Can HDAC/DNA methyltransferase inhibitors improve muscle function in a congenital myopathy caused by recessive RYR1 mutations?

Prof. Francesco Zorzato and Prof. Susan Treves, University Hospital Basel

Abstract

Defects of the calcium release channel (ryanodine receptor; RyR1) are causative of several congenital muscle disorders including malignant hyperthermia (MH; MIM #145600), central core disease (CCD; MIM #11700), certain forms of multi-minicore disease (MmD; MIM # 255320) and centronuclear myopathy (CNM; MIM # 255320). Experimental results indicate that RYR1 mutations mainly cause four types of channel defects: one class (linked to MH) causes the channels to become hypersensitive to activation by electrical and pharmacological stimuli. The second class (linked to CCD) results in leaky channels leading to depletion of Ca2+ from sarcoplasmic reticulum (SR) stores. The third class (also linked to CCD) causes excitation–contraction uncoupling, whereby activation of the voltage sensor Cav1.1 is unable to cause release of calcium from the SR. The fourth class linked to recessive mutations, is associated with a decrease of mutant RyR1 channels content on SR membranes. However, the pathological mechanism leading to reduced RyR1 expression and content is presently unknown. 

Our working hypothesis is that the presence of recessive RYR1 mutations (as well as recessive mutations in other genes associated with congenital myopathies including SELENON) activates an epigenetic loop leading to a reduced expression of RyR1. Indeed, muscles of patients with recessive RYR1 mutations exhibit striking changes at the level of protein and mRNA expression, including increased content of HDAC-4 and HDAC-5, depletion of muscle specific miR-1 and -133, as well as depletion of miR-22 and -124 two miRs that specifically bind to the 3’UTR of the human RYR1. Additionally, the increased content of HDAC-4 and -5 correlates with a hyper methylation of internal RYR1 CpG islands. The combination of these epigenetic modifications could be responsible for the depletion of RyR1 seen in muscles of these patients. 

The main aim of this project is to understand (i) why recessive RYR1 mutations are accompanied by epigenetic modifications, (ii) whether the epigenetic machinery represents a pharmacological target. In order to answer these questions we will use a mouse model we have developed carrying recessive RYR1 mutations isogenic with RYR1 mutations found in a severely affected MmD patient. We think that this project is important not only because it will help elucidate the pathophysiology of diseases linked to recessive mutations but also because it will help develop pharmacological therapies to improve the quality of life of patients with disorders leading to a decrease of RyR1 expression in skeletal muscle sarcoplasmic reticulum.