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

Expansions of repeat DNA tracts cause >70 diseases, and ongoing expansions in brains exacerbate disease. During expansion mutations, single-stranded DNAs (ssDNAs) form slipped-DNAs. We find the ssDNA-binding complexes canonical replication protein A (RPA1, RPA2, and RPA3) and Alternative-RPA (RPA1, RPA3, and primate-specific RPA4) are upregulated in Huntington disease and spinocerebellar ataxia type 1 (SCA1) patient brains. Protein interactomes of RPA and Alt-RPA reveal unique and shared partners, including modifiers of CAG instability and disease presentation. RPA enhances in vitro melting, FAN1 excision, and repair of slipped-CAGs and protects against CAG expansions in human cells. RPA overexpression in SCA1 mouse brains ablates expansions, coincident with decreased ATXN1 aggregation, reduced brain DNA damage, improved neuron morphology, and rescued motor phenotypes. In contrast, Alt-RPA inhibits melting, FAN1 excision, and repair of slipped-CAGs and promotes CAG expansions. These findings suggest a functional interplay between the two RPAs where Alt-RPA may antagonistically offset RPA's suppression of disease-associated repeat expansions, which may extend to other DNA processes. [Display omitted] • RPA and primate-specific Alt-RPA are upregulated in HD and SCA1 patient brains • RPA/Alt-RPA differentially interact with CAG instability and disease modifiers • RPA enhances and Alt-RPA inhibits correct repair of slipped-CAG intermediates • RPA prevents and Alt-RPA promotes disease-worsening somatic repeat expansions A balance between expression, function, and protein-protein interactions of single-stranded DNA-binding proteins canonical RPA and primate-specific Alternative-RPA regulates somatic trinucleotide CAG repeat expansions associated with neurodegeneration in Huntington Disease and Spinocerebellar Ataxia Type 1. [ABSTRACT FROM AUTHOR]