Malignant gliomas have an extremely poor prognosis with median survival rates of less than 2 years. Although most frequent in older adults, these malignancies are the third leading cause of cancer deaths in persons 15 to 34 years of age. Moreover, the incidence of malignant gliomas is increasing in those older than 65. Alkylating agents, when used in single agent or combination chemotherapy along with surgery and radiation, are the most effective antitumor drugs for the treatment of adult gliomas. However, intrinsic and acquired resistance to alkylating agents limits their usefulness. The broad, long-term objective is to define the contribution of DNA repair mechanisms to glioma resistance to chemotherapeutic agents (methylating and chloroethylating agents), and to identify strategies to combat resistance. The applicants have shown that resistance of 9 human glioma cell lines to alkylating agents is based on a mechanism(s) in addition to the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). Their hypothesis is that the 3-methyladenine-DNA glycosylase (3-MAG), an enzyme which initiates base excision repair of n3-methyladenine and other N-alkylpurines, also contributes to resistance. To confirm this hypothesis, they will demonstrate that N3-methyladenine, the principal substrate of 3-MAG, is lethal to human glioma cells by correlating adduct removal with sensitivity to MeOSO2(CH2)2-lexitropsin, a recently developed alkylator that methylates almost exclusively at the N3 atom of adenine. They will also use antisense mRNA expression to suppress 3-MAG in previously analyzed glioma lines in which we have quantitated alkylating agent cytotoxicity and the contribution of MGMT to resistance. They will quantitate the effect of antisense expression on 3-MAG mRNA and enzyme levels, and on chloroethylating and methylating agent cytotoxicity in the absence and presence of O6-benzylguanine, a substrate analog inhibitor of MGMT. In related work, they will quantitate 3-MAG, together with MGMT, in newly diagnosed and recurrent brain tumors, and asses the relationship of enzyme levels to response to alkylating agent therapy. The tissue studies, together with the in vitro work, will aid them in identifying potentially effective alkylating agent/inhibitor therapies. If their hypothesis is correct, inhibitors of 3-MAG might eventually be tested clinically, either with or without concurrent inhibition of MGMT with O6-benzylguanine. It is a logical expectation that a combination of differentially targeted inhibitors might effectively potentiate alkylating agent chemotherapy for adult gliomas. This is a revised application. The long-term goals of the project are to define the contribution of DNA repair mechanisms to glioma resistance to alkylating chemotherapeutic agents.
Three specific aims are outlined to test the hypothesis that 3-methyladenine DNA glycosylase, the enzyme that initiates base excision repair at N3-meAd and other N-alkyl purines, contributes to alkylating agent resistance.
Specific aim 1 will attempt to establish that unrepaired N3-meAd DNA adducts are toxic to human glioma cells.
Specific aim 2 will determine whether modulation of 3-MAG levels affects sensitivity to methylating and chloroethylating agents.
Specific aim 3 will measure the levels of 3-MAG and MGMT in newly diagnosed and recurrent brain tumors. In an effort to define the role of 3-MAG in clinical drug resistance, enzyme levels will be correlated with response to alkylator adjuvant therapy and clinical course.
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