Dosage compensation for an X-linked gene in pre-implantation mouse embryos

Abstract

THERE is strong evidence that one of the two X chromosomes is inactive in somatic cells of adult female mammals, so that the effective gene dosage per cell for X-linked loci is the same in XX females as in XY males or XO females. This system of dosage compensation is thought to be established soon after implantation¹—certainly both X chromosomes seem to be potentially active in the blastocyst². To test for dosage compensation before implantation one can study the activity of an X-chromosome-coded enzyme that is synthesised during pre-implantation development, as a result of de novo transcription, in male and female embryos. Two such enzymes are considered to be hypoxanthine phosphoribosyl transferase (HGPRT EC 2.4.2.8) and α-galactosidase^3,4. Epstein³ found that groups of unfertilised eggs from XX mothers had twice the HGPRT activity found in those from XO mothers, indicating that both X chromosomes are functional during oogenesis^3,5. By the morula and blastocyst stage, HGPRT activities had increased strikingly and the twofold difference had disappeared, implying that the enzyme present was no longer maternal in origin. Because pooled male and female embryos were assayed it was impossible to determine whether dosage compensation was occurring. Adler et al.⁴ measured α-galactosidase activity in single pre-implantation embryos, expecting that the presence of both female and male embryos would give rise to either a bimodal or a unimodal distribution of activities in the absence, or presence, respectively, of dosage compensation. The assay was insufficiently precise to give unequivocal results. We have now developed a single-embryo assay for HGPRT of sufficient precision to establish that the distribution of activities in single embryos, half of which we expect to be XX and half XY, is unimodal at both the eight-cell and the blastocyst stage. We conclude that dosage compensation occurs during pre-implantation development.