C010 Investigating the role of FAN1 variants and MLH1 binding in HTT CAG repeat instability

<h3>Background</h3> Huntington’s disease is a neurodegenerative disease caused by an expanded CAG repeat in exon 1 of the Huntingtin gene, HTT. Although the biggest driver of disease onset is HTT CAG repeat length, DNA damage response (DDR) genes, including FAN1 and MLH1, have emerged as major disease modulators. FAN1 plays an important role in protecting against somatic CAG repeat expansion and recent data from our lab suggests that this may in part be due to its interaction with MLH1. FAN1 binds to MLH1 via two binding domains – an MLH1-interacting protein (MIP) box and an MLH1-interacting motif (MIM). Mutation of residues in the MIP box, also known as the SPYF motif, modulates the FAN1/MLH1 interaction and CAG repeat instability. It has been found that phosphorylation of the serine (S126) in this site may disrupt the binding of FAN1 to MLH1, leading to increased HTT CAG instability. <h3>Aims</h3> This project aims to explore the effect of FAN1-S126 phosphorylation on HTT CAG repeat instability and mechanisms behind this. <h3>Methods</h3> An iPSC line derived from a juvenile HD patient carrying 125 CAG repeats has been utilised. CRISPR-Cas9 has been used to introduce mutations into FAN1’s SPYF motif, including a phospho-mimetic mutation (S126D) and a mutation preventing phosphorylation (P127R). Results Clones carrying S126D and P127R mutations have been successfully generated. FAN1-S126D clones carrying the phospho-mimetic mutation show reduced FAN1-MLH1 binding and increased somatic instability at the CAG repeat in both iPSCs and iPSC-derived neurons. The effect of the P127R mutation is currently being explored in iPSCs and iPSC-derived neurons. <h3>Conclusions</h3> Phosphorylation at Ser-126 modulates the FAN1-MLH1 interaction in an HD iPSC cell model. This may present a potential therapeutic avenue for modulating HTT CAG expansion.