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Increased DNA synthesis and repair-enzyme expression in lymphocytes from patients with chronic lymphocytic leukemia
R Geleziunas1, A McQuillan, A Malapetsa
1Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec, Canada.
Abstract:
Resistance to the nitrogen mustards in patients with chronic lymphocytic leukemia (CLL) correlates with an enhanced removal of melphalan-induced DNA interstrand cross-links. This finding suggests that DNA repair enzymes may be involved in this process. The activity of 3-methyladenine-DNA glycosylase, which can release altered bases, including adducts at the N-7 position of guanine, was increased significantly in lymphocytes from patients with resistant CLL compared with those from untreated CLL patients. Since glycosylase activity varies with cell proliferation, the amount of [3H]thymidine incorporated into DNA was determined and found to be elevated almost threefold in lymphocytes from patients with resistant CLL. The ratio of glycosylase activity to level of thymidine incorporation did not differ between these two groups of patients. Northern blot analysis of ERCC1 gene (a putative DNA repair enzyme involved in nucleotide excision repair) expression in lymphocytes from patients with CLL revealed multiple gene transcripts (1.1, 3.4, and 3.8 kilobases). In addition, analysis of two samples revealed the presence of a 2.6-kilobase transcript. The 2.6-kilobase transcript was recognized by specific RNA probes that hybridize to antisense ERCC1 transcripts. Levels of expression of the 1.1-kilobase protein encoding transcript in lymphocytes from patients with resistant CLL were increased twofold to threefold above those of untreated patients with CLL. These results indicate that increased expression of ERCC1 and increased activity of 3-methyladenine-DNA glycosylase occur with the development of resistance to the nitrogen mustards in patients with CLL, suggesting a role for enhanced DNA repair in this process.
Insights
Resistance to nitrogen mustards in chronic lymphocytic leukemia (CLL) is linked to enhanced DNA repair. Increased activity of 3-methyladenine-DNA glycosylase and ERCC1 gene expression suggest DNA repair enzymes play a role in drug resistance.
Area of Science:
- Molecular Biology
- Oncology
- Genetics
Background:
- Nitrogen mustards are chemotherapy agents used for chronic lymphocytic leukemia (CLL).
- Drug resistance in CLL can be a significant clinical challenge.
- Enhanced DNA repair mechanisms are hypothesized to contribute to chemotherapy resistance.
Purpose of the Study:
- To investigate the role of DNA repair enzymes in nitrogen mustard resistance in CLL patients.
- To examine the activity of 3-methyladenine-DNA glycosylase and ERCC1 gene expression in resistant CLL cells.
Main Methods:
- Assessed melphalan-induced DNA interstrand cross-link removal in CLL lymphocytes.
- Measured 3-methyladenine-DNA glycosylase activity and [3H]thymidine incorporation.
- Utilized Northern blot analysis to evaluate ERCC1 gene expression.
Main Results:
- Lymphocytes from resistant CLL patients showed enhanced removal of melphalan-induced DNA interstrand cross-links.
- Significantly increased 3-methyladenine-DNA glycosylase activity was observed in resistant CLL lymphocytes.
- Elevated ERCC1 gene expression, particularly the 1.1-kilobase transcript, was found in resistant CLL cells.
Conclusions:
- Increased expression of ERCC1 and heightened 3-methyladenine-DNA glycosylase activity are associated with nitrogen mustard resistance in CLL.
- These findings suggest that enhanced DNA repair pathways contribute to the development of drug resistance in chronic lymphocytic leukemia.