Type 8 long QT syndrome: pathogenic variants in CACNA1C-encoded Cav1.2 cluster in STAC protein binding site

Abstract

Aims Pathogenic gain-of-function variants in CACAN1C cause type-8 long QT syndrome (LQT8). We sought to describe the electrocardiographic features in LQT8 and utilize molecular modelling to gain mechanistic insights into its genetic culprits. Methods and results Rare variants in CACNA1C were identified from genetic testing laboratories. Treating physicians provided clinical information. Variant pathogenicity was independently assessed according to recent guidelines. Pathogenic (P) and likely pathogenic (LP) variants were mapped onto a 3D modelled structure of the Cav1.2 protein. Nine P/LP variants, identified in 23 patients from 19 families with non-syndromic LQTS were identified. Six variants, found in 79% of families, clustered to a 4-residue section in the cytosolic II–III loop region which forms a region capable of binding STAC SH3 domains. Therefore, variants may affect binding of SH3-domain containing proteins. Arrhythmic events occurred in similar proportions of patients with II–III loop variants and with other P/LP variants (53% vs. 48%, P = 0.41) despite shorter QTc intervals (477 ± 31 ms vs. 515 ± 37 ms, P = 0.03). A history of sudden death was reported only in families with II–III loop variants (60% vs. 0%, P = 0.03). The predominant T-wave morphology was a late peaking T wave with a steep descending limb. Exercise testing demonstrated QTc prolongation on standing and at 4 min recovery after exercise. Conclusion The majority of P/LP variants in patients with CACNA1C-mediated LQT8 cluster in an SH3-binding domain of the cytosolic II–III loop. This represents a ‘mutation hotspot’ in LQT8. A late-peaking T wave with a steep descending limb and QT prolongation on exercise are commonly seen.

Update on the Molecular Genetics of Timothy Syndrome

“…CACNA1C gene distinct from those described throughout this review. A significant number of individuals with a CACNA1C variant do present with non-syndromic LQT8, but do not have Timothy Syndrome (11)(12)(13). To our knowledge, there is no quantification of non-syndromic LQT8 individuals to compare with those affected by TS.…”

Voltage-Gated Ca2+-Channel α1-Subunit de novo Missense Mutations: Gain or Loss of Function – Implications for Potential Therapies

“…In LQT8 families Cav1.2 channel variants were found to cluster within four-residues (857-860, a PRPR-motif, not illustrated) in the cytosolic II-III loop region and enhance channel activity through a so far unknown mechanism. Recently, crystal structures and homology modeling provided compelling evidence that this region binds SH3-domain-containing proteins (e.g., STAC) and LQT8 variants are expected to weaken this interaction (Mellor et al, 2019). Another example is the slow inactivation of the Cav1.2-G406R type 1 mutation, which appears to be mediated by cyclin-dependent kinase 5 (CDK-5; Song et al, 2017).…”

“…In addition to TS, GOF Cav1.2 mutations can also cause a non-syndromic form of LQT8. LQT8 can be regarded as part of a CACNA1C disease spectrum because with some variants the clinical phenotype can be either LQT8 alone (G402S, Mellor et al, 2019), LQT with syndactyly but without other TStypical features (G402S, I1186T; Fröhler et al, 2014;Wemhöner et al, 2015) or Timothy syndrome. Somatic mosaicism and/or genetic background can explain these variable phenotypes.…”

“…This has recently been shown for a cluster of variants (affecting residues P857, R858, R860; not illustrated) located within the long cytoplasmic II-III -linker without obvious role in channel gating and conductance. However, this proline-rich region serves as a binding site for SH3-domain containing proteins, such as STAC proteins, known to affect the expression and function of L-type channels (Mellor et al, 2019;Flucher and Campiglio, 2019). For a complete list of LQT8 mutations see recent reviews (Giudicessi and Ackerman, 2016;Zhang et al, 2018;Marcantoni et al, 2020).…”

Structure and function of STAC proteins: Calcium channel modulators and critical components of muscle excitation–contraction coupling

“…Although Ca V 1.1 is also a known target for MH (106), so far no mutations have been identified in the STAC3 binding site within the II-III loop. Interestingly, such mutations have been found in Ca V 1.2: several patients with type 8 long QT syndrome harbour mutations in a short segment of the Ca v 1.2 II-III loop, which maps directly to the STAC binding site (107). Unexpectedly, these patients experience arrhythmic symptoms and sudden death.…”