INVIVO MUTATIONS IN HUMAN BLOOD-CELLS - BIOMARKERS FOR MOLECULAR EPIDEMIOLOGY

Mutations arising in vivo in recorder genes of human blood cells provi de biomarkers for molecular epidemiology by serving as surrogates for cancer-causing genetic changes. Current markers include mutations of t he glycophorin-A (GPA) or hemoglobin (Hb) genes, measured in red blood cells, or mutations of the hypoxanthine-guanine phosphoribosyltransfe rase (hprt) or HLA genes, measured in T-lymphocytes. Mean mutant frequ encies (variant frequencies) for normal young adults are approximately : Hb (4 x 10(-8)) < hprt (5 x 10(-6)) = GPA (10 x 10(-6)) < HLA (30 x 10(-6)). Mutagen-exposed individuals show decided elevations. Molecula r mutational spectra are also being defined. For the hprt marker syste m, about 15% of background mutations are gross structural alterations of the hprt gene (e.g., deletions); the remainder are point mutations (e.g., base substitutions or frameshifts). Ionizing radiations result in dose-related increases in total gene deletions. Large deletions may encompass several megabases as shown by co-deletions of linked marker s. Possible hprt spectra for defining radiation and chemical exposures are being sought. In addition to their responsiveness to environmenta l mutagens/carcinogens, three additional findings suggest that the in vivo recorder mutations are relevant in vivo surrogates for cancer mut ations. First, a large fraction of GPA and HLA mutations show exchange s due to homologous recombination, an important mutational event in ca ncer. Second, hprt mutations arise preferentially in dividing T-cells, which can accumulate additional mutations in the same clone, reminisc ent of the multiple hits required in the evolution of malignancy. Fina lly fetal hprt mutations frequently have characteristic deletions of h prt exons 2 and 3, which appear to be mediated by the VDJ recombinase that rearranges the T-cell receptor genes during thymic ontogeny. Ille gitimate events such as these also appear to occur in human leukemias.