V(D)J RECOMBINASE-MEDIATED HPRT MUTATIONS IN PERIPHERAL-BLOOD LYMPHOCYTES OF NORMAL-CHILDREN

Somatic mutations in the hypoxanthine-guanine phosphoribosyltransferas e (hprt) gene are rare occurrences in T-lymphocytes of normal individu als. Lacking pathogenic significance, these events can serve as report ers for assessing environmental genotoxicity. The present molecular an alyses 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 similar to 20 Kb deletions encompassing exons 2 and 3. The fr equency of these specific changes are dramatically decreased in older children. Sequence analysis of these deletion breakpoint and joining r egions reveal the molecular hallmarks of V(D)J recombinase-mediated re combination events. This early childhood hprt mutational spectrum is q uite distinct from the adult background spectrum but similar to that r eported previously for newborns, as determined in lymphocytes from pla cental 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-ce ll 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 pres ent study. This newly found age-frequency distribution of V(D)J recomb inase-mediated hprt mutations correlates with the age-frequency distri bution of childhood acute lymphocytic leukemia. A significant number o f these malignancies, including acute T-cell leukemia, are also charac terized by V(D)J recombinase-mediated recombinations but in critical r egions of the genome, hprt, therefore, captures a pathogenic mutagenic mechanism as a harmless mistake which, when it occurs in other geneti c regions, may result in malignancy.