Mismatch binding proteins and tolerance to alkylating agents in human cells
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1Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Herts., Great Britain.
Abstract:
The Mex- (Mer-) phenotype of human cells is characterised by a sensitivity to agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU). The hypersensitivity of Mex- cells is a consequence of their failure to express the DNA-repair enzyme m6-Gua-DNA methyltransferase. Resistance to MNNG and MNU may be acquired by Mex- cells either by reexpression of a methyltransferase function or by an ill-defined process of tolerance in which the cytotoxic potential of m6-Gua is circumvented without the altered base being removed from DNA. It has been suggested that tolerance might involve an altered mismatch correcting function. We have investigated proteins which recognise and bind specifically to DNA fragments containing single-base mismatches. Cell-free extracts of a Burkitt's lymphoma cell line (Raji) contain two such mismatch binding activities. Neither protein appears to have a high affinity for m6-Gua-containing base pairs. The data indicate that m6-Gua-containing base pairs might be poor substrates for mismatch repair processes in human cells.
Insights
Human cells with the Mex- (Mer-) phenotype are sensitive to DNA damaging agents like MNNG and MNU due to a lack of the m6-Gua-DNA methyltransferase repair enzyme. This study suggests mismatch repair may not efficiently process the resulting DNA damage.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- The Mex- (Mer-) phenotype in human cells confers sensitivity to DNA alkylating agents like N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU).
- This hypersensitivity arises from the deficient expression of the DNA repair enzyme, O6-methylguanine-DNA methyltransferase (MGMT).
- Acquired resistance in Mex- cells can occur through MGMT re-expression or a tolerance mechanism bypassing the cytotoxic effects of O6-methylguanine (m6-Gua) without its removal.
Purpose of the Study:
- To investigate the role of mismatch repair in cellular tolerance to DNA damage.
- To identify and characterize proteins involved in recognizing and binding to DNA with single-base mismatches.
Main Methods:
- Analysis of cell-free extracts from a human Burkitt's lymphoma cell line (Raji).
- Investigation of proteins that specifically bind to DNA fragments containing single-base mismatches.
Main Results:
- Two distinct mismatch-binding activities were identified in Raji cell extracts.
- These identified proteins did not show high affinity for DNA base pairs containing O6-methylguanine (m6-Gua).
Conclusions:
- The findings suggest that O6-methylguanine (m6-Gua)-containing base pairs may be inefficient substrates for mismatch repair pathways in human cells.
- This implies that mismatch repair might not be the primary mechanism underlying the tolerance observed in Mex- cells.
- Further research is needed to fully elucidate the tolerance mechanisms against DNA alkylating agents.