Fragile X-Associated Tremor-Ataxia Syndrome: Linking Ca2+ Dysregulation and DNA Damage Responses

Abstract

Fragile X-associated tremor-ataxia syndrome (FXTAS) is a late-onset, neurodegenerative disorder that affects carriers of the premutation CGG-repeat allele (preCGG; 55-200 repeats) of the fragile X mental retardation 1 (FMR1) gene. Early abnormal growth and Ca2+ dynamics in FMR1 premutation preCGG knock-in mice and human iPSCs-derived preCGG neurons precede neurodegeneration and FXTAS. Whether intra-nuclear inclusions containing DNA damage response (DDR) proteins, pathologic hallmark of FXTAS, causally link Ca2+ dysregulation with abnormal neuronal growth and survival is unknown. We hypothesize a role for CdK5 and ATM in FXTAS pathogenesis, two key signaling kinases involved in both DDR and synaptic signaling. FMRP expression in postnatal day 0 (P0), 6-week- and 6-month-old preCGG mouse brain is reduced 60-, 50- and 30% compared to wt, respectively. In primary hippocampal neurons, FMRP reduction is more pronounced in preCGG soma (80%, p<0.0001) compared to neurites (94%, p=0.0177). Resting cytoplasmic calcium concentration ([Ca2+]i) in preCGG hippocampal neurons is chronically elevated 3-fold compared to wt at 7 DIV and 14 DIV, and can be reversed by acute exposure to 10μM dantrolene. Elevated [Ca2+]i-dependent calpain activity (120%-140%) and p25/p35 (120%) at 6-weeks and 6-months in preCGG cortex indicate abnormal CdK5 regulation. CdK5 substrate ATM is upregulated 150 to 200% and P-Ser1981-ATM is 150% (p= p<0.01-0.007) of wt at P0 and 6 months in preCGG brain. We propose that chronic Ca2+ dysregulation amplifies Cdk5/ATM signaling pathway activity possibly linking impaired DDR and neurodegeneration leading to FXTAS with early abnormal [Ca2+]i and synaptic activity.