Non-Coding Data in KCNH2 mRNA Control Translation and Trafficking Efficiencies

Abstract

The KCNH2 gene encodes a K+ channel important to normal cardiac electrophysiology. Its significance is underscored by its link to both hereditary (locus LQT2) and acquired Long-QT cardiac arrhythmias. Over 400 deleterious mutations have been reported throughout the KCNH2 length. Why the channel is so susceptible to missense mutations is unclear. More than 50% of LQT2 mutations result in defective assembly and trafficking. There is evidence that even wild type channels are processed inefficiently. It has also been reported that the channel protein may reach the surface via atypical pathways. Most attention has logically focused on the protein for these processes.Much less is known about mRNA-dependent factors in channel processing. We analyzed the coding mRNA sequence of KCNH2. It has 66% GC content and 60 potential hair-pin loop segments. We re-synthesized the cDNA to achieve 50% GC-content and reduce the number of potential hair-pin loops while maintaining identical amino acid coding. Cellular expression of the codon-modified cDNA (CM-KCNH2) produced ionic currents comparable to native cDNA. Channel protein was expressed in a dramatically different pattern. Whereas the native protein (NT-KCNH2) is usually more abundant (immature form) in ER/Golgi compartments than on the surface, CM-KCNH2 showed a preponderance of the mature form, indicating channels at the surface. Immunofluorescence analysis confirmed this localization. From these results we hypothesize that KCNH2 translation efficiency, determined by RNA sequence-specific elements independent of coding, affect downstream protein assembly and/or trafficking_an unusual occurrence. As a corollary to this, we postulate a synergy between the inherently inefficient biosynthesis of KCNH2 and LQT2 mutations that contribute to the pathogenesis of hereditary LQT2. Further investigation of mechanisms underlying mRNA-dependent processing of KCNH2 channels may lead us to reconsider approaches to hereditary and acquired arrhythmia syndromes.