HUMAN IN-VIVO SOMATIC MUTATION MEASURED AT 2 LOCI - INDIVIDUALS WITH STABLY ELEVATED BACKGROUND ERYTHROCYTE GLYCOPHORIN-A (GPA) VARIANT FREQUENCIES EXHIBIT NORMAL T-LYMPHOCYTE HPRT MUTANT FREQUENCIES

A survey of glycophorin A (gpa) in vivo somatic cell mutation in a pop ulation of 394 healthy people from 8 to 77 years of age (mean age +/- SD 41 +/- 15 years) revealed a subset of 37 individuals with stably el evated allele-loss and/or allele-loss with duplication variant erythro cyte frequencies (V-f) exceeding 30 x 10(-6). These 37 individuals wit h gpa outlier V-f are significantly older (p < 0.001) than the remaind er of the larger study population from which they were drawn reflectin g a highly significant increase in the prevalence of these individuals in the population beyond age 40 years. A study of hprt mutant frequen cies (M-f) in the peripheral blood T-lymphocytes of 27 of these indivi duals, together with 15 matched control individuals with unremarkable gpa V-f. was undertaken to determine ii these subjects also displayed elevated mutation frequencies at this independent locus indicative of globally elevated somatic mutation. The hprt M-f in these 27 subjects (geometric mean 11.5 x 10(-6) (dispersion interval 5.8 x 10(-6) to 22. 8 x 10(-6))) was not significantly different from that observed in the 15 controls (geometric mean 12.1 x 10(-6) (dispersion interval 5.7 x 10(-6) to 25.5 x 10(-6))). These M-f are higher than typically reporte d values reflecting the older age distribution of these individuals (a rithmetic mean age +/- SD 53 +/- 12 and 50 +/- 16 years for the subjec ts and controls, respectively). Taken together, these data suggest tha t several genetic mechanisms may be responsible for producing the gpa outlier V-f observed in these subjects. The observation that hprt M-f were not increased indicates that the majority did not arise by a geno me-wide increased rate of somatic mutation detectable at both loci. Th e fixation and subsequent expansion of 'jackpot' mutations at the gpa locus occurring early in embryonic/fetal development also does not app ear to be a predominant mechanism. Some cases may result from a stable over-representation of gpa variant cells, perhaps associated with a m arked age-dependent decrease in the number of contributing erythroid s tem cells in the bone marrow. The subset that displays elevated allele -loss with duplication V-f involving both gpa alleles may represent in dividuals with increased rates of somatic recombination. Elevations ar ising by this mechanism are not detected in the hprt assay, but could be confirmed using an autosomal locus in vivo somatic cell mutation en dpoint such as thr hla-a assay. Of primary biological significance, th ese results demonstrate that genetic/stochastic processes leading to t he loss of heterozygosity of somatic cells occur ubiquitously in human s and in some individuals this level of somatic mosaicism can approach a frequency of 10(-3) at the gpa locus in erythroid lineage cells. (C ) 1998 Elsevier Science B.V.