Biochemical features of SERCA1 in Brody disease and identication of candidate genes in Brody syndrome

Brody disease is a rare skeletal muscle disorder transmitted as an autosomal recessive or dominant trait. The recessive inheritance is associated to mutation of ATP2A1 gene encoding the sarcoplasmic/endoplasmic reticulum calcium ATPase 1 (SERCA1), a protein that catalyzes the ATP dependent Ca2+ uptake from the cytosol to the lumen of sarcoplasmic reticulum. However, mutations in the ATP2A1 gene are missed in some patients with recessive inheritance and have never been found in patients with an autosomal dominant pattern suggesting the genetic heterogeneity of the disease. The term Brody syndrome has been recently proposed to designate patients with decreased SR Ca2+ATPase activity but without ATP2A1 mutation. The main clinical feature is the exercise-induced delay in muscle relaxation which causes painless muscle stiffness following contraction. Serum creatine kinase (CK) is normal or slightly increased, needle electromyography (EMG) records no myotonic and pseudomyotonic discharges during the exercise-induced muscle stiffness (“silent cramps”) and percussion myotonia is absent. Reduction of SR Ca2+ ATPase activity has been reported in all described cases, independently from the association with ATP2A1 mutation. Conversely, data on SERCA1 protein expression are still under debate. Indeed, in muscle of patients with BD, immunostaining for SERCA1 has been reported to be normal or reduced while immunoblot analysis,performed in just a few cases, documented a significant reduction in protein amount. In the first part of the study we performed molecular and biochemical analysis on muscle from 13 patients with Brody myopathy. Immunofluorescence studies of SERCA1 revealed similar staining pattern and intensity in muscle of controls and of patients with and without ATP2A1 mutation whereas, contrary to the expectations, immunoblot analysis after SDS-PAGE and 2D gel electrophoresis showed a significant reduction of SERCA1 protein in muscle of patients with BD as compared to muscle of patients with BS and of control subjects. Therefore, we hypothesized that the mutated protein could have different solubility features of the wild type one. However, the recovery of mutated SERCA1 was lower than that of the wild type protein, irrespectively the lysis buffer, suggesting that mutations detected in ATP2A1 gene did not affect the solubility properties of SERCA1 in the buffers we used for muscle homogenization The study of SERCA1 oligomerization revealed the shift from monomeric/dimeric forms to the high-oligomeric status of SERCA1 in muscle from patients with ATP2A1 mutation, providing a possible explanation of the reduced detection of SERCA1 monomer after denaturing electrophoresis. The present data provide also evidence that immunoblotting, in contrast to the immunohistochemistry, could be a useful tool for confirming the diagnosis of BD in patients with reduced SR Ca2+ ATPase activity. In this study we reported also the clinical and histological characterization of a new patient with Brody syndrome and document novel mutations in the ATP2A1 gene of a new patient and in a previously described case of Brody disease. Moreover, we study SERCA1 distribution within the sarcoplasmic reticulum revealing no remarkable changes in the localization of the protein in muscle from patients with BD and BS. In the second part of the study we analyzed the expression of SERCA1 isoforms and investigated SR Ca2+ ATPase activity in muscle of patients with myotonic dystrophy (type 1 and 2) and in hypothyroid myopathy. Indeed, data from the literature seem to support the alteration of SERCA1 expression and SR Ca2+ ATPase activity in patients with myotonic dystrophies and in hypothyroid mice. We observed no significant changes in SR Ca2+ ATPase activity, SERCA1 and SERCA2 expression in muscle from patients with myotonic dystrophies and with hypothyroid myopathy. Interestingly, we observed that the neonatal isoform SERCA1b is expressed in myotonic dystrophies, in particular at higher levels in type 2 than type 1 disease. Moreover, we provide data on SERCA1 and SERCA2 expression in neonatal muscle at different developmental stages (10, 20 days, one and four months after birth) revealing that, at all analyzed time of neonatal development, SERCA1 is expressed only in some fibers whereas nearly all fibers express SERCA2 up to 1 month after birth. In muscle from 10 and 20 days after birth, only a few fibers express SERCA1b. Finally, we showed that SERCA1b is the main SERCA1 isoforms expressed, together with SERCA2, in cultured human muscle fibers which therefore represent a good model to study neonatal muscle at early stages after birth. In the third, and last part of this study we focused on the identification of novel physiological interacting partners of SERCA1 that could be candidate genes responsible for Brody syndrome. Indeed, mutation in a gene encoding a protein which, by interacting with SERCA1, is able to regulate its function could account for the decreased SR Ca2+ ATPase activity in patients without ATP2A1 mutations. Two different approaches were used to identify novel SERCA1 binding partners. Sarcoplasmic/endoplasmic reticulum calcium ATPase 3 (SERCA3) has been identified as a reliable SERCA1-binding protein by affinity purification couple to mass spectrometry. The analysis of SERCA3 isoforms expression in skeletal muscle tissue led to identify SERCA3b as strongly expressed in type 2 muscle fibers, where SERCA1 is also located. Moreover, native/SDS-PAGE and co-immunoprecipitation experiments seem to support the existence of an interaction between SERCA1 and SERCA3b in skeletal muscle in physiological conditions. The second approach consisted in isolating SERCA1-protein complexes in native state from skeletal muscle and in protein identification by mass spectrometry. This strategy led to the identification of sarcalumenin, reticulon-2, Nogo/reticulon-4 and myoadenylate deaminase as putative SERCA1 interacting partners. A reduction of sarcalumenin expression was observed in muscle from four patients with Brody syndrome suggesting SLN gene as a candidate gene the disease.