title: Frequent HPRT mutations in paroxysmal nocturnal haemoglobinuria reflect T cell clonal expansion, not genomic instability.
abstract: Paroxysmal nocturnal haemoglobinuria (PNH) results from acquired mutations in the PIG-A gene of an haematopoietic stem cell, leading to defective biosynthesis of glycosylphosphatidylinositol (GPI) anchors and deficient expression of GPI-anchored proteins on the surface of the cell's progeny. Some laboratory and clinical findings have suggested genomic instability to be intrinsic in PNH; this possibility has been supported by mutation analysis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene abnormalities. However, the HPRT assay examines lymphocytes in peripheral blood (PB), and T cells may be related to the pathophysiology of PNH. We analysed the molecular and functional features of HPRT mutants in PB mononuclear cells from eleven PNH patients. CD8 T cells predominated in these samples; approximately half of the CD8 cells lacked GPI-anchored protein expression, while only a small proportion of CD4 cells appeared to derive from the PNH clone. The HPRT mutant frequency (Mf) in T lymphocytes from PNH patients was significantly higher than in healthy controls. The majority of the mutant T lymphocyte clones were of CD4 phenotype, and they had phenotypically normal GPI-anchored protein expression. In PNH patients, the majority of HPRT mutant clones were contained within the Vbeta2 T cell receptor (TCR) subfamily, which was oligoclonal by complementarity-determining region three (CDR3) size analysis. Our results are more consistent with detection of uniform populations of expanded T cell clones, which presumably acquired HPRT mutations during antigen-driven cell proliferation, and not due to an increased Mf in PNH. HPRT mutant analysis does not support underlying genomic instability in PNH.
authors.affiliation: Haematology Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892-1652, USA.
authors.forename: Guibin
authors.initials: G
authors.lastname: Chen
authors.forename: Weihua
authors.initials: W
authors.lastname: Zeng
authors.forename: Spencer
authors.initials: S
authors.lastname: Green
authors.forename: Neal S
authors.initials: NS
authors.lastname: Young
mesh_terms: D000328:Adult; D002478:Cells, Cultured; D003114:Colony-Forming Units Assay; D022801:Complementarity Determining Regions / Q000378:metabolism; D005260:Female; D042822:Genomic Instability / Q000235:genetics; D017261:Glycosylphosphatidylinositols / Q000097:blood / Q000172:deficiency; D006457:Hemoglobinuria, Paroxysmal / Q000235:genetics* / Q000276:immunology; D006801:Humans; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics*; D016130:Immunophenotyping; D008297:Male; D008875:Middle Aged; D009154:Mutation; D016693:Receptors, Antigen, T-Cell, alpha-beta / Q000235:genetics / Q000378:metabolism; D016176:T-Lymphocyte Subsets / Q000187:drug effects / Q000378:metabolism*; D013866:Thioguanine / Q000494:pharmacology
chemical_list: D022801:Complementarity Determining Regions; D017261:Glycosylphosphatidylinositols; D016693:Receptors, Antigen, T-Cell, alpha-beta; D007041:Hypoxanthine Phosphoribosyltransferase; D013866:Thioguanine