1b:I[76749,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 1c:I[3486,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 1d:I[41892,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 1e:I[17707,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 1f:I[66528,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 20:I[63176,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 21:I[87099,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 22:I[26200,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"Group"] 23:I[23365,["/_next/static/chunks/0383663887945ae7.js","/_next/static/chunks/df7e62179e4ae938.js","/_next/static/chunks/acf71181277ecf85.js","/_next/static/chunks/63561dcd578bd4db.js","/_next/static/chunks/5bf36002c846d2d8.js","/_next/static/chunks/b24fef6e58c9b2f5.js","/_next/static/chunks/ea1fda2cc52e974e.js"],"default"] 24:T514,Over 50 hereditary degenerative disorders are caused by expansions of short tandem DNA repeats (STRs). (GAA)_n repeat expansions are responsible for Friedreich's ataxia as well as late-onset cerebellar ataxias (LOCAs). Thus, the mechanisms of (GAA)_n repeat expansions attract broad scientific attention. To investigate the role of DNA nicks in this process, we utilized a CRISPR-Cas9 nickase system to introduce targeted nicks adjacent to the (GAA)_n repeat tract. We found that DNA nicks 5' of the (GAA)₁₀₀ run led to a dramatic increase in both the rate and scale of its expansion in dividing cells. Strikingly, they also promoted large-scale expansions of carrier- and large normal-size (GAA)_n repeats, recreating, in a model system, the expansion events that occur in human pedigrees. DNA nicks 3' of the (GAA)₁₀₀ repeat led to a smaller but significant increase in the expansion rate as well. Our genetic analysis implies that in dividing cells, conversion of nicks into double-strand breaks (DSBs) during DNA replication followed by DSB or fork repair leads to repeat expansions. Finally, we show

W. Shem Scott

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