Morphological Alterations of the Sarcotubular System in Permanent Myopathy of Hereditary Hypokalemic Periodic Paralysis with a Mutation in the CACNA1S Gene

We investigated the immunohistochemical localization of several proteins related to excitation-contraction coupling and ultrastructural alterations of the sarcotubular system in biopsied muscles from a father and a daughter in a family with permanent myopathy with hypokalemic periodic paralysis (PMPP) due to a mutation in calcium channel CACNA1S; p. R1239H hetero. Immunostaining for L-type calcium channels (LCaC) showed linear hyper-stained regions indicating proliferation of longitudinal t-tubules. The margin of vacuoles was positive for ryanodine receptor, LCaC, calsequestrin (CASQ) 1, CASQ 2, SR/ER Ca2+-ATPase (SERCA) 1, SERCA2, dysferlin, dystrophin, alpha-actinin, LC3, and LAMP 1. Electron microscopy indicated that the vacuoles mainly originated from the sarcoplasmic reticulum (SR). These findings indicate impairment of the muscle contraction system related to Ca2+ dynamics, remodeling of t-tubules and muscle fiber repair. We speculate that PMPP in patients with a CACNA1S mutation might start with abnormal SR function due to impaired LCaC. Subsequent induction of muscular contractile abnormalities and the vacuoles formed by fused SR in the repair process including autophagy might result in permanent myopathy. Our findings may facilitate prediction of the pathomechanisms of PMPP seen on morphological observation.