Joubert Syndrome Related Disorders (JSRDs) are autosomal recessive pleiotropic conditions sharing a peculiar cerebellar and brainstem malformation known as the "molar tooth sign" (MTS). Recently, mutations in a novel ciliary gene, RPGRIP1L, have been shown to cause both JSRDs and MeckelGruber syndrome. We searched for RPGRIP1L mutations in 120 patients with proven MTS and phenotypes representative of all JSRD clinical subgroups. Two homozygous mutations, the previously reported p.T615P in exon 15 and the novel c.2268_2269delA in exon 16, were detected in two out of 16 families with cerebello-renal presentation (~12%). Conversely, no pathogenic changes were found in patients with other JSRD phenotypes, suggesting that RPGRIP1L mutations are largely confined to the cerebello-renal subgroup, while they overall represent a rare cause of JSRD (<2%).
Ciliopathies are an expanding group of rare conditions characterised by multiorgan involvement, that are caused by mutations in genes encoding for proteins of the primary cilium or its apparatus. Among these genes, CEP290 bears an intriguing allelic spectrum, being commonly mutated in Joubert syndrome and related disorders (JSRD), Meckel syndrome (MKS), Senior-Loken syndrome and isolated Leber congenital amaurosis (LCA). Although these conditions are recessively inherited, in a subset of patients only one CEP290 mutation could be detected. To assess whether genomic rearrangements involving the CEP290 gene could represent a possible mutational mechanism in these cases, exon dosage analysis on genomic DNA was performed in two groups of CEP290 heterozygous patients, including five JSRD/MKS cases and four LCA, respectively. In one JSRD patient, we identified a large heterozygous deletion encompassing CEP290 C-terminus, that resulted in marked reduction of mRNA expression. No copy number alterations were identified in the remaining probands. The present work expands the CEP290 genotypic spectrum to include multiexon deletions. Although this mechanism does not appear to be frequent, screening for genomic rearrangements should be considered in patients in whom a single CEP290 mutated allele was identified.
We describe three sisters with Joubert syndrome, two of whom are monozygotic twins with highly discordant phenotypes. The twins were born at 34 weeks' gestation with discordant birthweights. Their anatomic, neurologic, and developmental status differs greatly: Twin B is able to walk, run, and is verbal, unlike Twin A who is wheelchair-bound, severely retarded, nonverbal, and autistic. Abnormal eye movements and retinal dysplasia are striking features in all three girls, but none has renal cysts seen by ultrasonography. Magnetic resonance images show the "molar tooth sign," the radiologic hallmark of Joubert syndrome, although only one twin, the most severely handicapped, has severe hypoplasia of the cerebellar hemispheres. Phenotypic differences between the twins could be attributable to postzygotic unequal division of the inner cell mass, unequal sharing of the venous return from a monochorionic placenta, mosaicism, or a mutation of a modifying gene.
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