Evolutionary origin of pathogenic GJB2 alleles in China

The frequency of the pathogenic allele of the autosomal recessive deafness gene GJB2 varies among different populations in the world, and accumulates to a sufficiently high frequency in certain population. The purpose of this study is to investigate the origin and evolution of GJB2 pathogenic alleles in Chinese deaf patients. Children with non-syndromic hearing loss, and their parents, from 295 families were recruited. Customized capture probes targeted at 943 single nucleotide polymorphisms (SNPs) related to GJB2 gene were designed for sequencing of genomic DNA in blood samples. Haplotypes carrying pathogenic allele were analyzed through linkage disequilibrium block building, ancestry tracing, and extended haplotype heterozygosity calculation. Two pathogenic GJB2 alleles, c.235delC (18.41%) and c.109G > A (15.57%), were observed in 867 donors. For c.235delC allele, three different core haplotypes with one major haplotype (97.32%) were found, and their core SNPs were 100% conserved. For c.109G > A allele, six different haplotypes with one major haplotype (93.28%) were found and the major c.109G > A allele evolved from a specific ancestral haplotype. Geographical origins of donors carrying GJB2 c.109G > A and c.235delC core haplotypes centered between Qinghai and Neimenggu. GJB2 c.235delC has long-range linkage disequilibrium. No positive selection signature was found for GJB2 c.235delC or c.109G > A in the studied population. In conclusion, we discovered a single origin of GJB2 c.235delC allele and multiple independent origins of GJB2 c.109G > A allele. Alternative to positive selection or multiple independent recurrent mutation event, population bottleneck effect might account for the observed high population frequency of these pathogenic alleles.