Abstract

In this review, we discuss how two evolutionarily conserved pathways at the interface of DNA replication and repair, template switching and break-induced replication, lead to the deleterious large-scale expansion of trinucleotide DNA repeats that cause numerous hereditary diseases. We highlight that these pathways, which originated in prokaryotes, may be subsequently hijacked to maintain long DNA microsatellites in eukaryotes. We suggest that the negative mutagenic outcomes of these pathways, exemplified by repeat expansion diseases, are likely outweighed by their positive role in maintaining functional repetitive regions of the genome such as telomeres and centromeres. Break-induced replication and template switching are conserved mechanisms of replication fork restart. They do not cause microsatellite instability in prokaryotes, but promote repeat expansion in eukaryotes. We suggest that TS and BIR persisted in eukaryotes despite their mutagenic potential because they help maintain long repetitive telomeres and centromeres.