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In mammals, X-chromosome dosage compensation is achieved by inactivating one X chromosome in female cells1. To test the hypothesis2 that genes on the silent X chromosome reactivate as a consequence of ageing, we examined the X-linked hypoxanthine phosphoribosyltransferase (HPRT) locus in 41 women who are heterozygous for mutations at this locus, leading to severe deficiency of the enzyme (Lesch-Nyhan syndrome3). We find that heterozygotes who are more than 10 yr old have an excess of HPRT+ skin fibroblast clones (59% rather than the 50% expected as a consequence of random X inactivation) but this excess does not increase with age. Further studies of eight of these heterozygotes show that the silent locus does not detectably reactivate spontaneously in culture, but only in response to treatment with 5-aza-2-deoxycytidine, a potent inhibitor of methylation. There is no age difference in the frequency of this reactivation as assayed by HATr clones, and a more sensitive autoradiographic assay shows only a twofold difference between young and old heterozy-gotes. Thus, age-related reactivation is not a feature of all X-linked loci, and may have species, tissue and locus-specific determinants.

The X chromosome in mammalian somatic cells is subject to unique regulation—usually genes on a single X chromosome are expressed while those on other X chromosomes are inactivated1. The X-locus for steroid sulphatase (STS; EC 3.1.6.2), the microsomal enzyme that catalyses the hydrolysis of various 3β-hydroxysteroid sulphates, is exceptional because it seems to escape inactivation. Evidence for this comes from fibroblast clones in females heterozygous for mutations that result in a severe deficiency of this enzyme in affected males; all clones from these heterozygotes have STS activity, and enzyme-deficient clones that are expected if the locus were subject to inactivation2, have not been found3. Further evidence that the STS locus escapes inactivation is that the human inactive X chromosome contributes STS activity to mouse–human hybrid cells4. On the basis of these hybrid studies5,6 the STS locus has been mapped to the distal half of the short arm (p22–pter) of the human X chromosome. Although the STS locus on both X chromosomes in human female cells is expressed, quantitative measurements of STS activity in males and females do not accurately reflect the sex differences in number of X chromosomes7–13 (Table 1). The ratio of mean values for normal females to that of normal males is greater than 1:1 but less than the ratio of 2:1 expected if STS loci on all X chromosomes were equally expressed. The incomplete dosage effect suggests that the STS locus on the inactive X chromosome might not be fully expressed. To test this hypothesis, we examined two heterozygotes for X-linked STS deficiency who were also heterozygous for the common electrophoretic variants of glucose-6-phosphate dehydrogenase (G6PD A and B). Studies of fibroblast clones from these females provide evidence, presented here, for differential expression of STS loci on the active and inactive X chromosome.

THE N-acetyl-β-D-glucosaminidase (hexosaminidase) activity in cultured human fibroblasts consists of at least three components (A, B and C)1–3. At least two of these (designated Hex A and Hex B) seem to be closely related, for (1) antiserum against Hex A reacts against Hex B, and vice versa4,5 (2) in Tay-Sachs disease only Hex A activity is deficient, accompanied by an increase in Hex B activity in certain tissues1,6, and (3) both Hex A and Hex B activities are missing in Sandhoff disease, another autosomal recessive disorder7. But in spite of several theories8–11, the precise relationship between these components remains unknown, as does the nature of the genetic and biochemical relationships between the two diseases. To investigate these questions we have fused Tay-Sachs with Sandhoff fibroblasts and obtained cultures containing heterokaryons which produce a hexosaminidase which is absent from the parent lines. It has the electrophoretic and heat lability characteristics of the Hex A found in normal fibroblasts.