Abstract 9741: A Novel Mutation Identified in a Large Multi-Generational Family With Long QT Syndrome Reveals Distinct, Disease-Specific Mutation Hot Spots within the CACNA1C- Encoded L-type Calcium Channel

Abstract

Background: Mutations in the CACNA1C-encoded L-type calcium channel have been associated with Timothy Syndrome (TS) with severe QT prolongation, cardiac hypertrophy, syndactyly, facial dysmorphisms, developmental delay, and sudden death by impaired channel inactivation and increase peak current. Recently, a small number of patients with CACNA1C mutations with only long QT syndrome (LQTS) have been described. Objective: To explore whether CACNA1C mutations associated with TS were mechanistically and clinically distinct from CACNA1C-mediated LQTS. Methods/Results: Illumina HiSeq 2000 platform-based whole exome sequencing with variant alignment using HGSC Mercury analysis and Atlas2 revealed a novel variant CACNA1C-L762F that co-segregated with affected patients of a large, multi-generational family with LQTS and sudden death. The proband presented at 3 years of age with out-of-hospital cardiac arrest and demonstrated QTc of 530. The mutation localized to the DII/DIII intracellular linker domain of the channel in a highly conserved region in close proximity to the 6th DI transmembrane domain. Whole cell patch clamp of heterologously expressed CACNA1C-L762F in TSA201 cells demonstrated slower inactivation tau and increased late current indicating its gain-of-function cellular electrophysiological phenotype. Comprehensive review and topological mapping of all described CACNA1C mutations associated with TS or LQTS revealed disease-specific hotspots. TS-associated mutations localized to cytoplasmic aspects of transmembrane domains while LQTS-associated mutations localized to cytoplasmic N- and C-terminal domains and linker regions. Probands hosting TS mutations demonstrated markedly prolonged QTc values compared to LQTS (618 vs 486 ms, p<0.001) and were younger at presentation (2.7 vs 20.1 years, p<0.002). Supporting these associations, the L762F mutation localized closer to the CACNA1C transmembrane domain than any other LQTS mutation and are associated with a QTc and age of diagnosis reflective of TS-associated mutations. Conclusions: The CACNA1C-L762F mutation is associated with biochemical and clinical features which highlight a molecular and clinical distinction between TS- and LQTS-associated CACNA1C mutations.