The hearing loss disorders attributable to genetic causes, are classified as non-syndromic or syndromic. Among the many disorders classified as syndromic hearing loss, the pathology varies widely, but, in non-syndromic hearing loss, the defect is generally sensorineural. Non-syndromic deafness is paradigm of genetic heterogeneity. More than 83 loci have been mapped, and 32 of the nuclear genes responsible for non-syndromic deafness have been identified. Autosomal recessive genes are responsible for about 77 % of the cases of hereditary non-syndromic deafness, with 39 loci and 16 different genes identified to date. This extreme genetic heterogeneity suggests that there are many different processes that can malfunction within inner ear to cause hearing loss.
In the present study, five families (A, B, C, D, E) from Punjabi speaking population of Pakistan, segregating non-syndromic recessive hearing loss have been described. The patients hearing loss was prelingual, varied from severe to profound, and was not caused by inflammatory middle ear disease or environmental factors. Linkage in the families was initially searched by using polymorphic micro satellite markers located within the genetic intervals of all the known hearing loss loci. On the bases of genotypes observed, linkage to the known loci was excluded in families A, Band E; while in families C and D linkage was established with known loci DFNB1 and DFNB7/11, respectively.
After excluding the disorder in three families from linkage to the known deafness chromosomal regions, a genome-wide screening was undertaken using the Mappairs Set of Microsatellite markers. In family A, genome-wide search with a panel of 181 markers, spaced at 20 cM intervals, detected the linkage between a novel hearing loss locus DFNB35 and markers on chromosome 14q24.1-q24.3. A maximum two-point LOD score of 5.3 and multipoint LOD score of 7.6 was obtained at marker D14S53. Saturation of the region with additional markers and examination of haplotypes defined a critical region of about 11.75 cM flanked by markers D14S588 and D14S59. The genetic interval for DFNB35 does not overlap with the regions of the previously mapped deafness loci on chromosome 14.
In families B and E, after failing to detect linkage with the panel of 181 markers, a second genome-wide search was performed by using 386 fluorscently labeled micro satellite markers, spaced at 10 cM intervals on the human genome. In family B, genetic linkage was established between a novel DFNB38 locus and markers on chromosome 6q26-q27. A maximum two-point LOD score of 3.62 and multipoint LOD score of 3.87 was obtained for marker D6S1599. Observed recombinants defined an interval of 10.1 cM for DFNB38. which lies between markers D6S980 and D6S 1719. The genetic interval for DFNB38 does not overlap with the regions of the previously mapped deafness loci on chromosome 6. In family E, significant evidence of linkage to the chromosomal region 7p14.l-7q11.22 was found with two-point LOD score of > 3.0 with marker D7S3046. Saturation of the region with additional markers and analysis of haplotypes of individuals carrying re combinations mapped the gene to an interval of 23.04 cM bounded by markers D7S2435 and D7S2209. Several lines of evidence presented, indicate that the hearing impairment locus identified in family E does not overlap with the closely linked DFNB39 locus.
In two of the five families presented in the current study, linkage was established to the previously known loci (DFNBl and DFNB7/11). In family C, sequencing of the coding exon of connexin 26 gene, which is responsible for hearing loss at DFNBl locus, revealed a G to A transition at nucleotide position 71, resulting in the conversion of tryptophan to premature termination codon, designated W24X. In family D, sequencing of all the 24 coding exons and splice junctions of TMCl gene at DFNB7/11 locus, revealed a homozygous G to A transition in all the affected individuals, which was present in heterozygous state in obligate carriers within the family. The G to A transition occurred at nucleotide position 2575, leading to a missense mutation and converting the lysine to glutamic acid residue, and was designated E679K.
While strong evidence of linkage of hearing loss to three new loci has been presented in the present study, the distribution of the recombination events in the families has only allowed assigning the genes to large genetic intervals. This being the case, the map locations will have to be substantially refined to identify the mutated gene. An alternative approach is to identify candidate genes mapping to the regions, which could be tested for disease specific mutations.
The work presented in the thesis has been published in the following articles.
1. Muhammad Ansar, Mohammad Amin ud Din, Muhammad Arshad, Muhammad Sohail, Mohammad Faiyaz ul Haque, Sayedul Haque, Wasim Ahmad, Suzanne M. Leal. A novel autosomal recessive non-syndromic deafness locus (DFNB35) maps to14q24.1-14q24.3 in large consanguineous kindred from Pakistan. European Journal of Human Genetics (2003) 11, 77 - 80.
2. Muhammad Ansar, Mohammad Ramzan, Thanh L. Pham, Kai Yan, Syed Muhammad Jamal, Sayedul Haque, Wasim Ahmad, Suzanne M. Leal. Localization of A Novel Autosomal Recessive Non-Syndromic Hearing Impairment Locus (DFNB38) to 6q26-q27 in a Consanguineous Kindred from Pakistan. Human Heredity (2003) 55,71-74.
3. Muhammad Ansar, Thanh L. Pham, Mohammad Amin ud Din, Muhammad Arshad Rafiq, Sayedul Haque, Kai Yan, Wasim Ahmad, Suzanne M. Leal. Mapping of DFNB, a gene for a non-syndromic autosomal recessive deafness, to chromosome 7p14.1-7q11.22 (Manuscript in preparation)