Understanding the role of the SNM1B and EXD2 in DNA damage repair

Thesis

Unrepaired, or misrepaired, DNA damage can be carcinogenic or mutagenic; thus functional DNA damage repair pathways are essential for the safeguarding of the genome.

SNM1B is a 5’ to 3’ exonuclease implicated in the repair of damaged DNA, particularly the repair of interstrand crosslinks. Genetic studies have identified SNPs in the SNM1B gene as related to cancer risk. One of these (rs11552449) encodes a single amino acid change, H61Y. This study shows that WT and H61Y SNM1B have comparable in vitro biochemical and biophysical characteristics. The structures of both WT and H61Y C-terminally truncated SNM1B (Δ-SNM1B) were solved to 2.8 and 3.1 Å, respectively, and reveal similar structural properties. The structure of WT Δ-SNM1B was also reported (to 1.8 Å) with two 2’-deoxy-5’- adenosine monophosphate molecules in the active site. The structure of SNM1B shows an accessible extended active site, which may facilitate the binding of a variety of non-canonical DNA substrates. Accordingly, in vitro, WT and H61Y SNM1B are able to exonucleolytically process a wide range of structurally diverse DNA substrates. By utilising SNM1B depleted cell lines, this study also shows that SNM1B is required for DNA repair in response to treatment with DNA-crosslinking genotoxic agents (including cisplatin and SJG-136).

This study also identifies the novel double strand break repair factor, EXD2, as having intrinsic 3’ to 5’ exonuclease activity. EXD2 was shown to have enzymatic activity on a variety of substrates in vitro, including replication fork intermediates, ‘nicked’ or ‘gapped’ DNA duplexes, and RNA based substrates. Together with the cellular data this suggests a role for EXD2 in nucleolytically processing RNA or DNA-based intermediates in damage repair pathways.

Authors: Baddock, H

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: DNA damage repair