Genetic analysis of autosomal recessive osteopetrosis in Chuvashiya: the unique splice site mutation in TCIRG1 gene spread by the founder effect

Abstract

The rare malignant disorder autosomal recessive osteopetrosis (OPTB) is one of the most prevalent autosomal recessive diseases in the Chuvash Republic of Russia. The purpose of this study was to determine the underlying molecular cause of osteopetrosis in Chuvashiya and to reveal the factors causing the unusual high frequency of the disease in this region. Having assumed a founder effect, we performed linkage disequilibrium (LD) mapping of the OPTB locus at the TCIRG1 region and found a unique splice site mutation c.807+5G>A in all Chuvashian OPTB patients studied. We then analyzed the mutational change in mRNA and detected an intron insertion within the mutant transcript, resulting in a frameshift and premature stop-codon formation (p.Leu271AspfsX231). A decreased expression of the mutant transcript was also detected, which may have been the result of nonsense-mediated decay. Real-time qPCR and MLPA melting curve analysis-based systems were designed and used for c.807+5G>A mutation screening. In addition to analyzing the gene frequency in Chuvashiya, we also estimated three other populations in the Volga-Ural region (Mari, Udmurt and Bashkir). We found a 1.68% prevalence in Chuvashiya (calculated disease frequency, 1/3500 newborns) and a 0.84% in the Mari population (1/14 000 newborns). The haplotype analysis revealed that all OPTB cases in Chuvashians and Marians originated from a single mutational event and the age of the mutation in Chuvashians was estimated to be approximately 890 years.

Figure 1

Haplotypes of chromosomes at polymorphic markers tightly linked to the TCIRG1 gene in OPTB Chuvashian families.

Figure 2

c.807+5G>A mutation in the TCIRG1 gene effect analysis. (a) RT-PCR analysis of heterozygous parents, m/N; individuals not bearing the mutation, N/N; the fragments of chromatograms obtained by sequencing of purified smaller (b) and larger (c) bands observed during (a). (d) RLFP analysis of dbSNP3808973:C>T in individuals double heterozygous for SNP and the mutation, m/N, and those heterozygous for SNP and not bearing the mutation, N/N; on the right a fluorescence intensity of bands is shown in the graph (it is obtained by Quantity One® version 4.2 program (Bio-Rad)).