Deadness, the lack of ability to hear, is the most prevalent sensory deficit in human populations (Mckusick, 1992). It can be divided into two groups; namely syndromic and nonsyndromic on the basis of associated phenotype other than deafness. To date, 123 loci and 37 of the corresponding genes for nonsyndromic deafness have been identified. Receive and dominant mutant alleles of above 14 genes have been reported to cause hoth syndromic and nonsyndromic deafness. Considering the intricacy of hearing process, it has been estimated that at least 300 human genes are involved in the hearing process (Friedman and Griffith 2003). Thus, search for new deafness loci/genes is indispensable for a better understanding of genetic and molecular basis of auditory functions and allied syndromes. The ratio of recessively inherited is 1.6 per 1000 in the Pakistan population, which in higher than world‚€™s average due to high consanguinity (Hussain and Bittles 1998, jabber et al . 1998).
The present study has two basic objectives; firstly to determine genotype/phenotype correlation for of the grevalent loci, DFNB2/USHIB, and secondly to identify new loci/genes involved in hearing impairment in Pakistani population. Fifty consanguineous families segregating deafness were identified and enrolled from different cities of Pakistan while twenty of them were selected for further molecular studies. All the families had three or more deaf individuals and showed recessive mode of inheritance. Clinical histories were obtained and pure one audiometry tests for air and bone conduction were performed at frequencies 250 to 8,000 hz on affected and unaffected members of these families. Vestibular function was evaluated by testing tandem gait ability, Romberg test and electronystagmography (ENG). Ocular funduscopy and electroretinography (ERG) was performed to detect the presence of retinopathy. Written informed consents were obtained and genomic DNA was isolated after collecting blood samples from affected, normal individuals, their parents, grandparents if alive, and other family members of related sibships.
Previously, recessive and dominant mutant alleles of MYO74A were reported be associated with both syndromic (USHIB) and ninsyndromic (DFNB2,DFNA11) deafness (Liu et al. 1997a; Liu et al. 1997; weal et al. 1997). However, clinical re-evaluation of all the reported DFNB2 families concluded that there is no polished evidence of mutations of MYO7A associated with nonsyndromic deafness DFNB2 (Zina et al. 2001, Astute et al. 2002). For the first objective and to eliminate the chaos about DFNB2, 270 families (20 enrolled + 250 families from CEMB repository) were screened for linkage to DFNB2/USHIB markers and as a result 11 families were found linked. Mutational analysis of DFNB2/USHIB linked families led to identification of 9 novel mutations in MYO7A gene. Amongst them, a novel mutation E1716del of MYO7A was found to be associated withDFNB2. This is the first clinically well documented example of MYO7A mutant allele associated with nonsyndromic deafness DFNB2. Furthermore, it is the first report describing mutation spectrum of MYO7A associated with USHIB and DFNB2 in Pakistan population. The results of these studies are being written up for publication.
To study the second objective seventeen families (remaining of twenty families) were analyzed for linkage by typing at least three STR markers for all the known recessive deafness loci (except DFNB2/ USHIB). Consequently, four families showed linkage to DFNB4/PDS and DFNB12/ USHID while thirteen remained unlinked. Mutational studies of SLC26A4 gene revealed a known mutation V239D (park el al. 2003) and a novel mutation Q446R in two families linked to DFNB4. The fact that a large number of families remained unlinked to known loci further supports the notion that still a large number of loci/genes remain undiscovered and instigate to identify more novel loci/genes associated with deafness.
Genome with was performed on seven selected unlinked families by using ABIPRISM¬ģ linked mapping set version 2.5 having 411 fluorescent dye-labeled micro satellites markers spaced at an average distance of 10 cm across the human the human genomic. Genome. Genome wide linkage analysis studies led to mapping of two novel nonsyndromic autosomal recessive deafness loci ‚€œDFNB51‚€Ě and ‚€œDFNB56‚€Ě as designated by human genome organization(HUGO) committee. DFNB51 and DFNB51 and DFFNB56 were mapped to chromosome 11p 13-p12 and 3q13.3-q21 on two set of families (PKDF240, PKDF407 and pkkf637, pkdf223) segregating recessively inherited, profound congenital deafness respectively. Moreover, sequencing of three candidate genes (SLCIA2, RAMP, and TRAF6) , percent in the critical linked region of DFNB51 was done but no potentially causative variant was identified (SHAIKH et al. 2005). Localization of DFNB51 and DFNB56 is the first step toward the identification of novel genes that will help to further reveal the genetics deafness.