Matthias Vorgerd

A Mutation in the Dimerization Domain of Filamin C Causes a Novel Type of Autosomal Dominant Myofibrillar Myopathy

Myofibrillar myopathy (MFM) is a human disease that is characterized by focal myofibrillar destruction and pathological cytoplasmic protein aggregations. In an extended German pedigree with a novel form of MFM characterized by clinical features of a limb-girdle myopathy and morphological features of MFM, we identified a co-segregating, heterozygous nonsense mutation (8130G-->A; W2710X) in the filamin c gene (FLNC) on chromosome 7q32.1. The mutation is the first found in FLNC and is localized in the dimerization domain of filamin c. Functional studies showed that, in the truncated mutant protein, this domain has a disturbed secondary structure that leads to the inability to dimerize properly. As a consequence of this malfunction, the muscle fibers of our patients display massive cytoplasmic aggregates containing filamin c and several Z-disk-associated and sarcolemmal proteins.

Mutations in CAV3 cause mechanical hyperirritability of skeletal muscle in rippling muscle disease

Hereditary rippling muscle disease (RMD) is an autosomal dominant human disorder characterized by mechanically triggered contractions of skeletal muscle. Genome-wide linkage analysis has identified an RMD locus on chromosome 3p25. We found missense mutations in positional candidate CAV3 (encoding caveolin 3; ref. 5) in all five families analyzed. Mutations in CAV3 have also been described in limb-girdle muscular dystrophy type 1C (LGMD1C; refs. 6,7), demonstrating the allelism of dystrophic and non-dystrophic muscle diseases.

Mitochondrial impairment in patients and asymptomatic mutation carriers of Huntington's disease

Huntingtons disease (HD) is an autosomal dominantly inherited neurodegenerative disorder caused by a CAG repeat expansion in the IT‐15 gene; however, it remains unknown how the mutation leads to selective neurodegeneration. Several lines of evidence suggest impaired mitochondrial function as a component of the neurodegenerative process in HD. We assessed energy metabolism in the skeletal muscle of 15 HD patients and 12 asymptomatic mutation carriers in vivo using 31P magnetic resonance spectroscopy. Phosphocreatine recovery after exercise is a direct measure of ATP synthesis and was slowed significantly in HD patients and mutation carriers in comparison to age‐ and gender‐matched healthy controls. We found that oxidative function is impaired to a similar extent in manifest HD patients and asymptomatic mutation carriers. Our findings suggest that mitochondrial dysfunction is an early and persistent component of the pathophysiology of HD.

Distinct muscle imaging patterns in myofibrillar myopathies

Objective: To compare muscle imaging findings in different subtypes of myofibrillar myopathies (MFM) in order to identify characteristic patterns of muscle alterations that may be helpful to separate these genetic heterogeneous muscular disorders. Methods: Muscle imaging and clinical findings of 46 patients with MFM were evaluated (19 desminopathy, 12 myotilinopathy, 11 filaminopathy, 1 αB-crystallinopathy, and 3 ZASPopathy). The data were collected retrospectively in 43 patients and prospectively in 3 patients. Results: In patients with desminopathy, the semitendinosus was at least equally affected as the biceps femoris, and the peroneal muscles were never less involved than the tibialis anterior (sensitivity of these imaging criteria to detect desminopathy in our cohort 100%, specificity 95%). In most of the patients with myotilinopathy, the adductor magnus showed more alterations than the gracilis muscle, and the sartorius was at least equally affected as the semitendinosus (sensitivity 90%, specificity 93%). In filaminopathy, the biceps femoris and semitendinosus were at least equally affected as the sartorius muscle, and the medial gastrocnemius was more affected than the lateral gastrocnemius. The semimembranosus mostly showed more alterations than the adductor magnus (sensitivity 88%, specificity 96%). Early adult onset and cardiac involvement was most often associated with desminopathy. In patients with filaminopathy, muscle weakness typically beginning in the 5th decade of life was mostly pronounced proximally, while late adult onset (>50 years) with distal weakness was more often present in myotilinopathy. Conclusions: Muscle imaging in combination with clinical data may be helpful for separation of distinct myofibrillar myopathy subtypes and in scheduling of genetic analysis.

Enhancement of inhibitory mechanisms in the motor cortex of patients with cerebellar degeneration: a study with transcranial magnetic brain stimulation

The excitatory state of the primary motor cortex can be studied by measuring either the postexcitatory inhibition after transcranial magnetic single stimulation (pI-S) or the refractory period with magnetic double stimulation (rP-D). The cerebellum may influence the excitability of the motor cortex by cerebellar inputs and outputs from side loops of transcortical projections. Therefore, we studied pI-S and rP-D in 24 patients with autosomal dominant cerebellar ataxia or idiopathic cerebellar ataxia, who were allocated to one group (Group A) with mild to moderate ataxia (n = 11) and to another group (Group B) with severe ataxia (n = 13). The results were compared with those obtained in 21 normal age-matched control subjects. The central motor conduction time (CMCT) was delayed in approximately half of the patients, demonstrating that the degenerative process, beyond the cerebellum, also affects the pyramidal tract. Mean CMCT was significantly delayed only in patients of Group B. pI-S was prolonged in 10 of our 24 patients; incidence of pathology in pI-S did not differ between the two patient groups. In 5 patients with normal CMCT, pathological pI-S results were found. Mean pI-S was prolonged in the whole patient group and in both subgroups as well. rP-D was prolonged in two patients of Group B only, but mean rP-D was significantly prolonged in the whole patient group. Prolonged postexcitatory inhibition and refractory period may be a consequence of a transient facilitation of cortical inhibitory interneurons, which results in a decreased excitability of primary motor cortex in patients with cerebellar degeneration.

De novo desmin mutation N116S is associated with arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited heart muscle disease, frequently accompanied by sudden cardiac death and terminal heart failure. Genotyping of ARVC patients might be used for palliative treatment of the affected family. We genotyped a cohort of 22 ARVC patients referred to molecular genetic screening in our heart center for mutations in the desmosomal candidate genes JUP, DSG2, DSC2, DSP and PKP2 known to be associated with ARVC. In 43% of the cohort, we found disease-associated sequence variants. In addition, we screened for desmin mutations and found a novel desmin-mutation p.N116S in a patient with ARVC and terminal heart failure, which is located in segment 1A of the desmin rod domain. The mutation leads to the aggresome formation in cardiac and skeletal muscle without signs of an overt clinical myopathy. Cardiac aggresomes appear to be prominent, especially in the right ventricle of the heart. Viscosimetry and atomic force microscopy of the desmin wild-type and N116S mutant isolated from recombinant Escherichia coli revealed severe impairment of the filament formation, which was supported by transfections in SW13 cells. Thus, the gene coding for desmin appears to be a novel ARVC gene, which should be included in molecular genetic screening of ARVC patients.

The novel neurofilament light (NEFL) mutation Glu397Lys is associated with a clinically and morphologically heterogeneous type of Charcot-Marie-Tooth neuropathy.

Charcot-Marie-Tooth disease comprises a heterogeneous group of hereditary neuropathies which fall into two main groups: demyelinating CMT1 with reduced nerve conduction velocity and axonal CMT2 with normal nerve conduction velocity. The neuropathological features correspond in most cases to this classification. Four genes were recently identified to cause autosomal dominant CMT2, including the neurofilament light gene. Thus far, only few mutations have been reported in neurofilament light involving eight amino acids of the gene. We identified a novel mutation, Glu397Lys, in a conserved motive signaling the end of the rod domain. The affected family members from three generations showed strikingly different clinical phenotypes, including weakness of the lower extremities, foot deformities, and deafness. The mutation was associated with nerve conduction velocities ranging from 27 m/s in a 25-year-old female to 43 m/s in an 82-year-old male in the lower extremity motor nerves. Sural nerve biopsies of two affected subjects were analyzed by light and electron microscopy. The pathological changes consisted of a reduction of predominantly large myelinated nerve fibers and various stages of onion bulb formation as typically seen in CMT1. This correlative study further confirms that neurofilament light gene mutations cause a wide clinical spectrum. Thus, analysis of the neurofilament light gene should not be restricted to pure axonal neuropathies.

Idebenone in patients with Friedreich ataxia

Friedreich ataxia (FA), the most common form of degenerative ataxia, is thought to be caused by respiratory deficiency due to mitochondrial iron accumulation and oxidative stress. Idebenone, a free-radical scavenger, protects mitochondrial function in in vitro models of FA. In a placebo-controlled crossover trial we studied the effect of idebenone on respiratory function in nine ambulant FA patients. (31)P magnetic resonance spectroscopy demonstrated mitochondrial impairment in vivo in skeletal muscle of all FA patients, but no recovery with idebenone. No effects were seen in clinical scores. Echocardiography did not confirm a preliminary study reporting improvement of FA-associated cardiomyopathy with idebenone.

A sporadic case of rippling muscle disease caused by a de novo caveolin-3 mutation

Objective: To determine the cause of sporadic rippling muscle disease (RMD) in a 24-year-old patient. Background: RMD is a rare myopathy characterized by percussion-induced rapid muscle contractions (PIRC), muscle mounding, and rippling waves. We have recently found that autosomal dominant RMD is caused by mutations in the caveolin-3 gene (CAV3) on chromosome 3p25. Possibly, increased activity of neuronal nitric oxide synthase (nNOS) contributes to the clinical characteristics of increased mechanical muscle hyperexcitability. Methods: Clinical examination, mutational analysis, and immunohistochemistry of muscle tissue were performed in a patient with sporadic RMD. Results: The authors observed a de novo CAV3 missense mutation Arg26Gln. Immunohistochemistry showed reduced caveolin-3 surface expression in a muscle biopsy. In addition, the authors found normal sarcolemmal nNOS expression and a reduced expression of α-dystroglycan in muscle fibers. Conclusions: These data confirm that RMD is caused by CAV3 mutations. Moreover, there is evidence that CAV3 mutations may also be found in patients without a positive family history of RMD.

Mitochondrial impairment of human muscle in Friedreich ataxia in vivo

Friedreich ataxia occurs due to mutations in the gene encoding the mitochondrial protein frataxin. This (31)P magnetic resonance spectroscopy study on the calf muscle of Friedreich ataxia patients provides in vivo evidence of a severe impairment of mitochondrial function. Mitochondrial adenosine triphosphate resynthesis was studied by means of the post-exercise recovery of phosphocreatine. After ischemic exercise in calf muscles of all patients, phosphocreatine recovery was dramatically delayed. Time constants of recovery correlated with mutations of the frataxin gene, the age of the patients, and disease duration. (31)P magnetic resonance spectroscopy represents the first expedient tool for monitoring therapeutic trials in Friedreich ataxia non-invasively.