Single-allele expression at an X-linked hyperuricemia locus in heterozygous human cells.

One type of control over genic expression involves repression of genes on all but one of the X chromosomes in somatic cells of mammals, according to the Lyon hypothesis' or a counterpart of this concept termed the single-active-X hypothesiS.2 (See refs. 3-6 for recent reviews.) The differentiation into a single "active" X and additional "inactive" X's (one in normal females) begins during cleavage. The two functional states of the X defined at that time appear to be stable during succeeding cell divisions; active X's replicate as such, and inactive X's produce only inactive replicas. The X that is active can be of maternal origin in some cells and of paternal origin in other cells of an individual. These statements imply that cells heterozygous for X-linked alleles will have the phenotype corresponding to either one allele or the other and that such single-allele expression7 will be manifested in clones derived from heterozygotes. Tests of this prediction can be made with clones of cultured cells that are heterozygous for Xlinked genes having phenotypes which are expressed in vitro. This aspect of the single-active-X control scheme has already been demonstrated in clones of cultured human cells that were heterozygous for alleles at the X-linked loci concerned with glucose-6-phosphate dehydrogenase (G6PD)7' 8 or with Hinter's syndrome.9 Similar studies of additional genes will be necessary to evaluate the hypotheses correctly and to discover how the control is effected. Primary hyperuricemia in boys is often the pathological expression of mutant alleles, jh, of a normal X-linked gene, Jh.10 The immediate biochemical lesion is a deficiency in the enzyme hypoxanthine-guanine-phosphoribosyl transferase ("PRTase"; E.C. 2.4.2.811), which converts the bases hypoxanthine and guanine into their ribonucleotides."' 13 The enzyme can be indirectly demonstrated in cultured fibroblasts by autoradiography of cells grown in a medium containing tritiated hypoxanthine. Fibroblasts grown from mutant individuals incorporate little or none of the radioactive compound into their nucleic acids and have few grains of reduced silver over them in autoradiographs.'4 The hypothesis of X-linked inheritance of hyperuricemia is supported by the observation of the disease in males only up to the present time and by the data from five published pedigrees"-7 (and our unpublished pedigree). Additional supporting evidence which suggests linkage of the Jh locus to the X-linked locus, Xga, is found in one pedigree."7 Assuming X-linkage of Jh, one would expect the locus to exhibit single-allele expression. Rosenbloom et al.14 have described the occurrence of phenotypically normal and phenotypically mutant cells in a culture derived from the skin of a female heterozygous for the mutant jh gene. We demonstrate here that clones of cells cultured from such heterozygotes maintain the same discreteness and are of two phenotypic classes, normal and mutant. These data provide additional support for the single-active-X hypothesis by