GR gene polymorphism is associated with inter-subject variability in response to glucocorticoids in patients with myasthenia gravis.

BACKGROUND AND PURPOSE: Glucocorticoids (GCs) are the mainstay treatment of myasthenia gravis (MG). However, wide inter-individual variability exists in the response to GCs. METHODS: A Chinese cohort of 257 MG patients treated with GCs was evaluated for the association between 19 single nucleotide polymorphisms in the GR gene and clinical response to the initial 3 month GC therapy. A quantitative MG score decreasing by ≥3 units or becoming zero was defined as sensitivity to GCs. RESULTS: The rs17209237* G allele was less frequent in the GC insensitive group compared with the GC sensitive group [P = 0.013, odds ratio (OR) 0.119]. The rs9324921* A allele was more frequent in the GC insensitive group than in the GC sensitive group (P = 0.046, OR 1.94). Carriers of the rs17209237 G allele were less frequent in the GC insensitive group than in the GC sensitive group (dominant model, P = 0.009). Carriers of the rs9324921 A allele were more frequent in the GC insensitive group than in the GC sensitive group (dominant model, P = 0.037). Multivariate logistic regression revealed that the rs17209237 G allele carrier (P = 0.037, OR 0.12) and disease duration before GC treatment (P = 0.011, OR 3.45) were independent factors that contributed to GC efficacy. CONCLUSION: rs17209237 in the GR gene was identified as an independent factor that contributes to GC efficacy in MG patients. The genetic variations of the GR gene may play a role in predicting response to GC treatment.

Incidence and mortality rates of myasthenia gravis and myasthenic crisis in US hospitals.

OBJECTIVE: To determine the incidence and mortality rates and predictors of death in myasthenia gravis (MG) and MG crisis in a large US cohort. METHODS: Our cohort was identified from the Nationwide Inpatient Sample database for the years 2000 through 2005 using ICD-9-CM codes. MG crisis was identified by the principal diagnosis code or by the presence of respiratory failure. The incidence of MG was stratified by age, ethnicity, and gender. Multivariate logistic regression analysis was used to identify predictors of mortality in MG. For trend analyses of immune intervention, we used the Cochrane-Armitage test. RESULTS: After data cleansing, 5,502 patients with MG were included. In women, the incidence of admission was two to three times higher during the first 5 decades. In men, the incidence of admission was higher during the sixth, seventh, and eighth decades. The annual incidence rate of MG was higher in black women (0.01 per 1,000 persons/year) compared to white women and white and black men (0.009, 0.008, and 0.007 per 1,000 persons/year). The overall in-hospital mortality rate was 2.2%, being higher in MG crisis (4.47%). Older age and respiratory failure were the predictors of death, with adjusted odds ratios of 9.28 (95% confidence interval [CI], 3.31, 26.0) and 3.58 (95% CI, 2.01, 6.38). The trend of i.v. immunoglobulin utilization has increased compared to plasma exchange and thymectomy (p < 0.0001). CONCLUSION: Myasthenia gravis (MG) is still a disease of young women and old men, as reflected by the hospital admission rates. In-hospital mortality of MG is low. Hospital utilization of i.v. immunoglobulin has significantly increased compared to plasma exchange and thymectomy.

Extraocular muscle is defined by a fundamentally distinct gene expression profile.

Skeletal muscle fibers are defined by patterned covariation of key traits that determine contractile and metabolic characteristics. Although the functional properties of most skeletal muscles result from their proportional content of a few conserved muscle fiber types, some, typically craniofacial, muscles exhibit fiber types that appear to lie outside the common phenotypic range. We analyzed gene expression profiles of three putative muscle classes, limb, masticatory, and extraocular muscle (EOM), in adult mice by high-density oligonucleotide arrays. Pairwise comparisons using conservative acceptance criteria identified expression differences in 287 genes between EOM and limb and/or masticatory muscles. Use of significance analysis of microarrays methodology identified up to 400 genes as having an EOM-specific expression pattern. Genes differentially expressed in EOM reflect key aspects of muscle biology, including transcriptional regulation, sarcomeric organization, excitation-contraction coupling, intermediary metabolism, and immune response. These patterned differences in gene expression define EOM as a distinct muscle class and may explain the unique response of these muscles in neuromuscular diseases.

Nitric oxide: biologic effects on muscle and role in muscle diseases.

Nitric oxide is a ubiquitous cell-signaling molecule involved in regulation of numerous homeostatic mechanisms and in mediation of tissue injury. Nitric oxide influences contraction, blood flow, and metabolism, as well as myogenesis. Nitric oxide exerts its influence by activation of guanylate cyclase and nitrosylation of proteins, which include glyceraldehyde-3-phosphate dehydrogenase, the ryanodine receptor and actomyosin ATPase. Skeletal muscle expresses all three isoforms of the nitric oxide synthase, including a muscle-specific splice variant; expression of the isoforms is fiber-type specific and influenced by age and disease. Nitric oxide produced with certain systemic conditions and local inflammation is likely toxic to skeletal muscle, either directly or in reactions with oxygen-derived radicals. Although nitric oxide impacts on many functions in muscle, its effects are subtle, and much work remains to be done to determine its importance in the pathogenesis of muscle diseases.

Nitric oxide synthase expression and effects of nitric oxide modulation on contractility of rat extraocular muscle.

Extraocular muscles (EOMs) are specialized skeletal muscles that are constantly active, generate low levels of force for cross sectional area, have rapid contractile speeds, and are highly fatigue resistant. The neuronal isoform of nitric oxide synthase (nNOS) is concentrated at the sarcolemma of fast-twitch muscles fibers, and nitric oxide (NO) modulates contractility. This study evaluated nNOS expression in EOM and the effect of NO modulation on lateral rectus muscle's contractility. nNOS activity was highest in EOM compared with diaphragm, extensor digitorum longus, and soleus. Neuronal NOS was concentrated to the sarcolemma of orbital and global singly innervated fibers, but not evident in the multi-innervated fibers. The NG-nitro-L-arginine methyl ester (L-NAME, a NOS inhibitor), increased submaximal tetanic and peak twitch forces. The NO donors S-nitroso-N-acetylcysteine (SNAC) and spermineNONOate reduced submaximal tetanic and peak twitch forces. The effect of NO on the contractile force of lateral rectus muscle is greater than previously observed on other skeletal muscle. NO appears more important in modulating contraction of EOM compared with other skeletal muscles, which could be important for the EOM's specialized role in generation of eye movements.

T cell recognition of muscle acetylcholine receptor in ocular myasthenia gravis.

We examined the proliferative response of blood CD4(+) cells to muscle acetylcholine receptor (AChR) subunits and the epitope repertoire of the epsilon and gamma subunits, in ocular myasthenia gravis (oMG) patients and healthy subjects. oMG patients seldom recognized all subunits. The frequency and intensity of recognition was the same for all subunits, irrespective of the disease duration. The responses in oMG were lower than in generalized myasthenia gravis. Healthy subjects had frequent, low responses to one or more subunits. oMG patients recognized several epitopes on the gamma and epsilon subunits, that partially overlapped those recognized in gMG. The subunits and epitopes recognized by individual oMG patients changed over time. Thus, oMG patients have minimal and unstable sensitization of anti-AChR CD4(+) cells, in agreement with their low and inconsistent synthesis of anti-AChR antibody.

Nitric oxide myotoxicity is age related.

Nitric oxide (NO) is generated under normal conditions in skeletal muscle and acts as a messenger that influences contractility, blood flow, and glucose metabolism. Excess NO generation may occur in pathological states, in particular inflammatory conditions. We demonstrate that incubation of rat extensor digitorum longus muscle with the NO donor, S-nitrosocysteine, leads to release of creatine kinase, a marker of muscle injury after a delay of 90 min. Muscle of old animals was more sensitive to the NO donor. Light microscopic analysis does not show abnormalities, with the exception of an increase in interfiber distance. Histological staining identified no pathological elevations of calcium. The study demonstrates the direct toxicity of NO to skeletal muscle, and that muscle of older animals is differentially susceptible to NO toxicity.

Eye muscle sparing by the muscular dystrophies: lessons to be learned?

The devastating consequences of the various muscular dystrophies are even more obvious when a muscle or muscle group is spared. The study of the exceptional cell or tissue responses may prove to be of considerable value in the analysis of disease mechanisms. The small muscles responsible for eye movements, the extraocular muscles, have functional and morphological characteristics that set them aside from other skeletal muscles. Notably, these muscles are clinically unaffected in Duchenne/Becker, limb-girdle, and congenital muscular dystrophies, pathologies due to a broken mechanical or signaling linkage between the cytoskeleton and the extracellular matrix. Uncovering the strategies used by the extraocular muscles to "naturally" protect themselves in these diseases should contribute to knowledge of both pathogenesis and treatment. We propose that careful investigation of the cellular determinants of extraocular muscle-specific properties may provide insights into how these muscles avoid or adapt to the cascade of events leading to myofiber degeneration in the muscular dystrophies.

Autoantibody testing in neuromuscular disorders, part I: peripheral neuropathies.

A large number of autoantibodies with reactivity to glycoconjugate components of peripheral nerve have been described in the last two decades. Serum autoantibodies to gangliosides and glycoproteins have received the most attention and have been implicated in a variety of sensory and motor neuropathy syndromes. Commercial laboratories market panels of autoantibodies and heavily promote them to clinicians as an important step in the evaluation of neuropathies of unclear etiology. Certainly, detection of these autoantibodies raises the possibility that peripheral nerve disturbance is immune-mediated and that immunosuppressive therapy may be effective treatment. However, most of these autoantibodies do not define a specific clinical syndrome, and patients with and without the antibody may present with an identical neuropathy pattern. Furthermore, a pathogenic role remains to be proven for essentially all of the autoantibodies. In this review, we summarize the peripheral nerve syndromes associated with autoantibodies and present a rational approach to autoantibody testing in routine practice.

Autoantibody Testing in Neuromuscular Disorders, Part II: Neuromuscular Junction, Hyperexcitability, and Muscle Disorders.

In myasthenia gravis and Lambert-Eaton myasthenic syndrome, antibodies against ion channels are clearly related to pathogenesis; however, in the inflammatory myopathies, a link to disease causation is not as evident, except as a marker of autoimmunity. Autoantibody testing in patients with neuromuscular junction and skeletal muscle disorders is useful in diagnosis and, in some diseases, prognosis. This review briefly summarizes methods of autoantibody testing and then proceeds to a clinician-friendly guide to autoantibody testing of neuromuscular junction disorders, hyperexcitability syndromes, and inflammatory muscle diseases.

Nitric oxide synthase in aging rat skeletal muscle.

The neuronal isoform of nitric oxide synthase (NOS) is expressed at high concentrations in skeletal muscle, and NO influences muscle contractility, glucose utilization, and free radical damage or protection. NOS activity and expression was evaluated in extensor digitorum longus (EDL), soleus, and diaphragm of 8 and 24 month old Fisher 344 rats. In 8-month-old animals, NOS activity was highest in EDL, which contained the highest percentage of NOS containing fibers, and was lowest in soleus. NOS activity and percentage of NOS containing fibers was significantly reduced in all muscle groups with age. To determine if NOS reduction correlated with free radical injury the level of lipid peroxidation, as measured by malonaldehyde equivalents, was determined. With age lipid peroxidation increased in EDL, was reduced in diaphragm, and showed a non-significant change in soleus. Therefore, a straightforward reduction of NOS activity does not correlate with lipid peroxidation. The reduction of NOS with age in skeletal muscle may be most significant for muscle metabolism and force production and be of limited significance for free radical metabolism.

Heme oxygenase-2 expression at rat neuromuscular junctions.

The neuromuscular junction is specialized for rapid transmission of electrical signals. Nitric oxide synthase (NOS) is concentrated at the junction, and NO modulates transmission and could influence signaling pathways. Increasing evidence suggests that carbon monoxide (CO) serves as a neurotransmitter, and heme oxygenase (HO), the enzyme that catalyzes the formation of CO, is often colocalized with NOS. Immunoreactivity for HO-2 was present at rat neuromuscular junctions of leg muscles and persisted in denervated muscle indicating the localization of the enzyme to the postsynaptic surface. In contrast, HO-2 immunoreactivity was absent from the en grappe and orbital en plaque endplates of extraocular muscle (EOM), while only the global en plaque endplates possessed HO-2 immunoreactivity. The difference between EOM and leg endplates may arise from EOM's unique physiology. The presence of HO-2 at neuromuscular junctions suggests CO could serve as a pre- and post-synaptic messenger.

Neuromuscular transmission defect caused by carbamazepine.

The clinical effect of carbamazepine (CBZ) on neuromuscular transmission is described in two children who presented in coma with diffuse hypotonia and areflexia following CBZ overdose. Repetitive nerve stimulation (RNS) showed a decremental response only at high-frequency stimulation. With supportive care, the patients made an uneventful recovery. Follow-up RNS was normal. This is the first report of a clinically evident neuromuscular transmission defect produced by CBZ. We postulate that CBZ's known effect on decreasing sodium channel depolarization produced a defect in neuromuscular transmission. The report emphasizes the contribution of RNS in the evaluation of coma of uncertain etiology, particularly in cases of possible intoxication, and the potential for CBZ to compromise neuromuscular transmission in normal individuals or in patients with a decreased neuromuscular transmission safety factor.

Disorders of neuromuscular junction ion channels.

Ion channel defects produce a clinically diverse set of disorders that range from cystic fibrosis and some forms of migraine to renal tubular defects and episodic ataxias. This review discusses diseases related to impaired function of the skeletal muscle acetylcholine receptor and calcium channels of the motor nerve terminal. Myasthenia gravis is an autoimmune disease caused by antibodies directed toward the skeletal muscle acetylcholine receptor that compromise neuromuscular transmission. Congenital myasthenias are genetic disorders, a subset of which are caused by mutations of the acetylcholine receptor. Lambert-Eaton myasthenic syndrome is an immune disorder characterized by impaired synaptic vesicle release likely related to a defect of calcium influx. The disorders will illustrate new insights into synaptic transmission and ion channel structure that are relevant for all ion channel disorders.