Expression of human hprt gene on the inactive X chromosome after DNA-mediated gene transfer

Abstract

The paternal or maternal mammalian X chromosome is inactivated (‘lyonization’) at random early in the development of the female embryo^1,2. Once established, the inactivation of the chromosome is maintained in the cell and in all its descendants. However, spontaneous local reactivation of the inactive X chromosome is found at low frequency in human–mouse hybrid cells^3,4. So far only the X_g locus^5,6 and the locus for microsomal steroid sulphatase (sts)⁷ (both assigned to the short arm of the X chromosome), have been shown to escape inactivation. The locus for hypoxanthine phosphoribosyltransferase (hprt) is susceptible to inactivation⁸ and humans heterozygous for HPRT deficiency (the Lesch–Nyhan syndrome, L–N)⁹, show a mosaic pattern of HPRT activity in their cultured fibroblasts⁸. To investigate the molecular basis of X inactivation, we have used DNA isolated from each of the two subpopulations of a L–N heterozygote, to transform cultured HPRT-deficient mouse fibroblasts. Transformation of the mouse cells with the inactivated human hprt⁺ gene occurred with essentially the same efficiency as with the active human hprt⁺ gene. Thus, reactivation of the lyonized human hprt locus is possible after interspecific DNA-mediated gene transfer.