Mutant Ataxin‐3 inhibits 3’ phosphatase activity of human polynucleotide kinase 3′‐phosphatase (PNKP)

Abstract

Ataxin‐3 (ATXN3), a polyglutamine repeat‐containing protein is mutated to abnormally expanded poly Q tracts in Spinocerebellar Ataxia type 3 SCA3 / Machado‐Joseph Disease (MJD). This disease is one of the most common dominantly inherited Ataxia worldwide; the molecular defect is CAG repeat expansion (from 14–41 to 55–82 repeats) in the ATXN3 coding region. However, how the expanded form gains its toxic function is poorly understood. Here, we show that, expanded poly Q form of human Ataxin‐3 (Q72), stably associates and inhibits 3’[prime]‐phosphatase activity of PNKP one of the major DNA end processing enzyme in mammalian cell. Activities of other single strand break repair (SSBR) proteins are not affected. A conditionally expressing pathological ATXN3 cells also showed reduced 3’[prime]‐phophatase activity of PNKP and subsequent accumulation of DNA strand breaks in nuclear genomes. Moreover, LA‐QPCR analysis of human MJD patients’ brain samples showed significant accumulation of DNA strand breaks that supported the cause. In summary, selective inhibition of 3’[prime]‐phosphatase activity of PNKP may be the signature mechanism contributing to accumulated DNA damage and pathophysiology of SCA3/MJD.