Antagonistic roles of canonical and alternative-RPA in disease-associated tandem CAG repeat instability

Ongoing CAG expansions in Spinocerebellar ataxia type 1 (SCA1) and Huntington disease (HD) brains exacerbate disease.  Expansions involve aberrant repair of mutagenic slipped-DNAs, formed from single-stranded DNAs.  Whether singlestranded DNA-binding proteins prevent or facilitate expansions is unknown.  We assessed canonical RPA (RPA1-RPA2-RPA3) and Alternative-RPA (RPA1-RPA4- RPA3), containing primate-specific RPA4.  RPA is essential for all DNA metabolism.  Alt-RPA has undefined functions.  Alt-RPA is upregulated 10-fold in HD and SCA1 patient brains.  RPA enhances, while Alt-RPA blocks, correct repair of slipped-CAGs.  RPA, but not Alt-RPA, efficiently binds and melts slipped-DNAs.  RPA enhances, while Alt-RPA blocks, removal of excess slipped-DNAs by FAN1 nuclease.  Protein-protein interactomes reveal unique and shared partners of RPA and Alt-RPA, including expansion-driving proteins.  RPA-overexpression in SCA1 mice inhibits CAG expansions in brains, rescuing neuron morphology and motor phenotypes.  Modulating repeat mutations is one example involving antagonistic Alt-RPA↔RPA interactions, illuminating questions as to which RPA-mediated processes are also modulated by AltRPA.