V(D)J recombinase-mediated HPRT mutations in peripheral blood lymphocytes of normal children

Somatic mutations in the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene are rare occurrences in T-lymphocytes of normal individuals. Lacking pathogenic significance, these events can serve as reporters for assessing environmental genotoxicity. The present molecular analyses of hprt mutations arising spontaneously in normal children show that 30-35% of the genomic hprt changes in children under 5 years of age have approximately 20 Kb deletions encompassing exons 2 and 3. The frequency of these specific changes are dramatically decreased in older children. Sequence analysis of these deletion breakpoint and joining regions reveal the molecular hallmarks of V(D)J recombinase-mediated recombination events. This early childhood hprt mutational spectrum is quite distinct from the adult background spectrum but similar to that reported previously for newborns, as determined in lymphocytes from placental cord blood. The present study also demonstrates that definition of sequences in the hprt deletion joining regions that are analogous to the N-nucleotide insertion hypervariable regions of rearranged T-cell receptor genes allows the same identification of in vivo clonality of mutants as does analysis of the T-cell receptor gene rearrangements themselves. These methods reveal an in vivo clonal amplification of a V(D)J recombinase-mediated hprt mutant clone in one child in the present study. This newly found age-frequency distribution of V(D)J recombinase-mediated hprt mutations correlates with the age-frequency distribution of childhood acute lymphocytic leukemia. A significant number of these malignancies, including acute T-cell leukemia, are also characterized by V(D)J recombinase-mediated recombinations but in critical regions of the genome. hprt, therefore, captures a pathogenic mutagenic mechanism as a harmless mistake which, when it occurs in other genetic regions, may result in malignancy.