An international, phase III, randomized trial of mycophenolate mofetil in myasthenia gravis.

BACKGROUND: This prospective, randomized, double-blind, placebo-controlled, phase III trial assessed the efficacy, safety, and tolerability of mycophenolate mofetil (MMF) as a steroid-sparing agent in patients with myasthenia gravis (MG). METHODS: Patients with acetylcholine receptor antibody-positive class II-IVa MG (MG Foundation of America [MGFA] criteria) taking corticosteroids for at least 4 weeks were randomized to MMF (2 g/day) or placebo for 36 weeks. The primary endpoint was a composite measure defined as achievement of minimal manifestations or pharmacologic remission (MGFA post-intervention status), with reduction of corticosteroid dose on a set schedule. Secondary endpoints included disease severity, quality-of-life scores, and safety. RESULTS: A total of 44% of MMF-treated (n = 88) and 39% of placebo-receiving (n = 88) patients achieved the primary endpoint (p = 0.541). Improvements in mean quantitative MG, MG activities of daily living, and 36-item Short-Form health survey scores were similar in both groups. Numbers of adverse events were similar in both groups. The most commonly reported adverse events in the MMF-treated group were headache (12.5%) and worsening of MG (11.4%), and in the placebo group, worsening of MG (20.5%) and diarrhea (10.2%). CONCLUSIONS: Initiation of mycophenolate mofetil (MMF) treatment was not superior to placebo in maintaining myasthenia gravis (MG) control during a 36-week schedule of prednisone tapering. There were no significant differences in the primary or secondary endpoints between the study groups. MMF was well tolerated and adverse events were consistent with previous studies. Experience from this large, international, multicenter, phase III study employing full MG Foundation of America guidelines will aid the design of future MG studies.

Thymectomy in the treatment of ocular myasthenia gravis.

BACKGROUND: Thymectomy is an effective and accepted treatment for myasthenia gravis, but thymectomy for ocular myasthenia gravis (Osserman stage I) is controversial. OBJECTIVE: To assess the efficacy and propriety of thymectomy for the treatment of ocular myasthenia gravis. METHODS: We conducted a review and follow-up of all patients who had thymectomy for the treatment of ocular myasthenia gravis between 1970 and 1998 at the University of California, Davis, Medical Center, and the University of Rome, "La Sapienza," Rome, Italy. Patient response to thymectomy was categorized as follows: cured, patients who became symptom-free and required no further medication; improved, patients who required less medication and whose symptoms were less severe; unchanged, patients whose symptoms and medications were the same; worse, patients who had more severe symptoms, needed more medication, or died. RESULTS: Sixty-one patients (mean age 37 years; range 14-73 years) were followed up for a mean duration of 9 years (range 0.5-29 years). Ocular myasthenia gravis with mixed and cortical thymomas, stages I to IV, occurred in 12 patients, and ocular myasthenia without thymomas occurred in 49 patients. Transsternal thymectomy (n = 55) and transcervical thymectomy (n = 6) resulted in cure in 31 (51%) patients, improvement in 12 (20%) patients, no change in 16 (26%) patients, and worsening of symptoms (including 1 postoperative death) in 2 patients. Patient outcomes were statistically independent of the duration of preoperative symptoms (mean 9.5 months), patient age, or the presence or absence of thymoma. In patients with ocular myasthenia, 70% were cured or improved after thymectomy; in the subgroup of patients with ocular myasthenia and thymoma, 67% were cured or improved. CONCLUSION: Thymectomy is an effective and safe treatment for patients with ocular myasthenia gravis.

Presynaptic congenital myasthenic syndrome due to quantal release deficiency.

OBJECTIVE: To provide clinical, electrophysiologic, and ultrastructural findings in three patients with a presynaptic congenital myasthenic syndrome (CMS). BACKGROUND: Familial infantile myasthenia and paucity of synaptic vesicles are the only two fully characterized CMS. We are describing here three patients with another form of presynaptic CMS characterized by deficiency of the action potential-dependent release without reduction of the spontaneous release of neurotransmitter from the nerve terminal. METHODS: The authors performed electromyography and anconeus muscle biopsies that included intracellular recordings and electron microscopy of the neuromuscular junction in three patients with presynaptic CMS. They also sequenced part of the P/Q-calcium alpha(1)-subunit gene (CACNA1A) and the acetylcholine receptor subunit (AChR) genes in these patients. RESULTS: In these patients there were additional neurologic findings including nystagmus and ataxia. In all three patients the end-plate potential quantal content (m) was markedly reduced but neither the amplitudes nor the frequencies of miniature end-plate potentials were diminished. Ultrastructurally, postsynaptic end-plate folds, nerve terminal size, and synaptic vesicle number were normal but double-membrane-bound sacs containing synaptic vesicles were present in the nerve terminal of all three patients. The screening of reported pathogenic mutations in the CACNA1A and a mutational analysis of AChR subunit genes were negative. CONCLUSION: This form of CMS appears to result only from a deficiency of the quantal release of neurotransmitter that may be due to an abnormal calcium mechanism or impaired endocytosis and recycling of synaptic vesicles.

Differential surface accessibility of alpha(187-199) in the Torpedo acetylcholine receptor alpha subunits.

We have probed the surface accessibility of residues alpha187 to alpha199 of the Torpedo acetylcholine receptor with monoclonal antibody 383C, which binds uniquely to these residues. However, 383C binds to only one of the two alpha subunits in the membrane-bound receptor, neither of the two subunits in carbamylcholine-desensitized receptor, and to both alpha subunits in Triton X-100 solubilized receptor. The kinetics of association and dissoci-ation of 383C with the peptide alpha(183-199) compared to those with the membrane-bound receptor suggest that all but a single hydrogen bond of affinity derives from contacts between this peptide and the monoclonal antibody paratope. Inhibition of 383C binding by alpha-bungarotoxin selectively directed to the alpha subunit correlated with the high-affinity d-tubocurarine binding site, along with a lack of inhibition by alpha-bungarotoxin directed to the alpha subunit correlated with the low-affinity d-tubocurarine binding site, suggests that the 383C epitope on the membrane-bound receptor resides on the alpha subunit associated with the high-affinity d-tubocurarine binding site. The results presented here suggest a structural basis for the differences between the two receptor acetylcholine binding sites.

Mapping the mAb 383C epitope to alpha 2(187-199) of the Torpedo acetylcholine receptor on the three-dimensional model.

Monoclonal antibody 383C is an anti-acetylcholine receptor antibody whose binding to the receptor is blocked by alpha-bungarotoxin and by carbamylcholine. Monoclonal antibody 383C binds to the alpha subunit of the Torpedo acetylcholine (ACh) receptor as well as to its V8-protease 20 kDa fragment that possesses the affinity alkylatable Cys192/193. In an epitope scanning experiment spanning the N-terminal 211 amino acid residues of the alpha subunit, 383C binds uniquely to three overlapping peptides; alpha(184-196), alpha(187-199) and alpha(190-202). These peptides span a cluster of amino acid residues implicated in the binding of acetylcholine, including Cys192/193. To map the location of these residues on the three-dimensional model of the ACh receptor, we have employed a combination of X-ray diffraction from oriented complexes of 383C with ACh receptor-enriched membrane vesicles and electron microscopy of negatively stained tubular arrays of 383C/receptor complexes. The X-ray diffraction study finds extra electron density in the presence of 383C centered 35 A above the synaptic side phosphate head groups. The electron micrographic images display extra stain exclusion from the antibody at a site adjacent to the alpha2 subunit on the periphery of the rosette clockwise to the alpha2 vertex. This mapping localizes several residues of the ACh receptor alpha subunit involved in the binding of acetylcholine. Despite these residues being present in both alpha subunits, only the alpha2 subunit is decorated with this monoclonal antibody.

Cluster of wound botulism in California: clinical, electrophysiologic, and pathologic study.

Over a period of 15 months we have seen 6 patients with long-standing history of subcutaneous heroin injections who experienced acute blurred vision, dysphagia, dysarthria, and generalized weakness. Decreased or absent deep tendon reflexes, pupillary abnormalities, incremental responses to fast repetitive nerve stimulation, and positive serology for Clostridia botulinum toxin A were found, but not in all cases. Muscle biopsies showed variable signs of neurogenic atrophy. In vitro electrophysiology studies revealed decreased end-plate potentials quantal content, confirming the presynaptic nature of the disorder. Mechanical ventilation was required in 5 patients. Half of the patients were treated with polyvalent antitoxiin. Prognosis was favorable, though recovery was slow. In conclusion, acute bulbar weakness with visual symptoms in patients with subcutaneous heroin abuse strongly suggets the possibility of wound botulism. High diagnostic suspicion combined with histology and in vitro electrophysiology confirmation of presynaptic failure, especially in seronegative cases, may significantly improve morbidity.

Myasthenia gravis: pathogenesis and treatment.

Much is known about the pathogenesis of MG. This information has provided a rational basis for treatment of the disease, which is very effective. On the other hand, present treatments are limited by significant adverse effects. Our knowledge of pathogenesis also provides clues for the development of new, effective, but less toxic treatments. In addition, knowledge of the autoimmune mechanisms in MG may be useful in devising better treatments for the many human autoimmune diseases that are less well understood.

Pathogenesis of hyperacute experimental autoimmune myasthenia gravis. Acetylcholine receptor/cholinergic site/receptor function/autoimmunity.

Three mAbs, mAbs 249E, 370, and 383C, directed against the alpha-bungarotoxin (alpha BgTx) binding site of the acetylcholine receptor (AChR) induce a hyperacute form of experimental autoimmune myasthenia gravis (EAMG), characterized by death within hours of mAb injection. To analyze the mechanisms of this effect, purified AChR-mAb complexes were investigated for their ability to bind the cholinergic agonist carbamoylcholine and to undergo agonist-induced activation of the cholinergic ionophore. The three mAbs inhibited carbamylcholine binding, and, conversely, their binding to AChR was inhibited by carbamylcholine. All three completely inhibited carbamylcholine-induced T1+ influxes to AChR-rich vesicles. These data indicate that the severe hyperacute EAMG induced by these mAbs results from blockage of AChR function and that the role of such potent Abs (even if present in small amounts) in the pathogenesis of human myasthenia gravis deserves further investigation.

Acquired myasthenia gravis. Immunopathology.

Much of the remarkable advance in our understanding of the immunopathology of MG relates to the availability of two gifts of nature that permit the ready purification of the antigen, AChR. Immunization of experimental animals with AChR has led to the development of the extremely faithful animal model, EAMG. Analysis of both EAMG and MG has revealed that the effector agents in this autoimmune disease are anti-AChR antibodies, whose production is regulated by anti-AChR CD4+ T cells. The pathogenic effects on neuromuscular transmission are mediated by antibody-induced antigenic modulation of end-plate AChR, end-plate membrane destruction through complement fixation and recruitment of inflammatory cells, and antibody-induced blockade of the function of the remaining AChR molecules. The origin of MG remains unknown. One theory proposes that dysregulation of the thymic control of tolerance plays an important role. An alternative hypothesis is that tolerance is broken as the result of a vigorous immune response directed against an invading microorganism that expresses a molecule that is similar to AChR, so-called molecular mimicry. This "normal" response eventually cross-reacts with self-AChR, resulting in the autoimmune damage. Our current knowledge of MG has suggested a number of possible sites of therapeutic intervention that are under active study. Future information concerning the origin of the disease will likely be useful in the design of more effective treatment for this and other related autoimmune diseases.

Transcervical thymectomy for myasthenia gravis.

The use of transcervical thymectomy in the treatment of myasthenia gravis remains controversial. We retrospectively reviewed our experience with this procedure to determine its usefulness in the management of myasthenia gravis. Fifty-three selected myasthenic patients without thymoma underwent transcervical thymectomy between 1977 and 1991. The mean age (27.5 +/- 1.5 years), duration of symptoms (2 +/- 1.0 years), and preoperative Osserman classification (13% class I, 53% class IIA, 28% class IIB, 6% class III) were consistent with previous reports. The average hospitalization was 3.0 +/- 0.3 days, but has been 1.6 +/- 0.2 days since 1987 (n = 14). There were no deaths, and no patients required mechanical ventilation for more than 24 hours. Average follow-up was 4.3 +/- 0.4 years with a range of 0 to 13 years. Eighty-one percent of patients are symptom free, and 9 of 21 (43%) are in complete remission at least 5 years postoperatively. One patient required a transsternal exploration for worsening symptoms. Clinical improvement continued over an extended period of time, and a statistically significant decrease in symptoms was evident comparing the first and sixth postoperative years. Patients were more likely to be improved or in remission if thymectomy was performed within the first year of the onset of symptoms (p < 0.05). Osserman classification, thymus histology, and patient age were not prognostic indicators. Transcervical thymectomy is effective surgical therapy for myasthenia gravis in selected patients without thymoma.

Neuromuscular transmission in amyotrophic lateral sclerosis.

The functional and structural characteristics of the neuromuscular junction were studied in anconeus muscle biopsies of 10 patients with amyotrophic lateral sclerosis (ALS). Intracellular recordings revealed decreased amplitudes of miniature endplate potentials (MEPPs). The MEPP frequencies were highly variable in ALS patients but the average MEPP frequency was not different from that of control patients. The mean quantal content of endplate potentials (m), the mean quanta available for immediate release (n), and the mean quantal stores (N) were all decreased. In contrast, the mean probability of quantal release (p) was normal and the mean probability of quantal store release (P) was surprisingly high at the majority of ALS endplates. Histologic evidence of denervation and small or absent nerve terminals were observed in all ALS patients. These functional and structural abnormalities of the neuromuscular junction may explain the fatigability and the electromyographic evidence of impaired neuromuscular transmission often encountered in ALS patients.