Hearing loss is a common sensory disorder that typically illustrates genetic heterogeneity in human populations. The incidence of congenital hearing loss is estimated at 1 in 1,000 births of which approximately 60 % of cases are attributed to genetic factors. Genetic hearing impairment can be classified as either syndromic or non-syndromic. Several hundred syndromes, for which hearing impairment is one of the clinical features, have been described. These account for 30% of hearing impairment cases with a genetic etiology. Among non-syndromic hereditary hearing impairments (NSHI), autosomal recessive inheritance predominates and accounts for approximately 75-80% of the cases, while autosomal dominant inheritance is observed in some 15% of cases. Non-syndromic hearing impairment is the most heterogeneous trait known and thus far over 90 loci have been mapped and 35 genes identified.
In the present study, eleven families (A-K) with syndromic and non-syndromic hearing impairment have been described. In seven families (A-G), with non-syndromic recessive deafness, affected individuals 'have prelingual profound hearing impairment. In other four families (H-K) affected individuals show severe to profound prelingual hearing loss associated with goiter (Pendred syndrome).
Linkage in the families was initially searched by using micro satellite markers corresponding to candidate genes involved in related autosomal recessive non-syndromic deafness phenotypes. Linkage was detected in nine families, i.e., C-K. In family C linkage was established to DFNB8/10 locus on chromosome 21q22.3. In families D-G linkage was established to DFNBl locus on chromosome 13. In families H-K it was detected on chromosome 7 at DFNB4 locus. In other two families (A and B) linkage to all the known loci was conclusively excluded, thus indicating the involvement of some novel loci, responsible for deafness in these families.
After excluding the disorder from linkage to known hearing loss loci in families A and B, genome-wide scan was performed by using the Mappairs sets of Microsatellite markers. In family A, screening of the human genome with markers spaced a t 10 c M intervals led to the identification of a new autosomal recessive non-syndromic hearing loss locus, DFNB39, on the telomeric side of chromosome 7q11.22-21.12. Two-point linkage analysis generated LOD score of 2.7 at marker D7S820. Multipoint linkage analysis using ALLEGRO yielded a significant lod score of 3.8 at marker D7S2212. Examination of haplotypes defined a critical region 19.8 cM and is flanked by markers D7S482 and D7S644. The genetic interval does not overlap with closely I inked autosomal recessive deafness locus DFNB44.
In family B, genetic linkage was established between a novel locus DFNB42 and markers located on chromosome 3q13.31-q22.3. Two-point linkage analysis generated a maximum LOD score of 3.2 for marker D3S4523 at recombination fraction zero, and multipoint linkage analysis produced a maximum LOD score of 3.7 at the same marker. Observed recombinants in the affected individuals defined an interval of 21.6 cM, which lies between D3S1765 and D3S2453.
In four families where linkage was established to DFNB 1 locus, sequencing of the single coding exon of GJB2 gene led to the identification of two nonsense and two missense mutations. In families E and F sequence analysis detected a G to A transition at nucleotide position 369 leading to premature termination codon (W77X). In families D and G, single base pair substitutions resulted in missense mutations V153I and R32H, respectively.
Families H-K showed linkage to Pendred syndrome locus on chromosome 7q31. These families were described as having Pendred syndrome. The loss of hearing in these families was present at birth, whereas the development of goiter was progressive in nature and develops after puberty. The goiter is variable in its severity.
In the study presented here, strong evidence of linkage of hearing loss in two families to two new loci has been presented. However, the distribution of recombination events in the families has only allowed the assigning of the genes to large genetic intervals. In order to identify the defective genes, the map locations have to be refined. An alternative approach is to identify the 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. M Wajid, AA Abbasi, M Ansar, TL Pham, K Van, S Haque, W Ahmad and SM Leal. 2003. DFNB39, a recessive form of sensorineural hearing impairment, maps to chromosome 7q11.22-q21.12. European Journal of Human Genetics 11: 812-815
2. M Wajid, M Aslam, TL Pham, M Ansar, S Haque, K Van, W Ahmad, SM Leal. A novel autosomal recessive non-syndromic deafness locus (DFNB42) maps to chromosome 3q13.31-q22.3. (American Journal of Medical Genetics-Manuscript No. 04-0355, In Press)
3. Regie Lyn P. Santos, Muhammad Wajid, Thanh L. Pham, Jawad Hussan, Ghazanfar Ali, Wasim Ahmad and Suzanne M. Leal. Low prevalence of Connexin 26 (GJB2) mutations in Pakistani families with autosomal recessive non-syndromic hearing impairment. (Clinical Genetics - Manuscript No. CGE-00318-2004.Rl, In Press)