Distinct pathological signatures in human cellular models of myotonic dystrophy subtypes.

Myotonic dystrophy (DM) is the most common autosomal dominant muscular dystrophy and encompasses both skeletal muscle and cardiac complications. DM is nucleotide repeat expansion disorder in which type 1 (DM1) is due to a trinucleotide repeat expansion on chromosome 19 and type 2 (DM2) arises from a tetranucleotide repeat expansion on chromosome 3. Developing representative models of DM in animals has been challenging due to instability of nucleotide repeat expansions, especially for DM2, which is characterized by nucleotide repeat expansions often greater than 5,000 copies. To investigate mechanisms of human DM, we generated cellular models of DM1 and DM2. We used regulated MyoD expression to reprogram urine-derived cells into myotubes. In this myogenic cell model, we found impaired dystrophin expression, in the presence of muscleblind-like 1 (MBNL1) foci, and aberrant splicing in DM1 but not in DM2 cells. We generated induced pluripotent stem cells (iPSC) from healthy controls and DM1 and DM2 subjects, and we differentiated these into cardiomyocytes. DM1 and DM2 cells displayed an increase in RNA foci concomitant with cellular differentiation. iPSC-derived cardiomyocytes from DM1 but not DM2 had aberrant splicing of known target genes and MBNL sequestration. High-resolution imaging revealed tight association between MBNL clusters and RNA foci in DM1. Ca2+ transients differed between DM1- and DM2 iPSC-derived cardiomyocytes, and each differed from healthy control cells. RNA-sequencing from DM1- and DM2 iPSC-derived cardiomyocytes revealed distinct misregulation of gene expression, as well as differential aberrant splicing patterns. Together, these data support that DM1 and DM2, despite some shared clinical and molecular features, have distinct pathological signatures.

Analysis of extracellular mRNA in human urine reveals splice variant biomarkers of muscular dystrophies.

Urine contains extracellular RNA (exRNA) markers of urogenital cancers. However, the capacity of genetic material in urine to identify systemic diseases is unknown. Here we describe exRNA splice products in human urine as a source of biomarkers for the two most common forms of muscular dystrophies, myotonic dystrophy (DM) and Duchenne muscular dystrophy (DMD). Using a training set, RT-PCR, droplet digital PCR, and principal component regression, we identify ten transcripts that are spliced differently in urine exRNA from patients with DM type 1 (DM1) as compared to unaffected or disease controls, form a composite biomarker, and develop a predictive model that is 100% accurate in our independent validation set. Urine also contains mutation-specific DMD mRNAs that confirm exon-skipping activity of the antisense oligonucleotide drug eteplirsen. Our results establish that urine mRNA splice variants can be used to monitor systemic diseases with minimal or no clinical effect on the urinary tract.

Creatine monohydrate supplementation does not increase muscle strength, lean body mass, or muscle phosphocreatine in patients with myotonic dystrophy type 1.

Creatine monohydrate (CrM) supplementation may increase strength in some types of muscular dystrophy. A recent study in myotonic muscular dystrophy type 1 (DM1) did not find a significant treatment effect, but measurements of muscle phosphocreatine (PCr) were not performed. We completed a randomized, double-blind, cross-over trial using 34 genetically confirmed adult DM1 patients without significant cognitive impairment. Participants received CrM (5 g, approximately 0.074 g/kg daily) and a placebo for each 4-month phase with a 6-week wash-out. Spirometry, manual muscle testing, quantitative isometric strength testing of handgrip, foot dorsiflexion, and knee extension, handgrip and foot dorsiflexion endurance, functional tasks, activity of daily living scales, body composition (total, bone, and fat-free mass), serum creatine kinase activity, serum creatinine concentration and clearance, and liver function tests were completed before and after each intervention, and muscle PCr/beta-adenosine triphosphate (ATP) ratios of the forearm flexor muscles were completed at the end of each phase. CrM supplementation did not increase any of the outcome measurements except for plasma creatinine concentration (but not creatinine clearance). Thus, CrM supplementation at 5 g daily does not have any effects on muscle strength, body composition, or activities of daily living in patients with DM1, perhaps because of a failure of the supplementation to increase muscle PCr/beta-ATP content.

Patients with dystrophinopathy show evidence of increased oxidative stress.

Duchenne muscular dystrophy (DMD) is associated with an increase in oxidative stress. We measured 24 h 8-hydroxy-2'-deoxyguanosine (8-OHdG) excretion in 24 patients with MD (DMD + Becker's MD), 23 with myotonic dystrophy, and 34 healthy controls. The 8-OHdG/creatinine ratio was higher in patients with dystrophinopathy ( upward arrow 48%, p <.01) but not myotonic dystrophy, as compared to healthy controls. These results indicate that 8-OHdG excretion can be used as a marker of oxidative stress in clinical trials with dystrophinopathy.

Lack of relationship of hypogonadism to muscle wasting in myotonic dystrophy.

Myotonic dystrophy is frequently associated with testicular atrophy. Since androgens may play a role in the maintenance of muscle mass, we have studied the levels of plasma testosterone and gonadotropins and of urinary 17-ketosteroids in 22 men with myotonic dystrophy, 36 normal men, and 16 men (control group) with muscle wasting. Results were correlated with muscle mass as estimated by creatinine excretion and total body potassium. Patients with myotonic dystrophy had significantly lower testosterone and higher gonadotropin levels than normal, and these changes were progressive in longitudinal studies. Testosterone levels were also lower than normal in disease control subjects. There was no correlation between low testosterone levels and diminished muscle mass in either myotonic dystrophy or disease control patients. The low plasma concentration of testosterone in men with myotonic dystrophy and other neuromuscular diseases does not appear to be directly related to their muscle wasting. This study does not exclude the possibility that an alteration in testosterone receptor or tissue effects may contribute to a loss of muscle tissue.

3-methylhistidine excretion in myotonic dystrophy.

3-Methylhistidine (3-MH) excretion reflects the rate of muscle protein catabolism, since 3-MH occurs almost exclusively in muscle actin and myosin and is not reutilized or catabolized. We studied 3-MH excretion in 9 patients with myotonic dystrophy, 8 normals, and 10 disease controls with Duchenne dystrophy and other disorders. 3-MH excretion was expressed relative to muscle mass as determined by both urinary creatinine and total body potassium (40K method). Absolute 3-MH excretion was decreased in myotonic dystrophy patients but was normal when related to muscle mass. The finding of normal 3-MH excretion in myotonic dystrophy suggests that the muscle wasting in this disorder results from impaired anabolic processes rather than accelerated muscle destruction.

3-Methylhistidine excretion as an index of myofibrillar protein catabolism in neuromuscular disease.

Myofibrillar protein catabolism has been calculated in a variety of neuromuscular diseases from the amount of 3-methylhistidine excreted in the urine. It was found to be significantly raised in Duchenne type muscular dystrophy, motor neurone disease, polymyositis, and thyrotoxic myopathy. In Becker type muscular dystrophy the level was slightly raised. It was normal in scapuloperoneal and limb girdle dystrophy, dystrophia myotonica, extrapyramidal disease, and multiple sclerosis. It was significantly decreased in hypothyroid myopathy.