Immunogenicity of P/Q-type calcium channel in small cell lung cancer:: investigation of α1 subunit polyglutamine expansion

The ectopic expression of neuronal P/Q-type voltage-gated calcium channels in small cell lung carcinoma (SCLC) is thought to induce antisynaptic autoimmunity in the paraneoplastic Lambert-Eaton myasthenic syndrome. The gene CACNLIA4, encoding the principal (alpha 1A) subunit of this calcium channel, is mutated in several inherited neurological disorders. One of these disorders (spinocerebellar ataxia, type 6, or SCA-6) involves the expansion of a trinucleotide (CAG) repeat unit. We hypothesized that a somatic CAG repeat instability of this gene in neoplastic cells might generate a non-self epitope capable of initiating autoimmunity to P/Q-type calcium channels. We therefore analyzed the CACNL1A4 gene in SCLC lines established from metastases derived from seven individual patients (four associated with Lambert-Eaton myasthenic syndrome, one associated with myasthenia gravis, and two not associated with autoimmunity). We compared their CAG repeat numbers (determined by polymerase chain reaction (PCR) amplification followed by separation of products on a 6 % polyacrylamide/8M urea gel) to published and to DNA ham a patient with SCA-6. The number of CAG repeats in SCLC DNA fell within a normal range whether or not;the neoplasm was complicated by neurological autoimmmunity Therefore, it is unlikely that somatically unstable CAG repeat units in the gene encoding the P/Q-type voltage-gated calcium channel account for this tumor protein's immunogenicity in the Lambert-Eaton myasthenic syndrome.