Genetic variability of the number of CAG repeats in the X-linked androgen receptor gene and human embryonic mortality

Distribution of the X-linked androgen receptor gene (AR) alleles with diffe rent number of CAG repeat units in the first exon was analyzed in spontaneo us abortuses having the 46,XX karyotype. The increased frequency of alleles with low number of trinucleotide repeat units in these specimens compared to adult healthy women was revealed. For more detailed analysis, all AR all eles were grouped into three classes: A, with a low CAG repeat number (21 t o 23); B, with a moderate repeat number (24 to 28); and C, with a high maxi mum repeat number (29 to 32). Statistically significant among-sample differ ences in the frequencies of AA genotypes, which prevailed in spontaneous ab ortuses, were revealed. The samples tested were also different by the mean squared distance between the AR allele lengths, which constituted 81.8 +/-0 .4 and 53.2 +/-0.5 in healthy women and spontaneous abortuses, respectively . Based on the data on inverse correlation between the length of the CAG re peat sequence and transcriptional activity of the androgen receptor gene, a hypothesis on selection against female individuals with the small number o f CAG repeats in the AR gene and high transcriptional activity of this locu s in prenatal period of human ontogeny is advanced.