delay GALNT3-CDG is one of the major causes of hyperphosphatemic familial tumoral calcinosis. These patients present with hyperphosphatemia from increased renal phosphate retention, recurrent painful calcified

13

subcutaneous masses. B3GALTL-CDG is responsible for Peters-plus syndrome. This disorder is characterized by eye anomalies including corneal opacities and irido-corneal adhesions, intellectual disabilities, seizures and mild dysmorphic facial features. It can be detected by the deficiency of O-fucosyl glycoamino acids in urine. Five disorders have been reported with defects in the glycosylation of glycolipids and GPI anchors including SIAT9-CDG (Amish infantile epilepsy) and GPI deficiencies due to PIGA, PIGM, PIGN or PIGV mutations. Any glycoprotein based CDG testing will not detect this group of disorders. 5.1. Glycolipid synthesis disorder SIAT9-CDG (Amish infantile epilepsy) is an infantile-onset epilepsy syndrome associated with developmental delay and blindness. SIAT9 encodes GM3 synthase. Eight patients within one Amish pedigree have been reported [60]. 5.2. GPI anchor synthesis disorders X-linked PIGA-CDG causes paroxysmal nocturnal hemoglobinuria (PNH), a potentially life-threatening hemolytic anemia associated with hemoglobinuria and thrombosis. PIGM-CDG is the first CDG identified in the GPI-anchor biosynthesis pathway. It is an autosomal recessive condition, characterized by venous thrombosis and seizures [61]. PIGN-CDG is a multiple congenital anomaly-hypotonia-seizure syndrome [62]. PIGV-CDG is characterized as a hyperphosphatasiaintellectual disability syndrome, also known as Mabry syndrome. The hyperphosphatasia is due to a secondary alkaline phosphatase deficiency, a GPI-anchored protein [63]. 5.3. Approaches to identifying new genetic defects in N-linked CDGs Molecular testing for the type of CDG is not successful then finding the defective step in the N-linked glycan synthetic pathway in an affected individual, and defining a novel type of N-linked CDG, requires a research approach. This mode of glycobiologic research is growing and becoming more innovative. Historically, fibroblasts and/or leukocytes from affected children or adults were assayed for the accumulation of LLOs, which when incorrectly made by the cell, may not be efficiently transferred to the protein. Once the

14

D. Krasnewich / Human glycosylation disorders

structure of these LLOs has been determined, a candidate gene, coding for the apparent defective enzyme, can be identified and sequencing can reveal causative sequence changes. Complementation assays have also been done in yeast or mammalian cell lines. Saccharomyces cerevisiae mutants have a strong historical presence in the research of the early defects in N-linked CDGs since their N-linked glycosylation pathway is similar to the human pathway. The resource of yeast mutants available in the scientific community have provided complementation systems to test potential deleterious mutations in orthologues to human proteins involved in glycosynthetic pathways. Normal human alleles rescue abnormal glycosylation of a marker protein while mutant alleles do not. Chinese hamster ovary cells have also been used for this purpose and several novel types of N-linked CDGs have been defined in using these model organisms. With the advent of whole exome and whole genome sequencing, a reverse approach to defining CDG types is now being reported in the literature. Recently, interpretation of the genomic sequence of an individual with an undiagnosed disorder by a sequencing lab yielded a variant in a gene in the glycan synthetic pathway. Glycobiologic laboratories may then use functional assays either enzyme or cell based to test whether this genetic variant impacts the glycosyltransferase or transporter in model organism systems. This approach was used to define mosaicism of the UDP-galactose transporter, SLC352A, in three unrelated families [18]. 5.4. Therapeutics There are only two types of CDG where carbohydrate therapy has been suggested to be beneficial, MPICDG (CDG-Ib) and SLC35C1 (CDG-IIc). MPI-CDG (CDG-Ib), characterized by hepatic-intestinal disease without developmental delay, is the most common type of CDG for which carbohydrate therapy is considered standard of care. Because this is a rare disorder and the natural history is quite variable and few individuals have been treated, careful monitoring and discussion among physicians treating these individuals are warranted [39]. In the first reported case, mannose normalized hypoproteinemia and coagulation defects and rapidly improved the protein-losing enteropathy and hypoglycemia [64]. In two children with MPI-CDG (CDGIb) treated from infancy with mannose, protein-losing

enteropathy and vomiting improved significantly; however, they were recently reported to have progressive liver fibrosis [65]. Individuals with SLC35C1-CDG (CDG-IIc) present with immunodeficiency, neutropenia and macrothrombocytopenia. In one affected individual fucose improved the fucosylation of glycoproteins and reduced recurrent infections [66]. Another concept that has been highlighted in CDG clinical stories is maternal compensation of monosaccharide substrate or other factors required for correct synthesis of N-linked oligosaccharides. There are two clinical examples. The first is the initial attempt at prenatal diagnosis in a family with a child with PMM2CDG (CDG-Ia) prior to the availability of molecular diagnostics. Periumbilical cord blood sampling of a fetus, with an affected sibling, showed normal serum transferrin glycosylation. While the expectation at that time, was that the fetus was unaffected continued sampling of the newborns blood at birth and for the following month continued to show normal transferrin glycosylation. However, at six weeks the infant’s serum transferrin showed abnormal glycosylation, cathodally migrating bands on isoelectric focusing, and, along with clinical findings, the child was diagnosed as affected with CDG. The authors hypothesized that maternal factors compensated for the fetal enzyme deficit which then showed true after 6 weeks without maternal placental impact. Another example is in CDG-Id, where a chorion cells from a 18 week affected fetus synthesized truncated LLO species despite blood samples from the fetus showing normal glycosylation of several plasma glycoproteins [67].

6. In closing Today, the momentum to explore glycobiology is escalating and there are clear roles for both basic scientists as well as clinicians in this field. Communication between these experts is the key to discovery, as the basic science of glycobiology is linked to the human biologic and physiologic principles unmasked by the clinical manifestations of CDGs. In this multi-disciplinary interface, key progress will take place to define the mechanisms of human glycosylation disorders and elucidate potential therapeutic interventions for those afflicted.

D. Krasnewich / Human glycosylation disorders

References [1] [2]

[3] [4]

[5]

[6]

[7]

[8]

[9]

[10]

[11] [12]

[13] [14] [15]

[16]

[17]

[18]

[19]

[20]

H.H. Freeze, Genetic defects in the human glycome. Nat Rev Genet 7, 537 (Jul, 2006). H. H. Freeze, Understanding human glycosylation disorders: biochemistry leads the charge. The Journal of biological chemistry 288, 6936 (Mar 8, 2013). J. Jaeken, G. Matthijs, Congenital disorders of glycosylation. Annual review of genomics and human genetics 2, 129 (2001). H. Stibler, J. Jaeken, Carbohydrate deficient serum transferrin in a new systemic hereditary syndrome. Archives of disease in childhood 65, 107 (Jan, 1990). J. Jaeken et al., Sialic acid-deficient serum and cerebrospinal fluid transferrin in a newly recognized genetic syndrome. Clinica chimica acta; international journal of clinical chemistry 144, 245 (Dec 29, 1984). J. Jaeken, Komrower Lecture. Congenital disorders of glycosylation (CDG): it’s all in it! Journal of inherited metabolic disease 26, 99 (2003). T. Hennet, Diseases of glycosylation beyond classical congenital disorders of glycosylation. Biochimica et biophysica acta 1820, 1306 (Sep, 2012). T. Hennet, From glycosylation disorders back to glycosylation: what have we learned? Biochimica et biophysica acta 1792, 921 (Sep, 2009). R. Kornfeld, S. Kornfeld, Assembly of asparagine-linked oligosaccharides. Annual review of biochemistry 54, 631 (1985). S. Wopereis, D. J. Lefeber, E. Morava, R. A. Wevers, Mechanisms in protein O-glycan biosynthesis and clinical and molecular aspects of protein O-glycan biosynthesis defects: A review. Clinical chemistry 52, 574 (Apr, 2006). J. Jaeken, Congenital disorders of glycosylation. Annals of the New York Academy of Sciences 1214, 190 (Dec, 2010). J. Jaeken, Congenital disorders of glycosylation (CDG): it’s (nearly) all in it! Journal of inherited metabolic disease 34, 853 (Aug, 2011). J. Jaeken, Congenital disorders of glycosylation. Handbook of clinical neurology 113, 1737 (2013). S. E. Sparks, D. M. Krasnewich, in GeneReviews, R. A. Pagonet al., Eds. (Seattle (WA), 1993). M. Tajiri, M. Kadoya, Y. Wada, Dissociation profile of protonated fucosyl glycopeptides and quantitation of fucosylation levels of glycoproteins by mass spectrometry. Journal of proteome research 8, 688 (Feb, 2009). D. Hess, J. J. Keusch, S. A. Oberstein, R. C. Hennekam, J. Hofsteenge, Peters Plus syndrome is a new congenital disorder of glycosylation and involves defective Omicronglycosylation of thrombospondin type 1 repeats. The Journal of biological chemistry 283, 7354 (Mar 21, 2008). T. Y. Heinonen, M. Maki, Peters’-plus syndrome is a congenital disorder of glycosylation caused by a defect in the beta1, 3glucosyltransferase that modifies thrombospondin type 1 repeats. Ann Med 41, 2 (2009). B. G. Ng et al., Mosaicism of the UDP-galactose transporter SLC35A2 causes a congenital disorder of glycosylation. American journal of human genetics 92, 632 (Apr 4, 2013). J. Jaeken, T. Hennet, H. H. Freeze, G. Matthijs, On the nomenclature of congenital disorders of glycosylation (CDG). Journal of inherited metabolic disease 31, 669 (Dec, 2008). K. D. Sparks SE, PMM2-CDG (CDG-Ia) In: Pagon RA, Adam MP, Bird TD, Dolan CR, Fong CT, Stephens K, editors. GeneReviews?[Internet]. Seattle (WA): University of Wash-

15

ington, Seattle; 1993-2013., (2005 Aug 15 [updated 2011 Apr 21].). [21] S. Grunewald, The clinical spectrum of phosphomannomutase 2 deficiency (CDG-Ia). Biochimica et biophysica acta 1792, 827 (Sep, 2009). [22] H. H. Freeze, E. A. Eklund, B. G. Ng, M. C. Patterson, Neurology of inherited glycosylation disorders. Lancet neurology 11, 453 (May, 2012). [23] G. Matthijs et al., Mutations in PMM2 that cause congenital disorders of glycosylation, type Ia (CDG-Ia). Human mutation 16, 386 (Nov, 2000). [24] S. Funke et al., Perinatal and early infantile symptoms in congenital disorders of glycosylation. American journal of medical genetics. Part A 161, 578 (Mar, 2013). [25] T. C. Iancu et al., The liver in congenital disorders of glycosylation: ultrastructural features. Ultrastructural pathology 31, 189 (May-Jun, 2007). [26] G. Damen, H. de Klerk, J. Huijmans, J. den Hollander, M. Sinaasappel, Gastrointestinal and other clinical manifestations in 17 children with congenital disorders of glycosylation type Ia, Ib, and Ic. J Pediatr Gastroenterol Nutr 38, 282 (Mar, 2004). [27] E. Strom, P. Stromme, J. Westvik, S. Pedersen, Renal cysts in the carbohydrate-deficient glycoprotein syndrome. Pediatr Nephrol 7, 253 (1993). [28] B. S. Miller, H. H. Freeze, New disorders in carbohydrate metabolism: congenital disorders of glycosylation and their impact on the endocrine system. Rev Endocr Metab Disord 4, 103 (Mar, 2003). [29] D. Krasnewich, K. O’Brien, S. Sparks, Clinical features in adults with congenital disorders of glycosylation type Ia (CDG-Ia). American journal of medical genetics. Part C, Seminars in medical genetics 145, 302 (Aug 15, 2007). [30] U. Kornak et al., Impaired glycosylation and cutis laxa caused by mutations in the vesicular H+-ATPase subunit ATP6V0A2. Nature genetics 40, 32 (Jan, 2008). [31] E. Leao-Teles, D. Quelhas, L. Vilarinho, J. Jaeken, De Barsy syndrome and ATP6V0A2-CDG. European journal of human genetics: EJHG 18, 526; author reply 526 (May, 2010). [32] E. A. Eklund et al., Congenital disorder of glycosylation Ic due to a de novo deletion and an hALG-6 mutation. Biochemical and biophysical research communications 339, 755 (Jan 20, 2006). [33] C. S. Kasapkara et al., SRD5A3-CDG: a patient with a novel mutation. European journal of paediatric neurology: EJPN: official journal of the European Paediatric Neurology Society 16, 554 (Sep, 2012). [34] C. Thiel et al., A new type of congenital disorders of glycosylation (CDG-Ii) provides new insights into the early steps of dolichol-linked oligosaccharide biosynthesis. The Journal of biological chemistry 278, 22498 (Jun 20, 2003). [35] C. E. Grubenmann et al., ALG12 mannosyltransferase defect in congenital disorder of glycosylation type lg. Human molecular genetics 11, 2331 (Sep 15, 2002). [36] E. Reynders et al., Golgi function and dysfunction in the first COG4-deficient CDG type II patient. Human molecular genetics 18, 3244 (Sep 1, 2009). [37] T. Lubke et al., Complementation cloning identifies CDG-IIc, a new type of congenital disorders of glycosylation, as a GDPfucose transporter deficiency. Nature genetics 28, 73 (May, 2001). [38] T. B. Willig et al., Macrothrombocytopenia with abnormal demarcation membranes in megakaryocytes and neutropenia

16

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

D. Krasnewich / Human glycosylation disorders with a complete lack of sialyl-Lewis-X antigen in leukocytes– a new syndrome? Blood 97, 826 (Feb 1, 2001). P. de Lonlay, N. Seta, The clinical spectrum of phosphomannose isomerase deficiency, with an evaluation of mannose treatment for CDG-Ib. Biochimica et biophysica acta 1792, 841 (Sep, 2009). C. Kranz et al., Congenital disorder of glycosylation type Ik (CDG-Ik): A defect of mannosyltransferase I. American journal of human genetics 74, 545 (Mar, 2004). E. A. Eklund et al., Molecular and clinical description of the first US patients with congenital disorder of glycosylation Ig. Molecular genetics and metabolism 84, 25 (Jan, 2005). P. T. Clayton, S. Grunewald, Comprehensive description of the phenotype of the first case of congenital disorder of glycosylation due to RFT1 deficiency (CDG In). Journal of inherited metabolic disease, (Mar 11, 2009). N. Rind et al., A severe human metabolic disease caused by deficiency of the endoplasmatic mannosyltransferase hALG11 leads to congenital disorder of glycosylation-Ip. Human molecular genetics 19, 1413 (Apr 15). C. Kranz et al., CDG-Id in two siblings with partially different phenotypes. American journal of medical genetics. Part A 143A, 1414 (Jul 1, 2007). X. Wu et al., Deficiency of UDP-GlcNAc:Dolichol Phosphate N-Acetylglucosamine-1 Phosphate Transferase (DPAGT1) causes a novel congenital disorder of Glycosylation Type Ij. Human mutation 22, 144 (Aug, 2003). M. Garshasbi et al., A defect in the TUSC3 gene is associated with autosomal recessive mental retardation. American journal of human genetics 82, 1158 (May, 2008). F. Molinari et al., Oligosaccharyltransferase-subunit mutations in nonsyndromic mental retardation. American journal of human genetics 82, 1150 (May, 2008). J. Tan, J. Dunn, J. Jaeken, H. Schachter, Mutations in the MGAT2 gene controlling complex N-glycan synthesis cause carbohydrate-deficient glycoprotein syndrome type II, an autosomal recessive disease with defective brain development. American journal of human genetics 59, 810 (Oct, 1996). C. M. De Praeter et al., A novel disorder caused by defective biosynthesis of N-linked oligosaccharides due to glucosidase I deficiency. American journal of human genetics 66, 1744 (Jun, 2000). M. A. Rafiq et al., Mutations in the alpha 1,2-mannosidase gene, MAN1B1, cause autosomal-recessive intellectual disability. American journal of human genetics 89, 176 (Jul 15, 2011). V. Cantagrel, D. J. Lefeber, From glycosylation disorders to dolichol biosynthesis defects: A new class of metabolic diseases. Journal of inherited metabolic disease 34, 859 (Aug, 2011). I. Martinez-Duncker et al., Genetic complementation reveals a novel human congenital disorder of glycosylation of type II,

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

[65]

[66]

[67]

due to inactivation of the Golgi CMP-sialic acid transporter. Blood 105, 2671 (Apr 1, 2005). A. Iolascon et al., Congenital dyserythropoietic anaemias: new acquisitions. Blood transfusion = Trasfusione del sangue 9, 278 (Jul, 2011). E. Reynders, F. Foulquier, W. Annaert, G. Matthijs, How Golgi glycosylation meets and needs trafficking: the case of the COG complex. Glycobiology 21, 853 (Jul, 2011). E. Mercuri et al., Congenital muscular dystrophies with defective glycosylation of dystroglycan: a population study. Neurology 72, 1802 (May 26, 2009). E. Mercuri, F. Muntoni, The ever-expanding spectrum of congenital muscular dystrophies. Annals of neurology 72, 9 (Jul, 2012). W. Wuyts et al., Mutations in the EXT1 and EXT2 genes in hereditary multiple exostoses. American journal of human genetics 62, 346 (Feb, 1998). Y. Li et al., Temtamy preaxial brachydactyly syndrome is caused by loss-of-function mutations in chondroitin synthase 1, a potential target of BMP signaling. American journal of human genetics 87, 757 (Dec 10, 2010). J. Tian et al., Loss of CHSY1, a secreted FRINGE enzyme, causes syndromic brachydactyly in humans via increased NOTCH signaling. American journal of human genetics 87, 768 (Dec 10, 2010). M. A. Simpson et al., Infantile-onset symptomatic epilepsy syndrome caused by a homozygous loss-of-function mutation of GM3 synthase. Nature genetics 36, 1225 (Nov, 2004). A. M. Almeida et al., Hypomorphic promoter mutation in PIGM causes inherited glycosylphosphatidylinositol deficiency. Nature medicine 12, 846 (Jul, 2006). G. Maydan et al., Multiple congenital anomalies-hypotoniaseizures syndrome is caused by a mutation in PIGN. Journal of medical genetics 48, 383 (Jun). D. Horn, P. Krawitz, A. Mannhardt, G. C. Korenke, P. Meinecke, Hyperphosphatasia-mental retardation syndrome due to PIGV mutations: expanded clinical spectrum. American journal of medical genetics. Part A 155A, 1917 (Aug, 2011). H. K. Harms et al., Oral mannose therapy persistently corrects the severe clinical symptoms and biochemical abnormalities of phosphomannose isomerase deficiency. Acta paediatrica 91, 1065 (2002). K. Mention et al., Development of liver disease despite mannose treatment in two patients with CDG-Ib. Molecular genetics and metabolism 93, 40 (Jan, 2008). T. Marquardt et al., Leukocyte adhesion deficiency II syndrome, a generalized defect in fucose metabolism. The Journal of pediatrics 134, 681 (Jun, 1999). J. Denecke et al., Congenital disorder of glycosylation type Id: clinical phenotype, molecular analysis, prenatal diagnosis, and glycosylation of fetal proteins. Pediatric research 58, 248 (Aug, 2005).

Copyright of Cancer Biomarkers is the property of IOS Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.