HGPRT activity changes in preimplantation mouse embryos

Abstract

TO compensate for unequal doses of genes on the X chromosomes of males and females, one of the X chromosomes in the somatic cells of mammalian females is inactive¹. This inactivation occurs early in development, although the exact time is unknown². Before X-chromosome inactivation, and in the absence of other dosage compensating mechanisms, female embryos with two X chromosomes would be expected to have twice as much activity for an X-linked enzyme as male embryos with only one X chromosome. In a litter with approximately an equal number of male and female embryos, the distribution of enzyme activity should have two equal-sized peaks separated by a factor of two. The change from a bimodal to unimodal distribution would indicate that X-chromosome inactivation had occurred. Early in development, the X-linked enzymes α-galactosidase (α-gal)³ and hypoxanthine guanine phosphoribosyltransferase (HGPRT)^4,5 are both derived from embryonic gene activity. α-gal was found to have a bimodal distribution at the morula stage³. For HGPRT, Monk and Kathuria⁶ found no bimodality at either the eight-cell or blastocyst stages, however further analysis revealed bimodality at certain stages⁷. Epstein et al.⁸ have found that females have twice as much HGPRT activity as males in early blastocysts. We present here evidence for the activity of both the maternal and paternal X chromosomes by the eight-cell stage, with inactivation initiated at blastulation.