Carina Wallgren-Pettersson

Mutations in amphiphysin 2 ( BIN1 ) disrupt interaction with dynamin 2 and cause autosomal recessive centronuclear myopathy

Centronuclear myopathies are characterized by muscle weakness and abnormal centralization of nuclei in muscle fibers not secondary to regeneration. The severe neonatal X-linked form (myotubular myopathy) is due to mutations in the phosphoinositide phosphatase myotubularin (MTM1), whereas mutations in dynamin 2 (DNM2) have been found in some autosomal dominant cases. By direct sequencing of functional candidate genes, we identified homozygous mutations in amphiphysin 2 (BIN1) in three families with autosomal recessive inheritance. Two missense mutations affecting the BAR (Bin1/amphiphysin/RVS167) domain disrupt its membrane tubulation properties in transfected cells, and a partial truncation of the C-terminal SH3 domain abrogates the interaction with DNM2 and its recruitment to the membrane tubules. Our results suggest that mutations in BIN1 cause centronuclear myopathy by interfering with remodeling of T tubules and/or endocytic membranes, and that the functional interaction between BIN1 and DNM2 is necessary for normal muscle function and positioning of nuclei.

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 90th 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.

Mutations in the β-tropomyosin (TPM2) gene – a rare cause of nemaline myopathy

Abstract Nemaline myopathy is a clinically and genetically heterogeneous muscle disorder. In the nebulin gene we have detected a number of autosomal recessive mutations. Both autosomal dominant and recessive mutations have been detected in the genes for α-actin and α-tropomyosin 3. A recessive mutation causing nemaline myopathy among the Old Order Amish has recently been identified in the gene for slow skeletal muscle troponin T. As linkage studies had shown that at least one further gene exists for nemaline myopathy, we investigated another tropomyosin gene expressed in skeletal muscle, the β-tropomyosin 2 gene. Screening 66 unrelated patients, using single strand conformation polymorphism analysis and sequencing, we found four polymorphisms and two heterozygous missense mutations. Both mutations affect conserved amino acids, and in both cases, the mutant allele is expressed. We speculate that the observed mutations affect the formation of the tropomyosin dimer and its actin-binding properties.

MTM1 mutations in X-linked myotubular myopathy

X‐linked myotubular myopathy (XLMTM; MIM# 310400) is a severe congenital muscle disorder caused by mutations in the MTM1 gene. This gene encodes a dual‐specificity phosphatase named myotubularin, defining a large gene family highly conserved through evolution (which includes the putative anti‐phosphatase Sbf1/hMTMR5). We report 29 mutations in novel cases, including 16 mutations not described before. To date, 198 mutations have been identified in unrelated families, accounting for 133 different disease‐associated mutations which are widespread throughout the gene. Most point mutations are truncating, but 26% (35/133) are missense mutations affecting residues conserved in the Drosophila ortholog and in the homologous MTMR1 gene. Three recurrent mutations affect 17% of the patients, and a total of 21 different mutations were found in several independent families. The frequency of female carriers appears higher than expected (only 17% are de novo mutations). While most truncating mutations cause the severe and early lethal phenotype, some missense mutations are associated with milder forms and prolonged survival (up to 54 years). Hum Mutat 15:393–409, 2000.

Muscle disease caused by mutations in the skeletal muscle alpha-actin gene (ACTA1).

Mutations in the skeletal muscle alpha-actin gene (ACTA1) associated with congenital myopathy with excess of thin myofilaments, nemaline myopathy and intranuclear rod myopathy were first described in 1999. At that time, only 15 different missense mutations were known in ACTA1. More than 60 mutations have now been identified. This review analyses this larger spectrum of mutations in ACTA1. It investigates the molecular consequences of the mutations found to date, provides a framework for genotype-phenotype correlation and suggests future studies in light of results of investigation of normal and mutant actin in other systems, notably the actin specific to the indirect flight muscles of Drosophila. The larger series confirms that the majority of ACTA1 mutations are dominant, a small number are recessive and most isolated cases with no previous family history have de novo dominant mutations. The severity of the disease caused ranges from lack of spontaneous movements at birth requiring immediate mechanical ventilation, to mild disease compatible with life to adulthood. Overall, the mutations within ACTA1 are randomly distributed throughout the protein. However, the larger series of mutations now available indicates that there may be clustering of mutations associated with some phenotypes, e.g. actin myopathy. This would suggest that interference with certain actin functions may be more associated with certain phenotypes, though the exact pathophysiology of the actin mutations remains unknown.

Nemaline myopathy with minicores caused by mutation of the CFL2 gene encoding the skeletal muscle actin-binding protein, cofilin-2.

Nemaline myopathy (NM) is a congenital myopathy characterized by muscle weakness and nemaline bodies in affected myofibers. Five NM genes, all encoding components of the sarcomeric thin filament, are known. We report identification of a sixth gene, CFL2, encoding the actin-binding protein muscle cofilin-2, which is mutated in two siblings with congenital myopathy. The probands muscle contained characteristic nemaline bodies, as well as occasional fibers with minicores, concentric laminated bodies, and areas of F-actin accumulation. Her affected sisters muscle was reported to exhibit nonspecific myopathic changes. Cofilin-2 levels were significantly lower in the probands muscle, and the mutant protein was less soluble when expressed in Escherichia coli, suggesting that deficiency of cofilin-2 may result in reduced depolymerization of actin filaments, causing their accumulation in nemaline bodies, minicores, and, possibly, concentric laminated bodies.

Mutations and polymorphisms of the skeletal muscle alpha-actin gene (ACTA1).

The ACTA1 gene encodes skeletal muscle α‐actin, which is the predominant actin isoform in the sarcomeric thin filaments of adult skeletal muscle, and essential, along with myosin, for muscle contraction. ACTA1 disease‐causing mutations were first described in 1999, when a total of 15 mutations were known. In this article we describe 177 different disease‐causing ACTA1 mutations, including 85 that have not been described before. ACTA1 mutations result in five overlapping congenital myopathies: nemaline myopathy; intranuclear rod myopathy; actin filament aggregate myopathy; congenital fiber type disproportion; and myopathy with core‐like areas. Mixtures of these histopathological phenotypes may be seen in a single biopsy from one patient. Irrespective of the histopathology, the disease is frequently clinically severe, with many patients dying within the first year of life. Most mutations are dominant and most patients have de novo mutations not present in the peripheral blood DNA of either parent. Only 10% of mutations are recessive and they are genetic or functional null mutations. To aid molecular diagnosis and establishing genotype–phenotype correlations, we have developed a locus‐specific database for ACTA1 variations (http://waimr.uwa.edu.au). Hum Mutat 30:1–11, 2009.

Proximal myotonic dystrophy—a family with autosomal dominant muscular dystrophy, cataracts, hearing loss and hypogonadism: heterogeneity of proximal myotonic syndromes?

We describe a family with an autosomal dominant, multisystem disorder, consisting of late-onset proximal muscular dystrophy, electrophysiological myotonia, cataracts, late-onset deafness and male hypogonadism. Four patients were available for clinical examinations. Examination of asymptomatic family members revealed another patient with bilateral cataracts but without definite muscle disorder. Five deceased members of the family had proximal muscle weakness, reportedly or confirmed in medical records. Molecular examination of genomic DNA showed no expansion of the unstable (CTG)n trinucleotide repeat on chromosome 19q13.3 associated with myotonic dystrophy (DM). Linkage to two loci implicated in other myotonic disorders, the muscle chloride channel (CLCN1) gene, and the muscle sodium channel (SCN4A) gene, was assessed and excluded. The clinical findings differ from those described in proximal myotonic myopathy (PROMM), in terms of the more severe muscle involvement with atrophy of affected muscles and the hearing loss. These findings suggest phenotypic and probably genetic heterogeneity among the proximal myotonic syndromes.

The myotubular myopathies: differential diagnosis of the X linked recessive, autosomal dominant, and autosomal recessive forms and present state of DNA studies.

Clinical differences exist between the three forms of myotubular myopathy. They differ regarding age at onset, severity of the disease, and prognosis, and also regarding some of the clinical characteristics. The autosomal dominant form mostly has a later onset and milder course than the X linked form, and the autosomal recessive form is intermediate in both respects. These differences are, however, quantitative rather than qualitative. Muscle biopsy studies of family members are useful in some cases, and immunohistochemical staining of desmin and vimentin may help distinguish between the X linked and autosomal forms. Determining the mode of inheritance and prognosis in individual families, especially those with a single male patient, still poses a problem. Current molecular genetic results indicate that the gene for the X linked form is located in the proximal Xq28 region. Further molecular genetic studies are needed to examine the existence of genetic heterogeneity in myotubular myopathy and to facilitate diagnosis.

Mutations in KLHL40 Are a Frequent Cause of Severe Autosomal-Recessive Nemaline Myopathy

Nemaline myopathy (NEM) is a common congenital myopathy. At the very severe end of the NEM clinical spectrum are genetically unresolved cases of autosomal-recessive fetal akinesia sequence. We studied a multinational cohort of 143 severe-NEM-affected families lacking genetic diagnosis. We performed whole-exome sequencing of six families and targeted gene sequencing of additional families. We identified 19 mutations in KLHL40 (kelch-like family member 40) in 28 apparently unrelated NEM kindreds of various ethnicities. Accounting for up to 28% of the tested individuals in the Japanese cohort, KLHL40 mutations were found to be the most common cause of this severe form of NEM. Clinical features of affected individuals were severe and distinctive and included fetal akinesia or hypokinesia and contractures, fractures, respiratory failure, and swallowing difficulties at birth. Molecular modeling suggested that the missense substitutions would destabilize the protein. Protein studies showed that KLHL40 is a striated-muscle-specific protein that is absent in KLHL40-associated NEM skeletal muscle. In zebrafish, klhl40a and klhl40b expression is largely confined to the myotome and skeletal muscle, and knockdown of these isoforms results in disruption of muscle structure and loss of movement. We identified KLHL40 mutations as a frequent cause of severe autosomal-recessive NEM and showed that it plays a key role in muscle development and function. Screening of KLHL40 should be a priority in individuals who are affected by autosomal-recessive NEM and who present with prenatal symptoms and/or contractures and in all Japanese individuals with severe NEM.