CONTRIBUTION OF O-6-METHYLGUANINE-DNA METHYLTRANSFERASE TO MONOFUNCTIONAL ALKYLATING-AGENT RESISTANCE IN HUMAN BRAIN TUMOR-DERIVED CELL-LINES

The DNA repair protein O-6-methylguanine-DNA methyltransferase (MGMT) has been implicated in resistance of human brain tumors to alkylating agents. We observed that 14 human medulloblastoma- and glioma-derived cell lines differ in sensitivity to the methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), as shown by their 28-fold range in 10% survival dose (LD(10)). By using the substrate analogue inhibitor O-6-benzylguanine (O-6-BG), we showed that the contribution of MGMT to resistance varies widely, as evidenced by 3- to 30-fold reductions in LD(10) among the lines, and varies up to 26-fold among subpopulations of individual lines. Importantly, variability in resistance, manifested as a 20-fold range in LD(10), persists after measurable MCMT is eliminated, disclosing differential contributions of other resistance mechanisms to survival. Cells exposed to MNNG while suspended in growth medium are more resistant than cells alkylated as subconfluent monolayers, and MGMT accounts for a smaller proportion of their resistance. Notably, the MGMT content of the lines is not statistically correlated with MNNG resistance or with potentiation of killing by O-6-BG, even though MGMT is a biochemically demonstrated determinant of resistance. In contrast, the same lines vary less in resistance to the ethylating agent N-ethylnitrosourea (ENU), and MCMT makes only a small contribution to resistance. Our results strongly indicate that resistance to both MNNG and ENU is multifactorial. (C) 1995 Wiley-Liss, Inc.