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Expansions of repetitive DNA sequences cause numerous human neurological and neuromuscular diseases. Ongoing repeat expansions in patients can exacerbate disease progression and severity. As pathogenesis is connected to repeat length, a potential therapeutic avenue is to modulate disease by manipulating repeat expansion size — targeting DNA, the root-cause of symptoms. How repeat instability is mediated by DNA replication, repair, recombination, transcription and epigenetics may explain its contribution to pathogenesis and give insights into therapeutic strategies to block expansions or induce contractions. [ABSTRACT FROM AUTHOR]

Mechanisms contributing to disease-associated trinucleotide repeat instability are poorly understood. DNA ligation is an essential step common to replication and repair, both potential sources of repeat instability. Using derivatives of DNA ligase I (hLigI)-deficient human cells (46BR.1G1), we assessed the effect of hLigI activity, overexpression, and its interaction with proliferating cell nuclear antigen (PCNA) upon the ability to replicate and repair trinucleotide repeats. Compared with LigI+/+, replication progression through repeats was poor, and repair tracts were broadened beyond the slipped-repeat for all mutant extracts. Increased repeat instability was linked only to hLigI overexpression and expression of a mutant hLigI incapable of interacting with PCNA. The endogenous mutant version of hLigI with reduced ligation activity did not alter instability. We distinguished the DNA processes through which hLigI contributes to trinucleotide instability. The highest levels of repeat instability were observed under the hLigI overexpression and were linked to reduced slipped-DNAs repair efficiencies. Therefore, the replication-mediated instability can partly be attributed to errors during replication but also to the poor repair of slipped-DNAs formed during this process. However, repair efficiencies were unaffected by expression of a PCNA interaction mutant of hLigI, limiting this instability to the replication process. The addition of purified proteins suggests that disruption of LigI and PCNA interactions influences trinucleotide repeat instability. The variable levels of age- and tissue-specific trinucleotide repeat instability observed in myotonic dystrophy patients and transgenic mice may be influenced by varying steady state levels of DNA ligase I in these tissues and during different developmental windows. [ABSTRACT FROM AUTHOR]