Group 3 Stony Brook University BME 301 Long
Group 3 Stony Brook University BME 301 Long QT Syndrome Type 2 (LQT 2) Scott Dow, Hui Zheng, Mohammad Reda El Mkhantar, Kamal James Monjur Khuda, Hanbei Zhang, Dandi Zheng, Anders Chan
Long QT Syndrome (LQTS) ● Comprises a number of disorders, all of which involve an elongated QT interval ● In this case, caused by delayed ventricular repolarization ● Not a problem per se, but is a risk factor for more dangerous complications, such as ventricular arrhythmias
LQT 2 Syndrome A specific type of LQTS caused by mutations of HERG gene, which codes for Kv 11. 1, the α-subunit that makes up the pores of Ikr channels
Ikr Channels & Kv 11. 1 ● Ikr is notable for its slow activation/deactivation, and fast inactivation/recovery ● Kv 11. 1 is the α-subunit of Ikr ● The Ikr channel consists of 4 identical αsubunits that together form the pore ● Each α-subunit comprises 6 transmembrane α helices labeled S 1 -S 6 ● S 4 is most important domain for voltage sensing (+AA every ~3 residues)
Comparing Missense Mutations in h. ERG ● Depending on the region affected, mutations can be pathological or benign ● Most likely harmful mutations are the ones on transmembrane/linker/pore region ● Mutations may also be nonsense, but these usually never leave the ER *N-terminus, Transmembrane/Linker/Pore, and CTerminus Domains
Refresher on the Cardiac AP 0. INa depolarizes the cell 1. Ito sets the plateau voltage 2. ICa-L & IKs balance to create a the plateau 3. IKr, IKs, and IK 1 repolarize Sherwood, L. (2012). Human Physiology, From Cells to Systems (8 th [revised] ed. ). Cengage Learning. ISBN 9781111577438. 4. Ik 1 enforces negative Vm
What kinds of loss in function? ● Reduction in number of channels in the cell membrane (changes effective gkr) ○ Due to reduced protein trafficking since quality control mechanisms prevent departure from ER ● Decreasingle channel conductance ○ Pore forming domains are mutated ● Changing probability of channel gates to be open ○ E. g. Deactivation can be accelerated by N or C-terminal mutations (these help regulate gating)
Reduced Ikr Conductance - So What? ● Reduction in gkr means a reduction in current ● With a reduction in Ikr, the slope of phase 3 of the AP is drastically reduced ● If repolarization is too slow (i. e. delayed)… EADs are possible!!!
What about Altered Gating? Recall that Two Things Characterize Ikr: 1. Slow activation and deactivation 2. Fast inactivation and recovery from inactivation Mutations affecting the function of either one of these gates could affect Ikr in numerous ways, including these that we found in the literature: ● Accelerated deactivation ● Voltage-shifted inactivation ● Voltage-shifted activation Any of these can also lead to EADs!
Early Afterdepolarizations ● Any depolarization that happens too early and interrupts another AP phase ● Can be caused by delayed repolarization as in LQT 2 (slope not steep enough) ● Calcium channels have a limited refractory period - if repolarization is delayed, they can fire again or even create a self-sustaining wave ● Ventricular tachyarrhythmias!!!
MATLAB Code Manipulations Input parameters Ik splitting and scaling factor in Ikr conductance(g_krscale)
MATLAB Code Manipulations Cont’d Voltage shift in activation (vshift_a) and scaling factor in deactivation (Bp) Voltage shift in inactivation (vshift_n)
Effect of gkr Changes on AP
Effects of Voltage Shifting Gates on AP
Effect of Accelerated Deactivation
Member contributions Research/Background MATLAB code PPT presentation Website Hui Zheng 1 1 1 0 Scott Dow (presenter) 1 1 1 0 Hanbei Zhang 1 1 0 0 Reda El Mkhantar 1 1 0 1 Kamal James 1 1 0 0 Dandi Zheng 1 0 0 1 Monjur Khuda (presenter) 1 0 Anders Chan 0 0 Website: http: //you. stonybrook. edu/bmeswagyolo/
References 1. Nakajima, T. , et al. , Novel mechanism of HERG current suppression in LQT 2: shift in voltage dependence of HERG inactivation. Circ Res, 1998. 83(4): p. 415 -22. 2. Anderson CL, Delisle BP, Anson BD, Kilby JA, Will ML, Tester DJ, Gong Q, Zhou Z, Ackerman MJ, January CT. Most LQT 2 mutations reduce Kv 11. 1 (h. ERG) current by a class 2 (trafficking-deficient) mechanism. Circulation. 2006 Jan 24; 113(3): 365 -73. 3. Morita, H. , Wu, J. , & Zipes, D. P. (2008). The QT syndromes: long and short. The Lancet, 372(9640), 750 -763. 4. Chen J, Zou A, Splawski I, Keating MT, Sanguinetti MC. Long QT syndrome-associated mutations in the Per-Arnt. Sim (PAS) domain of HERG potassium channels accelerate channel deactivation. J Biol Chem. 1999 Apr 9; 274(15): 101138. 5. Kapa, S. , Tester, D. J. , Salisbury, B. A. , Harris-Kerr, C. , Pungliya, M. S. , Alders, M. , … Ackerman, M. J. (2009). Genetic Testing for Long QT Syndrome - Distinguishing Pathogenic Mutations from Benign Variants. Circulation, 120(18), 1752– 1760. doi: 10. 1161/CIRCULATIONAHA. 109. 863076 6. Zeng, J. , Laurita, K. R. , Rosenbaum, D. S. , & Rudy, Y. (1995). Two Components of the Delayed Rectifier K+ Current in Ventricular Myocytes of the Guinea Pig Type Theoretical Formulation and Their Role in Repolarization. Circulation Research, 77(1), 140 -152. 7. Shimizu W, Moss AJ, Wilde AM, et al. Genotype-Phenotype Aspects of Type 2 Long QT Syndrome. J Am Coll Cardiol. 2009; 54(22): 2052 -2062. doi: 10. 1016/j. jacc. 2009. 08. 028.