Explaining Cardiac Arrhythmia
The KCNQ1 potassium channel, in complex with the KCNE1 regulatory protein, is responsible for a delayed outward flow of potassium ions during the repolarization phase of the cardiac myocyte action potential. Mutations that result in loss of function (LOF) of KCNQ1 are associated with type 1 long QT syndrome (LQTS), a hereditary heart condition associated with arrhythmias that can be life-threatening. Over 600 mutations of the gene encoding KCNQ1 are linked to LQTS, and many more “variants of unknown significance” (VUS) have been described. To better understand the impact of these mutations so as to enable physicians to predict their clinical relevance, Vanderbilt Basic Science investigators Chuck Sanders and Jarrod Smith along with their collaborators Jens Meiler (Chemistry), and Carlos Vanoye and Alfred George (Northwestern University) have explored the mechanisms by which LQTS-associated mutations lead to KCNQ1 channel dysfunction. They first developed a screen to quickly identify KCNQ1 LOF mutations. They next selected 51 of these variants for further study, including 17 mutations known to cause LQTS, 21 VUS mutations, and 13 mutations expected to be harmless. They then expressed each of the proteins in HEK293 cells. In each case, a Myc epitope insertion enabled the researchers to trace both total and cell surface levels of expression. They found that many LOF KCNQ1 mutations led to reduced overall expression and trafficking to the plasma membrane. Pharmacological blockade of proteasomal degradation increased the total expression of many of these proteins but not their localization to the cell surface. Two-dimensional nuclear magnetic resonance studies demonstrated that many of the poorly expressed and/or improperly localized proteins were structurally unstable. Molecular dynamics simulations identified the S0 segment of the KCNQ1 voltage sensor domain as the site of multiple interactions important to protein stability. Many LOF mutations affect residues that are either located in this segment or interact with residues in this segment. In summary, the researchers concluded that the most common cause of type 1 LQTS is mutation of the gene for KCNQ1 that leads to protein destabilization and subsequent proteasomal degradation. KCNQ1 is now one of only two membrane proteins linked to human disease for which instability is the documented cause of dysfunction for more than just a few mutations. The results suggest that new approaches to stabilize proteins via “pharmacological chaperones” may be of value for the treatment of patients suffering from type 1 LQTS. The work is published in the journal Science Advances [H. Huang, et al. (2018) Sci. Adv., 4, eaar2631].