AP endonuclease 1 prevents trinucleotide repeat expansion via a novel mechanism during base excision repair.
Bottom Line: Base excision repair (BER) of an oxidized base within a trinucleotide repeat (TNR) tract can lead to TNR expansions that are associated with over 40 human neurodegenerative diseases.Here, we further provide the first evidence that AP endonuclease 1 (APE1) prevented TNR expansions via its 3'-5' exonuclease activity and stimulatory effect on DNA ligation during BER in a hairpin loop.Coordinating with flap endonuclease 1, the APE1 3'-5' exonuclease activity cleaves the annealed upstream 3'-flap of a double-flap intermediate resulting from 5'-incision of an abasic site in the hairpin loop.
Affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA Biochemistry Ph.D. Program, Florida International University, Miami, FL 33199, USA.Show MeSH
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Mentions: A study from the Bambara group showed that APE1 can stimulate LIG I activity to promote ligation of a nick and facilitate the formation of repaired products during BER (25). Thus, it is possible that APE1 may also stimulate LIG I activity to promote the removal of a CAG repeat hairpin and facilitate the production of the repaired products during BER of a base lesion in a hairpin loop. To test this possibility, we determined the effects of APE1 on LIG I activity independent of its 3′-5′ exonuclease activity by measuring the production of the repaired products in the presence of low concentrations of APE1 (0.5 and 1 nM) that exhibited a low 3′-5′ exonuclease activity on the (CAG)3/(CAG)4 double-flap substrate (Supplementary Figure S1, lanes 3–4) and little exonucleolytic activity on the (CAG)7/(CAG)7 double-flap substrate (Supplementary Figure S1, lanes 9–10). We found that low concentrations of APE1 failed to alter the production of repaired products resulting from a short double-flap substrate (Figure 6, compare lanes 5–6 with lane 4 and lanes 8–9 with lane 7) indicating that APE1 did not significantly alter LIG I activity during removal of a small hairpin. However, the same concentrations of APE1 resulted in the production of the unexpanded product on the long (CAG)7/(CAG)7 double-flap substrate (Figure 6, compare lanes 14–15 with lane 13 and lanes 17–18 with lane 16). The results indicated that APE1 stimulated the ligation activity of LIG I, thereby promoting the production of the unexpanded repair product. Interestingly, we observed that the same concentrations of APE1 failed to stimulate the production of shortened repaired expanded products (Figure 6, lanes 14–15, lanes 17–18) suggesting that APE1 failed to stimulate the ligation of a nick by LIG I that was adjacent a hairpin structure.
Affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA Biochemistry Ph.D. Program, Florida International University, Miami, FL 33199, USA.