Oxidative Stress Drives Cell Cycle Stalling, Apoptosis and Metabolic Suppression in Cystatin B Deficient <scp>EPM1</scp> Patient <scp>iPSCs</scp>

Cystatin B (CSTB) is an inhibitor of cysteine proteases, particularly cathepsins. Biallelic loss-of-function mutations in the CSTB gene are causative for progressive myoclonic epilepsy type 1 (EPM1), a neurodegenerative disorder characterised by stimulus-sensitive myoclonus and generalised tonic-clonic seizures. Pathomechanisms underlying the disease progression include disturbed proteostasis, increased oxidative stress, neuroinflammation and increased neuronal apoptosis. CSTB downregulation can also lead to cell cycle defects and reduced cell proliferation. Conversely, overexpression of CSTB has been reported to drive proliferation and survival of cancer cells, emphasising the important role of CSTB in cell growth and survival. In the current study, we focused on the role of CSTB in regulating proliferation and survival of EPM1 patients' stem cells. We reprogrammed EPM1 patient fibroblasts into induced pluripotent stem cells (iPSCs), a highly proliferative cell type. Patient cells manifested poor growth and increased apoptosis. Further, EPM1 cells showed oxidative stress, increased lysosomal activity, increased DNA damage and suppressed metabolism. Treatment with antioxidants rescued the growth phenotype and activated metabolism, suggesting oxidative stress as the cause and suppressed metabolism as a protective response. Our data strengthen the central role of CSTB in supporting cell survival and growth. Elucidating the regulatory role of CSTB expression in cell growth can advance our understanding of the pathophysiological mechanisms underlying both EPM1 and cancer and could inform the development of novel therapeutic strategies.