O6-alkylguanine DNA-alkyltransferase is not a major determinant of sensitivity to 1,3-bis(2-chloroethyl)-1-nitrosourea in four medulloblastoma cell lines.

The protein O6-alkylguanine-DNA alkyltransferase (O6-AGT) has been implicated as a major determinant of resistance of diverse tumors to chloroethylnitrosoureas. To evaluate the contribution of O6-AGT to resistance of medulloblastomas to chloroethylnitrosoureas, we assessed the role of O6-AGT in determining (BCNU). Sensitivity to BCNU cytotoxicity, measured as dose dependent survival of soft agar colony forming ability, varied among the lines. Two lines (UW443 and UW228-3) displayed linear survival curves and comparable BCNU sensitivity (D37 ca. 140 microM). The other lines (UW228-2 and UW228-1) had biphasic survival curves indicating that each line was composed of two sub-populations that differed in BCNU sensitivity. The D37 for these sub-populations ranged from 51 microM to 253 microM. The O6-AGT activities of the cell lines, however, did not reflect their varied susceptibilities to BCNU as evidenced by a 9-fold difference in O6-AGT activity between UW443 and UW228-3. Moreover, elimination of O6-AGT activity by the inhibitor O6-benzylguanine did not appreciably increase sensitivity to BCNU compared with the response of other human tumor cells [Dolan et al. Cancer Res. 51:3367-3372, 1991]. Our results demonstrate that O6-AGT is not a major determinant of BCNU sensitivity in the four medulloblastoma lines.