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Intracellular Localization and intercellular heterogeneity of the human DNA repair protein O(6)-methylguanine-DNA
M Belanich1, T Randall, M A Pastor
1Applied Genetics Inc., Freeport, NY 11520 USA.
Abstract:
O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes alkyl adducts from DNA and may be important in tumor resistance to alkylation chemotherapy. MGMT was visualized in human cells and tumor tissues with monoclonal antibodies against MGMT and immunofluorescence microscopy, and fluorescent signals were quantified by digital image analysis. MGMT was found both in the cytoplasm and the nucleus, and in either locale the protein reacts with alkylated DNA bases and becomes inactivated and lost from the cell. Cell lines in culture and xenografts showed a broad normal distribution of nuclear MGMT levels, but human brain tumors often showed a skewed distribution, with a significant fraction of cells with high levels of MGMT. O(6)-Benzylguanine, a suicide substrate inactivator for MGMT activity, reduced MGMT in human cells and in a mouse xenograft to levels undetectable by antibody assay 1 h post-treatment. In melanoma specimens taken from a patient 3 h post-treatment with temozolomide, MGMT levels were reduced by 70%. This quantitative immunofluorescence assay can be used to monitor MGMT and it depletion in human tumors to improve the use of alkylating agents in cancer chemotherapy.
Insights
O(6)-Methylguanine-DNA methyltransferase (MGMT) repairs DNA damage. This study quantifies MGMT levels in tumors, showing high levels in brain tumors and successful reduction with O(6)-Benzylguanine and temozolomide.
Area of Science:
- Molecular Biology
- Oncology
- Biochemistry
Background:
- O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein crucial for removing alkyl adducts.
- MGMT activity influences tumor resistance to alkylation chemotherapy.
Purpose of the Study:
- To visualize and quantify MGMT protein levels in human cells and tumor tissues.
- To assess the impact of MGMT inactivators on protein levels in vitro and in vivo.
Main Methods:
- Monoclonal antibodies against MGMT and immunofluorescence microscopy were employed.
- Digital image analysis quantified fluorescent signals for MGMT detection.
- O(6)-Benzylguanine and temozolomide were used as MGMT inactivators.
Main Results:
- MGMT was detected in both cytoplasm and nucleus of human cells and tissues.
- Human brain tumors exhibited a skewed distribution of nuclear MGMT, with many cells showing high levels.
- O(6)-Benzylguanine treatment reduced MGMT to undetectable levels within 1 hour.
- Temozolomide treatment reduced MGMT levels by 70% in melanoma specimens within 3 hours.
Conclusions:
- A quantitative immunofluorescence assay can effectively monitor MGMT levels and depletion.
- Understanding MGMT levels is vital for optimizing alkylating agent chemotherapy in cancer treatment.