DNA strand breaks and repair synthesis in Yoshida sarcoma cells with differential sensitivities to bifunctional alkylating agents and UV light

The induction of DNA-strand breaks and repair synthesis has been examined in cultured Yoshida sarcoma cell lines sensitive (YS) and resistant (YR) to methylene dimethanesulphonate (MDMS). Using an alkaline DNA unwinding-hydroxylapatite technique, we were able to detect breaks in DNA immediately after MDMS treatment and at similar levels in both YS and YR cells. MDMS treatment and post-treatment incubation in the presence of 1-β-D-arabino-furanosylcytosine (araC) lead to a large increase in the numbers of breaks when compared with MDMS treatment alone which indicated that many of the DNA-strand breaks seen after MDMS treatment were intermediates in excision repair. The magnitude of break incidence with the araC treatment was again equal in YS and YR cells indicating that these 2 lines made enzymic incision next to MDMS-induced lesions with equal capacities.During incubation following MDMS treatment, the levels of DNA-strand breaks in YR cells were found to decrease more rapidly than in YS cells. Parallel DNA-repair synthesis estimations, using BND-cellulose chromatography, revealed that the increased rate of decline in breaks in YR cells was accompanied by an increase in repair-synthesis activity compared to YS cells. This was interpreted as indicating that an intermediate step in an excision-repair pathway for MDMS-induced lesions was relatively deficient in YS compared to YR cells.A similar difference in the rates of decline of DNA-strand breaks between YS and YR cells was also observed following treatment with UV light to which MDMS-resistant YR cells also display cross-resistance. However, no such difference was detected following treatment with the monofunctional alkylating agent, methyl methanesulphonate, to which YS and YR cells are equally sensitive. These results suggest that resistance to MDMS in the YR cell line is achieved by an increased efficiency in the gap-sealing component of the excision-repair process.