MUTAGENIC EFFECTS OF TUMORIGENIC NEUTRON RADIATION

A fraction of thymic lymphomas induced by high LET neutron radiation c ontains activating mutations (single-base substitutions) in the ros ge nes. To determine whether such mutations are the result of the interac tion of high LET radiation with cellular DNA, we have utilized an in v itro model system to screen and isolate neutron-radiation-induced muta nts. With that aim, we irradiated the PL61 hamster cell line with 0.4 MeV neutrons. This cell line contains linked copies of the gpt and neo (r) genes, which permits selection for large or small alterations, dep ending on the selection imposed. Mutants selected for large alteration s represented 98.2% of the total. When selection for small mutations w as imposed, 9 clones grew. The molecular and biochemical analysis of t hese clones revealed that 5 of them had identifiable mutations in the gpt gene, consisting of small insertions and deletions, but no single- base substitutions were detected. This represents the first sequence c haracterization of neutron-induced mutants. The results obtained are c onsistent with the notion that the ros point mutations identified in t he neutron-induced tumors are most likely detected due to the strong s elective advantage that they confer to the host cell, but they probabl y arose during tumor evolution, since they represent a negligible prop ortion of the total number of alterations induced by neutron radiation . (C) 1996 Wiley-Liss, Inc.