Progressive Myoclonus Epilepsy

Abstract Unverricht-Lundborg disease (ULD; EPM1) is an inherited neurodegenerative disorder characterized by onset at 6–15 years, stimulus-sensitive, action-activated myoclonus, epilepsy, and progressive neurological deterioration. It is caused by biallelic pathogenic variants in the CSTB gene, encoding a cystatin B. The most common of these is an unstable expansion of a dodecamer repeat element in the promoter region of the gene, leading to marked downregulation of CSTB expression. Total loss of CSTB is associated with severe neonatal-onset encephalopathy. A cystatin B–deficient mouse models the EPM1 disease relatively well. Myoclonic seizures, preceded by microglial activation, develop at one month of age and are followed by progressive gray and white matter degeneration and motor problems. CSTB is an inhibitor of cysteine proteases of the cathepsin family showing both nuclear and cytoplasmic localization with partial co-localization with lysosomal markers. CSTB function has been linked to protecting neurons from oxidative damage through an oxidative stress-responsive cystatin B-cathepsin B signaling pathway. In the nucleus CSTB has been shown to participate in regulation of cell cycle, and histone H3 tail proteolytic cleavage. Downstream effects of CSTB dysfunction have also been implicated in apopotosis, microglial dysfunction, inflammation, neurogenesis and synapse physiology. Despite the progress made, the exact disease mechanisms in EPM1 remain elusive. This chapter discusses the clinical features of EPM1 and recent advances in understanding its pathophysiology.