Proteasomal-dependent CHK1 degradation leads to DNA damage accumulation in ALS cellular model systems

Amyotrophic lateral sclerosis (ALS) is characterised by the aggregation of TDP-43 and mutant FUS in the cytoplasm of affected motor neurons. Accumulation of DNA damage is emerging as a novel hallmark of ALS. We recently showed that TDP-43 and FUS cytoplasmic inclusions (CIs) lead to DNA damage accumulation by altering DNA damage signalling and impairing the activation of the DNA damage response (DDR). However, the multiple molecular mechanisms contributing to DNA damage accumulation in affected motor neurons in ALS have not been fully elucidated. In recent years, chemical inhibition of the serine/threonine kinase CHK1 was shown to lead to accumulation of DNA breaks as well as increased apoptosis in differentiated cortical neurons. Mechanistically, CHK1 has been involved in DNA double strand break repair in non-dividing cells, by acting through the histone chaperone ASF1A. In this article, we show that cells bearing FUS and TDP-43 CIs show downregulation of CHK1 and ASF1A total proteins. Indeed, we could observe CHK1 downregulation associated with mutant FUS expression and induction of CIs both in cell lines as well as in patient-derived motor neuron progenitors and a FUS-ALS mouse model. Restoring nuclear levels of CHK1 and ASF1A via transient overexpression in cells bearing FUS CIs is sufficient to reduce DNA damage accumulation and to rescue DDR defects. Importantly, we show that the ubiquitin-proteasome system is the pathway responsible for CHK1 degradation in cells bearing FUS inclusions, since its inhibition restores CHK1 and ASF1A protein levels and reduces DNA damage accumulation. Our study, demonstrate that proteasomal dependent CHK1 and ASF1A downregulation contribute to accumulation of DNA damage in cells affected by FUS aggregates.