IDENTIFICATION OF NITROSOUREA-RESISTANT HUMAN RHABDOMYOSARCOMAS BY IN-SITU IMMUNOSTAINING OF O6-METHYLGUANINE-DNA METHYLTRANSFERASE

Cellular levels of O6-methylguanine-DNA methyltransferase (MGMT) corre late strongly with cellular resistance to carcinogenic and chemotherap eutic agents that produce adducts at the O6-position of guanine in DNA . Although biochemical and molecular assays can indicate the average M GMT content of tissues or tumors, they cannot distinguish mixed popula tions of cells, such as those that exist in tumor biopsy samples. We h ave determined MGMT at the cellular level in a panel of pediatric rhab domyosarcoma xenografts by in situ immunostaining with a human MGMT-sp ecific antibody employing a very sensitive procedure that involves bio tin-avidin coupled horseradish peroxidase with silver-enhanced diamino benzidine-nickel staining. Two xenograft tumor lines known to be MGMT- deficient were not stained, whereas the nuclei in three MGMT-expressin g lines were clearly stained. This is the first demonstration of an in situ procedure that discriminates drug-sensitive MGMT-deficient tumor s from drug-resistant MGMT expressing tumors. This procedure should pr ove useful, therefore, for predicting the susceptibility of tissues an d tumors to O6-guanine alkylating agents.