Consecutive mutational events in a TSHR allele of Arab families with resistance to thyroid stimulating hormone.

backgroundOur laboratory identified six distinct inactivating TSHR gene mutations in Arab tribes living in Israel. We recently reported three nucleotide substitutions in exon 3 producing p.[L89L;Q90P] and one in exon 9 of the same allele producing p.P264S in Family A. Family B, reported herein, harbors the identical mutation in exon 3 only. We set to determine whether the mutations have common ancestral origin.

methodsCoding regions of the TSHR were sequenced and flanking microsatellite markers spanning 5.3 cM were used for haplotyping.

resultsTwo siblings of Family B were compound heterozygous for TSHR gene mutations. The paternal allele contained the exon 3 mutation and the maternal allele harbored a mutation in exon 10 (p.L653V). We investigated the possibility of a founder effect with subsequent mutational events for the presence of the same exon 3 mutation in different families. The haplotype of the allele harboring the exon 3 mutation in Family B was identical to that of Family A, also harboring the exon 9 mutation on the same allele, indicating that the latter occurred subsequently. The ancestral wild-type TSHR was present in Family B, suggesting that the mutation in exon 3 was also new in the history of that population.

conclusionsIt is more likely that two consecutive mutational events occurred on the ancestral wild-type allele instead of a recombination bringing exon 3 and exon 9 mutations together on the same allele. New mutational events contribute to the high prevalence of TSHR mutations in this population in addition to a founder effect and limited gene pool due to inbreeding.