Paternal X-chromosome inactivation in human trophoblastic cells

Dosage compensation for X-chromosome-linked genes between male and female mammals occurs by inactivation of one of the two X chromosomes in the female. In somatic cells, either the paternal or the maternal X chromosome is randomly inactivated in a given cell. In contrast, in the extra-embryonic tissues of mice, the paternally-derived X chromosome is preferentially inactivated. The evidence for paternal X-chromosome inactivation in humans is controversial and remains to be clarified. In this study, we have developed a sensitive polymerase chain reaction (PCR) technique to investigate the methylation pattern of the X-linked androgen receptor (AR) gene. The 5' CpG island of this gene is methylated on the inactive X chromosome and hypomethylated on the active X chromosome in somatic cells. The paternal and the maternal alleles of the AR gene may be distinguished by a polymorphism in the number of CAG triplet repeats within the CpG island. As a source of human extra-embryonic tissue, we used chorionic villus (CV) samples from female conceptuses of 10-12 weeks gestation. From a tiny branch of a CV sample, two distinct cell lineages, the trophoblastic and mesodermal lineages, were dissected apart by trypsin digestion and micromanipulation and DNA was extracted separately from these purified tissues. Digestion of the DNA with the methylation-sensitive restriction enzyme, Hpall, followed by PCR amplification revealed that the paternal allele is preferentially methylated in trophoblastic cells, but not in mesodermal cells. These results strongly suggest that the paternal X chromosome is preferentially inactivated in the human extra-embryonic tissues early in development.