DNA expansions generated by human Polm on iterative sequences

Abstract

Polm is the only DNA polymerase equipped with template-directed and terminal transferase activities. Polm is also able to accept distortions in both primer and template strands, resulting in misinsertions and extension of realigned mismatched primer terminus. In this study, we propose a model for human Polm-mediated dinucleotide expansion as a function of the sequence context. In this model, Polm requires an initial dislocation, that must be subsequently stabilized, to generate large sequence expansions at different 5′-P-containing DNA substrates, including those that mimic non-homologous end-joining (NHEJ) intermediates. Our mechanistic studies point at human Polm residues His329 and Arg387 as responsible for regulating nucleotide expansions occurring during DNA repair transactions, either promoting or blocking, respectively, iterative polymerization. This is reminiscent of the role of both residues in the mechanism of terminal transferase activity. The iterative synthesis performed by Polο at various contexts may lead to frameshift mutations producing DNA damage and instability, which may end in different human disorders, including cancer or congenital abnormalities.