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.

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.

The gamma-subunit of the acetylcholine receptor is not expressed in the levator palpebrae superioris.

In extraocular muscle (EOM), expression of the gamma-subunit, which is associated with the fetal-type acetylcholine receptor (AChR), may offer a differential target for immune-mediated damage and could explain the preponderance of ocular manifestations caused by myasthenia gravis (MG). Using Poly(A)+ RNA hybridization, we investigated expression of the gamma-subunit in bovine levator palpebrae superioris (LP), a muscle also differentially involved by MG. There were no transcripts of the gamma-subunit of the AChR, but the epsilon-subunit, associated with the adult-type AChR, was present. The results indicate that the susceptibility of LP to MG is not mediated by gamma-subunit expression and suggest that multiterminal fibers in EOM may be the site of gamma-subunit expression.

Expression of acetylcholine receptor isoforms at extraocular muscle endplates.

PURPOSE: To determine the expression of fetal and adult acetylcholine receptor (AChR) isoforms among extraocular muscle (EOM) en plaque and en grappe endplates. METHODS: Antibodies against peptide fragments of the gamma- and epsilon-subunits of the fetal and adult AChRs and alpha-bungarotoxin were used in immunofluorescence experiments to stain rat neonatal leg, adult diaphragm, and extraocular rectus muscle endplates. RESULTS: Anti-epsilon antibodies intensely stained diaphragm endplates weakly stained rare neonatal endplates. Anti-gamma antibodies stained neonatal, but not diaphragm, endplates. Anti-epsilon antibodies bound to all en plaque and en grappe endplates of extraocular muscle. Anti-gamma antibodies bound to global and orbital en grappe endplates. All en plaque endplates of the orbital region and a subset of endplates in the global region stained with anti-gamma antibodies. CONCLUSIONS: All en grappe endplates and certain en plaque endplates of EOM are the only mature endplates that coexpress the adult and fetal AChR isoforms. The expression of both isoforms may be important to determine contractile properties, protein expression regulation, and EOM susceptibility to myasthenia gravis.

Nitric oxide synthase is concentrated at the skeletal muscle endplate.

NO performs a wide array of cell signaling functions. Neuronal NO synthase (nNOS) immunoreactivity and nicotinamide adenine dinucleotide phosphate diaphorase (NDP) activity, a marker of nNOS, were concentrated at adult rat neuromuscular junctions and persisted in denervated muscle indicating the localization of the enzyme to the postsynaptic surface. The concentration of nNOS at the muscle endplate suggests NO could serve as a messenger pre- and postsynapticly.

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.

Ryanodine receptor gene expression thymomas.

Ryanodine receptor (RyR) antibodies are present in sera of myasthenics with thymoma, and their titer correlates with morbidity and mortality. We investigated whether skeletal muscle RyR expression in thymic tissues could be the source of immune sensitization to the RyR. Skeletal muscle RyR gene expression was investigated using reverse transcription followed by semiquantitative polymerase chain reaction. Hyperplastic and normal thymuses expressed significant levels of RyR, but RyR gene transcripts was statistically less likely in thymomas than in hyperplastic and normal thymus (P < 0.05). The presence of RyR transcripts in thymomas did not correlate with myasthenic manifestations, thymic pathology, or serum RyR antibodies. We conclude that the skeletal muscle RyR in thymoma is not the inciting antigen for immune sensitization to RyR epitopes in thymoma-associated myasthenia gravis.