The role of autoantibodies in Lambert-Eaton myasthenic syndrome

Curses or Cures: A Review of the Numerous Benefits Versus the Biosecurity Concerns of Conotoxin Research

Conotoxins form a diverse group of peptide toxins found in the venom of predatory marine cone snails. Decades of conotoxin research have provided numerous measurable scientific and societal benefits. These include their use as a drug, diagnostic agent, drug leads, and research tools in neuroscience, pharmacology, biochemistry, structural biology, and molecular evolution. Human envenomations by cone snails are rare but can be fatal. Death by envenomation is likely caused by a small set of toxins that induce muscle paralysis of the diaphragm, resulting in respiratory arrest. The potency of these toxins led to concerns regarding the potential development and use of conotoxins as biological weapons. To address this, various regulatory measures have been introduced that limit the use and access of conotoxins within the research community. Some of these regulations apply to all of the ≈200,000 conotoxins predicted to exist in nature of which less than 0.05% are estimated to have any significant toxicity in humans. In this review we provide an overview of the many benefits of conotoxin research, and contrast these to the perceived biosecurity concerns of conotoxins and research thereof.

3,4-diaminopyridine base effectively treats the weakness of Lambert-Eaton myasthenia

Introduction: 3,4‐diaminopyridine has been used to treat Lambert‐Eaton myasthenia (LEM) for 30 years despite the lack of conclusive evidence of efficacy. Methods: We conducted a randomized double‐blind placebo‐controlled withdrawal study in patients with LEM who had been on stable regimens of 3,4‐diaminopyridine base (3,4‐DAP) for ≥ 3 months. The primary efficacy endpoint was >30% deterioration in triple timed up‐and‐go (3TUG) times during tapered drug withdrawal. The secondary endpoint was self‐assessment of LEM–related weakness (W‐SAS). Results: Thirty‐two participants were randomized to continuous 3,4‐DAP or placebo groups. None of the 14 participants who received continuous 3,4‐DAP had > 30% deterioration in 3TUG time versus 72% of the 18 who tapered to placebo (P < 0.0001). W‐SAS similarly demonstrated an advantage for continuous treatment over placebo (P < 0.0001). Requirement for rescue and adverse events were more common in the placebo group. Discussion: This trial provides significant evidence of efficacy of 3,4‐DAP in the maintenance of strength in LEM. Muscle Nerve57: 561–568, 2018

Paraneoplastic disorders of the central and peripheral nervous systems

Paraneoplatic neurologic syndromes (PNS) have been seminally defined as acute or subacute neurological syndromes resulting from nervous system dysfunction that is remote from the site of a malignant neoplasm or its metastases. However, in respect to our current understanding of their pathogenesis we may redefine these disorders as cancer-related dysimmune neurologic syndromes. We first deal with the epidemiology and the pathogenesis of PNS, then the different classic PNS are reviewed with clinical features according to the associated onconeuronal antibodies. Finally, therapeutic approaches are discussed.

Autoantibodies in neuromuscular transmission disorders

It is a great pleasure to be asked to honour the memory of Dr. Baldev Singh by reviewing the field of autoantibodies in myasthenia gravis and other neurotransmission disorders. The neuromuscular junction (NMJ) is the site of a number of different autoimmune and genetic disorders, and it is also the target of many neurotoxins from venomous snakes, spiders, scorpions and other species. The molecular organization of the NMJ is graphically represented in Figure 1A, where different ion channels, receptors and other proteins are shown. Four of the ion channels or receptors are directly involved in autoimmune diseases. This brief review will not only concentrate on these conditions but also illustrate how their study is helping us to understand the etiology of rare but treatable neurological syndromes of the central nervous system.

[Plasma exchange as a therapeutic option in neurological disorders]

Plasma exchange is a therapeutic procedure commonly used in various neurological disorders. Here we review its current role as a treatment option in diseases of the central and peripheral nervous system.

Autoantibody testing

Recent advances in neuroimmunology have led to improvements in the pathogenesis, diagnosis, prognosis, and treatment of many neuromuscular disorders. The value of autoantibody testing is increasing steadily in neurologic practice. Not all antibodies have a high yield in diagnosis. In some disorders, such as generalized adult onset of myasthenia gravis, Lambert-Eaton myasthenic syndrome,Miller Fisher syndrome, and multifocal motor neuropathy,autoantibody tests provide accurate diagnosis and can be considered biologic markers of these disorders.

Neuromuscular complications of cancer

Neuromuscular complications of cancer are common and can affect any component of the peripheral nervous system from peripheral nerve cell body to muscle. Perhaps the most common complication is a length-dependent symmetric axonal polyneuropathy that is often multifactorial in etiology, resulting from metabolic and treatment effects of the primary malignancy. However, neuromuscular disorders may also be the presenting complaint in many conditions, including disorders caused by malignant infiltration of nerve and disorders cause by paraneoplastic syndromes. Although many of the paraneoplastic conditions are poorly responsive to treatment, not all are, and one hopes that prompt diagnosis of the underlying malignancy will lead to improved patient outcome. Recognition of iatrogenic neuromuscular complications is also important to modify treatment protocols when possible and thus decrease the risk of long-term neurologic disability.

Differences in clinical features between the Lambert-Eaton myasthenic syndrome with and without cancer: an analysis of 227 published cases

To compare the clinical features of patients with the Lambert-Eaton myasthenic syndrome (LEMS) associated with carcinoma, with patients having LEMS but no cancer, reports on LEMS patients were analyzed systematically. Cancer was detected (CD group) in 62% of the 227 included cases. This CD group showed a male predominance (70%). No sex difference was found in patients in whom no cancer was detected (NCD group). Median age at onset of LEMS in the CD group was higher than in the NCD group (58 and 49.5 years, P<0.01). Median interval between onset of symptoms and diagnosis of LEMS was longest in NCD cases (P<0.001). CD patients had additional immunological disorders less frequently than NCD cases (6 and 27%, P<0.001). Symptoms distinguishing the CD group from the NCD group were weight loss (P<0.001) and need for prolonged artificial ventilation after anaesthesia (P<0.05). This analysis shows significant differences between CD and NCD cases of LEMS. The male predominance and higher age at onset in patients with a tumor probably reflects the characteristics of patients with small cell lung cancer. The high frequency of additional immunological disorders in patients without malignancy, together with the younger age at onset suggests a similar etiology as other non-paraneoplastic autoimmune diseases.

Remote neurologic manifestations of cancer

Paraneoplastic disorders may affect any part of the central or peripheral nervous systems. Although relatively uncommon, these disorders are a significant cause of neurologic morbidity for cancer patients. At least some paraneoplastic syndromes are believed to be caused by an autoimmune reaction against shared tumor-neural antigens. This article summarizes the clinical features of paraneoplastic disorders, the current evidence for autoimmunity, and guidelines for diagnosis and treatment.

Paraneoplastic anti-CV2 antibodies react with peripheral nerve and are associated with a mixed axonal and demyelinating peripheral neuropathy

Subacute sensory neuronopathy with anti-Hu antibodies is the best-characterized paraneoplastic peripheral neuropathy associated with carcinoma. Anti-CV2 antibodies, another group of paraneoplastic antibodies, react with a 66-kd brain protein belonging to the family of Ulip/CRMP proteins. The manifestations associated with anti-CV2 antibodies include cerebellar degeneration, uveitis, and peripheral neuropathy. Some of these patients also have anti-Hu antibodies. We have compared the clinical, electrophysiological, and pathological characteristics of the peripheral neuropathy in 9 patients with anti-CV2 antibodies (3 of whom also had anti-Hu antibodies) and 12 patients with only anti-Hu antibodies. Data for patients with anti-Hu antibodies alone indicated subacute sensory neuronopathy. Patients with anti-CV2 antibodies had a mixed axonal and demyelinating sensory motor neuropathy that was sometimes superimposed on subacute sensory neuronopathy when both anti-CV2 and anti-Hu antibodies were present. Unlike anti-Hu antibodies, anti-CV2 antibodies reacted with peripheral nerve antigens, as shown by their ability to bind to a 66-kd protein in human and rat nerve on Western blot analysis and to immunolabel peripheral nerve axons and sensory neurons on immunohistochemical study.

Molecular targets for autoimmune and genetic disorders of neuromuscular transmission

The neuromuscular junction is the target of a variety of autoimmune, neurotoxic and genetic disorders, most of which result in muscle weakness. Most of the diseases, and many neurotoxins, target the ion channels that are essential for neuromuscular transmission. Myasthenia gravis is an acquired autoimmune disease caused in the majority of patients by antibodies to the acetylcholine receptor, a ligand-gated ion channel. The antibodies lead to loss of acetylcholine receptor, reduced efficiency of neuromuscular transmission and muscle weakness and fatigue. Placental transfer of these antibodies in women with myasthenia can cause fetal or neonatal weakness and occasionally severe deformities. Lambert Eaton myasthenic syndrome and acquired neuromyotonia are caused by antibodies to voltage-gated calcium or potassium channels, respectively. In the rare acquired neuromyotonia, reduced repolarization of the nerve terminal leads to spontaneous and repetitive muscle activity. In each of these disorders, the antibodies are detected by immunoprecipitation of the relevant ion channel labelled with radioactive neurotoxins. Genetic disorders of neuromuscular transmission are due mainly to mutations in the genes for the acetylcholine receptor. These conditions show recessive or dominant inheritance and result in either loss of receptors or altered kinetics of acetylcholine receptor channel properties. Study of these conditions has greatly increased our understanding of synaptic function and of disease aetiology.

Acetylcholine receptors and myasthenia

Much progress has been made in the 26 years since initial studies of the first purified acetylcholine receptors (AChRs) led to the discovery that an antibody-mediated autoimmune response to AChRs causes the muscular weakness and fatigability characteristic of myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG). Now, the structure of muscle AChRs is much better known. Monoclonal antibodies to muscle AChRs, developed as model autoantibodies for studies of EAMG, were used for initial purifications of neuronal AChRs, and now many homologous subunits of neuronal nicotinic AChRs have been cloned. There is a basic understanding of the pathological mechanisms by which autoantibodies to AChRs impair neuromuscular transmission. Immunodiagnostic assays for MG are used routinely. Nonspecific approaches to immunosuppressive therapy have been refined. However, fundamental mysteries remain regarding what initiates and sustains the autoimmune response to muscle AChRs and how to specifically suppress this autoimmune response using a practical therapy. Many rare congenital myasthenic syndromes have been elegantly shown to result from mutations in muscle AChRs. These studies have provided insights into AChR structure and function as well as into the pathological mechanisms of these diseases. Evidence has been found for autoimmune responses even to some central nervous system neurotransmitter receptors, but only one neuronal AChR has so far been implicated in an autoimmune disease. Thus far, only two neuronal AChR mutations have been found to be associated with a rare form of epilepsy, but many more neuronal AChR mutations will probably be found to be associated with disease in the years ahead.