Defects in replicative DNA polymerases linked to cancer predisposition and tumour development

Somatic and germline mutations in the exonuclease domain of the replicative DNA polymerase epsilon (POLE-EDM) can contribute to cancer development. The ultramutator phenotype observed in the POLE-EDM tumours is thought to be due to alteration of the polymerase proofreading ability. Notably, while some POLE-EDM variants show a mutation rate similar to a 3’-5’ exonuclease-dead version, others have a mutation rate several orders of magnitude higher than exonuclease defective polymerases. Thus, it is likely that the mechanism of mutagenesis involves some processes other, or in addition to, loss of proofreading. As many POLE-EDM variants are unclassified, and to better assess their pathogenicity, we constructed in fission yeast carrying germline POLE-EDM variants identified in familial cases of colorectal cancer. Our combined in vitro and in vivo experimental approach confirmed that mutagenesis differs amongst POLE-EDM variants, and that this capability is not due to a simple loss of exonuclease activity. Although previous studies identified an unbalanced dNTP levels as the origin of hypermutator phenotypes, our data demonstrate that this does not apply in our study. Our exploration via deep-sequencing performed on hypermutated strains confirmed that concurrent POLE-EDM and mismatch repair (MMR) deficiency leads to a previously characterised specific mutational signature. Intriguingly, a second mutational signature related with coexisting polymerase delta alteration and MMR deficiency was also identified. Finally, further investigation via genetic interactions lead us to suspect the translesion synthesis polymerase polymerase kappa as a potential cause for the hypermutated phenotype and the mutational signatures observed. In summary, this work clarifies the tumour driver ability of germline POLE-EDMs, and has the potential to assist genetic counselling and POLE-EDM targeted treatments.