46 DETECTION OF HPRT MUTATION IN A HUMAN STEM CELL BEFORE DIFFERENTIATION INTO T OR B CELLS

Abstract

We have been measuring the frequency of in vivo HPRT mutant T cells in the peripheral blood of atomic bomb survivors in Hiroshima. The mean frequency of mutant T cells in 30 survivors (5.2 × 10−6) was significantly higher than that in 17 controls (3.4 × 10−6) and was found to correlate with the radiation dose estimates. In the course of this study, extremely high mutant frequency (2 × 10−4) was identified in one male survivor (59 y.o. radiation dose=199 rad). All the mutant colonies examined had the same chromosome aberration (20 q-) and the same alteration of DNA at the HPRT locus, indicating that all these mutants were derived from a single cell. However, the pattern of T cell receptor gene rearrangement was different between colonies. Furthermore, mutant B cells possessing the same aberration and HPRT gene alteration have also been cloned from this individual by using EB virus transformation. All these results indicate that all the mutant cells were derived from a single stem cell which can differentiate into at least T and B cells. Thus, a single mutational event in an undifferentiated stem cell produced a large number of mutant lymphocytes in periphery in the person studied here.