Sini Penttilä

Mutations affecting the cytoplasmic functions of the co-chaperone DNAJB6 cause limb-girdle muscular dystrophy

Limb-girdle muscular dystrophy type 1D (LGMD1D) was linked to chromosome 7q36 over a decade ago, but its genetic cause has remained elusive. Here we studied nine LGMD-affected families from Finland, the United States and Italy and identified four dominant missense mutations leading to p.Phe93Leu or p.Phe89Ile changes in the ubiquitously expressed co-chaperone DNAJB6. Functional testing in vivo showed that the mutations have a dominant toxic effect mediated specifically by the cytoplasmic isoform of DNAJB6. In vitro studies demonstrated that the mutations increase the half-life of DNAJB6, extending this effect to the wild-type protein, and reduce its protective anti-aggregation effect. Further, we show that DNAJB6 interacts with members of the CASA complex, including the myofibrillar myopathy–causing protein BAG3. Our data identify the genetic cause of LGMD1D, suggest that its pathogenesis is mediated by defective chaperone function and highlight how mutations in a ubiquitously expressed gene can exert effects in a tissue-, isoform- and cellular compartment–specific manner.

Mutations in the N-terminal Actin-Binding Domain of Filamin C Cause a Distal Myopathy

Linkage analysis of the dominant distal myopathy we previously identified in a large Australian family demonstrated one significant linkage region located on chromosome 7 and encompassing 18.6 Mbp and 151 genes. The strongest candidate gene was FLNC because filamin C, the encoded protein, is muscle-specific and associated with myofibrillar myopathy. Sequencing of FLNC cDNA identified a c.752T>C (p.Met251Thr) mutation in the N-terminal actin-binding domain (ABD); this mutation segregated with the disease and was absent in 200 controls. We identified an Italian family with the same phenotype and found a c.577G>A (p.Ala193Thr) filamin C ABD mutation that segregated with the disease. Filamin C ABD mutations have not been described, although filamin A and filamin B ABD mutations cause multiple musculoskeletal disorders. The distal myopathy phenotype and muscle pathology in the two families differ from myofibrillar myopathies caused by filamin C rod and dimerization domain mutations because of the distinct involvement of hand muscles and lack of pathological protein aggregation. Thus, like the position of FLNA and B mutations, the position of the FLNC mutation determines disease phenotype. The two filamin C ABD mutations increase actin-binding affinity in a manner similar to filamin A and filamin B ABD mutations. Cell-culture expression of the c.752T>C (p.Met251)Thr mutant filamin C ABD demonstrated reduced nuclear localization as did mutant filamin A and filamin B ABDs. Expression of both filamin C ABD mutants as full-length proteins induced increased aggregation of filamin. We conclude filamin C ABD mutations cause a recognizable distal myopathy, most likely through increased actin affinity, similar to the pathological mechanism of filamin A and filamin B ABD mutations.

Eight new mutations and the expanding phenotype variability in muscular dystrophy caused by ANO5

Objective: Description of 8 new ANO5 mutations and significant expansion of the clinical phenotype spectrum associated with previously known and unknown mutations to improve diagnostic accuracy. Methods: DNA samples of 101 patients in 95 kindreds at our quaternary referral center in Finland, who had undetermined limb-girdle muscular dystrophy (LGMD), calf distal myopathy, or creatine kinase (CK) elevations of more than 2,000 IU/L, were selected for ANO5 genetic evaluation, and the clinical findings of patients with mutations were retrospectively analyzed. Results: A total of 25 patients with muscular dystrophy caused by 11 different recessive mutations in the ANO5 gene were identified. The vast majority of mutations, 8 of 11, proved to be previously unknown new mutations. The most frequent mutation, c.2272C>T (p.R758C), was present in 20 patients. The phenotypes associated with this and the common European mutation, c.191dupA, varied from nearly asymptomatic high hyperCKemia to severe LGMD with consistently milder phenotypes in female patients. Conclusions: Mutations in ANO5 are a frequent cause of undetermined muscular dystrophy, with both distal and proximal presentation. Other types include high hyperCKemia, myalgia, or calf hypertrophy over decades without significant weakness, especially in female patients. Mutations are distributed all over the gene, indicating that muscular dystrophy caused by ANO5 can be expected to occur in all populations.

Novel mutations widen the phenotypic spectrum of slow skeletal/β-cardiac myosin (MYH7) distal myopathy.

Laing early onset distal myopathy and myosin storage myopathy are caused by mutations of slow skeletal/β‐cardiac myosin heavy chain encoded by the gene MYH7, as is a common form of familial hypertrophic/dilated cardiomyopathy. The mechanisms by which different phenotypes are produced by mutations in MYH7, even in the same region of the gene, are not known. To explore the clinical spectrum and pathobiology, we screened the MYH7 gene in 88 patients from 21 previously unpublished families presenting with distal or generalized skeletal muscle weakness, with or without cardiac involvement. Twelve novel mutations have been identified in thirteen families. In one of these families, the father of the proband was found to be a mosaic for the MYH7 mutation. In eight cases, de novo mutation appeared to have occurred, which was proven in four. The presenting complaint was footdrop, sometimes leading to delayed walking or tripping, in members of 17 families (81%), with other presentations including cardiomyopathy in infancy, generalized floppiness, and scoliosis. Cardiac involvement as well as skeletal muscle weakness was identified in nine of 21 families. Spinal involvement such as scoliosis or rigidity was identified in 12 (57%). This report widens the clinical and pathological phenotypes, and the genetics of MYH7 mutations leading to skeletal muscle diseases.

Late onset spinal motor neuronopathy is caused by mutation in CHCHD10.

A study was undertaken to identify the responsible gene defect underlying late onset spinal motor neuronopathy (LOSMoN/SMAJ; Online Mendelian Inheritance in Man #615048), an autosomal dominant disease mapped to chromosome 22q11.2.

Atypical phenotypes in titinopathies explained by second titin mutations

Several patients with previously reported titin gene (TTN) mutations causing tibial muscular dystrophy (TMD) have more complex, severe, or unusual phenotypes. This study aimed to clarify the molecular cause of the variant phenotypes in 8 patients of 7 European families.

Distinct distal myopathy phenotype caused by VCP gene mutation in a Finnish family

Inclusion body myopathy with Paget disease and frontotemporal dementia (IBMPFD) is caused by mutations in the valosin-containing protein (VCP) gene. We report a new distal phenotype caused by VCP gene mutation in a Finnish family with nine affected members in three generations. Patients had onset of distal leg muscle weakness and atrophy in the anterior compartment muscles after age 35, which caused a foot drop at age 50. None of the siblings had scapular winging, proximal myopathy, cardiomyopathy or respiratory problems during long-term follow-up. Three distal myopathy patients developed rapidly progressive dementia, became bedridden and died of cachexia and pneumonia and VCP gene mutation P137L (c.410C>T) was then identified in the family. Late onset autosomal dominant distal myopathy with rimmed vacuolar muscle pathology was not sufficient for exact diagnosis in this family until late-occurring dementia provided the clue for molecular diagnosis. VCP needs to be considered in the differential diagnostic work-up in patients with distal myopathy phenotype.

Mutations in HSPB8 causing a new phenotype of distal myopathy and motor neuropathy.

Objective: To report novel disease and pathology due to HSPB8 mutations in 2 families with autosomal dominant distal neuromuscular disease showing both myofibrillar and rimmed vacuolar myopathy together with neurogenic changes. Methods: We performed whole-exome sequencing (WES) in tandem with linkage analysis and candidate gene approach as well as targeted next-generation sequencing (tNGS) to identify causative mutations in 2 families with dominant rimmed vacuolar myopathy and a motor neuropathy. Pathogenic variants and familial segregation were confirmed using Sanger sequencing. Results: WES and tNGS identified a heterozygous change in HSPB8 in both families: c.421A > G p.K141E in family 1 and c.151insC p.P173SfsX43 in family 2. Affected patients had a distal myopathy that showed myofibrillar aggregates and rimmed vacuoles combined with a clear neurogenic component both on biopsy and neurophysiologic studies. MRI of lower limb muscles demonstrated diffuse tissue changes early in the disease stage progressing later to fatty replacement typical of a myopathy. Conclusion: We expand the understanding of disease mechanisms, tissue involvement, and phenotypic outcome of HSPB8 mutations. HSPB8 is part of the chaperone-assisted selective autophagy (CASA) complex previously only associated with Charcot-Marie-Tooth type 2L (OMIM 60673) and distal hereditary motor neuronopathy type IIa. However, we now demonstrate that patients can develop a myopathy with histologic features of myofibrillar myopathy with aggregates and rimmed vacuoles, similar to the pathology in myopathies due to gene defects in other compounds of the CASA complex such as BAG3 and DNAJB6 after developing the early neurogenic effects.

New phenotype and pathology features in MYH7-related distal myopathy

Laing distal myopathy is an autosomal dominant disease due to mutations in the gene encoding for the human slow-β myosin heavy chain, MYH7. Most reports describe it as a mild, early onset myopathy with involvement usually restricted to foot extensors, hand finger extensors and neck flexors, and unspecific findings on muscle biopsy. We identified the first two Italian families with Laing distal myopathy, harboring two novel mutations in the MYH7 gene and performed clinical, neurophysiological, pathological, muscle MRI and cardiological investigations on affected members from the two families. Subjects from one family presented a moderate-severe phenotype, with proximal together with distal involvement and even loss of ambulation at advanced age. One patient displayed atypical muscle biopsy findings including cytoplasmic bodies and myofibrillar myopathy-like features. Affected members from the second family shared a very mild phenotype, with weakness largely limited to long toe and foot extensors and/or late onset. No patient showed any sign of heart involvement. Our study significantly broadens the clinical and pathological spectrum of Laing distal myopathy. We suggest that MYH7 screening should be considered in undiagnosed late-onset distal myopathy or cytoplasmic body myopathy patients.

Co-segregation of DM2 with a recessive CLCN1 mutation in juvenile onset of myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is a common adult onset muscular dystrophy caused by a dominantly transmitted (CCTG)n expansion in intron 1 of the CNBP gene. In DM2 there is no obvious evidence for an intergenerational increase of expansion size, and no congenital cases have been confirmed. We describe the clinical and histopathological features, and provide the genetic and molecular explanation for juvenile onset of myotonia in a 14-year-old female with DM2 and her affected mother presenting with a more severe phenotype despite a later onset of symptoms. Histological and immunohistochemical findings correlated with disease severity or age at onset in both patients. Southern blot on both muscle and blood samples revealed only a small increase in the CCTG repeat number through maternal transmission. Fluorescence in situ hybridization, in combination with MBNL1 immunofluorescence on muscle sections, showed the presence of mutant mRNA and MBNL1 in nuclear foci; the fluorescence intensity and its area appeared to be similar in the two patients. Splicing analysis of the INSR, CLCN1 and MBNL1 genes in muscle tissue demonstrates that the level of aberrant splicing isoforms was lower in the daughter than in the mother. However, in the CLCN1 gene, a heterozygous mutation c.501C>G p.F167L was present in the daughter’s DNA and found to be maternally inherited. Biomolecular findings did not explain the unusual young onset in the daughter. The co-segregation of DM2 with a recessive CLCN1 mutation provided the explanation for the unusual clinical findings.