Mutations in SDCCAG8/NPHP10 Cause Bardet-Biedl Syndrome and Are Associated with Penetrant Renal Disease and Absent Polydactyly.

The ciliopathies are an expanding group of disorders caused by mutations in genes implicated in the biogenesis and function of primary cilia. Bardet-Biedl syndrome (BBS) is a model ciliopathy characterized by progressive retinal degeneration, obesity, polydactyly, cognitive impairment, kidney anomalies and hypogonadism. Mutations in SDCCAG8(NPHP10) were described recently in patients with nephronophthisis and retinal degeneration (Senior-Loken syndrome; SLS). Given the phenotypic and genetic overlap between known ciliopathy genes, we hypothesized that mutations in SDCCAG8 might also contribute alleles to more severe, multisystemic ciliopathies. We performed genetic and phenotypic analyses of 2 independent BBS cohorts. Subsequent to mutation screening, we made a detailed phenotypic analysis of 5 families mutated for SDCCAG8 (3 homozygous and 2 compound heterozygous mutations) and conducted statistical analyses across both cohorts to examine possible phenotype-genotype correlations with mutations at this locus. All patients with mutations in SDCCAG8 fulfilled the diagnostic criteria for BBS (retinal degeneration, obesity, cognitive defects, renal failure, hypogonadism). Interestingly, none of the patients with primary SDCCAG8 mutations had polydactyly, a frequent but not obligatory BBS feature. In contrast, the same patients displayed early-onset renal failure, obesity, as well as recurrent pulmonary and ENT infections. Comparison of the phenotypes of these families with our entire BBS cohort indicated that renal impairment and absent polydactyly correlated significantly with causal SDCCAG8 mutations. Thus, SDCCAG8 mutations are sufficient to cause BBS in 1-2% of our combined cohorts, and define this gene as the sixteenth BBS locus (BBS16). The absence of polydactyly and the concomitant, apparently fully penetrant association with early kidney failure represents the first significant genotype-phenotype correlation in BBS that potentially represents an indicator for phenotype-driven priority screening and informs specific patient management.

Hippocampal dysgenesis and variable neuropsychiatric phenotypes in patients with Bardet-Biedl syndrome underline complex CNS impact of primary cilia.

The Bardet-Biedl syndrome (BBS) is a rare ciliopathy clinically defined by the association of retinitis pigmentosa, polydactyly, obesity, kidney disease and cognitive impairment. The cognitive functioning, behavioral phenotype, prevalence of psychiatric diseases and memory performances of a cohort of 34 patients with BBS were evaluated and a systemic brain magnetic resonance imaging (MRI) was performed. The patients' cognitive functioning was of marked variable efficiency ranging from normal to disabling performances. Neuropsychological disorders such as slow thought process, attention difficulties and obsessive-compulsive traits were observed. Our main finding was hippocampal dysgenesis, diagnosed by MRI, found in 42.31% of the patients in this cohort. Moreover, we show that BBS proteins are expressed in the human hippocampus and in the human brain in the normal subject. Recent literature in the murine model shows that hippocampal neurogenesis, in particular in the adult mouse, requires an intact primary cilia. These results encourage us to further investigate the possible role of BBS proteins in the hippocampus and related central nervous system structures.

The Renpenning syndrome spectrum: new clinical insights supported by 13 new PQBP1-mutated males.

Since the first reports of polyglutamine-binding protein 1 (PQBP1) mutations in Renpenning syndrome and related disorders, the spectrum of PQBP1-linked clinical manifestations has been outlined from rare published case reports. The phenotypic description is often obtained from medical archives, and therefore, heterogeneous. Moreover, some aspects such as brain imaging or cognitive and behavioral functioning are rarely described. In this study, 13 PQBP1-mutated French patients were subjected to a standardized clinical, cognitive and behavioral assessment. Physical measurements of their relatives were also collected. We report on a recognizable clinical and radiological phenotype. All patients presented with microcephaly, leanness and mild short stature, relative to familial measurements. Three new clinical features are described: upper back progressive muscular atrophy, metacarpophalangeal ankylosis of the thumb and velar dysfunction. The specific facial dysmorphic features included at least four of the following signs: long triangular face, large ridged nose, half-depilated eyebrows, dysplastic or protruding ears and rough slightly sparse hair. An over-aged appearance was noticed in elderly patients. Cortical gyrification was normal based on available magnetic brain imaging of six patients. PQBP1-linked microcephaly (or Renpenning syndrome) is an X-linked mental retardation syndrome, which has clinically recognizable features.

Identification of 28 novel mutations in the Bardet-Biedl syndrome genes: the burden of private mutations in an extensively heterogeneous disease.

Bardet-Biedl syndrome (BBS), an emblematic disease in the rapidly evolving field of ciliopathies, is characterized by pleiotropic clinical features and extensive genetic heterogeneity. To date, 14 BBS genes have been identified, 3 of which have been found mutated only in a single BBS family each (BBS11/TRIM32, BBS13/MKS1 and BBS14/MKS4/NPHP6). Previous reports of systematic mutation detection in large cohorts of BBS families (n > 90) have dealt only with a single gene, or at most small subsets of the known BBS genes. Here we report extensive analysis of a cohort of 174 BBS families for 12/14 genes, leading to the identification of 28 novel mutations. Two pathogenic mutations in a single gene have been found in 117 families, and a single heterozygous mutation in 17 families (of which 8 involve the BBS1 recurrent mutation, M390R). We confirm that BBS1 and BBS10 are the most frequently mutated genes, followed by BBS12. No mutations have been found in BBS11/TRIM32, the identification of which as a BBS gene only relies on a single missense mutation in a single consanguineous family. While a third variant allele has been observed in a few families, they are in most cases missenses of uncertain pathogenicity, contrasting with the type of mutations observed as two alleles in a single gene. We discuss the various strategies for diagnostic mutation detection, including homozygosity mapping and targeted arrays for the detection of previously reported mutations.

Confirmation of TFAP2A gene involvement in branchio-oculo-facial syndrome (BOFS) and report of temporal bone anomalies.

Branchio-oculo-facial syndrome (BOFS) is an autosomal-dominant condition characterized by three main features, respectively: branchial defects, ocular anomalies, and craniofacial defects including cleft lip and/or palate (CL/P). We report on one family with three affected, and two sporadic cases that have been found to carry missense mutations in the newly reported BOFS gene: TFAP2A. This report confirms the involvement of this transcription factor in this developmental syndrome with clinical variability. Moreover, we present CT scan temporal bone anomalies in the familial cases, related to branchial arch defects, highlighting the importance of radiological investigations for differential diagnosis.