Antibodies against the β subunit of voltage-dependent calcium channels in Lambert–Eaton Myasthenic Syndrome

Abstract

Lambert–Eaton myasthenic syndrome is an autoimmune disease that impairs neuromuscular transmission. Several studies suggest that neurotransmitter release is reduced by an immune response directed against the calcium channel complex of nerve terminals. The immunoglobulin G fractions from Lambert–Eaton myasthenic syndrome patients immunoprecipitate solubilized neuronal N- and P/Q-type channels and in certain cases brain, skeletal and cardiac muscle L-type channels [El Far O. et al. (1995) J. Neurochem. 64, 1696–1702; Lennon V. A. and Lambert E. H. (1989) Mayo Clin. Proc. 64, 1498–1504; Sher E. et al. (1989) Lancet ii, 640–643; Suenaga A. et al. (1996) Muscle Nerve 19, 1166–1168]. These channel immunoprecipitation assays are considered as useful for the diagnosis of this syndrome. In this study, we demonstrate that two predominant neuronal voltage-dependent calcium channel β subunits ( β 3 and β 4, of mol. wt 58 000) are general targets of Lambert–Eaton myasthenic syndrome autoantibodies. Of 20 disease sera tested, 55% were able to immunoprecipitate 35S-labeled β subunits. All five patients affected with small-cell lung carcinoma were positive for the β-subunit immunoprecipitation assay. Interestingly, only a fraction of the β-subunit-positive sera was also able to immunoprecipitate N- and P/Q-type channels, suggesting that several of the β-subunit epitopes are masked in native channels. In accordance with this observation, we found that several β-positive sera were able to prevent the interaction between calcium channel α 1 and β subunits in vitro. In cases where sera were able to immunoprecipitate β subunits, N- and P/Q-type channels, the immunoprecipitation of both channel types was either partially or entirely mediated by β-subunit antibodies. Our results suggest that assays based on the immunoprecipitation of β subunits can be used as an additional test to assist in the diagnosis of Lambert–Eaton myasthenic syndrome.