Complementation of aprataxin deficiency by base excision repair enzymes in mitochondrial extracts

Mitochondrial aprataxin (APTX) protects the mitochondrial genome from the consequence of ligase failure by removing the abortive ligation product, i.e. the 5'-adenylate (5'-AMP) group, during DNA replication and repair. In the absence of APTX activity, blocked base excision repair (BER) intermediates containing the 5'-AMP or 5'-adenylated-deoxyribose phosphate (5'-AMP-dRP) lesions may accumulate. In the current study, we examined DNA polymerase (pol) γ and pol β as possible complementing enzymes in the case of APTX deficiency. The activities of pol β lyase and FEN1 nucleotide excision were able to remove the 5'-AMP-dRP group in mitochondrial extracts from APTX-/- cells. However, the lyase activity of purified pol γ was weak against the 5'-AMP-dRP block in a model BER substrate, and this activity was not able to complement APTX deficiency in mitochondrial extracts from APTX-/-Pol β-/- cells. FEN1 also failed to provide excision of the 5'-adenylated BER intermediate in mitochondrial extracts. These results illustrate the potential role of pol β in complementing APTX deficiency in mitochondria.