Mitochondrial abnormalities in extraocular muscles in myotonic dystrophy

AMPK is mitochondrial medicine for neuromuscular disorders

Duchenne muscular dystrophy (DMD), myotonic dystrophy type 1 (DM1), and spinal muscular atrophy (SMA) are the most prevalent neuromuscular disorders (NMDs) in children and adults. Central to a healthy neuromuscular system are the processes that govern mitochondrial turnover and dynamics, which are regulated by AMP-activated protein kinase (AMPK). Here, we survey mitochondrial stresses that are common between, as well as unique to, DMD, DM1, and SMA, and which may serve as potential therapeutic targets to mitigate neuromuscular disease. We also highlight recent advances that leverage a mutation-agnostic strategy featuring physiological or pharmacological AMPK activation to enhance mitochondrial health in these conditions, as well as identify outstanding questions and opportunities for future pursuit.

A single dose of exercise stimulates skeletal muscle mitochondrial plasticity in myotonic dystrophy type 1

AIM

Myotonic dystrophy type 1 (DM1) is the second most common muscular dystrophy after Duchenne and is the most prevalent muscular dystrophy in adults. DM1 patients that participate in aerobic exercise training experience several physiological benefits concomitant with improved muscle mitochondrial function without alterations in typical DM1-specific disease mechanisms, which suggests that correcting organelle health is key to ameliorate the DM1 pathology. However, our understanding of the molecular mechanisms of mitochondrial turnover and dynamics in DM1 skeletal muscle is lacking.

METHODS

Skeletal muscle tissue was sampled from healthy and DM1 mice under sedentary conditions and at several recovery time points following an exhaustive treadmill run.

RESULTS

We demonstrate that DM1 patients exhibit an imbalance in the transcriptional apparatus for mitochondrial turnover and dynamics in skeletal muscle. Additionally, DM1 mice displayed elevated expression of autophagy and mitophagy regulators. A single dose of exercise successfully enhanced canonical exercise molecular pathways and skeletal muscle mitochondrial biogenesis despite failing to alter the cellular pathology in DM1 mice. However, treadmill running stimulated coordinated organelle fusion and fission signaling, as well as improved alternative splicing of Optic atrophy 1. Exercise also evoked autophagy and mitophagy pathways in DM1 skeletal muscle resulting in the normalized expression of autophagy- and lysosome-related machinery responsible for the clearance of dysfunctional organelles.

CONCLUSION

Collectively, our data indicate that mitochondrial dynamics and turnover processes in DM1 skeletal muscle are initiated with a single dose of exercise, which may underlie the adaptive benefits previously documented in DM1 mice and patients.

Aerobic exercise elicits clinical adaptations in myotonic dystrophy type 1 patients independent of pathophysiological changes

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a complex life-limiting neuromuscular disorder characterized by severe skeletal muscle atrophy, weakness, and cardio-respiratory defects. Exercised DM1 mice exhibit numerous physiological benefits that are underpinned by reduced CUG foci and improved alternative splicing. However, the efficacy of physical activity in patients is unknown.

METHODS

Eleven genetically diagnosed DM1 patients were recruited to examine the extent to which 12-weeks of cycling can recuperate clinical, and physiological metrics. Furthermore, we studied the underlying molecular mechanisms through which exercise elicits benefits in skeletal muscle of DM1 patients.

RESULTS

DM1 was associated with impaired muscle function, fitness, and lung capacity. Cycling evoked several clinical, physical, and metabolic advantages in DM1 patients. We highlight that exercise-induced molecular and cellular alterations in patients do not conform with previously published data in murine models and propose a significant role of mitochondrial function in DM1 pathology. Lastly, we discovered a subset of small nucleolar RNAs (snoRNAs) that correlated to indicators of disease severity.

CONCLUSION

With no available cures, our data supports the efficacy of exercise as a primary intervention to partially mitigate the clinical progression of DM1. Additionally, we provide evidence for the involvement of snoRNAs and other noncoding RNAs in DM1 pathophysiology.

TRIAL REGISTRATION

This trial was approved by the HiREB committee (#7901) and registered under ClinicalTrials.gov (NCT04187482).

FUNDING

This work was primarily supported by Neil and Leanne Petroff. This study was also supported by a Canadian Institutes of Health Research Foundation Grant to MAT (#143325).

Limited diagnostic value of enzyme analysis in patients with mitochondrial tRNA mutations

We evaluated the diagnostic value of respiratory chain (RC) enzyme analysis of muscle in adult patients with mitochondrial myopathy (MM). RC enzyme activity was measured in muscle biopsies from 39 patients who carry either the 3243A>G mutation, other tRNA point mutations, or single, large-scale deletions of mtDNA. Findings were compared with those obtained from asymptomatic relatives with the 3243A>G mutation, myotonic dystrophy patients, and healthy subjects. Plasma lactate concentration, maximal oxygen uptake, and ragged-red fibers/cytochrome c-negative fibers in muscle were also determined. Only 10% of patients with the 3243A>G point mutation had decreased enzyme activity of one or more RC complexes, whereas this was the case for 83% of patients with other point mutations and 62% of patients with deletions. Abnormal muscle histochemistry was found in 65%, 100%, and 85% of patients, respectively, in these three groups. The results indicate that RC enzyme analysis in muscle is not a sensitive test for MM in adults. In these patients, abnormal muscle histochemistry appears to be a better predictor ofMM.

Eye movement disorders in myotonic dystrophy type 1

CONCLUSIONS

No definite sign was found of central oculomotor system disorders being independent of saccadic slowing because (1) diminished maximum slow phase velocity of the optokinetic nystagmus (OKNspv) was closely related to saccadic slowing (p<0.01, r=0.59), (2) maximum frequency of optokinetic nystagmus (OKNfq) was normal, (3) visual suppression (VS) change was mild, and (4) the diminished maximum slow phase velocity of the caloric nystagmus (CNspv) seen in some patients is explained by both peripheral and central vestibular involvement. These findings support the extraocular muscle hypothesis.

OBJECTIVE

To assess whether eye movement disorders seen in patients with myotonic dystrophy type 1 (DM1) are caused by central oculomotor system involvement or extraocular muscle damage.

PATIENTS AND METHODS

Oculomotor functions and their correlation with (CTG)n length were studied in 29 DM1 patients and 12 age-matched controls.

RESULTS

Values for saccadic velocity (p<0.005), maximum OKNspv (p<0.005), and maximum CNspv (p<0.01) in the patient group were markedly lower than in the control group. VS of caloric nystagmus in the patient group was slightly lower than that in the controls. No significant difference was found between the two groups in the maximum OKNfq. Patients with greater (CTG)n lengths had lower saccadic velocities (p<0.01, r=0.71).

Coenzyme Q10, exercise lactate and CTG trinucleotide expansion in myotonic dystrophy

Steinert's myotonic dystrophy (DM) is a genetic autosomal dominant disease and the most frequent muscular dystrophy in adulthood. Although causative mutation is recognized as a CTG trinucleotide expansion on 19q13.3, pathogenic mechanisms of multisystem involvement of DM are still under debate. It has been suggested that mitochondrial abnormalities can occur in this disease and deficiency of coenzyme Q 10 (CoQ10) has been considered one possible cause for this. The aim of this investigation was to evaluate, in 35 DM patients, CoQ10 blood levels and relate them to the degree of CTG expansion as well as to the amount of lactate production in exercising muscle as indicator of mitochondrial dysfunction. CoQ10 concentrations appeared significantly reduced with respect to normal controls: 0.85 +/- 0.25 vs. 1.58 +/- 0.28 microg/ml (p < 0.05). Mean values of blood lactate were significantly higher in DM patients than controls (p < 0.05) both in resting conditions (2.9 +/- 0.55 vs. 1.44 +/- 1.11 mmol/L) and at the exercise peak (6.77 +/- 1.79 vs. 4.90 +/- 0.59 mmol/L), while exercise lactate threshold was anticipated (30-50% vs. 60-70% of the predicted normal maximal power output, p < 0.05). Statistical analysis showed that serum CoQ10 levels were significantly (p < 0.05) inversely correlated with both CTG expansion degree and lactate values at exercise lactate threshold level. Our data indicates the occurrence of reduced CoQ10 levels in DM, possibly related to disease pathogenic mechanisms associated with abnormal CTG trinucleotide amplification.

Specific detection of deleted mitochondrial DNA by in situ hybridization using a chimera probe

We report a new method for the specific detection of the mutant mitochondrial DNA (mtDNA) that contains the 4977-bp deletion. We designed an oligonucleotide probe that was designated the 'ATP8/ND5 Chimera' probe: its 5'- and 3'-portions correspond to the ATP8 gene and the ND5 gene, respectively, and its middle portion includes the 13-bp direct repeat sequence that flanks the 4977-bp deletion. By Southern blot analysis, this chimeric probe specifically detected the deleted mtDNA, even in the presence of both normal mtDNA and other mtDNA deletions. The specificity of the probe was further confirmed by in situ hybridization of muscle fibers from patients with Kearns-Sayer syndrome who carry the deleted DNA in the heteroplasmic state. The deleted mtDNA was markedly accumulated in cytochrome-c oxidase (COX)-deficient ragged-red fibers. In tissues where multiple deleted mtDNAs were detected, such as muscle tissues from a patient with myotonic dystrophy and from an aged individual, the in situ hybridization detected a small number of muscle fibers that contained the deleted mtDNA. These results indicate that in situ hybridization using this chimera probe is a useful and specific method for detecting a small amount of deleted mtDNA.

Ocular myopathy without ophthalmoplegia can be a form of mitochondrial myopathy

To characterize muscle pathology in 3 cases affected by ocular myopathy with eyelid ptosis and upper facial weakness, but without ophthalmoplegia, light microscopy and ultrastructural study were performed on levator palpebrae, orbicularis oculi and deltoid muscle biopsies. While levator palpebrae proved uninformative because of the massive fibrous degeneration of muscle, orbicularis oculi biopsies showed histochemical and ultrastructural alterations indicating a mitochondrial involvement, resembling that reported in ocular mitochondrial myopathies (OMM). On the other hand very mild aspecific findings were observed in deltoid. We suggest that these cases with ocular myopathy and without ophthalmoplegia should be considered a partial or initial form of OMM.

Eye movement disorder: an early expression of the myotonic dystrophy gene?

Eye movement recording (EMR) has been performed in 5 asymptomatic myotonic dystrophy (MyD) gene carriers, 7 mildly affected MyD patients, and 23 age- and sex-matched healthy controls. The purpose of the study was to evaluate whether eye movement abnormalities are an early expression of the MyD gene, and to determine the value of this procedure for detection of otherwise asymptomatic gene carriers. EMR did not reveal any abnormalities in the asymptomatic group, but in the mildly affected group showed significantly (P less than 0.01) decreased maximum velocities of the saccades, compared with controls. The results indicate that EMR may aid in the detection of mildly affected MyD patients. However, true presymptomatic diagnosis with EMR has not yet proven possible.