Spinal muscular atrophy (SMA) is a neurodegenerative disease associated with severe muscle atrophy and weakness in the limbs and trunk. We report interim efficacy and safety outcomes as of March 29, 2019 in 25 children with genetically diagnosed SMA who first received nusinersen in infancy while presymptomatic in the ongoing Phase 2, multisite, open-label, single-arm NURTURE trial. Fifteen children have two SMN2 copies and 10 have three SMN2 copies. At last visit, children were median (range) 34.8 [25.7-45.4] months of age and past the expected age of symptom onset for SMA Types I or II; all were alive and none required tracheostomy or permanent ventilation. Four (16%) participants with two SMN2 copies utilized respiratory support for ≥6 h/day for ≥7 consecutive days that was initiated during acute, reversible illnesses. All 25 participants achieved the ability to sit without support, 23/25 (92%) achieved walking with assistance, and 22/25 (88%) achieved walking independently. Eight infants had adverse events considered possibly related to nusinersen by the study investigators. These results, representing a median 2.9 years of follow up, emphasize the importance of proactive treatment with nusinersen immediately after establishing the genetic diagnosis of SMA in presymptomatic infants and emerging newborn screening efforts.

The phenotypes in myotonic dystrophy types 1 and 2 (DM1 and DM2) are similar, suggesting a shared pathophysiologic mechanism. DM1 is caused by expansion of a CTG repeat in the DMPK gene. Pathogenic effects of this mutation are likely to be mediated, at least in part, by the expanded CUG repeat in mutant mRNA. The mutant transcripts are retained in the nucleus in multiple discrete foci. We investigated the possibility that DM2 is also caused by expansion of a CTG repeat or related sequence. Analysis of DNA by repeat expansion detection methods, and RNA by ribonuclease protection, did not show an expanded CTG or CUG repeat in DM2. However, hybridization of muscle sections with fluorescence-labeled CAGrepeat oligonucleotides showed nuclear foci in DM2 similar to those seen in DM1. Nuclear foci were present in all patients with symptomatic DM1 (n = 9) or DM2 (n = 9) but not in any disease controls or healthy subjects (n = 23). The foci were not seen with CUG-or GUC-repeat probes. Foci in DM2 were distinguished from DM1 by lower stability of the probe-target duplex, suggesting that a sequence related to the DM1 CUG expansion accumulates in the DM2 nucleus. Muscleblind proteins, which interact with expanded CUG repeats in vitro, localized to the nuclear foci in both DM1 and DM2. These results support the idea that nuclear accumulation of mutant RNA is pathogenic in DM1, suggest that a similar disease process occurs in DM2, and point to a role for muscleblind in the pathogenesis of both disorders.

Periodic paralyses (PPs) are rare neuromuscular disorders caused by mutations in skeletal muscle sodium, calcium, and potassium channel genes. PPs include hypokalemic paralysis, hyperkalemic paralysis, and Andersen‐Tawil syndrome. Common features of PP include autosomal dominant inheritance, onset typically in the first or second decades, episodic attacks of flaccid weakness, which are often triggered by diet or rest after exercise. Diagnosis is based on the characteristic clinic presentation then confirmed by genetic testing. In the absence of an identified genetic mutation, documented low or high potassium levels during attacks or a decrement on long exercise testing support diagnosis. The treatment approach should include both management of acute attacks and prevention of attacks. Treatments include behavioral interventions directed at avoidance of triggers, modification of potassium levels, diuretics, and carbonic anhydrase inhibitors. Muscle Nerve 57: 522–530, 2018

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Myotonic dystrophy types 1 (DM1) and 2 (DM2) are similar yet distinct autosomal-dominant disorders characterized by muscle weakness, myotonia, cataracts, and multiple organ involvement, including the brain. One key difference between DM1 and DM2 is that a congenital form has been described for DM1 only. Expression of RNA transcripts containing pathogenic repeat lengths produces defects in alternative splicing of multiple RNAs, sequesters specific repeat-binding proteins, and ultimately leads to developmentally inappropriate splice products for a particular tissue. Whether brain pathology in its entirety in adult DM1 and DM2 is caused by interference in RNA processing remains to be determined. This review focuses on the similarities and differences between DM1 and DM2 with respect to neuropsychological, neuropathological, and neuroimaging data relating to cerebral involvement, with special emphasis on the clinical relevance and social consequences of such involvement.

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Andersen syndrome (AS) is a rare, inherited disorder characterized by periodic paralysis, long QT (LQT) with ventricular arrhythmias, and skeletal developmental abnormalities. We recently established that AS is caused by mutations in KCNJ2, which encodes the inward rectifier K+ channel Kir2.1. In this report, we characterized the functional consequences of three novel and seven previously described KCNJ2 mutations using a two-microelectrode voltage-clamp technique and correlated the findings with the clinical phenotype. All mutations resulted in loss of function and dominant-negative suppression of Kir2.1 channel function. In mutation carriers, the frequency of periodic paralysis was 64% and dysmorphic features 78%. LQT was the primary cardiac manifestation, present in 71% of KCNJ2 mutation carriers, with ventricular arrhythmias present in 64%. While arrhythmias were common, none of our subjects suffered sudden cardiac death. To gain insight into the mechanism of arrhythmia susceptibility, we simulated the effect of reduced Kir2.1 using a ventricular myocyte model. A reduction in Kir2.1 prolonged the terminal phase of the cardiac action potential, and in the setting of reduced extracellular K+, induced Na+/Ca2+ exchanger–dependent delayed afterdepolarizations and spontaneous arrhythmias. These findings suggest that the substrate for arrhythmia susceptibility in AS is distinct from the other forms of inherited LQT syndrome

Context Non-dystrophic myotonias (NDM) are rare diseases caused by mutations in skeletal muscle ion channels. Patients experience delayed muscle relaxation causing functionally-limiting stiffness and pain. Mexiletine-induced sodium channel blockade reduced myotonia in case studies and one single blind trial. As is common in rare diseases, larger studies of safety and efficacy have not previously been considered feasible. Objective To determine the effects of mexiletine for symptoms and signs of myotonia in NDM. Design, Setting, and Participation Fifty-nine patients with NDM participated in a randomized, double-blind, placebo-controlled two-period crossover study conducted between December 23, 2008 and March 30, 2011 at 7 neuromuscular referral centers in 4 countries, as part of the NIH-funded Rare Disease Clinical Research Network. Intervention Oral 200 mg mexiletine or placebo capsules three times daily for 4 weeks, followed by the opposite intervention for 4 weeks, with 1 week wash-out between periods. Main Outcome Measures Patient-reported stiffness recorded on an interactive voice response diary (IVR) was the primary endpoint (1 ‘minimal’ to 9 ‘worst ever experienced’). Secondary endpoints included IVR-reported changes in pain, weakness, and tiredness, clinical myotonia assessment, quantitative grip myotonia, Individualized Neuromuscular Quality of Life (INQoL, percent of maximal detrimental impact), SF-36, electrophysiological exercise testing, and needle EMG. Results Mexiletine significantly improved patient-reported stiffness on the IVR. Because of a statistically significant interaction between treatment and period for this outcome, primary endpoint is presented by period (period 1 means were mexiletine 2.53 versus placebo 4.21, difference −1.68, 95% Confidence Interval [CI] −2.66, −0.706, P<0.001; period 2 means were mexiletine 1.60 versus placebo 5.27, difference −3.68, 95% CI −3.85, −0.139, P=0.04). Mexiletine improved the INQoL QOL score (mexiletine 14.0, placebo 16.7, difference −2.69, 95% CI −4.07, −1.30, P<0.001) and decreased handgrip myotonia on clinical exam (seconds: mexiletine 0.164, placebo 0.494, difference −0.330, 95% CI −0.633, −0.142, P<0.001). The most common adverse effect was gastrointestinal (9 mexiletine, 1 placebo). Two participants experienced transient cardiac effects that did not require stopping the study (1 placebo, 1 mexiletine). One serious adverse event was determined to be not study-related. Conclusion In this preliminary study of patients with NDM, the use of mexiletine compared with placebo resulted in improved patient-reported stiffness over 4 weeks of treatment, despite some concern about the maintenance of blinding. Trial Registration Clinicaltrials.gov identifier: NCT 00832000