Genetic analysis of Tunisian families with Usher syndrome type 1: toward improving early molecular diagnosis.

PURPOSE: Usher syndrome accounts for about 50% of all hereditary deaf-blindness cases. The most severe form of this syndrome, Usher syndrome type I (USH1), is characterized by profound congenital sensorineural deafness, vestibular dysfunction, and retinitis pigmentosa. Six USH1 genes have been identified, MYO7A, CDH23, PCDH15, USH1C, SANS, and CIB2, encoding myosin VIIA, cadherin-23, protocadherin-15, harmonin, scaffold protein containing ankyrin repeats and a sterile alpha motif (SAM) domain, and calcium- and integrin-binding member 2, respectively. METHODS: In the present study, we recruited four Tunisian families with a diagnosis of USH1, together with healthy unrelated controls. Affected members underwent detailed audiologic and ocular examinations. We used the North African Deafness (NADf) chip to search for known North African mutations associated with USH. Then, we selected microsatellite markers covering USH1 known loci to genotype the DNA samples. Finally, we performed DNA sequencing of three known USH1 genes: MYO7A, PCDH15, and USH1C. RESULTS: Four biallelic mutations, all single base changes, were found in the MYO7A, USH1C, and PCDH15 genes. These mutations consist of a previously reported splicing defect c.470+1G>A in MYO7A, three novel variants, including two nonsense (p.Arg3X and p.Arg134X) in USH1C and PCDH15, respectively, and one frameshift (p.Lys615Asnfs*6) in MYO7A. CONCLUSIONS: We found a remarkable genetic heterogeneity in the studied families with USH1 with a variety of mutations, among which three were novel. These novel mutations will be included in the NADf mutation screening chip that will allow a higher diagnosis efficiency of this extremely genetically heterogeneous disease. Ultimately, efficient molecular diagnosis of USH in a patient's early childhood is of utmost importance, allowing better educational and therapeutic management.

Identification of candidate regions for a novel Usher syndrome type II locus.

PURPOSE: Chronic diseases affecting the inner ear and the retina cause severe impairments to our communication systems. In more than half of the cases, Usher syndrome (USH) is the origin of these double defects. Patients with USH type II (USH2) have retinitis pigmentosa (RP) that develops during puberty, moderate to severe hearing impairment with downsloping pure-tone audiogram, and normal vestibular function. Four loci and three genes are known for USH2. In this study, we proposed to localize the gene responsible for USH2 in a consanguineous family of Tunisian origin. METHODS: Affected members underwent detailed ocular and audiologic characterization. One Tunisian family with USH2 and 45 healthy controls unrelated to the family were recruited. Two affected and six unaffected family members attended our study. DNA samples of eight family members were genotyped with polymorphic markers. Two-point and multipoint LOD scores were calculated using Genehunter software v2.1. Sequencing was used to investigate candidate genes. RESULTS: Haplotype analysis showed no significant linkage to any known USH gene or locus. A genome-wide screen, using microsatellite markers, was performed, allowing the identification of three homozygous regions in chromosomes 2, 4, and 15. We further confirmed and refined these three regions using microsatellite and single-nucleotide polymorphisms. With recessive mode of inheritance, the highest multipoint LOD score of 1.765 was identified for the candidate regions on chromosomes 4 and 15. The chromosome 15 locus is large (55 Mb), underscoring the limited number of meioses in the consanguineous pedigree. Moreover, the linked, homozygous chromosome 15q alleles, unlike those of the chromosome 2 and 4 loci, are infrequent in the local population. Thus, the data strongly suggest that the novel locus for USH2 is likely to reside on 15q. CONCLUSIONS: Our data provide a basis for the localization and the identification of a novel gene implicated in USH2, most likely localized on 15q.