omega-conotoxin-(conus-magus) and Lambert-Eaton-Myasthenic-Syndrome

The Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease in which autoantibodies are directed against voltage-gated calcium channels (VGCCs) at presynaptic nerve terminals. We first demonstrated the presence of P/Q-type and N-type VGCCs in digitonin extracts prepared from human and rabbit cerebellum using the specific ligands 125I-omega-conotoxin MVIIC (125I-omega-CmTx) and 125I-omega-conotoxin GVIA (125I-omega-CgTx), respectively. We then tested sera from 72 LEMS patients' 25 with proven small cell lung cancer (SCLC) and 66 healthy or other neurological, SCLC or autoimmune disease controls in an immunoprecipitation assay using 125I-omega-CmTx-labelled (P/Q-type) VGCCs in human cerebellar extract. Sixty-six of 72 LEMS serum samples (91.7%) were positive for the presence of VGCC antibodies, as defined as a titre greater than 3 standard deviations above the mean for the healthy controls (n = 22). Rabbit cerebellar extract as antigen gave similar results (r = 0.94, P < 0.001, n = 30). By contrast, only 24/72 (33%) LEMS sera were positive in the assay for anti-N-type VGCC antibodies using 125I-omega-CgTx. All these 24 were also positive in the 125I-omega-CmTx assay. All healthy and disease control sera were negative in both assays. The anti-P/Q-type VGCC antibody titres did not correlate with an electrophysiological index of disease severity across individuals; however, longitudinal studies in a LEMS patient with SCLC receiving chemotherapy, and in a non-SCLC LEMS patient receiving immunosuppressive therapy showed an inverse relation between antibody titre and disease severity. These results support the view that anti-P/Q-type VGCC antibodies are implicated in the motor disorder in LEMS, and show that the omega-CmTx radioimmunoassay is a highly specific and sensitive means of detecting them.

Topics: Adolescent; Adult; Aged; Aged, 80 and over; Animals; Autoantibodies; Binding Sites; Calcium Channel Blockers; Calcium Channels; Carcinoma, Small Cell; Case-Control Studies; Cerebellum; Female; Humans; Lambert-Eaton Myasthenic Syndrome; Lung Neoplasms; Male; Middle Aged; Mollusk Venoms; omega-Conotoxins; Peptides; Rabbits; Radioligand Assay

We investigated the heterogeneity of anti-voltage-gated calcium channel (VGCC) antibodies in the Lambert-Eaton myasthenic syndrome (LEMS) using a small-cell lung carcinoma line (MB) derived from an LEMS patient. Four of 13 LEMS patients had raised titers of anti-125I-omega-conotoxin-labeled (N-type) VGCCs, measured by radioimmunoassay using line MB as the source of antigen. Antagonists for L-type (nitrendipine and nifedipine) and N-type (omega-conotoxin) VGCCs inhibited K(+)-stimulated (voltage-dependent) Ca2+ flux into this line--by 22% for L-type and 2% for N-type at maximum concentration. Inhibition by the LEMS IgGs, by contrast, ranged from 46 to 78% at a concentration of 2 mg/ml. These differing effects on Ca2+ flux inhibition by LEMS IgGs on the one hand and by L- and N-type channel antagonists on the other, taken together with the observation that many of the sera failed to react with omega-conotoxin-labeled (N-type) channels in the immunoprecipitation assay, suggest that in many LEMS patients the autoantibodies target other VGCC subtypes besides L- or N-types, and that these are important in inducing the myasthenic disorder.

Topics: Autoantibodies; Calcium; Calcium Channel Blockers; Calcium Channels; Carcinoma, Small Cell; Cell Line; Humans; Immunoglobulin G; Kinetics; Lambert-Eaton Myasthenic Syndrome; Lung Neoplasms; Middle Aged; omega-Conotoxins; Peptides; Potassium; Tumor Cells, Cultured

We have tested 36 patients with the Lambert-Eaton myasthenic syndrome for serum antibodies to voltage-gated calcium channels by using an immunoprecipitation assay with [125I] omega-conotoxin-labeled voltage-gated calcium channels extracted from a human neuroblastoma cell line, SKN-SH. Forty-four percent of these patients had significant levels of antibody (30-1,466 pM) compared with healthy control individuals (less than 15 pM). The incidence of positive sera in patients without associated small cell lung carcinoma (61%) was greater than in those patients with small cell lung carcinoma (28%). Results correlated strongly with results obtained using voltage-gated calcium channels extracted from the small cell lung carcinoma line, MAR5. Anti-voltage-gated calcium channel antibody titers did not correlate with disease severity across individuals, but longitudinal studies in 2 patients receiving immunosuppressive therapy showed a clear inverse relation between antibody titer and an electromyographic index of disease severity. The incidence of positive sera among patients with other neurological disorders was not significant, but 8 of 12 patients with rheumatoid arthritis or systemic lupus erythematosus had raised titers (30-82 pM). We conclude that the antibodies detected in this assay are heterogeneous and that some of them are likely to be implicated in this disorder of neuromuscular transmission. The assay should prove useful as an additional diagnostic aid in patients with Lambert-Eaton myasthenic syndrome.

Topics: Action Potentials; Adolescent; Adult; Aged; Autoantibodies; Calcium Channels; Female; Humans; Kinetics; Lambert-Eaton Myasthenic Syndrome; Male; Middle Aged; omega-Conotoxins; Peptides, Cyclic; Precipitin Tests; Tumor Cells, Cultured