Different mechanisms of radiation-induced loss of heterozygosity in two human lymphoid cell lines from a single donor

Allelic loss is an important mutational mechanism in human carcinogenesis. Loss of heterozygosity (LOH) at an autosomal locus is one outcome of the re pair of DNA double-strand breaks (DSBs) and can occur by deletion or by mit otic recombination, We report that mitotic recombination between homologous chromosomes occurred in human lymphoid cells exposed to densely ionizing r adiation. We used cells derived from the same donor that express either nor mal TP53 (TK6 cells) or homozygous mutant TP53 (WTK1 cells) to assess the i nfluence of TP53 on radiation-induced mutagenesis, Expression of mutant TP5 3 (Met 237 Ile) was associated with a small increase in mutation frequencie s at the hemizygous HPRT (hypoxanthine phosphoribosyl transferase) Locus, b ut the mutation spectra were unaffected at this locus, In contrast, WTK1 ce lls (mutant TP53) were 30-fold more susceptible than TK6 cells (wild-type T P53) to radiation-induced mutagenesis at the TK1 (thymidine kinase) locus. Gene dosage analysis combined with microsatellite marker analysis showed th at the increase in TK1 mutagenesis in WTK1 cells could be attributed, in pa rt, to mitotic recombination, The microsatellite marker analysis over a 64- cM region on chromosome 17q indicated that the recombinational events could initiate at different positions between the TK1 locus and the centromere. Virtually all of the recombinational LOH events extended beyond the TK1 loc us to the most telomeric marker. In general, longer LOH tracts were observe d in mutants from WTK1 tells than in mutants from TK6 cells. Taken together , the results demonstrate that the incidence of radiation-induced mutations is dependent on the genetic background of the cell at risk, on the locus e xamined, and on the mechanisms for mutation available at the locus of inter est.