Life-threatening congestive heart failure as the presentation of centronuclear myopathy

Mouse myofibers lacking the SMYD1 methyltransferase are susceptible to atrophy, internalization of nuclei and myofibrillar disarray

The Smyd1 gene encodes a lysine methyltransferase specifically expressed in striated muscle. Because Smyd1-null mouse embryos die from heart malformation prior to formation of skeletal muscle, we developed a Smyd1 conditional-knockout allele to determine the consequence of SMYD1 loss in mammalian skeletal muscle. Ablation of SMYD1 specifically in skeletal myocytes after myofiber differentiation using Myf6(cre) produced a non-degenerative myopathy. Mutant mice exhibited weakness, myofiber hypotrophy, prevalence of oxidative myofibers, reduction in triad numbers, regional myofibrillar disorganization/breakdown and a high percentage of myofibers with centralized nuclei. Notably, we found broad upregulation of muscle development genes in the absence of regenerating or degenerating myofibers. These data suggest that the afflicted fibers are in a continual state of repair in an attempt to restore damaged myofibrils. Disease severity was greater for males than females. Despite equivalent expression in all fiber types, loss of SMYD1 primarily affected fast-twitch muscle, illustrating fiber-type-specific functions for SMYD1. This work illustrates a crucial role for SMYD1 in skeletal muscle physiology and myofibril integrity.

Centronuclear (myotubular) myopathy

Centronuclear myopathy (CNM) is an inherited neuromuscular disorder characterised by clinical features of a congenital myopathy and centrally placed nuclei on muscle biopsy.

The incidence of X-linked myotubular myopathy is estimated at 2/100000 male births but epidemiological data for other forms are not currently available.

The clinical picture is highly variable. The X-linked form usually gives rise to a severe phenotype in males presenting at birth with marked weakness and hypotonia, external ophthalmoplegia and respiratory failure. Signs of antenatal onset comprise reduced foetal movements, polyhydramnios and thinning of the ribs on chest radiographs; birth asphyxia may be the present. Affected infants are often macrosomic, with length above the 90^th centile and large head circumference. Testes are frequently undescended. Both autosomal-recessive (AR) and autosomal-dominant (AD) forms differ from the X-linked form regarding age at onset, severity, clinical characteristics and prognosis. In general, AD forms have a later onset and milder course than the X-linked form, and the AR form is intermediate in both respects.

Mutations in the myotubularin (MTM1) gene on chromosome Xq28 have been identified in the majority of patients with the X-linked recessive form, whilst AD and AR forms have been associated with mutations in the dynamin 2 (DNM2) gene on chromosome 19p13.2 and the amphiphysin 2 (BIN1) gene on chromosome 2q14, respectively. Single cases with features of CNM have been associated with mutations in the skeletal muscle ryanodine receptor (RYR1) and the hJUMPY (MTMR14) genes.

Diagnosis is based on typical histopathological findings on muscle biopsy in combination with suggestive clinical features; muscle magnetic resonance imaging may complement clinical assessment and inform genetic testing in cases with equivocal features. Genetic counselling should be offered to all patients and families in whom a diagnosis of CNM has been made.

The main differential diagnoses include congenital myotonic dystrophy and other conditions with severe neonatal hypotonia.

Management of CNM is mainly supportive, based on a multidisciplinary approach. Whereas the X-linked form due to MTM1 mutations is often fatal in infancy, dominant forms due to DNM2 mutations and some cases of the recessive BIN1-related form appear to be associated with an overall more favourable prognosis.

Cardiac complications of childhood myopathies

The management of individuals with a neuromuscular disorder is usually focused on the skeletal muscle weakness and resulting complications, such as respiratory failure. Long-term prognosis of a number of neuromuscular conditions is, however, also determined by the type and severity of cardiac involvement. Early recognition and treatment of the cardiovascular complications are part of the task of the multidisciplinary team involved in the care of these patients. Although for several of the common conditions, there is general consensus on the cardiac investigations and treatments, in the rarer disorders, evidence-based recommendations are not available, and suggestions from experts provide an acceptable solution. This review summarizes the recent advances in our understanding of the pathogenesis and phenotypic diversity of cardiac complications associated with pediatric myopathies and provides a rational framework for planning the monitoring and therapeutic intervention in individual conditions.

Centronuclear myopathy and cardiomyopathy requiring heart transplant

Centronuclear myopathy has been extremely rarely associated with cardiomyopathy, which can lead to heart failure and premature death. We report the case of a 3.5-year-old girl with early-onset dilated cardiomyopathy, biventricular hypertrophy and histologic features suggestive of centronuclear myopathy. After unsuccessful medical treatment for heart failure, she underwent cardiac transplantation at the age of 4.5 years. Results of a skeletal muscle biopsy showed increased central nuclei and perinuclear vacuolations with aggregates of mitochondria. Examination of the heart at the time of transplantation confirmed a diagnosis of dilated cardiomyopathy. Histologic results revealed hypertrophic myocardiocytes, focal areas of infarction and endocardial fibroelastosis, most prominently in the left ventricle. Although cardiomyopathy is commonly associated with other childhood myopathies, to our knowledge, this is the youngest patient reported with centronuclear myopathy presenting with heart failure caused by cardiomyopathy, and the first patient to successfully undergo cardiac transplantation. One year after the heart transplant, there were no signs of rejection. We recommend detailed cardiac assessment with regular follow-up for children with histologic features consistent with centronuclear myopathy.

[Cardiac involvement in neuromuscular diseases]

Many neuromuscular disorders involve the heart, occasionally with overt clinical disease. Muscular dystrophies (dystrophinopathies, limb girdle muscular dystrophy, Emery-Dreifuss muscular dystrophy, Steinert's myotonic dystrophy), congenital myopathies, inflammatory myopathies and metabolic diseases (glycogenosis, periodic paralysis, mitochondrial diseases) may produce dilated or hypertrophic cardiomyopathy and heart rhythm or conduction disturbances. Furthermore the heart is commonly involved in some hereditary and degenerative diseases (Friedreich's ataxia and Kugelberg-Welander syndrome) and acquired (Guillain-Barré syndrome) or inherited (Refsum's disease and Charcot-Marie-Tooth syndrome) polyneuropathies. A cardiologist's high clinical suspicion and a simple but systematic skeletal muscle and peripheral nerve investigation, including muscle enzymes quantification, neurophysiological study and muscle biopsy, are necessary for an accurate diagnosis. In selected patients, more sophisticated biochemical and genetic analysis will be necessary. In most cases, endomyocardial biopsy is not essential for the diagnosis.

A follow-up study of congenital non-progressive myopathies

To determine the development and prognosis of patients with the moderate congenital form of congenital non-progressive myopathies (CNM), we investigated 55 patients with CNM by questionnaires. This patient group included 18 with nemaline myopathy, 11 with central core disease, 3 with myotubular myopathy, 15 with congenital fiber type disproportion and 3 with minicore disease. As to motor development, almost all patients had learned to walk alone by 3 years of age, averaging 2 2/12 years. Central nervous system involvement including mental retardation was seen in 24% of the patients, predominantly in patients with myopathy. The level of mental retardation was not related to the severity of muscle weakness. Patients with central core disease had a higher frequency of skeletal deformities. CNM were not necessarily benign in their prognosis as previously thought, but sometimes showed progressive deterioration leading to death. In 16% of patients, progressive deterioration in muscle strength and respiratory function became manifest after once the patient became ambulant. Even in the benign congenital form, seven of 55 patients died from respiratory or cardiac failures by 20 years of age. Therefore evaluation of cardiopulmonary function at regular intervals is important in the continuing care of patients with CNM.