Custom Next-Generation Sequencing Identifies Novel Mutations Expanding the Molecular and clinical spectrum of isolated Hearing Impairment or along with defects of the retina, the thyroid, and the kidneys.

BACKGROUND: In the Tunisian population, the molecular analysis of hearing impairment remains based on conventional approaches, which makes the task laborious and enormously expensive. Exploration of the etiology of Hearing Impairment and the early diagnosis of causal mutations by next-generation sequencing help significantly alleviate social and economic problems. METHODS: We elaborated a custom SureSelectQXT panel for next-generation sequencing of the coding sequences of 42 genes involved in isolated hearing impairment or along with defects of the retina, the thyroid, and the kidneys. RESULTS: We report eight pathogenic variants, four of which are novel in patients with isolated hearing impairment, hearing impairment, and renal tubular acidosis, Usher syndrome and Pendred syndrome. Functional studies using molecular modeling showed the severe impact of the novel missense mutations on the concerned proteins. Basically, we identified mutations in nuclear as well as mitochondrial genes in a Tunisian family with isolated hearing impairment, which explains definitely the phenotype detected since 2006. CONCLUSION: Our results expanded the mutation spectrum and genotype-phenotype correlation of isolated and syndromic hearing loss and also emphasized the importance of combining both targeted next-generation sequencing and detailed clinical evaluation to elaborate a more accurate diagnosis for hearing impairment and related phenotypes especially in North African populations.

Posterior microphthalmia and nanophthalmia in Tunisia caused by a founder c.1059_1066insC mutation of the PRSS56 gene.

Congenital microphthalmia (CMIC) is a common developmental ocular disorder characterized by a small, and sometimes malformed, eye. Posterior microphthalmia (PM) and nanophthalmia are two rare subtypes of isolated CMIC characterized by extreme hyperopia due to short axial length and elevated lens/eye volume ratio. While nanophthalmia is associated with a reduced size in both anterior and posterior segments, PM involves a normal-size anterior chamber but a small posterior segment. Several genes encoding transcription and non-transcription regulators have been identified in different forms of CMIC. MFRP gene mutations have, for instance, been associated with nanophthalmia, and mutations in the recently identified PRSS56 gene have been linked to PM. So far, these two forms of CMIC have been associated with 9 mutations in PRSS56. Of particular interest, a c.1059_1066insC mutation has recently been reported in four Tunisian families with isolated PM and one Tunisian family with nanophthalmia. Here, we performed a genome-wide scan using a high density single nucleotide polymorphism (SNP) array 50 K in a large consanguineous Tunisian family (PM7) affected with PM and identified the same causative disease mutation. A total of 24 polymorphic markers spanning the PRSS56 gene in 6 families originating from different regions of Tunisia were analyzed to investigate the origin of the c.1059_1066insC mutation and to determine whether it arose in a common ancestor. A highly significant disease-associated haplotype, spanning across the 146 kb of the 2q37.1 chromosome, was conserved in those families, suggesting that c.1059_1066insC arose from a common founder. The age of the mutation in this haplotype was estimated to be around 1,850 years. The identification of such 'founder effects' may greatly simplify diagnostic genetic screening and lead to better prognostic counseling.

Screening of the DFNB3 locus: identification of three novel mutations of MYO15A associated with hearing loss and further suggestion for two distinctive genes on this locus.

Recessive mutations of MYO15A are associated with nonsyndromic hearing loss (HL) in humans (DFNB3) and in the shaker-2 mouse. Human MYO15A has 66 exons and encodes unconventional myosin XVA. Analysis of 77 Tunisian consanguineous families segregating recessive deafness revealed evidence of linkage to microsatellite markers for DFNB3 in four families. In two families, sequencing of MYO15A led to the identification of two novel homozygous mutations: a nonsense (c.4998C>A (p.C1666X) in exon 17 and a splice site mutation in intron 54 (c.9229 + 1G>A). A novel mutation of unknown significance, c.7395 + 3G>C, was identified in the third family, and no mutation was found in the fourth family. In conclusion, we discovered three novel mutations of MYO15A, and our data suggest the possibility that there are two distinct genes at the DFNB3 locus.

TMC1 but not TMC2 is responsible for autosomal recessive nonsyndromic hearing impairment in Tunisian families.

Hereditary nonsyndromic hearing impairment (HI) is extremely heterogeneous. Mutations of the transmembrane channel-like gene 1 (TMC1) have been shown to cause autosomal dominant and recessive forms of nonsyndromic HI linked to the loci DFNA36 and DFNB7/B11, respectively. TMC1 is 1 member of a family of 8 genes encoding transmembrane proteins. In the mouse, MmTmc1 and MmTmc2 are both members of Tmc subfamily A and are highly and almost exclusively expressed in the cochlea. The restricted expression of Tmc2 in the cochlea and its close phylogenetic relationship to Tmc1 makes it a candidate gene for nonsyndromic HI. We analyzed 3 microsatellite markers linked to the TMC1 and TMC2 genes in 85 Tunisian families with autosomal recessive nonsyndromic HI and without mutations in the protein-coding region of the GJB2 gene. Autozygosity by descent analysis of 2 markers bordering the TMC2 gene allowed us to rule out its association with deafness within these families. However, 5 families were found to segregate deafness with 3 different alleles of marker D9S1837, located within the first intron of the TMC1 gene. By DNA sequencing of coding exons of TMC1 in affected individuals, we identified 3 homozygous mutations, c.100C-->T (p.R34X), c.1165C-->T (p.R389X) and the novel mutation c.1764G-->A (p.W588X). We additionally tested 60 unrelated deaf Tunisian individuals for the c.100C-->T mutation. We detected this mutation in a homozygous state in 2 cases. This study confirms that mutations in the TMC1 gene may be a common cause for autosomal recessive nonsyndromic HI.