Scapuloperoneal muscular dystrophy phenotype due to TRIM32-sarcotubular myopathy in South Dakota Hutterite

Case report: A novel patient presenting TRIM32-related limb-girdle muscular dystrophy

Limb-girdle muscular dystrophy autosomal recessive 8 (LGMDR8) is a rare clinical manifestation caused by the presence of biallelic variants in the TRIM32 gene. We present the clinical, molecular, histopathological, and muscle magnetic resonance findings of a novel 63-years-old LGMDR8 patient of Italian origins, who went undiagnosed for 24 years. Clinical exome sequencing identified two TRIM32 missense variants, c.1181G > A p.(Arg394His) and c.1781G > A p.(Ser594Asp), located in the NHL1 and NHL4 structural domains, respectively, of the TRIM32 protein. We conducted a literature review of the clinical and instrumental data associated to the so far known 26 TRIM32 variants, carried biallelically by 53 LGMDR8 patients reported to date in 20 papers. Our proband's variants were previously identified only in three independent LGMDR8 patients in homozygosis, therefore our case is the first in literature to be described as compound heterozygous for such variants. Our report also provides additional data in support of their pathogenicity, since p.(Arg394His) is currently classified as a variant of uncertain significance, while p.(Ser594Asp) as likely pathogenic. Taken together, these findings might be useful to improve both the genetic counseling and the diagnostic accuracy of this rare neuromuscular condition.

Tripartite Motif-Containing Protein 32 (TRIM32): What Does It Do for Skeletal Muscle?

Tripartite motif-containing protein 32 (TRIM32) is a member of the tripartite motif family and is highly conserved from flies to humans. Via its E3 ubiquitin ligase activity, TRIM32 mediates and regulates many physiological and pathophysiological processes, such as growth, differentiation, muscle regeneration, immunity, and carcinogenesis. TRIM32 plays multifunctional roles in the maintenance of skeletal muscle. Genetic variations in the TRIM32 gene are associated with skeletal muscular dystrophies in humans, including limb-girdle muscular dystrophy type 2H (LGMD2H). LGMD2H-causing genetic variations of TRIM32 occur most frequently in the C-terminal NHL (ncl-1, HT2A, and lin-41) repeats of TRIM32. LGMD2H is characterized by skeletal muscle dystrophy, myopathy, and atrophy. Surprisingly, most patients with LGMD2H show minimal or no dysfunction in other tissues or organs, despite the broad expression of TRIM32 in various tissues. This suggests more prominent roles for TRIM32 in skeletal muscle than in other tissues or organs. This review is focused on understanding the physiological roles of TRIM32 in skeletal muscle, the pathophysiological mechanisms mediated by TRIM32 genetic variants in LGMD2H patients, and the correlations between TRIM32 and Duchenne muscular dystrophy (DMD).

A novel homozygous exon2 deletion of TRIM32 gene in a Chinese patient with sarcotubular myopathy: A case report and literature review

Sarcotubular myopathy (STM) is a rare autosomal recessive myopathy caused by TRIM32 gene mutations. It is predominantly characterized by the weakness of the proximal limb and mild to moderate elevation of creatine kinase levels. In this study, we describe a 50-year-old Chinese man who exhibited a proximal-to-distal weakness in the muscles of the lower limbs and who had difficulty standing up from a squat position. The symptoms gradually became more severe. He denied a history of cognitive or cardiological problems. The patient’s parents and children were healthy. Histopathological examination revealed dystrophic changes and irregular slit-shaped vacuoles containing amorphous materials. Whole-exome sequencing consisting of protein-encoding regions of 19,396 genes was performed, the results of which identified one novel homozygous 2kb deletion chr9.hg19: g.119460021_119461983del (exon2) in the TRIM32 gene. This was confirmed at the homozygous state with quantitative real-time polymerase chain reaction. Here, we present a Chinese case of STM with one novel mutation in TRIM32 and provide a brief summary of all known pathogenic mutations in TRIM32.

Altered myogenesis and premature senescence underlie human TRIM32-related myopathy

TRIM32 is a E3 ubiquitin -ligase containing RING, B-box, coiled-coil and six C-terminal NHL domains. Mutations involving NHL and coiled-coil domains result in a pure myopathy (LGMD2H/STM) while the only described mutation in the B-box domain is associated with a multisystemic disorder without myopathy (Bardet-Biedl syndrome type11), suggesting that these domains are involved in distinct processes. Knock-out (T32KO) and knock-in mice carrying the c.1465G > A (p.D489N) involving the NHL domain (T32KI) show alterations in muscle regrowth after atrophy and satellite cells senescence. Here, we present phenotypical description and functional characterization of mutations in the RING, coiled-coil and NHL domains of TRIM32 causing a muscle dystrophy. Reduced levels of TRIM32 protein was observed in all patient muscle studied, regardless of the type of mutation (missense, single amino acid deletion, and frameshift) or the mutated domain. The affected patients presented with variable phenotypes but predominantly proximal weakness. Two patients had symptoms of both muscular dystrophy and Bardet-Biedl syndrome. The muscle magnetic resonance imaging (MRI) pattern is highly variable among patients and families. Primary myoblast culture from these patients demonstrated common findings consistent with reduced proliferation and differentiation, diminished satellite cell pool, accelerated senescence of muscle, and signs of autophagy activation.

Suppression of Trim32 Enhances Motor Function Repair after Traumatic Brain Injury Associated with Antiapoptosis

To investigate the role of Trim32 in traumatic brain injury (TBI), adult male Sprague Dawley (SD) rats and mice were randomly divided into sham (n = 6) and TBI groups ( n = 24), respectively. Then, mice were assigned into Trim32 knockout mice (Trim32-KO [+/-]) and wild-type (WT) littermates. The TBI model used was the Feeney free-falling model, and neurological function was evaluated after TBI using a neurological severity score (NSS). Reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunohistochemistry were used to investigate the expression of Trim32 in the damaged cortex. Cell apoptosis in the cortex was detected by terminal-deoxynucleoitidyl transferase-mediated dUTP nick end labeling (TUNEL) staining. Moreover, Trim32-KO (+/-) mice were used to determine the effect of Trim in neurological repair after TBI. Results showed the NSS scores in TBI rats were significantly increased from day 1 to day 11 postoperation, compared with the sham group. Trim32 messenger RNA (mRNA) expression in the cortex was significantly increased at 7 d after TBI, while the level of Tnr and cytochrome c oxidase polypeptide 5A mRNA didn't exhibit significant changes. In addition, Western blot was used to detect the level of Trim32 protein in the cortex. Trim32 expression was significantly increased at 7 d after TBI, and immunoreactive Trim32-positive cells were mainly neurons. Moreover, Trim32-KO (+/-) mice with TBI had lower NSS scores than those in the WT group from day 1 to day 11 postoperation. Meanwhile, Trim32-KO (+/-) mice had a decreased number of TUNEL-positive cells compared with the control group at 3 d postoperation. Protein 73 (p73) decreased at 7 d postoperation in Trim32-KO (+/-) mice with TBI, when compared with WT mice with TBI. Our study is the first to confirm that suppression of Trim32 promotes the recovery of neurological function after TBI and to demonstrate that the underlying mechanism is associated with antiapoptosis, which may be associated with p73.

New era in genetics of early-onset muscle disease: Breakthroughs and challenges

Early-onset muscle disease includes three major entities that present generally at or before birth: congenital myopathies, congenital muscular dystrophies and congenital myasthenic syndromes. Almost exclusively there is weakness and hypotonia, although cases manifesting hypertonia are increasingly being recognised. These diseases display a wide phenotypic and genetic heterogeneity, with the uptake of next generation sequencing resulting in an unparalleled extension of the phenotype-genotype correlations and "diagnosis by sequencing" due to unbiased sequencing. Perhaps now more than ever, detailed clinical evaluations are necessary to guide the genetic diagnosis; with arrival at a molecular diagnosis frequently occurring following dialogue between the molecular geneticist, the referring clinician and the pathologist. There is an ever-increasing blurring of the boundaries between the congenital myopathies, dystrophies and myasthenic syndromes. In addition, many novel disease genes have been described and new insights have been gained into skeletal muscle development and function. Despite the advances made, a significant percentage of patients remain without a molecular diagnosis, suggesting that there are many more human disease genes and mechanisms to identify. It is now technically- and clinically-feasible to perform next generation sequencing for severe diseases on a population-wide scale, such that preconception-carrier screening can occur. Newborn screening for selected early-onset muscle diseases is also technically and ethically-achievable, with benefits to the patient and family from early management of these diseases and should also be implemented. The need for world-wide Reference Centres to meticulously curate polymorphisms and mutations within a particular gene is becoming increasingly apparent, particularly for interpretation of variants in the large genes which cause early-onset myopathies: NEB, RYR1 and TTN. Functional validation of candidate disease variants is crucial for accurate interpretation of next generation sequencing and appropriate genetic counseling. Many published "pathogenic" variants are too frequent in control populations and are thus likely rare polymorphisms. Mechanisms need to be put in place to systematically update the classification of variants such that accurate interpretation of variants occurs. In this review, we highlight the recent advances made and the challenges ahead for the molecular diagnosis of early-onset muscle diseases.

The ubiquitin ligase tripartite-motif-protein 32 is induced in Duchenne muscular dystrophy

Activation of the proteasome pathway is one of the secondary processes of cell damage, which ultimately lead to muscle degeneration and necrosis in Duchenne muscular dystrophy (DMD). In mdx mice, the proteasome inhibitor bortezomib up-regulates the membrane expression of members of the dystrophin complex and reduces the inflammatory reaction. However, chronic inhibition of the 26S proteasome may be toxic, as indicated by the systemic side-effects caused by this drug. Therefore, we sought to determine the components of the ubiquitin-proteasome pathway that are specifically activated in human dystrophin-deficient muscles. The analysis of a cohort of patients with genetically determined DMD or Becker muscular dystrophy (BMD) unveiled a selective up-regulation of the ubiquitin ligase tripartite motif-containing protein 32 (TRIM32). The induction of TRIM32 was due to a transcriptional effect and it correlated with disease severity in BMD patients. In contrast, atrogin1 and muscle RING-finger protein-1 (MuRF-1), which are strongly increased in distinct types of muscular atrophy, were not affected by the DMD dystrophic process. Knock-out models showed that TRIM32 is involved in ubiquitination of muscle cytoskeletal proteins as well as of protein inhibitor of activated STAT protein gamma (Piasγ) and N-myc downstream-regulated gene, two inhibitors of satellite cell proliferation and differentiation. Accordingly, we showed that in DMD/BMD muscle tissue, TRIM32 induction was more pronounced in regenerating myofibers rather than in necrotic muscle cells, thus pointing out a role of this protein in the regulation of human myoblast cell fate. This finding highlights TRIM32 as a possible therapeutic target to favor skeletal muscle regeneration in DMD patients.

Association of a Novel ACTA1 Mutation With a Dominant Progressive Scapuloperoneal Myopathy in an Extended Family

IMPORTANCE

New genomic strategies can now be applied to identify a diagnosis in patients and families with previously undiagnosed rare genetic conditions. The large family evaluated in the present study was described in 1966 and now expands the phenotype of a known neuromuscular gene.

OBJECTIVE

To determine the genetic cause of a slowly progressive, autosomal dominant, scapuloperoneal neuromuscular disorder by using linkage and exome sequencing.

DESIGN, SETTING, AND PARTICIPANTS

Fourteen affected individuals in a 6-generation family with a progressive scapuloperoneal disorder were evaluated. Participants were examined at pediatric, neuromuscular, and research clinics from March 1, 2005, to May 31, 2014. Exome and linkage were performed in genetics laboratories of research institutions.

MAIN OUTCOMES AND MEASURES

Examination and evaluation by magnetic resonance imaging, ultrasonography, electrodiagnostic studies, and muscle biopsies (n = 3). Genetic analysis included linkage analysis (n = 17) with exome sequencing (n = 7).

RESULTS

Clinical findings included progressive muscle weakness in an initially scapuloperoneal and distal distribution, including wrist extensor weakness, finger and foot drop, scapular winging, mild facial weakness, Achilles tendon contractures, and diminished or absent deep tendon reflexes. Both age at onset and progression of the disease showed clinical variability within the family. Muscle biopsy specimens demonstrated type I fiber atrophy and trabeculated fibers without nemaline rods. Analysis of exome sequences within the linkage region (4.8 megabases) revealed missense mutation c.591C>A p.Glu197Asp in a highly conserved residue in exon 4 of ACTA1. The mutation cosegregated with disease in all tested individuals and was not present in unaffected individuals.

CONCLUSIONS AND RELEVANCE

This family defines a new scapuloperoneal phenotype associated with an ACTA1 mutation. A highly conserved protein, ACTA1 is implicated in multiple muscle diseases, including nemaline myopathy, actin aggregate myopathy, fiber-type disproportion, and rod-core myopathy. To our knowledge, mutations in Glu197 have not been reported previously. This residue is highly conserved and located in an exposed position in the protein; the mutation affects the intermolecular and intramolecular electrostatic interactions as shown by structural modeling. The mutation in this residue does not appear to lead to rod formation or actin accumulation in vitro or in vivo, suggesting a different molecular mechanism from that of other ACTA1 diseases.