The Neuroradiology Journal 20: 421-425, 2007
www. centauro. it
The Expanding Joubert Spectrum E.M. VALENTE*,**, F. BRANCATI*,***, B. DALLAPICCOLA*,**** * CSS-Mendel Institute; Rome, Italy ** Department of Medical and Surgical Pediatric Sciences, University of Messina; Messina, Italy *** CeSI, Aging Research Centre, and Department of Biomedical Sciences, G. d’Annunzio University Foundation; Chieti, Italy **** Department of Experimental Medicine and Pathology, University La Sapienza; Rome, Italy
Key words: Joubert syndrome; molar tooth sign; cerebellar vermis hypoplasia; cerebello-oculo-renal syndromes; Meckel syndrome
Introduction Joubert syndrome (JS [MIM 213300]) is an autosomal recessive condition characterized by hypotonia, ataxia, psychomotor delay, oculomotor apraxia and neonatal breathing abnormalities. The neuroradiological hallmark of JS is a complex midbrain-hindbrain malformation known as the “molar tooth sign” (MTS) originating from the association of cerebellar vermis hypo/aplasia, horizontally-oriented and thickened superior cerebellar peduncles and a deepened interpeduncular fossa (figure 1). These clinical and neuroradiological features are shared by at least eight distinct syndromes, termed Joubert Syndrome Related Disorders (JSRDs), which additionally present with pleiotropic involvement, mainly of the eyes and kidneys 1. The four major subgroups of JSRDs include (1) the classical form (MIM 213300), largely restricted to brain involvement also occasionally displaying additional features such as postaxial polydactyly, retinopathy, or rarely renal involvement; (2) the oculo-renal form (referred to as JSRD-SLS or CORS), associating JS neurological features with the Senior-Löken syndrome (SLS [MIM 266900]) phenotype of nephronophthisis (NPH) and retinal dystrophy (either Leber congenital amaurosis – LCA – or retinitis pigmentosa); (3) the subgroup with preaxial or mesaxial polydactlyly and oro-facial defects such as lobulated tongue, notched upper lip or cleft lip/palate, known as the OroFacial-Digital type VI syndrome (OFDVI, or Varadi-Papp syndrome [MIM 277170]); (4) the subgroup with choroidoretinal coloboma and hepatic fibrosis, referred to as the Cerebellar vermis hypo/aplasia, Oligophrenia, Ataxia,
ocular Coloboma, Hepatic fibrosis syndrome (COACH [MIM 216360]). Intriguingly, some JSRDs present striking phenotypic overlap with other syndromes lacking the MTS, including SLS, isolated NPH and Meckel syndrome (MKS). NPH is characterized by interstitial renal fibrosis and progressive cystic degeneration often leading to renal failure. Juvenile NPH, with onset usually in the second decade of life, represents the most common cause of end stage renal failure in childhood requiring dialysis or renal transplantation. Occasionally, the age at onset can be within the first five years of life with rapid progression (infantile NPH). Some patients also display associated features such as retinopathy (defining the SLS = NPH + retinopathy), hepatic fibrosis, situs inversus and variable neurological signs including mental retardation and ataxia. MKS is a severe autosomal recessive condition which is often lethal prenatally or at birth, characterized by multicystic kidney and hepatic duct dysplasia with liver bile duct proliferation, occipital meningo-encephalocele and hydrocephalus, cerebellar and posterior fossa abnormalities (including vermis agenesis), polydactyly, ocular colobomas, situs inversus and other malformations. Some fetuses present incomplete, Meckel-like phenotypes characterized by isolated vermis hypoplasia or normal brain, few tubular medullary cysts, or with normal or discrete liver changes, showing a continuum of clinical spectrum between JSRD and MKS. The striking degree of phenotypic overlap between JSRD, NPH, SLS and MKS has found support in the recent findings on the molecular genetic basis of these syndromes, which have 421
The Expanding Joubert Spectrum
E.M. Valente
Table 1 Genes and Loci Causative of JSRD and Overlap with Other Ciliopathies
Locus
Gene/Protein
NPH
SLS
JBTS
MKS
MIM nNumber
9q34
–
–
–
JBTS1
–
%213300
11p12-q13
–
–
–
JBTS2
MKS2?
%608091 %603194?
6q23
AHI1/Jouberin
–
–
JBTS3
–
#608629
*608894
2q13
Nephrocystin
NPHP1
SLSN1
JBTS4
–
#256100
*607100
12q21
CEP290
NPHP6
SLSN6
JBTS5
MKS4
#610188-9 *610142
8q24
TMEM67/Meckelin
–
–
JBTS6
MKS3
#607361
16q
FTM/RPGRIP1L
NPHP8
–
JBTS7
MKS5
highlighted three major concepts: 1) a wide genetic heterogeneity mirrors the clinical heterogeneity in all conditions, with several genes and loci being progressively identified; 2) the clinical overlap is paralleled by a marked genetic overlap, with mutations in the same gene possibly causing all JSRD, MKS and NPH phenotypes (table 1); 3) all disease genes so far identified encode proteins expressed in the primary cilium or its apparatus (basal body and centrosome). The amazing finding that virtually all proteins causative of JSRD, MKS and NPH (as well as other cystic kidney disorders such as polycystic kidney disease and other pleiotropic syndromes such as Bardet-Biedl syndrome (BBS)) are due to mutations in genes encoding ciliary proteins, has truly represented a revolution in the field 2. Primary cilia are single, microtubule-based structures growing out from basal bodies or centrosomes that have been highly conserved throughout evolution. These organelles are found in several tissues, including epithelium of renal tubules and bile ducts, retinal photoreceptors and developing neurons, and function as modular cellular sensors that can detect a wide variety of physical and chemical stimuli of a mechanic, osmotic, photonic, hormonal and olfactory nature, their specificity depending on cell type 3. In particular, in the developing brain, cilia play a major role in controlling axonal migration and polarity, necessary for correct neural tube closure and CNS development 4. This supports a unifying hypothesis for the pathogenetic mechanisms underlying the different clinical manifestations observed in ciliopathies, including cystic dysplasia of kidneys and bile ducts, retinopa422
*609884
not assigned
thy and congenital brain malformations such as encephalocele and the MTS. The tight network of interactions among distinct ciliary proteins suggests they act in a common pathway to guarantee the correct functioning of the primary cilium in the context of each specific cell type. While this hypothesis can justify the fact that mutations in different genes give rise to the same phenotype, the mechanisms underlying the extreme phenotypic variability associated to mutations in the same gene are still unclear. For instance, NPHP1 homozygous deletions, which represent the most common cause of isolated juvenile NPH, have been also detected in patients with SLS and in JSRD patients with invariable renal involvement and occasional retinopathy. Mutations in the CEP290 gene cause a peculiar JSRD phenotype associated with NPH and LCA, but the clinical spectrum is extremely large ranging from isolated LCA to the severe MKS presentation. Similarly, mutations in the MKS3 gene, first identified in MKS patients, have been also shown to cause pure JS without renal or retinal involvement and a novel gene, RPGRIP1L, has been recently identified also causing both JSRD and MKS phenotypes. A list of genes/loci causing different forms of JSRD and their involvement in overlapping syndromes are presented in table 1. Yet, due to the small number of JSRD cases linked to each genetic determinant, their phenotypic spectrum has not been fully delineated, hampering the development of efficient molecular diagnostic protocols. The following paragraphs will present a summary of the current body of knowledge related to each JBTS gene, mutation frequency and phenotypic spectrum.
www. centauro. it
The Neuroradiology Journal 20: 421-425, 2007
A
Figure 1 Brain magnetic resonance imaging of a patient with JSRD. A) Sagittal T1-weighted image demonstrating thickened superior cerebellar peduncles running horizontally toward the brain stem; b) Axial T1-weighted image at the pontine level showing thickened superior cerebellar peduncles and umbrella-shaped fourth ventricle, giving the appearance of a molar tooth.
Linkage Analysis and Existing JBTS Loci JBTS1
This locus was mapped to 9q34.3 in two consanguineous pedigrees with a classical JS phenotype. We have identified three further consanguineous families mapping to the whole JBTS1 interval, also showing a pure JS phenotype with or without retinopathy. Interestingly, the proband from two of these families had developmental delay but no mental retardation, showing this feature is not pathognomonic of JS 5.
B
fied. At difference from JBTS1, the JBTS2 phenotype is associated with multiorgan involvement of kidney, retina, and liver, in addition to the central nervous system features, and results in extreme phenotypic variability and often in severe phenotypes overlapping with MKS. One of the linked family allowed refining the linked interval to 13.6 Mb between markers D11S1344 and D11S4191, still encompassing the centromere 5. The MKS2 locus, identified by linkage analysis in some MKS families, maps to an overlapping region on the chr 11 long arm, suggesting that, as for CEP290, MKS3 and RPGRIP1L, the two conditions are allelic at this locus 6.
JBTS2
In 2002 we mapped the JBTS2 locus to the pericentromeric region of chromosome 11 (11p12-q13) in a large consanguineous Sicilian family. Dr Gleeson’s lab independently mapped the same locus in three Asian families. The minimum interval spanned 17.2 Mb including about 5 Mb of centromeric DNA. Subsequently two other JBTS2-linked families were identi-
Mutation Analysis of Known Genes AHI1 - JBTS3
The JBTS3 locus was identified in 2004 from a large consanguineous Turkish pedigree with classical JS, although one patient also displayed spasticity, microcephaly and seizures. Loss of function mutations in the AHI1 gene were in423
The Expanding Joubert Spectrum
dependently identified by two distinct groups in several JBTS3-linked families, presenting a phenotype of either pure JS (occasionally with retinal dysplasia) or JS associated with polymicrogyria and corpus callosum abnormalities. Jouberin (JBN), the product of AHI1, had unknown function and was expressed in the developing brain (mostly cerebellum and cerebral cortex). The AHI1 gene comprises 29 coding exons. We have so far screened this gene in 137 JSRD patients, including 41 probands recruited in Rome and 96 probands from Dr Gleeson’s cohort. Deleterious mutations were found in ten families with pure JS or JS plus retinal and/or additional central nervous system abnormalities, with an overall frequency of 7.3%. The phenotypic group of JS plus retinopathy showed the highest mutation frequency, with five out of 23 tested probands bearing AHI1 mutations (21.7%). Various forms of retinopathy could be detected, ranging from retinitis pigmentosa with mild visual reduction to the more severe form of LCA. Conversely, no mutations were found in 37 patients with kidney involvement, in five patients with liver disease, and in seven patients with OFD6 syndrome 7. Overall, AHI1 mutations seem to cause JS variably associated with retinopathy or other CNS malformations, but other researchers have reported very rare AHI1-positive cases with NPH or with optical colobomas. Thus, screening of larger cohorts of patients are needed to fully evaluate the AHI1 phenotypic spectrum and to establish molecular diagnostic guidelines. NPHP1 - JBTS4
NPH is a frequent clinical feature associated with the MTS. Isolated NPH represents the most frequent cause of renal failure in adolescence and is due to mutations in five distinct genes (NPHP1-5) all encoding ciliary proteins. Approximately 80% of patients with isolated juvenile NPH demonstrate a homozygous deletion of about 290 kb that encompasses the NPHP1 gene. We and others have reported few NPHP1 deleted patients showing neurological signs and the MTS 8,9. All of them had NPH (either clinically manifest or detected through a DDAVP test) and some had pigmentary retinopathy. Overall, NPHP1 homozygous deletions seem to be a rare cause of JSRD and are always associated with renal and occasionally with retinal disease. Interestingly, the appearance of the MTS in the five JSRD patients with NPHP1 deletion appears to be remarkably similar, showing moderate cerebellar vermis 424
E.M. Valente
hypoplasia and elongated but not thickened superior cerebellar peduncles 8,9. This peculiar “milder” presentation of the MTS might be specifically associated with NPHP1 deletions, but this requires confirmation in additional cases. CEP290 - JBTS5
In collaboration with Dr Gleeson’s group at UCSD, we mapped a novel locus to chr12 (JBTS5), generating a maximum LOD score of 3.46. Positive LOD scores for markers in the same chromosome 12 region were obtained in an additional six smaller families. After saturation of the region with densely spaced STS markers and definition of the minimal candidate interval, bioinformatic analysis was performed to select candidate genes. The CEP290 gene appeared to be an excellent candidate as it encoded a 2480 aa protein known to be expressed in the centrosome, an organelle crucial for the primary cilium formation. Indeed, independent homozygous CEP290 mutations were identified in five of the six families. Four of these resulted in either nonsense or frame-shift mutations, while one resulted in a nonconservative missense substitution affecting a highly conserved residue. All mutations segregated with the disease and were not found in 400 control chromosomes 10. At the same time, mutations in CEP290 were independently identified in eight distinct families. The phenotype associated with CEP290 mutations is fairly homogeneous, being nearly always characterized by JS features with typical MTS, associated with NPHP and retinopathy of variable severity (usually LCA), defining the JSRD-SLS phenotype. About 50% of JSRDSLS cases have mutations in CEP290, defining a strong genotype-phenotype correlate. Rare cases with CEP290 mutations have been reported who lack either NPH (cerebello-retinal phenotype) or retinal involvement (cerebellorenal phenotype) 11. To add complexity, CEP290 mutations have been identified as causative of both isolated LCA (pure retinal phenotype), and of MKS 12,13. MKS3 - JBTS6
A recent work has demonstrated MKS3 mutations not only in fetuses with diagnosis of MKS but also in patients with pure JS and neuroradiologically proven MTS 14. Only three JS patients have been reported so far bearing MKS3 mutations and screens of large JSRD cohorts have not been performed yet, thus the phenotypic spectrum still has to be elucidated.
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The Neuroradiology Journal 20: 421-425, 2007
RPGRIP1L - JBTS7
Conclusions
The RPGRIP1L gene has been very recently identified as causative of JSRD and MKS by two distinct research groups. So far, two MKS and eight JSRD families have been reported which harbour mutations in this gene 15,16. Despite the number of mutated cases is still low, there appears to be a striking genotype-phenotype correlate, since all JSRD cases with RPGRIP1L mutations had a consistent cerebellorenal phenotype without retinal involvement. This correlation warrants further confirmation in larger groups of JSRD patients. The RPGRIP1L gene also encodes for a protein localised to basal bodies and centrosomes that interacts with other ciliary proteins encoded by NPH and JBTS genes.
No therapies for JSRDs are available at the moment. The genetic heterogeneity, the lack of a clear clinical-genetic classification, and the “splitting and lumping” (one gene = distinct phenotypes and one phenotype = distinct genes) make genetic counseling difficult. The detailed characterization of a growing number of patients with mutations in known genes, the identification of novel JBTS genes and the functional characterization of their product within the ciliary network represent crucial steps to pave the way for a novel genetically-driven classification of these disorders and better management of patients.
References 1 Gleeson JG, Keeler LC, Parisi MA et Al: Molar tooth sign of the midbrain-hindbrain junction: occurrence in multiple distinct syndromes. Am J Med Genet 125: 125-134, 2004. 2 Hildebrandt F, Otto E: Cilia and centrosomes: a unifying pathogenic concept for cystic kidney disease? Nat Rev Genet 6: 928-940, 2005. 3 Singla V, Reiter JF: The primary cilium as the cell’s antenna: signaling at a sensory organelle. Science 313: 629-633, 2006. 4 Louie CM, Gleeson JG: Genetic basis of Joubert syndrome and related disorders of cerebellar development. Hum Mol Genet 14 Spec N.2: R235-242, 2005. 5 Valente EM, Marsh SE, Castori M et Al: Distinguishing the four genetic causes of Jouberts syndrome-related disorders. Ann Neurol 57: 513-519, 2005. 6 Roume J, Genin E, Cormier-Daire V et Al: A gene for Meckel syndrome maps to chromosome 11q13. Am J Hum Genet 63: 1095-1101, 1998. 7 Valente EM, Brancati F, Silhavy JL et Al: AHI1 gene mutations cause specific forms of Joubert syndrome-related disorders. Ann Neurol 59: 527-534, 2006. 8 Castori M, Valente EM, Donati MA et Al: NPHP1 gene deletion is a rare cause of Joubert syndrome related disorders. J Med Genet 42: e9, 2005. 9 Tory K, Lacoste T, Burglen L et Al: High NPHP1 and NPHP6 mutation rate in patients with Joubert syndrome and nephronophthisis: potential epistatic effect of NPHP6 and AHI1 mutations in patients with NPHP1 mutations. J Am Soc Nephrol 18: 1566-1575, 2007. 10 Valente EM, Silhavy JL, Brancati F et Al: Mutations in CEP290, which encodes a centrosomal protein, cause pleiotropic forms of Joubert syndrome. Nat Genet 38: 623-625, 2006. 11 Brancati F, Barrano G, Silhavy JL et Al: CEP290 Mutations Are Frequently Identified in the Oculo-Renal Form of Joubert Syndrome Related Disorders. Am J Hum Genet 81: 104-113, 2007.
12 den Hollander AI, Koenekoop RK, Yzer S et Al: Mutations in the CEP290 NPHP6. gene are a frequent cause of Leber congenital amaurosis. Am J Hum Genet 79: 556-561, 2006. 13 Baala L, Audollent S, Martinovic J et Al: Pleiotropic effects of CEP290 (NPHP6) mutations extend to MeckelGruber syndrome. Am J Hum Genet 81: 170-179, 2007. 14 Baala L, Romano S, Khaddour R et Al: The MeckelGruber syndrome gene, MKS3, is mutated in Joubert syndrome. Am J Hum Genet 80: 186-194, 2007. 15 Delous M, Baala L, Salomon R et Al: The ciliary gene RPGRIP1L is mutated in cerebello-oculo-renal syndrome (Joubert syndrome type B) and Meckel syndrome. Nat Genet 2007. 16 Arts HH, Doherty D, van Beersum SE et Al: Mutations in the gene encoding the basal body protein RPGRIP1L, a nephrocystin-4 interactor, cause Joubert syndrome. Nat Genet 2007.
E.M. Valente, MD, PhD CSS-Mendel Institute viale Regina Margherita 261 00198 Rome - Italy Tel.: +39 06 44160537 Fax: +39 0644160548 E-Mail: [email protected]
425
www. centauro. it
The Expanding Joubert Spectrum E.M. VALENTE*,**, F. BRANCATI*,***, B. DALLAPICCOLA*,**** * CSS-Mendel Institute; Rome, Italy ** Department of Medical and Surgical Pediatric Sciences, University of Messina; Messina, Italy *** CeSI, Aging Research Centre, and Department of Biomedical Sciences, G. d’Annunzio University Foundation; Chieti, Italy **** Department of Experimental Medicine and Pathology, University La Sapienza; Rome, Italy
Key words: Joubert syndrome; molar tooth sign; cerebellar vermis hypoplasia; cerebello-oculo-renal syndromes; Meckel syndrome
Introduction Joubert syndrome (JS [MIM 213300]) is an autosomal recessive condition characterized by hypotonia, ataxia, psychomotor delay, oculomotor apraxia and neonatal breathing abnormalities. The neuroradiological hallmark of JS is a complex midbrain-hindbrain malformation known as the “molar tooth sign” (MTS) originating from the association of cerebellar vermis hypo/aplasia, horizontally-oriented and thickened superior cerebellar peduncles and a deepened interpeduncular fossa (figure 1). These clinical and neuroradiological features are shared by at least eight distinct syndromes, termed Joubert Syndrome Related Disorders (JSRDs), which additionally present with pleiotropic involvement, mainly of the eyes and kidneys 1. The four major subgroups of JSRDs include (1) the classical form (MIM 213300), largely restricted to brain involvement also occasionally displaying additional features such as postaxial polydactyly, retinopathy, or rarely renal involvement; (2) the oculo-renal form (referred to as JSRD-SLS or CORS), associating JS neurological features with the Senior-Löken syndrome (SLS [MIM 266900]) phenotype of nephronophthisis (NPH) and retinal dystrophy (either Leber congenital amaurosis – LCA – or retinitis pigmentosa); (3) the subgroup with preaxial or mesaxial polydactlyly and oro-facial defects such as lobulated tongue, notched upper lip or cleft lip/palate, known as the OroFacial-Digital type VI syndrome (OFDVI, or Varadi-Papp syndrome [MIM 277170]); (4) the subgroup with choroidoretinal coloboma and hepatic fibrosis, referred to as the Cerebellar vermis hypo/aplasia, Oligophrenia, Ataxia,
ocular Coloboma, Hepatic fibrosis syndrome (COACH [MIM 216360]). Intriguingly, some JSRDs present striking phenotypic overlap with other syndromes lacking the MTS, including SLS, isolated NPH and Meckel syndrome (MKS). NPH is characterized by interstitial renal fibrosis and progressive cystic degeneration often leading to renal failure. Juvenile NPH, with onset usually in the second decade of life, represents the most common cause of end stage renal failure in childhood requiring dialysis or renal transplantation. Occasionally, the age at onset can be within the first five years of life with rapid progression (infantile NPH). Some patients also display associated features such as retinopathy (defining the SLS = NPH + retinopathy), hepatic fibrosis, situs inversus and variable neurological signs including mental retardation and ataxia. MKS is a severe autosomal recessive condition which is often lethal prenatally or at birth, characterized by multicystic kidney and hepatic duct dysplasia with liver bile duct proliferation, occipital meningo-encephalocele and hydrocephalus, cerebellar and posterior fossa abnormalities (including vermis agenesis), polydactyly, ocular colobomas, situs inversus and other malformations. Some fetuses present incomplete, Meckel-like phenotypes characterized by isolated vermis hypoplasia or normal brain, few tubular medullary cysts, or with normal or discrete liver changes, showing a continuum of clinical spectrum between JSRD and MKS. The striking degree of phenotypic overlap between JSRD, NPH, SLS and MKS has found support in the recent findings on the molecular genetic basis of these syndromes, which have 421
The Expanding Joubert Spectrum
E.M. Valente
Table 1 Genes and Loci Causative of JSRD and Overlap with Other Ciliopathies
Locus
Gene/Protein
NPH
SLS
JBTS
MKS
MIM nNumber
9q34
–
–
–
JBTS1
–
%213300
11p12-q13
–
–
–
JBTS2
MKS2?
%608091 %603194?
6q23
AHI1/Jouberin
–
–
JBTS3
–
#608629
*608894
2q13
Nephrocystin
NPHP1
SLSN1
JBTS4
–
#256100
*607100
12q21
CEP290
NPHP6
SLSN6
JBTS5
MKS4
#610188-9 *610142
8q24
TMEM67/Meckelin
–
–
JBTS6
MKS3
#607361
16q
FTM/RPGRIP1L
NPHP8
–
JBTS7
MKS5
highlighted three major concepts: 1) a wide genetic heterogeneity mirrors the clinical heterogeneity in all conditions, with several genes and loci being progressively identified; 2) the clinical overlap is paralleled by a marked genetic overlap, with mutations in the same gene possibly causing all JSRD, MKS and NPH phenotypes (table 1); 3) all disease genes so far identified encode proteins expressed in the primary cilium or its apparatus (basal body and centrosome). The amazing finding that virtually all proteins causative of JSRD, MKS and NPH (as well as other cystic kidney disorders such as polycystic kidney disease and other pleiotropic syndromes such as Bardet-Biedl syndrome (BBS)) are due to mutations in genes encoding ciliary proteins, has truly represented a revolution in the field 2. Primary cilia are single, microtubule-based structures growing out from basal bodies or centrosomes that have been highly conserved throughout evolution. These organelles are found in several tissues, including epithelium of renal tubules and bile ducts, retinal photoreceptors and developing neurons, and function as modular cellular sensors that can detect a wide variety of physical and chemical stimuli of a mechanic, osmotic, photonic, hormonal and olfactory nature, their specificity depending on cell type 3. In particular, in the developing brain, cilia play a major role in controlling axonal migration and polarity, necessary for correct neural tube closure and CNS development 4. This supports a unifying hypothesis for the pathogenetic mechanisms underlying the different clinical manifestations observed in ciliopathies, including cystic dysplasia of kidneys and bile ducts, retinopa422
*609884
not assigned
thy and congenital brain malformations such as encephalocele and the MTS. The tight network of interactions among distinct ciliary proteins suggests they act in a common pathway to guarantee the correct functioning of the primary cilium in the context of each specific cell type. While this hypothesis can justify the fact that mutations in different genes give rise to the same phenotype, the mechanisms underlying the extreme phenotypic variability associated to mutations in the same gene are still unclear. For instance, NPHP1 homozygous deletions, which represent the most common cause of isolated juvenile NPH, have been also detected in patients with SLS and in JSRD patients with invariable renal involvement and occasional retinopathy. Mutations in the CEP290 gene cause a peculiar JSRD phenotype associated with NPH and LCA, but the clinical spectrum is extremely large ranging from isolated LCA to the severe MKS presentation. Similarly, mutations in the MKS3 gene, first identified in MKS patients, have been also shown to cause pure JS without renal or retinal involvement and a novel gene, RPGRIP1L, has been recently identified also causing both JSRD and MKS phenotypes. A list of genes/loci causing different forms of JSRD and their involvement in overlapping syndromes are presented in table 1. Yet, due to the small number of JSRD cases linked to each genetic determinant, their phenotypic spectrum has not been fully delineated, hampering the development of efficient molecular diagnostic protocols. The following paragraphs will present a summary of the current body of knowledge related to each JBTS gene, mutation frequency and phenotypic spectrum.
www. centauro. it
The Neuroradiology Journal 20: 421-425, 2007
A
Figure 1 Brain magnetic resonance imaging of a patient with JSRD. A) Sagittal T1-weighted image demonstrating thickened superior cerebellar peduncles running horizontally toward the brain stem; b) Axial T1-weighted image at the pontine level showing thickened superior cerebellar peduncles and umbrella-shaped fourth ventricle, giving the appearance of a molar tooth.
Linkage Analysis and Existing JBTS Loci JBTS1
This locus was mapped to 9q34.3 in two consanguineous pedigrees with a classical JS phenotype. We have identified three further consanguineous families mapping to the whole JBTS1 interval, also showing a pure JS phenotype with or without retinopathy. Interestingly, the proband from two of these families had developmental delay but no mental retardation, showing this feature is not pathognomonic of JS 5.
B
fied. At difference from JBTS1, the JBTS2 phenotype is associated with multiorgan involvement of kidney, retina, and liver, in addition to the central nervous system features, and results in extreme phenotypic variability and often in severe phenotypes overlapping with MKS. One of the linked family allowed refining the linked interval to 13.6 Mb between markers D11S1344 and D11S4191, still encompassing the centromere 5. The MKS2 locus, identified by linkage analysis in some MKS families, maps to an overlapping region on the chr 11 long arm, suggesting that, as for CEP290, MKS3 and RPGRIP1L, the two conditions are allelic at this locus 6.
JBTS2
In 2002 we mapped the JBTS2 locus to the pericentromeric region of chromosome 11 (11p12-q13) in a large consanguineous Sicilian family. Dr Gleeson’s lab independently mapped the same locus in three Asian families. The minimum interval spanned 17.2 Mb including about 5 Mb of centromeric DNA. Subsequently two other JBTS2-linked families were identi-
Mutation Analysis of Known Genes AHI1 - JBTS3
The JBTS3 locus was identified in 2004 from a large consanguineous Turkish pedigree with classical JS, although one patient also displayed spasticity, microcephaly and seizures. Loss of function mutations in the AHI1 gene were in423
The Expanding Joubert Spectrum
dependently identified by two distinct groups in several JBTS3-linked families, presenting a phenotype of either pure JS (occasionally with retinal dysplasia) or JS associated with polymicrogyria and corpus callosum abnormalities. Jouberin (JBN), the product of AHI1, had unknown function and was expressed in the developing brain (mostly cerebellum and cerebral cortex). The AHI1 gene comprises 29 coding exons. We have so far screened this gene in 137 JSRD patients, including 41 probands recruited in Rome and 96 probands from Dr Gleeson’s cohort. Deleterious mutations were found in ten families with pure JS or JS plus retinal and/or additional central nervous system abnormalities, with an overall frequency of 7.3%. The phenotypic group of JS plus retinopathy showed the highest mutation frequency, with five out of 23 tested probands bearing AHI1 mutations (21.7%). Various forms of retinopathy could be detected, ranging from retinitis pigmentosa with mild visual reduction to the more severe form of LCA. Conversely, no mutations were found in 37 patients with kidney involvement, in five patients with liver disease, and in seven patients with OFD6 syndrome 7. Overall, AHI1 mutations seem to cause JS variably associated with retinopathy or other CNS malformations, but other researchers have reported very rare AHI1-positive cases with NPH or with optical colobomas. Thus, screening of larger cohorts of patients are needed to fully evaluate the AHI1 phenotypic spectrum and to establish molecular diagnostic guidelines. NPHP1 - JBTS4
NPH is a frequent clinical feature associated with the MTS. Isolated NPH represents the most frequent cause of renal failure in adolescence and is due to mutations in five distinct genes (NPHP1-5) all encoding ciliary proteins. Approximately 80% of patients with isolated juvenile NPH demonstrate a homozygous deletion of about 290 kb that encompasses the NPHP1 gene. We and others have reported few NPHP1 deleted patients showing neurological signs and the MTS 8,9. All of them had NPH (either clinically manifest or detected through a DDAVP test) and some had pigmentary retinopathy. Overall, NPHP1 homozygous deletions seem to be a rare cause of JSRD and are always associated with renal and occasionally with retinal disease. Interestingly, the appearance of the MTS in the five JSRD patients with NPHP1 deletion appears to be remarkably similar, showing moderate cerebellar vermis 424
E.M. Valente
hypoplasia and elongated but not thickened superior cerebellar peduncles 8,9. This peculiar “milder” presentation of the MTS might be specifically associated with NPHP1 deletions, but this requires confirmation in additional cases. CEP290 - JBTS5
In collaboration with Dr Gleeson’s group at UCSD, we mapped a novel locus to chr12 (JBTS5), generating a maximum LOD score of 3.46. Positive LOD scores for markers in the same chromosome 12 region were obtained in an additional six smaller families. After saturation of the region with densely spaced STS markers and definition of the minimal candidate interval, bioinformatic analysis was performed to select candidate genes. The CEP290 gene appeared to be an excellent candidate as it encoded a 2480 aa protein known to be expressed in the centrosome, an organelle crucial for the primary cilium formation. Indeed, independent homozygous CEP290 mutations were identified in five of the six families. Four of these resulted in either nonsense or frame-shift mutations, while one resulted in a nonconservative missense substitution affecting a highly conserved residue. All mutations segregated with the disease and were not found in 400 control chromosomes 10. At the same time, mutations in CEP290 were independently identified in eight distinct families. The phenotype associated with CEP290 mutations is fairly homogeneous, being nearly always characterized by JS features with typical MTS, associated with NPHP and retinopathy of variable severity (usually LCA), defining the JSRD-SLS phenotype. About 50% of JSRDSLS cases have mutations in CEP290, defining a strong genotype-phenotype correlate. Rare cases with CEP290 mutations have been reported who lack either NPH (cerebello-retinal phenotype) or retinal involvement (cerebellorenal phenotype) 11. To add complexity, CEP290 mutations have been identified as causative of both isolated LCA (pure retinal phenotype), and of MKS 12,13. MKS3 - JBTS6
A recent work has demonstrated MKS3 mutations not only in fetuses with diagnosis of MKS but also in patients with pure JS and neuroradiologically proven MTS 14. Only three JS patients have been reported so far bearing MKS3 mutations and screens of large JSRD cohorts have not been performed yet, thus the phenotypic spectrum still has to be elucidated.
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The Neuroradiology Journal 20: 421-425, 2007
RPGRIP1L - JBTS7
Conclusions
The RPGRIP1L gene has been very recently identified as causative of JSRD and MKS by two distinct research groups. So far, two MKS and eight JSRD families have been reported which harbour mutations in this gene 15,16. Despite the number of mutated cases is still low, there appears to be a striking genotype-phenotype correlate, since all JSRD cases with RPGRIP1L mutations had a consistent cerebellorenal phenotype without retinal involvement. This correlation warrants further confirmation in larger groups of JSRD patients. The RPGRIP1L gene also encodes for a protein localised to basal bodies and centrosomes that interacts with other ciliary proteins encoded by NPH and JBTS genes.
No therapies for JSRDs are available at the moment. The genetic heterogeneity, the lack of a clear clinical-genetic classification, and the “splitting and lumping” (one gene = distinct phenotypes and one phenotype = distinct genes) make genetic counseling difficult. The detailed characterization of a growing number of patients with mutations in known genes, the identification of novel JBTS genes and the functional characterization of their product within the ciliary network represent crucial steps to pave the way for a novel genetically-driven classification of these disorders and better management of patients.
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12 den Hollander AI, Koenekoop RK, Yzer S et Al: Mutations in the CEP290 NPHP6. gene are a frequent cause of Leber congenital amaurosis. Am J Hum Genet 79: 556-561, 2006. 13 Baala L, Audollent S, Martinovic J et Al: Pleiotropic effects of CEP290 (NPHP6) mutations extend to MeckelGruber syndrome. Am J Hum Genet 81: 170-179, 2007. 14 Baala L, Romano S, Khaddour R et Al: The MeckelGruber syndrome gene, MKS3, is mutated in Joubert syndrome. Am J Hum Genet 80: 186-194, 2007. 15 Delous M, Baala L, Salomon R et Al: The ciliary gene RPGRIP1L is mutated in cerebello-oculo-renal syndrome (Joubert syndrome type B) and Meckel syndrome. Nat Genet 2007. 16 Arts HH, Doherty D, van Beersum SE et Al: Mutations in the gene encoding the basal body protein RPGRIP1L, a nephrocystin-4 interactor, cause Joubert syndrome. Nat Genet 2007.
E.M. Valente, MD, PhD CSS-Mendel Institute viale Regina Margherita 261 00198 Rome - Italy Tel.: +39 06 44160537 Fax: +39 0644160548 E-Mail: [email protected]
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