long qt syndrome type 3 - arrhythmia alliance · long qt syndrome type 3 • 1.7‐8% genotyped...

Long QT Syndrome Type 3: Lessons from mouse models

Dr Glyn Thomas MRCP PhDConsultant Electrophysiologist

Bristol Heart Institute

Inherited Long QT Syndrome

• Prolonged QT interval

• Syncope & Sudden Cardiac Death

• Ventricular Arrhythmias (TdP)

From: “Ion Channels and Disease” F.M. Ashcroft. Academic Press 2000

Inherited Channel MutationsLQTS Chromosome Gene Protein Ion Current Trigger

1 11p15.5 KCNQ1 KvLQT1 IKS Exercise, emotion

2 7q35‐36 KCNH2 HERG IKr Rest, emotion, exercise

3 3p24‐21 SCN5A Nav1.5 INa Rest, emotion, sleep

4 4q24‐27 ANK2 Ankyrin‐B INa‐Kr, INa‐Ca, Ina Exercise

5 21q22 KCNE1 MinK IKS Exercise, emotion

6 21q22 KCNE2 MiRP1 IKr Rest, exercise

7 17q23 KCNJ2 Kir2.1 IK1 Rest, exercise

8 12p13.3 CACNA1C Cav1.2 Ica Exercise, emotion

9 3p25.3 CAV3 Caveolin‐3 Ina Non‐exertional, sleep

10 11q23.3 SCN4B Navβ4 Ina Exercise, post‐partum

Long QT syndrome type 3

• 1.7‐8% genotyped patients

• Gain of function mutation in the Scn5a gene

• Encodes the α‐(pore‐forming) subunit of the cardiac Na+ channel

• Increased late Na+current (INa)

Roden D. N Engl J Med 2008;358:169‐176

0

‐100

mV

EAD

L‐type Ca2+ channel ‘window’

‘Conditioned’ AP

Normal AP

‘Window’ current hypothesis

INa

Genotype‐specific triggers for cardiac events

Schwartz, P. J. et al. Circulation 2001;103:89‐95

QTc = 498, 497, 506 ms p = NS

Lethal cardiac events according to 3 classified triggers in 3 genotypes

Schwartz, P. J. et al. Circulation 2001;103:89‐95

Zareba W et al. N Engl J Med 1998;339:960‐965

Cumulative Probability of a Cardiac EventIn LQT1, LQT2, and LQT3 Groups

LQT1 53%, LQT2 29%, LQT3 6%

Zareba W et al. N Engl J Med 1998;339:960‐965

Cumulative Probability of Death in LQT1, LQT2, and LQT3 Groups

20% of events fatal in LQT3 vs 4% in LQT1 & LQT2 (P<0.001)

Priori, S. G. et al. JAMA 2004;292:1341‐1344.

Event‐Free Survival in β‐blocker treatedpatients

Cardiac events: 32% in LQT3 vs 10% in LQT1 & 23% in LQT2

(P<0.001)

Etheridge, S. P. et al. J Am Coll Cardiol 2007;50:1335‐1340

Indications for Device Implantation

Schwartz, P. J., C. Spazzolini, et al. (2009). All LQT3 patients need an ICD: true or false? Heart Rhythm 6(1): 113‐20

LQT3 patients need ICDs:True or False?

3 Died despite β‐blockers

3 Died Aged 14, 16, 26 years

2 syncopal episodes9 ± 6 years

QTc: 650, 648, 550, 525ms

QTc: 540 ± 29ms450 ± 42ms

QTc: 497 ± 25ms

Poor prognosis in patients with events in the first year of life

2 months QTc 648 ms

1 week QTc 650 ms

2 months QTc 550 & 700 ms

Schwartz, P. J., C. Spazzolini, et al. (2009). All LQT3 patients need an ICD: true or false? Heart Rhythm 6(1): 113‐20

Rare Genetic Variants in LQTS Genes Identified in 26 SIDS Cases

Arnestad, M., L. Crotti, et al. (2007). Prevalence of long‐QT syndrome gene variants in sudden infant death syndrome. Circulation 115(3): 361‐7.

Life‐threatening neonatal arrhythmia

Kehl, H.‐G. et al. Circulation 2004;109:e205‐e206

Preterm female infant 35 weeks gestational age

Mice as Models for Investigating Cardiac Arrhythmias

Advantages

• Ease of molecular manipulation

• Common physiological structures

• Action potential heterogeneity

• Similar ionic currents

• Demonstrable atrial and ventricular arrhythmias

Disadvantages

• Mouse heart rate 600‐700/min

• Shorter action potential duration without plateau

• Ito is predominant repolarizing current (cf IKr & IKs)

• Markedly different ECG appearance

Mouse Models of LQT3

• Mechanism of arrhythmogenesis

• Effect of β‐blocker therapy

• Other therapeutic options

Mechanism of Arrhythmogenesis

• ΔKPQ 1505‐1507 deletion in channel inactivation domain by homologous recombination in ES cells

Head, C. E., Balasubramaniam, R., Thomas, G., Goddard, C. A., Lei, M., Colledge, W. H., Grace, A. A. & Huang, C. L.‐H. (2005). Paced Electrogram Fractionation Analysis of Arrhythmogenic Tendency in KPQ Scn5a Mice. Journal of Cardiovascular Electrophysiology 16, 1329‐1340.

Current Clamp (‐ 80mV)

Head, C. E., R. Balasubramaniam, Thomas G et al. (2005). "Paced Electrogram Fractionation Analysis of Arrhythmogenic Tendency in KPQ Scn5a Mice." Journal of Cardiovascular Electrophysiology 16(12): 1329‐1340.

1. Triggered Activity

Thomas, G., I. S. Gurung, et al. (2007). "Effects of L‐type Ca2+ channel antagonism on ventricular arrhythmogenesis in murine hearts containing a modification in the Scn5a gene modelling human long QT syndrome 3." J Physiol 578(Pt 1): 85‐97.

Scn5a+/Δ VT* *

100ms

100ms

WT alone

ENDOM‐cell

EPI

BASE

APEX

LA

LV

RA

RV

2. Re‐entry facilitated by transmural dispersion of repolarization (spatial)

EPI

ENDO

S1 S1 S2Orthodrom

ic impulse

Optical Mapping of Mouse re‐entrant VT

Salama, G. and B. London (2007). "Mouse models of long QT syndrome." J Physiol 578(Pt 1): 43‐53

Time

Record

Pace

S1 S1 S2 S1 S1 S2

S1S2 reduced on eachoccasion by 1 millisecond

Paced Electrogram Fractionation Analysis of LQT3 Mouse

Increased Electrogram Fractionation in LQT3 Mice


Transmural dispersion of repolarization

Anumonwo, J. M., Y. N. Tallini, et al. (2001). "Action potential characteristics and arrhythmogenic properties of the cardiac conduction system of the murine heart." Circ Res

Wild Type Mouse

Increased TDR in LQT3 Mice

Thomas, G., M. J. Killeen, et al. (2008). "Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome." Acta Physiol (Oxf) 192(4): 505‐17.

Proposed Arrhythmogenesis in LQTS

Early After Depolarizations

Prolonged QT Interval

Dispersion of repolarization (Spatial and temporal)

Re‐entrant excitation

Triggered ActivityFunctional conduction block

pVT (TdP)

Effect of β‐blocker therapy

Protective effect of β‐adrenoreceptor agonist isoproterenol

Nuyens, D., M. Stengl, et al. (2001). "Abrupt rate accelerations or premature beats cause life‐threatening arrhythmias in mice with long‐QT3 syndrome." Nat Med 7(9): 1021‐7.

“Scn5aΔ/+ are mice are sensitive to rate accelerations or premature beats… β‐blockers would suppress these triggers during sudden sympathetic surges “

“ However, β‐receptor activation was anti‐arrhythmic , possibly shortens the action potential, reduces transmural dispersion”

Propranolol increases electrogram fractionation


Propranolol suppresses EADs in LQT3 Mice

Thomas, G., M. J. Killeen, et al. (2008). "Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome." Acta Physiol (Oxf) 192(4): 505‐17

Propranolol increases TDR in LQT3 Mice

Thomas, G., M. J. Killeen, et al. (2008). "Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome." Acta Physiol (Oxf) 192(4): 505‐17

Propranolol causes AV block in Ambulant LQT3 Mice

Fabritz, L., M. Emmerich, et al. (2005). "Aggravation of bradycardia by propranolol in deltaKPQ SCN5A (Long QT3) mice." European Heart Journal 26 (Abstract Supplement): S299

100 ms

SCn5a Δ/+ KPQ Mouse

What other potential drugs?

Therapeutic Targets

0

‐100

mV

Normal AP

INaIK

Mexiletine

Effect of Mexiletine on QTc in LQT3 Patients

Schwartz, P. J., S. G. Priori, et al. (1995). "Long QT syndrome patients with mutations of the SCN5A and HERG genes have differential responses to Na+ channel blockade and to increases in heart rate. Implications for gene‐specific therapy." Circulation 92(12): 3381‐6.

Mexiletine shortens APD during bradycardia in LQT3 Mouse Model

Fabritz, L., P. Kirchhof, et al. (2003). "Effect of pacing and mexiletine on dispersion of repolarisation and arrhythmias in DeltaKPQ SCN5A (long QT3) mice." Cardiovasc Res 57(4): 1085‐93.

Mexiletine prevents inducible pVT in LQT3 Mouse Model


Nifedipine

Scn5a+/Δ + 1 µM nifedipine

C

Scn5a+/Δ VT

A

* *

100ms

100ms

WT alone WT + 1 µM nifedipine

B

D

100ms

100ms

Results

Effects of L-type Ca2+ channel antagonism on ventricular arrhythmogenesis in ΔKPQ Scn5a (long QT 3) murine hearts (2006). Thomas, G, Killeen, MJ, Gurung IS, Hakim, P, Goddard, CA, Colledge WH, Grace, AA, Huang, C L-H. J Physiol (In press)

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+ 100 nM Nifed

Scn5a+/Δ

+ 1 µM Nifed

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Scn5a+/Δ

+ 300 nM Nifed

Results

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-10 -9 -8 -7 -6 -50

20

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100

log [nifedipine] M

Results• IC50 = 79 nM

• Nifedipine exerts IC50block of ICa-L at 50nM at holding potential of -40 mV in isolated guinea pig myocytes1

1. Shen, J. B., Jiang, B. & Pappano, A. J. (2000). Comparison of L-type calcium channel blockade by nifedipine and/or cadmium in guinea pig ventricular myocytes. J Pharmacol Exp Ther 294, 562-570.

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f hea

rts

VT

No VT

CResults

-40

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+ 1 µM Nifed

time

(ms)

Epi APD90

Endo APD90

ΔAPD90

ResultsP>0.05

B

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-20

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s)

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+ 1 µM Nifed

Epi APD90

Endo APD90

ΔAPD90

ResultsP>0.05

Figure 5

Scn5a+/Δ

WT

WTScn5a+/Δ Scn5a+/Δ+ 1 µM nifedepine

WT+ 1 µM nifedepine

VER

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s)

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Results

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cur

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A]

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Control 0.3 μM Nif

-40 mV

10 mV

300 nM Nifedipine inhibits L-type Ca2+ current in ΔKPQ Scn5a KPQ mice

Control

Nifed (300nM)

Results

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sity

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/pF]

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300 nM Nifedipine does not inhibit Na+ current in ΔKPQ Scn5a KPQ mice

Control

Nifed (300nM)

Nicorandil

Effect of Nicorandil on Clinical TWA26 year old Female with nocturnal TdPQTc 620 ms

Fujimoto, Y., H. Morita, et al. (1999). "Nicorandil abolished repolarisation alternans in a patient with idiopathic long QT syndrome." Heart 82(5): e8.

6 mg IV (182 ng/ml)

Effect of Nicorandil on Human MAPs

Fujimoto, Y., H. Morita, et al. (1999). "Nicorandil abolished repolarisation alternans in a patient with idiopathic long QT syndrome." Heart 82(5): e8.

MAPs recorded from inferoseptal LV

Anti‐arrhythmic effects of Nicorandil in LQT3 Mouse Model

Hothi, S. S., S. W. Booth, et al. (2008). "Arrhythmogenic substrate and its modification by nicorandil in a murine model of long QT type 3 syndrome." Prog Biophys Mol Biol 98(2‐3): 267‐80.

Nicorandil reduces spatial TDR


Nicorandil reduces temporal TDR


Untreated SCN5a +/Δ SCN5a +/Δ & Nicorandil 20 µM


Nicorandil reduces temporal TDR

Summary

• LQT3 remains lethal in high risk groups

• Identifiable lower risk groups

• Triggered initiation maintained via re‐entry (spatial/temporal heterogeneity of repolarization)

• Anti‐adrenergic strategy reduces ‘triggers’

• Mexiletine/nifedipine/nicorandil offer anti‐arrhythmic effects

Acknowledgements

• Dr Andrew Grace

• Prof Chris Huang

• Dr Matthew Killeen

• Dr Sandeep Hothi

ECG Characteristics



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