UBC Theses and Dissertations
Voltage-gated potassium ion channels : evolution, functional variation and human disease Jackson, Heather Ann
Hyperpolarization-activated cyclic nucleotide gated (HCN) channels are structurally similar to voltage gated potassium channels and play pivotal roles in cellular pacemaking. Their physiological relevance is illustrated by the fact that genetic mutations in HCN channels are associated with cardiac arrhythmias. In this study, we performed in-depth evolutionary analyses of HCN channels and functionally characterized the biophysical properties of two novel HCN clones (ciHCNa and ciHCNb) from the urochordate, Ciona intestinalis; a species emerging at the pivotal evolutionary period of invertebrate and vertebrate divergence that occurred approximately 550MYA. We have expanded the list of known HCN sequences by identifying and annotating 31 novel genes from invertebrates, urochordates, fish, amphibians, birds, and mammals. Our data suggest that the four vertebrate HCN isoforms arose via three duplication and diversification events from a single ancestral gene following the divergence of urochordates. Functional analyses of the two ciHCN channels further support this evolutionary trajectory, suggesting that the common single HCN ancestor of urochordates and vertebrates had a mammalian-like channel phenotype. Lineage-specific duplication and diversification events and 550MY of independent evolution has lead to two Ciona HCN channels with distinct biophysical properties. The voltage-gated potassium channels, HERG and KCNQ1, play a key role in cardiac repolarization. Mutations in these delayed rectifier channels are also associated with cardiac arrhythmias, including long QT syndrome and sudden death. Taking advantage of the >200 disease mutations in both of these channels, we performed the first quantitative evolutionary and chemical severity analysis of arrhythmia associated mutations (AAMs). Unlike non-synonymous polymorphisms (nsSNPs), AAMs are preferentially located to the evolutionarily conserved and functionally important sites and regions within HERG and KCNQ1. The mutations are also chemically more severe than changes which occur throughout evolution. In conjunction with previous studies, our findings suggest that novel disease-associated mutations can be identified by surveying the naturally occurring variation that exists among species. Overall, this thesis contributes to the current knowledge of the interdependent relationships that exist among ion channel evolution, ion channel function, and human disease.
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