Characterisation of voltage-gated calcium channels and detection of their autoantibodies
The Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder of impaired neuromuscular transmission. It is associated with small cell lung carcinoma (SCLC) in 60% of patients. There is considerable evidence that the defect in LEMS is caused by autoantibodies to voltage-gated calcium channels (VGCCs) on the nerve terminal. Two VGCC subtypes were demonstrated in cultured neuronal cell lines by the technique of K+-induced 45Ca2+ influx: one subtype was inhibited by dihydropyridines (DHPs), the other by ω-conotoxin (ωCgTx). These may correspond to L and N channels previously described for neuronal tissue. The presence of these VGCC subtypes was confirmed by radioligand binding studies using [3 Hj-PN200-11 0 and 125 I-d)ωCgTx respectively. Pooled LEMS IgG inhibited K+-induced Ca2+ flux by 40% in SKNSH (human neuroblastoma) cells, while control IgG had no effect. The same LEMS pool reduced the density of 125I-ωCgTx binding sites in SKNSH and MAR5 (human SCLC) cells by 57% and 43% respectively. These results provide further evidence that LEMS antibodies cause a loss of functional VGCCs. 78 LEMS and 88 control sera were tested for anti-VGCC antibodies by the precipitation of 125I-ωCgTx-labelled VGCCs extracted from SKNSH cells. 42% of LEMS sera had significant levels of antibody (30-1466pM) compared to the healthy controls (<31pM). There was a high correlation between these results and those obtained using antigen extracted from MAR5 cells. Raised antibody titres (30-82pM) were also found among SCLC patients (47%) and patients with rheumatoid arthritis or systemic lupus erythematosus (56%). The incidence of positive sera was not significant among patients with other neurological disorders, including myasthenia gravis. Antibody titre did not correlate with disease severity across individuals. However, longitudinal studies in two LEMS patients showed an inverse relation between antibody titre and an electromyographic index of disease severity. Some of the antibodies detected may, therefore, be implicated in the neurological symptoms of LEMS. The assay may be a useful aid for the diagnosis of LEMS in some patients.