HEREDITARY OCCURRENCE OF NEMALINE MYOPATHY

Autosomal dominantly inherited myopathy likely caused by the TNNT1 variant p.(Asp65Ala)

Nemaline myopathies (NEM) are genetically and clinically heterogenous. Biallelic or monoallelic variants in TNNT1, encoding slow skeletal troponin T1 (TnT1), cause NEM. We report a 2-year-old patient and his mother carrying the heterozygous TNNT1 variant c.194A>C/p.(Asp65Ala) that occurred de novo in the mother. Both had muscle hypotrophy and muscle weakness. Muscle pathology in the proband's mother revealed slow twitch type 1 fiber hypotrophy and fast twitch type 2 fiber hypertrophy that was confirmed by a reduced ratio of slow skeletal myosin to fast skeletal myosin type 2a. RT-PCR and immunoblotting data demonstrated increased levels of high-molecular-weight TnT1 isoforms in skeletal muscle of the proband's mother that were also observed in some controls. In an overexpression system, complex formation of TnT1-D65A with tropomyosin 3 (TPM3) was enhanced. The previously reported TnT1-E104V and TnT1-L96P mutants showed reduced or no co-immunoprecipitation with TPM3. Our studies support pathogenicity of the TNNT1 p.(Asp65Ala) variant. This article is protected by copyright. All rights reserved.

Troponin Variants in Congenital Myopathies: How They Affect Skeletal Muscle Mechanics

The troponin complex is a key regulator of muscle contraction. Multiple variants in skeletal troponin encoding genes result in congenital myopathies. TNNC2 has been implicated in a novel congenital myopathy, TNNI2 and TNNT3 in distal arthrogryposis (DA), and TNNT1 and TNNT3 in nemaline myopathy (NEM). Variants in skeletal troponin encoding genes compromise sarcomere function, e.g., by altering the Ca²⁺ sensitivity of force or by inducing atrophy. Several potential therapeutic strategies are available to counter the effects of variants, such as troponin activators, introduction of wild-type protein through AAV gene therapy, and myosin modulation to improve muscle contraction. The mechanisms underlying the pathophysiological effects of the variants in skeletal troponin encoding genes are incompletely understood. Furthermore, limited knowledge is available on the structure of skeletal troponin. This review focusses on the physiology of slow and fast skeletal troponin and the pathophysiology of reported variants in skeletal troponin encoding genes. A better understanding of the pathophysiological effects of these variants, together with enhanced knowledge regarding the structure of slow and fast skeletal troponin, will direct the development of treatment strategies.

TNNT1 nemaline myopathy: natural history and therapeutic frontier

We describe the natural history of 'Amish' nemaline myopathy (ANM), an infantile-onset, lethal disease linked to a pathogenic c.505G>T nonsense mutation of TNNT1, which encodes the slow fiber isoform of troponin T (TNNT1; a.k.a. TnT). The TNNT1 c.505G>T allele has a carrier frequency of 6.5% within Old Order Amish settlements of North America. We collected natural history data for 106 ANM patients born between 1923 and 2017. Over the last two decades, mean age of molecular diagnosis was 16 ± 27 days. TNNT1 c.505G>T homozygotes were normal weight at birth but failed to thrive by age 9 months. Presenting neonatal signs were axial hypotonia, hip and shoulder stiffness, and tremors, followed by progressive muscle weakness, atrophy and contractures. Affected children developed thoracic rigidity, pectus carinatum and restrictive lung disease during infancy, and all succumbed to respiratory failure by 6 years of age (median survival 18 months, range 0.2-66 months). Muscle histology from two affected children showed marked fiber size variation owing to both Type 1 myofiber smallness (hypotrophy) and Type 2 fiber hypertrophy, with evidence of nemaline rods, myofibrillar disarray and vacuolar pathology in both fiber types. The truncated slow TNNT1 (TnT) fragment (p.Glu180Ter) was undetectable in ANM muscle, reflecting its rapid proteolysis and clearance from sarcoplasm. Similar functional and histological phenotypes were observed in other human cohorts and two transgenic murine models (Tnnt1-/- and Tnnt1 c.505G>T). These findings have implications for emerging molecular therapies, including the suitably of TNNT1 gene replacement for newborns with ANM or other TNNT1-associated myopathies.

Novel autosomal dominant TNNT1 mutation causing nemaline myopathy

Background

Nemaline myopathy (NEM) is one of the three major forms of congenital myopathy and is characterized by diffuse muscle weakness, hypotonia, respiratory insufficiency, and the presence of nemaline rod structures on muscle biopsy. Mutations in troponin T1 (TNNT1) is 1 of 10 genes known to cause NEM. To date, only homozygous nonsense mutations or compound heterozygous truncating or internal deletion mutations in TNNT1 gene have been identified in NEM. This extended family is of historical importance as some members were reported in the 1960s as initial evidence that NEM is a hereditary disorder.

Methods

Proband and extended family underwent Sanger sequencing for TNNT1. We performed RT‐PCR and immunoblot on muscle to assess TNNT1 RNA expression and protein levels in proband and father.

Results

We report a novel heterozygous missense mutation of TNNT1 c.311A>T (p.E104V) that segregated in an autosomal dominant fashion in a large family residing in the United States. Extensive sequencing of the other known genes for NEM failed to identify any other mutant alleles. Muscle biopsies revealed a characteristic pattern of nemaline rods and severe myofiber hypotrophy that was almost entirely restricted to the type 1 fiber population.

Conclusion

This novel mutation alters a residue that is highly conserved among vertebrates. This report highlights not only a family with autosomal dominant inheritance of NEM, but that this novel mutation likely acts via a dominant negative mechanism.

Nemaline myopathy: current concepts. The ENMC International Consortium and Nemaline Myopathy

Images

A gene for autosomal recessive nemaline myopathy assigned to chromosome 2q by linkage analysis

Clinical genetic evidence suggests the existence of an autosomal recessive form of congenital nemaline myopathy in addition to the autosomal dominant one(s). One mutation in an Australian kindred has been identified as causing an autosomal dominant form of the disease. This mutation in the alpha-tropomyosin gene TPM3 has previously been excluded as causing autosomal recessive nemaline myopathy. We searched systematically for genetic linkage to autosomal recessive nemaline myopathy (NEM2) by studying microsatellite marker alleles in seven multiplex families from Finland, Denmark, Wales, England and The Netherlands. Significant evidence of linkage was found to markers of chromosome 2q, the highest multipoint lod score value being 5.34 for the marker D2S151. Recombinant genotypes in affected individuals demarcate the the region in which the NEM2 gene is likely to reside as a 13 cM region between the markers D2S150 and D2S142. These results confirm the existence of at least one distinctive form of autosomal recessive nemaline myopathy and provide a basis for the identification of its gene.

Alpha-actinin in nemaline bodies in congenital nemaline myopathy: immunological confirmation by light and electron microscopy

To elucidate the protein composition of the nemaline bodies present in the muscle fibres of patients with congenital nemaline myopathy (CNM), we studied muscle biopsies with monoclonal antibodies against alpha-actinin and desmin in combination with a modified Gomori trichrome method. Electron microscopy of immunolabelled resin embedded sections was used for cytochemical localisation of alpha-actinin and desmin. Light microscopy of sections immunolabelled for alpha-actinin showed a cross-striation of the muscle fibres corresponding to the Z band pattern, focal thickening of the Z bands and additional reactivity with a granular pattern corresponding to the presence of nemaline bodies. Labelling of desmin did not show a similar pattern. Electron microscopy confirmed the presence of alpha-actinin in the nemaline bodies and Z bands, whereas desmin was only found in intermediate filaments around the Z bands. Western blots showed single, sharp alpha-actinin bands indistinguishable from normal. Our results provide direct evidence for the presence of alpha-actinin in nemaline bodies and a lack of quantitative or qualitative differences between the alpha-actinin of normal and CNM muscle.

Genetics of congenital nemaline myopathy: a study of 10 families

In order to investigate the inheritance in congenital nemaline myopathy (CNM), we studied the family histories and pedigrees of 13 patients with CNM from 10 families, and the 20 patients, by physical examination, single fibre electromyography, ultrasonography of muscles, measurement of serum creatine kinase, muscle biopsy, and electrophoresis of muscle proteins. None of the parents was affected. In three families there were two affected children. Of the parents, 15 showed deficiency of type 2B muscle fibres, and all except one father showed some other minor neuromuscular abnormality. These may represent heterozygous manifestations of recessive gene. Most of the ancestors came from sparsely populated rural communities in the west of Finland. We conclude that in the Finnish CNM patients, the mode of inheritance appears to be recessive. Apart from a few instances of dominant inheritance, most cases published also seem compatible with recessive inheritance.

Electromyography in congenital nemaline myopathy

To clarify the discrepancies between earlier reports of electromyography (EMG) in congenital nemaline myopathy (CNM), conventional electromyography was done on 13 patients with CNM, and results were compared with those of 18 earlier EMG examinations of the same patients. Fiber density was measured in 10 patients with a computerized method and neuromuscular jitter in 3 with single-fiber EMG. With age, the EMG abnormality progressed, and "neuropathic" EMG features developed in distal muscles. In 9 of 10 patients fiber density was higher than normal. In two of three patients jitter was abnormal. Motor (13 of 13 patients) and sensory (3 of 3 patients) nerve conduction velocities were normal. Our results seem to explain the conflicting reports of EMG in CNM. We conclude that active degeneration and regeneration of muscle fibers takes place in CNM and suggest that the "neuropathic" motor unit potentials seen in our patients may be secondary to myopathic disease activity.

Pathology of congenital nemaline myopathy. A follow-up study

This study was undertaken to review the development over 5-18 years of pathologic changes in 13 patients (4 male and 9 female) with congenital nemaline myopathy. Follow-up biopsies were compared with earlier biopsies and with published normal values as to quantity and location of nemaline bodies, secondary signs of myopathy, and in 6 patients as to muscle fiber type and size. Biopsy findings were correlated with the mobility and muscle power of the patient. The main differences in myofiber maturation in the patients as compared with normal myofiber maturation were: (1) deficient differentiation of type 2 fibers, (2) further increase of variation in fiber size with age, and (3) skewing in early adulthood of fiber size distribution curves toward the atrophic end. In ambulant patients, this skew seemed to be compensated with a population of hypertrophic fibers. The nemaline bodies tended to be located beneath the sarcolemma in the younger patients and inside the muscle fibers in the older patients. The quantity of nemaline bodies seemed to have increased with age. The clinical deterioration and the defective myofiber maturation in the patients together with an increase in internal nuclei and endomysial fat or fibrosis indicate an active disease process. This speaks against the generally held view that congenital nemaline myopathy is static.

Congenital nemaline myopathy: two patients with consanguineous parents, one with a progressive course

Clinical, morphological and genetic data are presented on two unrelated children with congenital nemaline myopathy. In one of these, the weakness and hypotonia were progressive. The parents of both children were second cousins. These cases together with those already published suggest that the frequency of consanguineous marriages in parents of children with nemaline myopathy is increased. This is a further argument in favour of an autosomal recessive type of congenital nemaline myopathy in addition to the autosomal dominant variety.

Early infant death in nemaline (rod) myopathy

Congenital 'floppy infant' syndrome with very early death is uncommon. It is here described in a girl. Histopathological examination of a muscle biopsy at the age of 10 days showed rods and infiltration of inflammatory cells. Death occurred at 5 weeks of age. Electron microscopy of the necropsy material showed widened Z disks and rods of Z disk density with a typical transverse periodicity of 18-20 nm. The differential diagnosis between nemaline (rod) myopathy and infantile polymyositis is discussed. Nemaline (rod) myopathy should always be considered in cases of the congenital 'floppy infant' syndrome.

Fatal neonatal nemaline myopathy

Nemaline myopathy was first reported in 1963 and has been considered to be a congenital, non-progressive myopathy with weakness since birth. However, severe forms leading to death in infancy have been rarely reported. Recently we necropsied a female infant with fatal neonatal nemaline myopathy who required mechanical ventilatory support immediately following delivery. She had suffered from recurrent pneumonia and died at five months of age. Light and electron microscopic studies on both muscle biopsy and autopsy specimens were diagnostic of nemaline myopathy. Characteristic rod-like structures were demonstrated within skeletal muscles, and accumulations of thin filaments were seen in numerous muscle fibers. The origin of these structures in the Z-disks and other morphologic features were the same as those of the rods occurring in congenital cases of rod myopathy. No involvement was observed in the cardiac muscle or in the smooth muscle of the gastrointestinal tract. It is likely that involvement of the skeletal muscles of the pharyngeal areas, intercostal spaces, and diaphragm might have contributed to the difficulty in swallowing and respiratory distress.

Genetics of congenital nemaline myopathy

Family patterns for 50 reported probands with congenital nemaline myopathy were compared with expected patterns derived from various possible genetic hypotheses. The disease had a high mortality in childhood but remained clinically stationary after this period. Some normal relatives showed nemaline rods on muscle biopsy. Chromosomes were normal in the two cases in which they were examined. Prenatal exposures appeared irrelevant to this disease. Autosomal recessive and X-linked recessive or dominant modes of inheritance were not compatible with the observed patterns, which could be explained by an autosomal dominant mode with a reduced penetrance. Normal relatives who carried rods were presumably unaffected heterozygotes of the same gene. The genetic ratio (the proportion of affected siblings) was 0.3, being short of the expected value, 0.5, probably because of the presence of these asymptomatic rod-bearing heterozygotes. While the pressure of natural selection was great in the patients who died or were severely disabled, the gene could be passed to the next generation by mildly affected patients or heterozygotes who remained unaffected.

Fatal neonatal nemaline myopathy with multiple congenital anomalies

The clinical course and autopsy findings in a patient with fatal neonatal nemaline myopathy are described. A hypotonic infant required mechanical ventilatory support immediately following delivery and developed progressive congestive heart failure. He had mild facial dysmorphism, a high-arched palate, clinodactyly, short first metacarpals, abnormal dermatoglyphics, simian creases, and bilateral talipes varus. Light and electron microscopic study of a muscle biopsy was diagnostic of nemaline myopathy. Autopsy revealed a papillary muscle anomaly, myocardial scarring, and hepatic fibrosis. The severe clinical impairment in this infant and the unusual associated anomalies are compared with other examples of nemaline myopathy. Nemaline myopathy is a cause of respiratory insufficiency in the neonatal period.

Nemaline (rod) myopathy: the need for histochemical evaluation of affected families

Histochemical changes in the mother of a patient with nemaline myopathy were used to identify her as the gene carrier even though rod-bodies were not present in her muscle biopsy and she was not weak. The patient and her mother both had marked type I fiber predominance with large groups of type I fibers present. Histochemical changes known to occur in nemaline myopathy include smallness and predominance of type I fibers. Such changes support the concept that this disease may result from subtle defects in innervation since fiber types are determined by innervation. Although this disease is thought to be transmitted by autosomal dominant mode, lack of male-to-male transmission and a predominance of female cases in the literature suggest that this may be (1) an X-linked dominant, (2) a sex-influenced autosomal dominant, or (3) an autosomal dominant which is semilethal in males. The family described here is the first in which a presumably affected parent showed only the histochemical change without rod-bodies, thus emphasizing the importance of histochemical evaluation of relatives' biopsies for genetic counseling.

Autosomal dominant "spheroid body myopathy"

A slowly progressive autosomal dominant neuromuscular disease--termed spheroid body myopathy--is described in four successive generations and documented by muscle biopsies in five patients of two generations. With an onset in adolescence, the disease proceeds to some motor incapacitation, but life span is apparently not shortened. The salient morphologic feature is the presence of spheroid bodies, chiefly occurring in type 1 myofibers. Ultrastructurally, these spheroid bodies are composed of tiny filaments but are devoid of organelles; in some cases they resemble cytoplasmic bodies. "Smearing in the 1-band" is a frequent and early finding. At a later age, signs of denervation are also present, both clinically and in muscle biopsies. The clinical and morphologic features justify the designation of this neuromuscular condition as a distinct entity.

Characteristics of myosin in nemaline myopathy

Electron-microscopic, morphometric, histochemical and biochemical studies were carried out on muscle biopsies from a patient with the characteristic clinical and pathological findings of nemaline myopathy. The mean fiber diameter was decreased, and the vastus lateralis muscle biopsy consisted exclusively of slow twitch (Type I) fibers. Quantitative biochemical investigations revealed significantly low calcium uptake and ATPase activity of the fragmented sarcoplasmic reticulum and decreased myosin ATPase activity. The electrophoretogram of myosin showed an abnormality in the light chain pattern which could not be explained by a disproportion of normal fiber types.