RYR1-Related Congenital Myopathies via the RYR1 Gene

Congenital myopathy refers to a group of neuromuscular disorders that most often present in childhood and have characteristic histopathological features in skeletal muscle. Based on histopathology, the three main types of congenital myopathy are core myopathy, centronuclear myopathy, and nemaline rod myopathy. Pathological RYR1 gene variants are causative for core myopathy, centronuclear myopathy, and core/rod myopathy. Cores are areas of the muscle that abnormally lack oxidative and glycolytic enzymatic activity. These areas of clearing reflect an absence of mitochondria in the muscle. Cores are further classified as central cores, which run the length of the muscle cell, or minicores, which are wider than they are long. Nemaline rods are rod-shaped structures in the sarcoplasm that contain material from the degenerated Z-disk.

Central core disease (CCD) is a static or slowly progressive myopathy that demonstrates a wide range of clinical severity and age of onset, but uniform histopathology. In the newborn period, CCD results in a floppy infant with feeding difficulties, respiratory insufficiency, myopathic facies, and extraocular muscle weakness. Skeletal abnormalities include talipes equinovarus and congenital dislocation of the hips. Childhood-onset CCD presents with mild proximal weakness involving hip and axial muscles and delayed motor development. Skeletal findings such as scoliosis and equinovarus are other complications (Jungbluth et al. 2007). Cases of adult-onset CCD with generally milder clinical symptoms have been described (Duarte et al. 2011). One representative patient had normal arm and leg strength and normal muscle bulk, but was easily fatigued and had difficulty maintaining an upright posture (Jungbluth et al. 2009). Intrafamilial variability of disease severity and pathology is also known (Sewry et al. 2002). Imaging of muscle groups in CCD patients reveals a distinct pattern of involvement (Fischer et al. 2006). The most severely affected muscles are gluteus maximus, medial and anterior compartments of the thigh muscles, and soleus and lateral gastrocnemius muscles of the lower leg.

The other congenital myopathies associated with the RYR1 gene demonstrate a much broader spectrum of histopathology and diagnostic imaging patterns than does CCD (Wilmshurst et al. 2010; Klein et al. 2011; Mercuri, E. et al. 2010). Associated histopathological findings include multiminicores, centrally placed nuclei, and congenital fiber type disproportion.

RYR1-related minicore myopathy is also known as multiminicore myopathy with external ophthalmoplegia because involvement of extraocular muscles is a nearly universal finding (Jungbluth et al. 2000; Monnier et al. 2009). Clinical signs include severe neonatal hypotonia, respiratory problems, feeding difficulties, frequent respiratory infections, delayed motor development, scoliosis, kyphoscoliosis, contractures, and myopathic facies (Wilmshurst et al. 2010).

RYR1-related centronuclear myopathy is associated with infantile-onset proximal weakness, external ophthalmoplegia, and bulbar involvement, followed by progressive improvement of symptoms. Histopathological findings in muscle biopsied from young patients include internalized nuclei and type 1 fiber predominance. However, when biopsied later in life, two-thirds of patients in one study had central cores or minicores in their muscle (Wilmshurst et al. 2010).

Congenital fiber type disproportion is a genetically heterogeneous disorder defined as hypotrophy of type 1 fibers compared to type 2 fibers. Patients with RYR1-related CFTD exhibit generalized hypotonia, motor delays, recurrent respiratory infections, a weak cry, ptosis, ophthalmoplegia, facial weakness, scoliosis, and contractures (Clarke et al. 2010).

The clinically most severe cases of RYR1-related myopathy present with decreased fetal movement, hypotonia, poor feeding, respiratory involvement, arthrogryposis, and ophthalmoplegia (Bharucha-Goebel et al. 2013). Additional features include fractures or hip dislocation at birth. In the published cohort of patients, the severe neonatal onset cases with dominant acting RYR1 mutations had classic central cores on muscle biopsy, while those with recessive acting RYR1 mutations had notable histologic variability, including fibrosis, variation in fiber size, and nuclear internalization.RYR1 is also involved in exercise-induced rhabdomyolysis and myalgia (Dlamini et al. 2013). Many of the RYR1 mutations identified in these patients are documented to be associated with malignant hyperthermia risk. Patients that span a broad range of ages presenting with symptoms of exercise or heat-induced rhabdomyolysis and sometimes myalgia have been reported (Dlamini et al. 2013).

Malignant hyperthermia susceptibility is another well documented RYR1 disorder.

The RYR1 gene encodes the skeletal muscle isoform of the ryanodine receptor. The RYR1 protein functions to regulate calcium release at the sarcoplasmic reticulum by serving as the calcium-induced, voltage-gated receptor, as well as the channel through which calcium flows. RYR1-related congenital myopathies are inherited as autosomal dominant and recessive disorders. Central core disease is typically dominant, with de novo and inherited mutations being well documented (Klein et al. 2012). The vast majority of CCD-causing mutations result in amino acid substitutions that lie in the C-terminus of the RYR1 protein (Zhou et al. 2007). The other RYR1-related congenital myopathies are most often inherited as autosomal recessive disorders, although exceptions exist. Recessive RYR1 myopathy cases are typically found to have one null allele and a second variant that results in an amino acid substitution or, as in the most severe neonatal forms, a second null allele (Bevilacqua et al. 2011; Monnier et al. 2008).

In a study to address the prevalence of congenital myopathy in a U.S. pediatric population, Amburgey et al. (2011) found RYR1 to be the most common overall cause. Thirteen of 46 cases were found to have RYR1 mutations for an estimated prevalence of 1:90,000. Similarly, RYR1 was found to be responsible for 26 of 44 congenital myopathy cases in a European cohort (Maggi et al. 2013). Clinical sensitivity is reported to be in excess of 90% in cases of CCD (Wu et al. 2006), and RYR1 mutations have been shown to be one of the most common causes of CFTD (Clarke et al. 2010). In a clinically and histologically well defined cohort of multiminicore patients, three of five families were found to have RYR1 mutations (Jungbluth et al. 2005). Analytical sensitivity is high with Next-Generation sequencing or Sanger sequencing because most RYR1 causative mutations are detectable by these methods.

Clinical sensitivity for gross deletion/duplication testing is likely low because, to date, only one gross deletion has been reported (Monnier et al. 2008).

This test provides full coverage of all coding exons of the RYR1 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).