Molecular analyses of in vivo hprt mutations in human T-lymphocytes I. Studies of low frequency ‘spontaneous’ mutants by Southern blots

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Abstract
Fifty wild-type and 164 in vivo-derived hprt mutant T-cell clones obtained from eight non-mutagen-exposed adult males with mutant frequency values in the normal range (usually < 10 × 10−6) were studied by Southern blot analyses to determine frequency and extent of gross structural alterations in the hprt gene. Sixteen (9.8%) of the mutant clones showed hprt changes. No site or type of lesion predominated. Relative frequencies of gross structural alterations in the recovered hprt mutants did not differ among the eight individuals, within limits detectable by the study. DNA from 201 of these 214 clones was also studied with a T-cell receptor (TCR) β gene probe as a marker for independence of in vivo-derived clones. Some clones were also studied with a TCR γ gene probe. Ninety-four percent of wild-type and 89% of the hprt mutants were found to originate from independent in vivo precursors. Therefore, most of the recovered hprt mutants in the study were presumably derived from separate in vivo mutations. For non-mutagenized adults with normal mutant frequencies, in vivo mutant frequencies are thus reasonable approximations of in vivo mutation frequencies, although elsewhere we show that this is not necessarily true for individuals with grossly elevated mutant frequencies.