Objective: To identify human genes encoding neuronal voltage-gated calcium channel gamma subunits corresponding to the gamma subunit recently described in mice with hereditary epilepsy, confirm their expression in adult brain, and determine whether a gamma subunit resides in human chromosomal regions with marker genes linked to convulsions.
Material and methods: Human homologues of a recently described mouse gamma subunit were sought by a computerized search of National Center for Biotechnology Information databases. Sequences of interest were mapped electronically to specific chromosomal regions. The proximity of these chromosomal regions to markers linked to convulsive disorders was determined by a MEDLINE search. Predicted gamma subunit sequences were amplified by polymerase chain reaction from a human adult brain complementary DNA (cDNA) library and were subcloned, and full-length sequences were determined or confirmed. Secondary structure and transmembrane regions were predicted by using the TMpredict program.
Results: Two putative isoforms (human gamma-2 and gamma-3 calcium channel subunits) were identified, cloned, and sequenced. They mapped to chromosomes 22 and 16, respectively. A marker very close to the gamma-3 gene gives maximal lod scores for an autosomal dominant syndrome of familial infantile convulsions and paroxysmal choreoathetosis.
Conclusion: Genomic DNA sequence data already existed in GenBank (partial for gamma-2 and complete for gamma-3) for the two putative neuronal calcium channel gamma subunits that we identified. By cloning both of these molecules from a cerebellar cDNA library, we demonstrated for the first time their expression in human brain. The gamma-3 gene is adjacent to a marker for a convulsive disorder. The convulsive phenotype of gamma channelopathies occurring in mice makes the human gamma subunit genes attractive candidate genes for hereditary convulsive disorders.