The cytotoxic drug 6-mercaptopurine (6-MR b-thiopurine) and the closely related immunosuppressive agent azathioprine (6-(1-methyl-4-nitro-5-irnidazolylthio)purine) act as purine antagonists (Fig. 1). They interfere with biochemical processes involving endogenous purines, which are essential components of DNA, RNA, and certain co-enzymes. Both mercaptopurine and azathioprine have cytotoxic and immunosuppressive properties that are variously due to the inhibition of the synthesis of proteins, DNA, and RNA, with additional effects attributable to the reactive thiot group. Mercaptopurine was initially evaluated for the treatment of leukaemia in the early 1950s [Burchenal et al. 1953]. Its potential value as an immunosuppressive was demonstrated by Schwartz et al. (1958), who showed that in laboratory animals injected with an antigen, treatment with mercaptopurine prevented the development of an antibody response. Azathioprine, a \"masked\" form of mercaptopurine, was found to be a more potent immunosuppressive agent in dogs [Calne et al. 1962], and this led to its introduction in clinical transplantation in the early 1960s [Murray et al. 1963]. Despite the widespread use of these compounds for over three decades precise details of their molecular basis of action in man are lacking.

The clinical pharmacology of 6-mercaptopurine.

This book has five chapters covering peroxisomal diseases, X-linked immunodeficiencies, genetic mutations affecting human lipoproteins and their receptors and enzymes, genetic aspects of cancer, and Gaucher disease. The chapter on peroxisomes covers their discovery, structure, functions, disorders, etc. The chapter on X-linked immunodeficiencies discusses such diseases as agammaglobulinemia, severe combined immunodeficiency, Wiskott-Aldrich syndrome, animal models, linkage analysis, etc. Apolipoprotein formation, synthesis, gene regulation, proteins, etc. are the main focus of chapter 3. The chapter on cancer covers such topics as oncogene mapping and the molecular characterization of some recessive oncogenes. Gaucher disease is covered from its diagnosis, classification, and prevention, to its organ system involvement and molecular biology.

Advances in Human Genetics

To explore the role of DNA methylation in maintaining dosage compensation of X chromosome-linked genes and in regulating the transcriptional activity of "housekeeping" genes, we characterized DNA methylation of active, inactive, and derepressed alleles at the locus for hypoxanthine phosphoribosyltransferase (HPRT) on the human X chromosome. The methylation of Hpa II and Hha I sites in HPRT alleles on the active X chromosome was the same in all tissues. The consensus pattern includes hypomethylation of 5' clustered sites and extensive methylation of the 3' sequence. The striking feature of methylation of inactive X-chromosome alleles is nonuniformity and less extensive hypomethylation of the 5' cluster. Analysis of HPRT alleles reactivated in response to 5-azacytidine showed at least partial restoration of the consensus pattern. These observations indicate that methylation of housekeeping genes on the X chromosome is the same as that of autosomal ones and that the overall pattern and methylation of multiple sites within a cluster may cooperate to facilitate transcription. Furthermore, the fidelity of methylation of the active allele and the extensive drift in methylation of the inactive allele suggest that mechanisms involved in X-chromosome dosage compensation may be directed at the active rather than inactive X chromosome.

https://www.pnas.org/content/pnas/81/9/2806.full.pdf

Methylation of the hypoxanthine phosphoribosyltransferase locus on the human X chromosome: implications for X-chromosome inactivation.

Glucose-6-phosphate dehydrogenase (G6PD) is a cytoplasmic enzyme that is essential for a cell's capacity to withstand oxidant stress. G6PD deficiency is the commonest enzymopathy of humans, affecting over 400 million persons worldwide. The geographical correlation of its distribution with the historical endemicity of malaria suggests that 66PD deficiency has risen in frequency through natural selection by malaria. This is supported by data from in vitro studies that demonstrate impaired growth of P. falciparum parasites in G6PD-deficient erythrocytes. Attempts to confirm that G6PD deficiency is protective in field studies of malaria have yielded conflicting results, but recent results from large case control studies conducted in East and West Africa provide strong evidence that the most common African G6PD deficiency variant, G6PD A-, is associated with a significant reduction in the risk of severe malaria for both G6PD female heterozygotes and male hemizygotes. The effect of female homozygotes on severe malaria remains unclear but can probably be assumed to be similar to that of comparably deficient male hemizygotes.

/pdf/glucose-6-phosphate-dehydrogenase-deficiency-and-malaria-2v0kjvhrxk.pdf

Glucose-6-phosphate dehydrogenase deficiency and malaria

We report the identification and characterization of a family of repeated restriction fragments whose molecular organization is apparently specific to the human X chromosome. This fragment, identified as an ethidium bromide-staining 2.0 kilobase (kb) band in BamHI-digested DNA from a Chinese hamster-human somatic cell hybrid containing a human X chromosome, has been cloned into pBR325 and characterized. The 2.0 kb repeated family has been assigned to the Xp11 leads to Xq12 region on the X by Southern blot analysis of somatic cell hybrids and is predominantly arranged in tandem clusters of up to seven 2.0 kb monomers. Homologous DNA sequences, not organized as 2.0 kb BamHI fragments, are found elsewhere on the X chromosome and on at least some autosomes, but are not found on the Y chromosome. From a dosing experiment using various amounts of the cloned repeat, we estimate that there are 5,000-7,500 copies of the 2.0 kb BamHI repeat per haploid genome. Since the vast majority, if not all, of these are confined to the X chromosome, this repeated DNA family must account for 5-10% of all X chromosome DNA and must constitute the major sequence component of the pericentromeric region of the X.

Isolation and characterization of a major tandem repeat family from the human X chromosome

We report a unique and complex karyotypic rearrangement involving chromosomes X, 3, 7, and 21. Blood cells and fibroblasts from the proband do not express the maternal allele for glucose-6-phosphate dehydrogenase (G6PD), providing biochemical evidence for nonrandom expression of X-linked genes in balanced X-autosome translocations. The break point on the X chromosome, at the junction of Xq27-Xq28, separates the loci for hypoxanthine phosphoribosyltransferase (HPRT) and G6PD. Studies of mouse-human hybrids derived from the proband's cells indicate that G6PD, at q28, is clearly distal to all other X loci now assigned. From these and previous studies, we can localize HPRT to that segment between Xq26 and Xq27. The studies also provide further evidence for the stability of the inactive X phenotype in hybrid cells.

https://www.pnas.org/content/pnas/77/5/2810.full.pdf

Localization of loci for hypoxanthine phosphoribosyltransferase and glucose-6-phosphate dehydrogenase and biochemical evidence of nonrandom X chromosome expression from studies of a human X-autosome translocation.

https://www.cell.com/cell/pdf/0092-8674(82)90175-1.pdf

Derepression with decreased expression of the G6PD locus on the inactive X chromosome in normal human cells

https://www.cell.com/cell/pdf/0092-8674(80)90117-8.pdf

Cristina E. Mareni

Papers

Localization of loci for hypoxanthine phosphoribosyltransferase and glucose-6-phosphate dehydrogenase and biochemical evidence of nonrandom X chromosome expression from studies of a human X-autosome translocation.

Derepression with decreased expression of the G6PD locus on the inactive X chromosome in normal human cells

Isolation and characterization of cloned DNA sequences that hybridize to the human X chromosome.