Study Of Molecular Mechanism Of Autosomal Dominant Nonsyndromic Hereditary Hearing Loss And Pfeiffer Syndrome

Hearing impairment is a common sensory disorder in human. It is caused by both environmental factors(medical factor,environment exposure, injury,medicine) and genetic factors. About 60% of deafness cases is attributed to genetic defects.In this study, by means of SNP typing and microsatellite marker (STR) mapping,we successfully mapped and identified causative genes in two Chinese families with autosomal-dominant nonsyndromic low-frequency hearing loss.We conducted the screening of two exons of WFS1 gene in sporadic cases with low and middle frequency hearing loss. We have also identified the causative gene for one patient with Pfeiffer syndrome.Part 1:Identifacation of causative genes in two families with autosomal-dominant nonsyndromic low-frequency hearing loss family and in 37 sporadic casesIn family HB-S037 we have mapped the disease locus on on chromosome 11q13.4-q14.1 region between D11S1314 and D11S4166 (two-point lod-score of 4.18) by applying the Affymetrix 5.0 SNP Genechip and linkage analysis.By direct sequencing of candidate genes in the critical interval, we identified a novel heterozygous missense mutation c.2011G>A in exonl7 of MYO7A,resulting in amino acid change of G671S, which was faithfully cosegregated with hearing loss in the family HB-S037. This mutation was absent in 100 unrelated control DNA samples of Chinese origin.In famalily BJ-L046 we have mapped the disease locus on chromosome 4p 12.0-12.40 by applying the Affymetrix 6.0 SNP Genechip and linkage analysis.By direct sequencing of candidate genes in mapping region,we have identified a novel heterozygous missense mutation c.2086 C>T in exon8 of WFS1, resulting in amino acid change of H696Y, which was faithfully cosegregated with hearing loss in this family. The mutation was absent in 100 unrelated control DNA samples of Chinese origin. To determine the genetic loading of WFS1 mutation in sporadic cases with low and middle frequency hearing loss,we have screened the WFS1 mutations.in exon 5 and 8 of 37 individuals with non-syndrome low and middle frequency hearing impairment. Single nucleotide variations were present in 14 out of 37 patients, but only heterozygous missense c.2108G> A identified in one patient, resulting in amino acid change of R703H,while the other changes are single nucleotide polymorphisms (SNPs). In addition, the mutation was absent in 100 unrelated control DNA samples of Chinese origin. This study suggests that the necessity of WFS1 screening in non-syndromic low frequency sporadic cases.Part 2:Mutation screening in one patient with Pfeiffer syndromePfeiffer syndrome is a rare autosomal dominantly inherited disorder that associates craniosynostosis,broad and deviated thumbs and big toes,and partial syndactyly on hands and feet. Hydrocephaly may be found occasionally, along with severe ocular proptosis, ankylosed elbows, abnormal viscera, and slow development. Based on the severity of the phenotype, Pfeiffer syndrome is divided into three clinical subtypes. Pfeiffer syndrome affects about 1 in 100,000 individuals. The disorder can be caused by mutations in the fibroblast growthfactor receptor genes FGFR1,FGFR2 and FGFR3. Molecular genetic testing is important to confirm the diagnosis. In this study, we have indentified the causative gene of a family diagnosed as Pfeiffer syndrome I subtype. By direct sequencing of candidate genes (FGFR1, FGFR2, FGFR3), we identified a heterozygous missense mutation c.1021 A>C in exon8 of FGFR2, resulting in amino acid change of T341P. The mutation was absent in his parents. We used molecular modeling to construct three-dimensional representation of the Ig-3 domain of FGFR2 based on the crystallographic coordinates of telokin, a myosin light chain kinase homolog, an approach that has been used previously. Cys342 mutation is the hotspot mutaton of FGFR2 T341 lie close to the disulfide-bonded cysteines, the T341P mutation would alter theβ-strand containing Cys342, which would be expected to disrupt its bonding with Cys278. From this analysis, it is apparent that the noncysteine craniosynostosis mutations function through disruption of the Ig3 disulfide bond, creating free cysteine residues that can form intermolecular disulfide bonds resulting in receptor dimerization and activation. We presumed the mutaton probably resulting from cytochimera or fresh mutation. Mutation screening can provide solid information for genetic counseling in this family.