Alterations of the hprt gene in human in vivo-derived 6-thioguanine-resistant T lymphocytes

Investigations into the extent and significance of somatic gene mutations occurring in vivo in humans have been hampered by the lack of a means of unambiguously defining the mutational origin of in vivo -derived variant cells. Several years ago we proposed that 6-thioguanine-resistant T lymphocytes, present at low frequencies in human peripheral blood, might be useful markers of in vivo somatic mutation 1,2 . We and others have since described methods for the isolation and study of these unusual cells 3–5 . The thioguanine-resistant T cells stably lack hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity, suggesting that they are somatic equivalents in normal individuals to cells from individuals with the X-chromosomal hprt Lesch-Nyhan germinal mutation 6,7 . We now report that in vivo -derived thioguanine-resistant T-cell colonies from a single normal individual show a variety of hprt structural alterations, as determined by Southern blot analysis. This finding demonstrates unequivocally that these cells are genetic mutants and validates their use for fundamental and applied mutational studies in humans.