Pre-clinical treatment of mouse models carrying recessive Ryr1 mutations with HDAC/DNA methyltransferase inhibitors.

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

Abstract (Lay summary see below)

Both dominantly and recessively inherited RYR1 mutations have been identified in patients with congenital myopathies and are often associated with different phenotypes. Dominant RYR1 mutations are the underlying cause of malignant hyperthermia/rhabdomyolysis/ exertional heat intolerance and central core disease. Recessive mutations are found in more severely affected patients presenting with multi-minicore disease and centronuclear myopathy. From a functional point of view dominant mutations affect the biophysical properties of the ryanodine receptor 1 (RyR1) calcium channel, whereas recessive mutations don’t, though their presence is accompanied by a significant reduction in the content of RyR1 protein in patients’ muscles. Reduced RyR1 content in the sarcoplasmic reticulum membrane would cause a decrease of calcium release during excitation contraction coupling, leading to weak muscles. These results have important implications namely, (i) the development of a single therapeutic strategy to improve muscle function in patients bearing RYR1 mutation is not plausible and (ii) the type of treatment will depend on the type(s) of mutations.
The pathological mechanism leading to reduced RyR1 content in patient’s muscle is unknown. Our working hypothesis is that the presence of recessive RYR1 mutations (compound heterozygous with a mis-sense+ frameshift/nonsense mutation and homozygous mis-sense) activates an epigenetic loop involving chromatin modifying enzymes, leading to reduced expression of RyR1. Thus, pharmacologically targeting chromatin modifying enzymes, including histone de-acetylases (HDACs) and DNA methyl transferase (DNMTs) may represent a valid therapeutic approach. In order to investigate this in a pre-clinical setting, we have created a mouse model knocked in for compound heterozygous Ryr1 mutations identified in severely affected child, namely the DKI mouse carrying a frameshift mutation in Ex36+ a mis-sense mutation in Ex91. 
In this project  we will address the  following specific aims: 1)  establish whether treatment of the DKI mice with HDAC/DNMT inhibitors (the drugs alone or in combination) results in improved muscle function; 2) elucidate the molecular and biochemical mechanisms leading to the improved muscle function brought about by HDAC/DNMT inhibitor treatment.
The results of this study will have important implications especially since they will promote the development of pharmacological therapies to improve muscle function and thus the quality of life of patients with disorders leading to a decrease of RyR1 expression in their skeletal muscles.

Lay summary

Questo progetto rappresenta la continuazione del progetto condotto nel 2019-2020, in cui sono stati ricercati i meccanismi che conducono ad una ridotta espressione della proteina RyR1 in pazienti con malattie muscolari congenite, portatori di mutazioni recessive nel gene RYR1. 
Utilizzando il modello murino Ryr1 DKI che abbiamo creato nel nostro laboratorio, ci proponiamo ora di saggiare se alcune molecole in grado di inibire i meccanismi intracellulari che abbiamo identificato, sono in grado di ristabilire la funzione muscolare nei modelli murini. I risultati di questo studio sono importanti per sviluppare terapie farmacologiche per pazienti che soffrono di miopatie congenite quali la miopatia multiminicore, la miopatia centronucleare e la miopatia da disproporzione congenita del tipo di fibra.