Stéphane Dubois

HOMOZYGOTES CARRYING AN AUTOSOMAL DOMINANT TIGR MUTATION DO NOT MANIFEST GLAUCOMA

nature genetics volume 19 august 1998 319 Autosomal dominant disorders typically result in more severe clinical manifestations in mutant homozygotes than in heterozygotes1. Huntington disease is the only reported non-neoplastic human pathology in which no phenotypic variance has been detected between these two types of carriers, and where the mutant allele is truly dominant over its wild-type counterpart2. Primary openangle glaucoma (POAG), a leading cause of blindness worldwide, is characterized by progressive degeneration of the optic nerve and is usually associated with intraocular hypertension3 (OHT). Several loci involved in glaucoma have been localized4,5. Recently, mutations in the trabecular meshwork-inducible glucocorticoid response (TIGR) gene, also known as myocilin6, mapping to the GLC1A locus at 1q23−q25, were identified in families affected by autosomal dominant POAG (refs 7−10). We investigated a FrenchCanadian family, pedigree GV-001, in which POAG was transmitted as an autosomal dominant trait linked to the GLC1A locus11. The large size of this kindred and its relative isolation in eastern Québec allowed us to assess the effects of a TIGR mutation present in double dosage in four adult homozygotes. These individuals were asymptomatic for the disorder, with POAG affecting only the heterozygotes. The pedigree currently contains 622 individuals, of which 83 manifested either juvenile-onset (JOAG), a subset of POAG that has an early age at onset3, or adultonset POAG. Ten individuals also displayed OHT, which often preceded POAG by several years11. A marriage in branch GV-510 between two affected seconddegree cousins, mother V-1 and her spouse, father V-2 (Fig. 1a), resulted in 10 children, now 33−50 years of age. It was expected that these siblings would carry two wild-type copies of TIGR in a proportion of approximately 25%, and approximately 75% would harbour at least one mutant allele. It was also anticipated that a high proportion of these adults would be affected. Phenotypic evaluation showed only two sibs to be affected: son VI-3 and daughter VI-4 (Fig. 1a). The other eight sibs disHomozygotes carrying an autosomal dominant TIGR mutation do not manifest glaucoma

Exome sequencing identifies mutations in the gene TTC7A in French-Canadian cases with hereditary multiple intestinal atresia

Background Congenital multiple intestinal atresia (MIA) is a severe, fatal neonatal disorder, involving the occurrence of obstructions in the small and large intestines ultimately leading to organ failure. Surgical interventions are palliative but do not provide long-term survival. Severe immunodeficiency may be associated with the phenotype. A genetic basis for MIA is likely. We had previously ascertained a cohort of patients of French-Canadian origin, most of whom were deceased as infants or in utero. The goal of the study was to identify the molecular basis for the disease in the patients of this cohort. Methods We performed whole exome sequencing on samples from five patients of four families. Validation of mutations and familial segregation was performed using standard Sanger sequencing in these and three additional families with deceased cases. Exon skipping was assessed by reverse transcription-PCR and Sanger sequencing. Results Five patients from four different families were each homozygous for a four base intronic deletion in the gene TTC7A, immediately adjacent to a consensus GT splice donor site. The deletion was demonstrated to have deleterious effects on splicing causing the skipping of the attendant upstream coding exon, thereby leading to a predicted severe protein truncation. Parents were heterozygous carriers of the deletion in these families and in two additional families segregating affected cases. In a seventh family, an affected case was compound heterozygous for the same 4bp deletion and a second missense mutation p.L823P, also predicted as pathogenic. No other sequenced genes possessed deleterious variants explanatory for all patients in the cohort. Neither mutation was seen in a large set of control chromosomes. Conclusions Based on our genetic results, TTC7A is the likely causal gene for MIA.

Glaucoma-associated WDR36 Variants Encode Functional Defects in a Yeast Model System

Primary open-angle glaucoma (POAG) is a leading cause of blindness worldwide. POAG is associated with a characteristic progression of changes to ocular morphology and degeneration at the optic nerve head with the loss of visual fields. Physical mapping efforts identified genomic loci in which to search for causative POAG gene mutations. WDR36, at locus GLC1G, was initially identified as a gene with a low frequency of non-synonymous sequence variations that were exclusive to adult-onset POAG patients. It has since been shown that rare WDR36 sequence variants are also present in the normal population at similarly low frequencies. The lack of a consistent genotype:phenotype correlation prompted us to investigate the functional consequences of WDR36 sequence variations. WDR36 is involved in rRNA processing, a critical step in ribosome biogenesis, and is very similar to yeast Utp21p which is a member of the small subunit (SSU) processome complex responsible for maturation of 18S rRNA. We, therefore, developed a yeast model system to test the functional and phenotypic consequences of POAG-associated sequence variants introduced into UTP21. Alone, the POAG variants did not produce any significant defects in cell viability or rRNA processing. However, when combined with disruption of STI1 (which synthetically interacts with UTP21), 5 of the 11 tested variants had increased or decreased cell viability which corresponded to reduced or elevated levels of pre-rRNA, respectively. These results demonstrate that, in the correct genetic background, WDR36 sequence variants can lead to an altered cellular phenotype, supporting the theory that WDR36 participates in polygenic forms of glaucoma.