Templated Insertions: A Smoking Gun for Polymerase Theta-Mediated End Joining

A recognized source of disease-causing genome alterations is erroneous repair of broken chromosomes, which can be executed by two distinct mechanisms: non-homologous end joining (NHEJ) and the recently discovered polymerase theta-mediated end joining (TMEJ) pathway. While TMEJ has previously been considered to act as an altemative mechanism backing up NHEJ, recent work points to a role for TMEJ in the repair of replication-associated DNA breaks that are excluded from repair through homologous recombination. Because of its mode of action, TMEJ is intrinsically mutagenic and sometimes leaves behind a recognizable genomic scar when joining chromosome break ends (i.e., 'templated insertions'). This review article focuses on the intriguing observation that this polymerase theta signature is frequently observed in disease alleles, arguing for a prominent role of this double-strand break repair pathway in genome diversification and disease-causing spontaneous mutagenesis in humans.