FAN1 exo- not endo-nuclease pausing on disease-associated slipped-DNA repeats: A mechanism of repeat instability.

Ongoing inchworm-like CAG and CGG repeat expansions in brains, arising by aberrant processing of slipped DNAs, may drive Huntington's disease, fragile X syndrome, and autism. FAN1 nuclease modifies hyper-expansion rates by unknown means. We show that FAN1, through iterative cycles, binds, dimerizes, and cleaves slipped DNAs, yielding striking exo-nuclease pauses along slip-outs: 5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'. CAG excision is slower than CTG and requires intra-strand A·A and T·T mismatches. Fully paired hairpins arrested excision, whereas disease-delaying CAA interruptions further slowed excision. Endo-nucleolytic cleavage is insensitive to slip-outs. Rare FAN1 variants are found in individuals with autism with CGG/CCG expansions, and CGG/CCG slip-outs show exo-nuclease pauses. The slip-out-specific ligand, naphthyridine-azaquinolone, which induces contractions of expanded repeats in vivo, requires FAN1 for its effect, and protects slip-outs from FAN1 exo-, but not endo-, nucleolytic digestion. FAN1's inchworm pausing of slip-out excision rates is well suited to modify inchworm expansion rates, which modify disease onset and progression.

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Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.

Déclaration de conflit d'intérêts

Declaration of interests A patent on methods of treating diseases associated with repeat instability has been filed (application no. 16/325,066) by The Hospital for Sick Children and Osaka University. A patent on methods and compositions for interfering with FAN1 has been filed (application no. 63/184,146) by The Hospital for Sick Children and University of Zurich.