Differential expression of steroid sulphatase locus on active and inactive human X chromosome

• Published in: Nature (London) 1982-10, Vol.299 (5886), p.838-840
• Main Authors: Migeon, Barbara R, Shapiro, Larry J, Norum, Robert A, Mohandas, Thuluvancheri, Axelman, Joyce, Dabora, Rebecca L
• Format: Article
• Language: English
• Subjects: Dosage Compensation, Genetic; Female; Genes; Glucosephosphate Dehydrogenase - metabolism; Heterozygote; Humanities and Social Sciences; Humans; letter; Male; multidisciplinary; Pedigree; Science; Science (multidisciplinary); Sex Chromosomes; Steryl-Sulfatase; Sulfatases; X Chromosome
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/299838a0

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 inactivated 1 . The X-locus for steroid sulphatase (STS; EC 3.1.6.2), the microsomal enzyme that catalyses the hydrolysis of various 3 β -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 inactivation 2 , have not been found 3 . Further evidence that the STS locus escapes inactivation is that the human inactive X chromosome contributes STS activity to mouse–human hybrid cells 4 . On the basis of these hybrid studies 5,6 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 chromosomes 7–13 (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.