Defects in Base Excision Repair (BER) are Linked to Changes in Cell Cycling in Saccharomyces cerevisiae

Date

2021-08

Authors

Nepal, Yogesh

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Abstract

The DNA present in cells can be damaged by exogenous factors such as radiation and chemical agents and is also damaged by endogenous agents such as free radicals. To prevent mutations and carcinogenesis, cells can use various DNA repair pathways, depending upon the type of lesion in the DNA. One major repair pathway, called base excision repair (BER), involves recognition of base damage by a glycosylase like Ogg1 and removal of the nucleotide by endonucleases such as Apn1. Another pathway, homologous recombination (HR), repairs DNA double-strand breaks. DNA damage activates a surveillance mechanism called a DNA damage checkpoint response (DDR), causing the cell to arrest cycling until it repairs the damage. Previous work in this lab showed that impaired HR in yeast cells led to a persistent DDR response and high levels of G2 cells. A subsequent yeast mutant library screen revealed that HR and BER mutants, but not other repair pathway mutants, have high levels of G2 cells during normal growth. In this project, the effects of inactivating BER pathway genes have been investigated. New BER-defective apn1 and ogg1 mutants were constructed using both MATa and MATα yeast strains. Each mutant exhibited high levels of G2/M cells during log phase growth. Inactivation of DDR genes RAD9 or CHK1 in the BER mutants abolished the high G2/M phenotype. Levels of G2/M cells were increased synergistically in apn1 apn2 double mutants, which also exhibited greatly increased cell sizes. BER- HR- apn1 rad52 and ogg1 rad52 double mutants displayed higher levels of G2/M cells than the single mutants, suggesting that the causes of the cell cycle responses were different. These results demonstrate that cells form spontaneous base damage in DNA during normal growth that must be repaired by the BER pathway. Reduction of repair by this pathway leads to accumulation of unrepaired lesions that induce a persistent DNA damage checkpoint response.

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Keywords

base excision repair, cell cycling, Saccharomyces cerevisiae

Citation

Nepal, Y. (2021). Defects in base excision repair (BER) are linked to changes in cell cycling in Saccharomyces cerevisiae (Unpublished thesis). Texas State University, San Marcos, Texas.

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