1. Group 3 Stony Brook University BME 301 Long QT Syndrome Type 2 (LQT2)
Scott Dow, Hui Zheng, Mohammad Reda El Mkhantar, Kamal James
Monjur Khuda, Hanbei Zhang, Dandi Zheng, Anders Chan

2. 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

3. LQT2 Syndrome
A specific type of LQTS caused by mutations of HERG gene, which codes for Kv11.1, the α-subunit that makes up the pores of Ikr channels

4. Ikr Channels & Kv11.1
Ikr is notable for its slow activation/deactivation, and fast inactivation/recovery
Kv11.1 is the α-subunit of Ikr
The Ikr channel consists of 4 identical α-subunits that together form the pore
Each α-subunit comprises 6 transmembrane α helices labeled S1-S6
S4 is most important domain for voltage sensing (+AA every ~3 residues)

5. Comparing Missense Mutations in hERG
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 C-Terminus Domains

6. 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 IK1 repolarize
4. Ik1 enforces negative Vm
Sherwood, L. (2012). Human Physiology, From Cells to Systems (8th [revised] ed.). Cengage Learning. ISBN

7. 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
Decreasing single 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)

8. 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!!!

9. What about Altered Gating?
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!
Recall that Two Things Characterize Ikr:
Slow activation and deactivation
Fast inactivation and recovery from inactivation

10. Early Afterdepolarizations
Any depolarization that happens too early and interrupts another AP phase
Can be caused by delayed repolarization as in LQT2 (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!!!

11. MATLAB Code Manipulations
Input parameters
Ik splitting and scaling factor in Ikr conductance(g_krscale)

12. MATLAB Code Manipulations Cont’d
Voltage shift in activation (vshift_a) and scaling factor in deactivation (Bp)
Voltage shift in inactivation (vshift_n)

13. Effect of gkr Changes on AP

14. Effects of Voltage Shifting Gates on AP

15. Effect of Accelerated Deactivation

16. Member contributions Website: http://you.stonybrook.edu/bmeswagyolo/
Research/Background
MATLAB code
PPT presentation
Website
Hui Zheng 1
Scott Dow (presenter)
Hanbei Zhang
Reda El Mkhantar
Kamal James
Dandi Zheng
Monjur Khuda
(presenter)
Anders Chan
Website:

17. References
Nakajima, T., et al., Novel mechanism of HERG current suppression in LQT2: shift in voltage dependence of HERG inactivation. Circ Res, (4): p
Anderson CL, Delisle BP, Anson BD, Kilby JA, Will ML, Tester DJ, Gong Q, Zhou Z, Ackerman MJ, January CT. Most LQT2 mutations reduce Kv11.1 (hERG) current by a class 2 (trafficking-deficient) mechanism. Circulation Jan 24;113(3):
3. Morita, H., Wu, J., & Zipes, D. P. (2008). The QT syndromes: long and short.The Lancet, 372(9640),
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 Apr 9;274(15):
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: /CIRCULATIONAHA
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),
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): doi: /j.jacc