The KCNH2 gene (human ether-a-go-go-related gene, hERG) encodes the Kv11.1 K+ channel that conducts the rapidly activating delayed rectifier K+ current (IKr) in the heart. The Kv11.1 channel contributes to the repolarization of cardiac action potentials and plays an important role in both inherited and drug-induced forms of long QT syndrome. Several alternatively processed mRNA isoforms have been identified in KCNH2. Two of these isoforms, Kv11.1a and Kv11.1a-USO, are expressed at high levels in the heart. We have shown that expression of Kv11.1a and Kv11.1a-USO are generated by alternative splicing and polyadenylation of KCNH2 pre-mRNA. The full-length Kv11.1a isoform is produced by the splicing of intron 9 and polyadenylation at a poly(A) site in exon 15, whereas Kv11.1a-USO is generated by polyadenylation at a poly(A) site in intron 9. Because only the Kv11.1a isoform is functional, the alternative processing of KCNH2 pre-mRNA represents a post-transcriptional mechanism that regulates Kv11.1 channel function. More importantly, disruption of this regulation leads to long QT syndrome. The relative expression of Kv11.1a and Kv11.1a-USO is developmentally regulated in the heart. The mechanisms underlying the regulation of Kv11.1 isoform expression are not fully understood. In the present application, we propose to study the molecular mechanisms underlying alternative splicing and polyadenylation of KCNH2 pre-mRNA and the role of alternative splicing and polyadenylation in regulation of Kv11.1 isoform expression during heart development and in inherited and drug-induced long QT syndrome. The specific aims are: Aim 1: To identify the cis-acting elements that regulate alternative splicing and polyadenylation of KCNH2 pre-mRNA and to develop strategies to restore expression of the functional Kv11.1a isoform disrupted by a long QT syndrome-associated mutation. Aim 2: To study effects of RNA binding proteins CELF1, CELF2 and SF3B1 on regulation of Kv11.1 isoform expression. Aim 3: To study mechanisms of developmental regulation of Kv11.1 isoform expression in a humanized knock-in mouse model. Aim 4: To study effects of SF3B1 inhibitors, a new class of anticancer drugs, on Kv11.1 isoform expression. The results from these studies will provide detailed information about the molecular mechanisms of alternative splicing and polyadenylation of KCNH2 pre-mRNA and how this process is regulated in normal and disease conditions. Elucidating these mechanisms will strengthen our understanding of post-transcriptional regulation of Kv11.1 channel expression and have important implications in the assessment of arrhythmogenic liability during drug development.