Toward a new Therapeutic Strategy in the Treatment of Timothy Syndrome

Abstract

Timothy Syndrome (TS) is a multisystem disorder, featuring neurological impairment and life-threatening arrhythmias which are often fatal in early childhood. The basis of TS is a single point mutation (either G406R or G402S) within the CaV1.2 L-type Ca2+ channel. These mutations can occur in either of the mutually exclusive exons 8 or 8a, such that the expression level of each exon contributes significantly to the overall severity of the disease. We have modeled this dependence on gene dosage in an established adult ventricular myocyte model, and found that there is a non-linear dependence of the action potential duration on the fraction of TS channels, resulting in a threshold for arrhythmogenesis. To explore this experimentally at the cell-network level, we variably expressed TS channels in cultured adult guinea-pig ventricular myocytes. Indeed, conservative expression of TS channels yielded graded action-potential prolongation, but a small further increase led to significant arrhythmia. Such non-linear dependence on channel expression imparts an important principle for therapeutics: a small shift in the complement of mutant versus wild type channels may impart a significant clinical improvement. A possible therapeutic strategy would therefore employ methods to alter the splice expression pattern of the mutually exclusive exons 8 and 8a. Here, we demonstrate just such a shift in the expression pattern of exons 8 versus 8a in cultured myocytes. This represents an exciting new treatment strategy, where suppression of the TS containing exon is accompanied by a corresponding increase in non-TS channels. The potential benefits for TS patients are significant and may serve as a model system for developing a new therapeutic strategy for any channelopathy in which the mutation occurs within a mutually exclusive exon.