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Relevance of DNA repair to carcinogenesis and cancer therapy
M F Rajewsky1, J Engelbergs, J Thomale
1Institute of Cell Biology (Cancer Research) [IFZ], University of Essen Medical School, Germany.
Abstract:
DNA-reactive carcinogens and anticancer drugs induce many structurally distinct cytotoxic and potentially mutagenic DNA lesions. The capability of normal and malignant cells to recognize and repair different DNA lesions is an important variable influencing the risk of mutation and cancer as well as therapy resistance. Using monoclonal antibody-based immunoanalytical assays, very low amounts of defined carcinogen-DNA adducts can be quantified in bulk genomic DNA, individual genes, and in the nuclear DNA of single cells. The kinetics of DNA repair can thus be measured in a lesion-, gene-, and cell type-specific manner, and the DNA repair profiles of malignant cells can be monitored in individual patients. Even structurally very similar DNa lesions may be repaired with extremely different efficiency. The miscoding DNA alkylation products O6-methylguanine (O6-MeGua) and O6-ethylguanine (O6-EtGua), for example, differ only by one CH2 group. These lesions are formed in DNA upon exposure to N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), both of which induce mammary adenocarcinomas in female rats at high yield. Unrepaired O6-alkylguanines cause transition mutations via mispairing during DNA replication. O6-MeGua is repaired at a similar slow rate in transcribed (H-ras, beta-actin) and inactive genes (IgE heavy chain; bulk DNA) of the target mammary epithelia (which express the repair protein O6-alkylguanine-DNA alkyltransferase at a very low level). O6-EtGua, however, via an alkyltransferase-independent mechanism, is excised approximately 20 times faster than O6-MeGua from the transcribed genes selectively. Correspondingly, G:C-->A:T transitions arising from unrepaired O6-MeGua at the second nucleotide of codon 12 (GGA) of the H-ras gene are frequently found in MeNU-induced mammary tumors, but are absent in their EtNU-induced counterparts.
Insights
Cells repair DNA damage differently based on the lesion type. This study shows distinct repair rates for similar DNA lesions, impacting mutation and cancer risk. Understanding DNA repair profiles is key for cancer therapy.
Area of Science:
- Molecular Biology
- Cancer Research
- Toxicology
Background:
- DNA-reactive agents cause diverse DNA lesions, influencing cancer risk and therapy outcomes.
- Cellular DNA repair mechanisms are crucial for preventing mutations and maintaining genomic stability.
- Assessing DNA repair kinetics in specific genes and cell types is vital for understanding individual cancer susceptibility and treatment response.
Purpose of the Study:
- To investigate the differential repair kinetics of structurally similar DNA lesions, O6-methylguanine (O6-MeGua) and O6-ethylguanine (O6-EtGua).
- To determine if DNA repair efficiency varies between transcribed and inactive genes, and between different cell types.
- To correlate specific DNA lesions with mutation patterns in carcinogen-induced mammary tumors.
Main Methods:
- Utilizing monoclonal antibody-based immunoassays to quantify carcinogen-DNA adducts in genomic DNA, specific genes, and single cells.
- Measuring the kinetics of DNA repair for O6-MeGua and O6-EtGua in mammary epithelia following exposure to N-methyl-N-nitrosourea (MeNU) and N-ethyl-N-nitrosourea (EtNU).
- Analyzing DNA repair rates in transcribed (H-ras, beta-actin) and inactive (IgE heavy chain) genes, as well as bulk DNA.
Main Results:
- O6-MeGua was repaired slowly and uniformly across transcribed and inactive genes in mammary epithelia.
- O6-EtGua was excised approximately 20 times faster than O6-MeGua from transcribed genes via an alkyltransferase-independent pathway.
- Mammary tumors induced by MeNU frequently showed G:C-->A:T transitions in the H-ras gene, associated with unrepaired O6-MeGua, while EtNU-induced tumors lacked these mutations.
Conclusions:
- Structural similarity of DNA lesions does not predict similar repair efficiency.
- Selective and rapid repair of O6-EtGua from transcribed genes influences mutation outcomes.
- Differential DNA repair mechanisms contribute to distinct mutation profiles observed in tumors induced by related carcinogens.