The psychiatric risk gene CACNA1C encodes distinct full-length isoforms in human brain, heart and aorta.

CACNA1C is a risk gene for multiple psychiatric disorders and the CaV1.2 protein it encodes is a potential target for their treatment. It is known that alternative start exon usage and splicing of CACNA1C RNA results in multiple transcript isoforms. However, the identity of full-length isoforms, and their relative abundance between tissues, remains unknown. Such information is important for understanding the molecular mechanisms of illness associations and to advance the potential for CaV1.2 isoform-selective drugs. In this study we compared full-length isoforms expressed in human brain with those expressed in heart and aorta using PCR-targeted long-read nanopore amplicon sequencing. We also performed 5'-RACE to profile which CACNA1C transcription start exons are used. CACNA1C RNA splicing revealed distinct tissue-enriched isoforms in brain, heart and aorta, with the results corroborated by publicly available short-read RNA-seq data. For example, splicing of mutually exclusive exons 21 and 22 differentiated isoforms expressed in the brain from those in the heart and aorta. 5'-RACE identified a novel start exon, exon 1d, which was detected in brain and aorta but not heart, and which predicts channels with a truncated N-terminus. The full-length isoforms identified here, from both the previously known start exons and the novel start exon, are likely to impact on the function and the pharmacology of the encoded CaV1.2 channel. Identification of brain-enriched isoforms provides the possibility of designing drugs preferentially targeting CaV1.2 for psychiatric indications.