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Cellular mechanisms of ventricular arrhythmias in a mouse model of Timothy syndrome (long QT syndrome 8).

ABSTRACT: Ca(2+) flux through l-type CaV1.2 channels shapes the waveform of the ventricular action potential (AP) and is essential for excitation-contraction (EC) coupling. Timothy syndrome (TS) is a disease caused by a gain-of-function mutation in the CaV1.2 channel (CaV1.2-TS) that decreases inactivation of the channel, which increases Ca(2+) influx, prolongs APs, and causes lethal arrhythmias. Although many details of the CaV1.2-TS channels are known, the cellular mechanisms by which they induce arrhythmogenic changes in intracellular Ca(2+) remain unclear. We found that expression of CaV1.2-TS channels increased sarcolemmal Ca(2+) "leak" in resting TS ventricular myocytes. This resulted in higher diastolic [Ca(2+)]i in TS ventricular myocytes compared to WT. Accordingly, TS myocytes had higher sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) spark activity, larger amplitude [Ca(2+)]i transients, and augmented frequency of Ca(2+) waves. The large SR Ca(2+) release in TS myocytes had a profound effect on the kinetics of CaV1.2 current in these cells, increasing the rate of inactivation to a high, persistent level. This limited the amount of influx during EC coupling in TS myocytes. The relationship between the level of expression of CaV1.2-TS channels and the probability of Ca(2+) wave occurrence was non-linear, suggesting that even low levels of these channels were sufficient to induce maximal changes in [Ca(2+)]i. Depolarization of WT cardiomyocytes with a TS AP waveform increased, but did not equalize [Ca(2+)]i, compared to depolarization of TS myocytes with the same waveform. We propose that CaV1.2-TS channels increase [Ca(2+)] in the cytosol and the SR, creating a Ca(2+)overloaded state that increases the probability of arrhythmogenic spontaneous SR Ca(2+) release.

SUBMITTER: Drum BM

PROVIDER: S-EPMC3903114 | biostudies-literature | 2014 Jan

REPOSITORIES: biostudies-literature

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Project description:BackgroundMutations in the CACNA1C gene-encoding for the major Ca2+ channel of the heart-may exhibit a variety of clinical manifestations. These include typical or atypical Timothy syndromes (TS) which are associated with multiple organ manifestations, and cardiac involvement in form of malignant arrhythmias, QTc prolongation, or AV block. "Cardiac only" Timothy syndrome (COTS) shows no extracardiac manifestation, whereas some CACNA1C gene mutations are associated with QTc prolongation alone (isolated long QT syndrome 8, LQT8).MethodsA systematic search of the literature reporting cases of CACNA1C gene mutation associated syndromes, including TS, COTS and isolated LQT8 via major databases published from 2004 through 2019 was performed. Detailed patient-level phenotypic and genotypic characteristics, as well as long-term outcome measures were collected and compared between pre-specified patient groups, defined both on phenotype and genotype.ResultsA total of 59 TS, 6 COTS, and 20 isolated LQT8 index cases were identified. Apart of syndactyly or baldness, there were no major differences regarding clinical manifestations or outcome measures between TS subtypes, either defining TS subtypes on the genotype or based on the phenotype. Both subtypes were characterized by an extreme degree of QTc prolongation (median ≥600 ms) which were reflected in high major adverse cardiac event rate. On the other hand, there were marked differences between TS, COTS, and isolated LQT8. Timothy syndrome was characterized by a much earlier disease onset, much more pronounced QTc prolongation and much higher mortality rate than COTS or isolated LQT8. Similar differences were observed comparing CACNA1C exon 8/8A vs. non-exon 8/8A mutation carriers. TS showed a high degree of genetic homogeneity, as the p.Gly406Arg mutation either in exon 8 or exon 8A alone was responsible for 70% of the cases.ConclusionsClinical phenotypes associated with mutations in the CACNA1C gene show important clinical differences. Timothy syndrome is associated with the most severe clinical phenotype and with the highest risk of morbidity and mortality. However, distinguishing TS subtypes, in any form, are not supported by our data.Systematic review registration[https://www.crd.york.ac.uk/prospero/], identifier [CRD42020184737].

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Cellular mechanisms of ventricular arrhythmias in a mouse model of Timothy syndrome (long QT syndrome 8).

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