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Endonucleolytic activity for gamma-irradiated DNA in normal and ataxia telangiectasia fibroblast cell extracts
Abstract:
The increased sensitivity of ataxia telangiectasia cells towards ionizing radiation may be related to their inability to incise DNA near sites of radiation-induced base damages. When compared to 3 unaffected controls, crude extracts from 5 lines of fibroblast cells derived from ataxia telangiectasia patients were capable of incising gamma-irradiated DNA to the same extent as normal cells as determined in a nicking assay, using the circular replicative form of phiX174. However, the types of alterations introduced into DNA by gamma-irradiation could be distinguished from sites of base loss due to depurination or depyrimidination and from sites of base modification by OsO4. The specific endonuclease involved was demonstrated to be distinct from the apurinic endonuclease by its rate of temperature inactivation.
Insights
Ataxia telangiectasia (AT) cells show increased sensitivity to ionizing radiation. Research indicates AT cell extracts can incise irradiated DNA, but the specific mechanism distinguishing radiation damage remains unclear.
Area of Science:
- Molecular Biology
- Genetics
- Radiation Biology
Background:
- Ataxia telangiectasia (AT) is a genetic disorder characterized by increased sensitivity to ionizing radiation.
- This sensitivity is hypothesized to stem from an impaired ability to repair DNA damage, specifically at sites of radiation-induced base modifications.
Purpose of the Study:
- To investigate the DNA incising activity in cell extracts from ataxia telangiectasia patients.
- To determine if AT cells can process radiation-induced DNA damage similarly to normal cells.
- To characterize the specific endonuclease involved in DNA incision near radiation-induced lesions.
Main Methods:
- A nicking assay was employed using the circular replicative form of phiX174 DNA.
- Crude cell extracts from AT fibroblast cell lines and unaffected controls were used.
- Gamma-irradiated DNA and DNA with base modifications (OsO4) or base loss (depurination/depyrimidination) were analyzed.
- Temperature inactivation rates were used to distinguish the endonuclease activity.
Main Results:
- Crude extracts from AT cells exhibited similar DNA incising activity on gamma-irradiated DNA compared to normal cell extracts.
- The assay could differentiate between DNA alterations caused by gamma-irradiation and those from base loss or OsO4 modification.
- The specific endonuclease responsible for incision was found to be distinct from apurinic endonuclease based on temperature inactivation profiles.
Conclusions:
- The DNA incising activity in AT cell extracts is comparable to normal cells for gamma-irradiated DNA.
- The findings suggest that the defect in AT cells might not be a general inability to incise damaged DNA but rather a specific issue with recognizing or processing certain types of radiation-induced lesions.
- The identified endonuclease is distinct from known repair enzymes like apurinic endonuclease, highlighting a potentially novel player in DNA damage response.