Emerging drivers of DNA repeat expansions

Expansions of short tandem repeats (STRs) are the cause of a class of human hereditary disorders called repeat expansion diseases (REDs). Most REDs are neurodegenerative or neurodevelopmental diseases such as Huntington's disease, myotonic dystrophy, fragile X syndrome, and Friedreich's ataxia. Some common neurodegenerative diseases, including Alzheimer's and Parkinson's disease, have also been associated with STR expansions. Many cellular processes such as meiotic recombination, DNA replication, and mismatch repair have been shown to promote STR instability. However, STR instability is likely the result of a variety of factors, and many questions regarding this phenomenon remain to be answered. In this review, we summarize recent studies that propose DNA single-strand breaks as drivers of large-scale STR instability, in both dividing and non-dividing cells, and discuss additional evidence that supports this model. We also highlight the FANCD2- and FANCI-associated nuclease 1 protein, which was shown to be the strongest genetic modifier of several REDs.