MYASTHENIA & RELATED DISORDERS: P.37 Autophagy affected in patients with hypokalemic periodic paralysis

Abstract

Hypokalemic periodic paralysis (hypoPP) is an autosomal dominant rare disorder that is caused by mutations in the genes for voltage gated calcium channel CaV1.1 (CACNA1S) and NaV1.4 (SCN4A). Patients with hypoPP may suffer from periodic paralysis alone, periodic paralysis with weakness or permanent weakness. HypoPP has been known to be associated with vacuolar myopathy for decades but the cause of this has not been known. We have investigated a cohort of 14 hypoPP patients with the R528H mutation in the CACNA1S gene for vacuoles and their content. The R528H mutation has previously been demonstrated to cause a Ca2+ leak through CaV1.1 into the cytoplasm, possibly causing disturbance in the calcium homeostasis. Histology and transmission electron microscopy revealed that vacuoles had a very heterogenous content of glycogen, fibrils and autophagosomes/endosomes. Further investigation demonstrated autophagosomes and endosomes were arrested in the autophagic processing pathway between the early and late stages of maturation, as well as large non-fused lysosomes. Expression analysis of proteins associated with endosomes, autophagosomes and lysosomes showed a significant decrease in levels of proteins involved in autophagy such as Rab5, Rab7, p62/SQSTM1, LC3I, Atg5 and LAMP1 in patients with hypoPP suggesting a systemic effect on the protein expression at the gene level, which decreased mRNA expression levels confirmed, possibly through a master regulator of autophagy gene expression. The findings suggest that changes in calcium homeostasis leads to disruptions in autophagic flux and abnormally large endosomes, autophagosomes and lysosomes that contribute to the vacuoles seen in patients with hypoPP.