Mapping the End Points of Large Deletions Affecting the hprt Locus in Human Peripheral Blood Cells and Cell Lines

We have examined the extent of HPRT^- total gene deletions in three mutant collections: spontaneous and X-ray-induced deletions in TK6 human B lymphoblasts, and HPRT^- deletions arising in vivo in T cells. A set of 13 Xq26 STS markers surrounding hprt and spanning approximately 3.3 Mb was used. Each marker used was observed to be missing in at least one of the hprt deletion mutants analyzed. The largest deletion observed encompassed at least 3 Mb. Nine deletions extended outside of the mapped region in the centromeric direction (>1.7 Mb). In contrast, only two telomeric deletions extended to marker 342R (1.26 Mb), and both exhibited slowed or limited cell growth. These data suggest the existence of a gene, within the vicinity of 342R, which establishes the telomeric limit of recoverable deletions. Most (25/41) X-ray-induced total gene deletion mutants exhibited marker loss, but only 1/8 of the spontaneous deletions encompassed any Xq26 markers (P = 0.0187). Furthermore, nearly half (3/8) of the spontaneous 3′ total deletion breakpoints were within 14 kb of the hprt coding sequence. In contrast, 40/41 X-ray-induced HPRT^- total deletions extended beyond this point (P = 0.011). Although the overall representation of total gene deletions in the in vivo spectrum is low, 4/5 encompass Xq26 markers flanking hprt. This pattern differs significantly from spontaneous HPRT^- large deletions occurring in vitro (P = 0.032) but resembles the spectrum of X-ray-induced deletions.