Comparison of the Base Excision and Direct Reversal Repair Pathways for Correcting 1, N 6 -Ethenoadenine in Strongly Positioned Nucleosome Core Particles.

1,N-6-ethenoadenine (epsilon A) is a mutagenic lesion and biomarker observed in numerous cancerous tissues. Two pathways are responsible for its repair: base excision repair (BER) and direct reversal repair (DRR). Alkyladenine DNA glycosylase (AAG) is the primary enzyme that excises epsilon A in BER, generating stable intermediates that are processed by downstream enzymes. For DRR, the Fe(II)/alpha-ketoglutarate-dependent ALKBH2 enzyme repairs epsilon A by direct conversion of epsilon A to A. While the molecular mechanism of each enzyme is well understood on unpackaged duplex DNA, less is known about their actions on packaged DNA. The nucleosome core particle (NCP) forms the minimal packaging unit of DNA in eukaryotic organisms and is composed of 145-147 base pairs wrapped around a core of eight histone proteins. In this work, we investigated the activity of AAG and ALKBH2 on epsilon A lesions globally distributed at positions throughout a strongly positioned NCP. Overall, we examined the repair of epsilon A at 23 unique locations in packaged DNA. We observed a strong correlation between rotational positioning of epsilon A and AAG activity but not ALKBH2 activity. ALKBH2 was more effective than AAG at repairing occluded epsilon A lesions, but only AAG was capable of full repair of any epsilon A in the NCP. However, notable exceptions to these trends were observed, highlighting the complexity of the NCP as a substrate for DNA repair. Modeling of binding of the repair enzymes to NCPs revealed that some of these observations can be explained by steric interference caused by DNA packaging. Specifically, interactions between ALKBH2 and the histone proteins obstruct binding to DNA, which leads to diminished activity. Taken together, these results support in vivo observations of alkylation damage profiles and contribute to our understanding of mutational hotspots.