Functional Annotation of Human
Ion Channel Variants
Alfred L. George, Jr., M.D. Department of Pharmacology
Genetic Arrhythmia Syndromes
Congenital Long QT Syndrome KCNQ1, KCNH2, SCN5A, KCNE1, KCNE2, ANK2, SCN4B, CAV3, AKAP9, SNTA1
Brugada Syndrome SCN5A, GPD1L, CACNA1C, CACNB2, SCN1B, KCNE3, SCN3B
Andersen Syndrome KCNJ2
Timothy Syndrome CACNA1C
Familial Cardiac Conduction Diseases SCN5A
Short QT Syndrome KCNQ1, KCNH2, KCNJ2
Catecholaminergic Polymorphic VT RYR2, CASQ2, TRND
Familial and Lone Atrial Fibrillation KCNQ1, KCNE2, KCNA5, SCN5A, SCN10A, NPPA
Congenital Long QT Syndrome
Clinical Features • Congenital prolongation of the rate-corrected QT interval QTc 440 msec in symptomatic individuals or QTc 460 in asymptomatic individuals • Recurrent syncope • Cardiac arrest or torsade de pointes • Family history of unexplained and premature sudden death • Incomplete penetrance and variable expressivity • Congenital deafness (recessive Jervell, Lange-Nielson)
• Molecular diagnostics - technically challenging (13 genes)
QT > 440 ms
QT < 440 ms
P
Q
R
S
T
P
Q
R
S
T
Normal ECG
Congenital LQTS
Congenital Long QT Syndrome (LQTS)
Torsades de pointes
Congenital Long QT Syndrome
Genetic Heterogenity in Congenital LQTS
Gene Current Frequency Trigger
LQT1 KCNQ1 ↓ IKs 45% Exercise, emotion
LQT2 HERG ↓ IKr 40% Sudden auditory stimuli
LQT3 SCN5A ↑ INa 10% Sleep
LQT4 Ank2 complex Rare
LQT5 KCNE1 ↓ IKs Rare
LQT6 KCNE2 ↓ IKr Rare
LQT7 KCNJ2 ↓ IK1 Rare
LQT8 CACNA1C ↑ ICa Rare
I Na
I Ca
I Ks
I Kr
I to
I K1
Na +
Ca 2+
K +
extracellular
intracellular
KCNQ1 and KCNE1 Generate IKs
IKs
Slow delayed rectifier current
in heterologous cells 11
KCNQ1
11p15.5
LQT1
21 KCNE1
21q22.1
LQT5
7
Mutations in Genetic Arrhythmias
Gene Mutations in HGMD
KCNQ1 602
KCNH2 910
SCN5A 802
RYR2 308
CACNA1C 57
Genetic Testing for LQTS
• Commercial genetic testing for LQTS has become standard-of-care
• Most variants discovered in LQTS cases are missense
• Most new variants have never been seen before (‘private mutations’)
• Most variants have uncertain functional consequences
• Diagnostic labs reports many ‘variants of uncertain significance’
• No reliable methods to distinguish benign from pathogenic variants
9
Classification of Mutations
Variant of Unknown
Significance Pathogenic
Likely
Pathogenic Benign
Likely
Benign
Strategies for Variant Annotation
Computational (in silico)
Experimental (in vitro)
Experimental (in vivo)
Variants without clear pathogenicity confound genetic diagnosis
2015
Strong evidence for pathogenicity:
Conventional patch clamp:
- 16 measurements/person/day
- days-weeks per mutation
Functional Annotation of Ion Channels Variants
Patch clamp electrophysiology is the gold standard
Functional Annotation of Ion Channels Variants
Automated patch clamp:
- 768 measurements per hour
- multiple variants per day
SyncroPatch 768PE
Patch clamp electrophysiology is the gold standard
Electroporation Automated patch-clamp
Functional Analysis of KCNQ1 Variants by Automated Electrophysiology
Overcoming need for stable cell lines
Electroporation of KCNQ1 and KCNE1 evaluated by flow cytometry
Transient Co-expression of KCNQ1 and KCNE1
Viability: 84.9% Percent Co-transfected: 79.1%
CMV
KCNQ1
IRES2EGFP
Kan R
KCNQ1
CMV
KCNE1
IRES2
DsRed2Kan R
KCNE1
Automated Electrophysiology of IKS (Transient transfection of CHO with KCNQ1/KCNE1)
500 ms
Manual Patch SyncroPatch
Normalized to peak current measured at +60 mV
Automated Electrophysiology of IKS (Transient transfection of CHO with KCNQ1/KCNE1)
Manual Patchn = 18
-50-60 -40 -20 20 40 60
I, pA/pF
Vm, mV
500
400
200
300SyncroPatch
n = 153
-80
100
Vm, (mV)I
/ I m
ax
1.0
0.8
0.6
0.4
0.2
0.0
0 40 6020-20-40-60-80
Manual Patchn = 18
V1/2
= 22.5 +/- 0.8 mV
SyncroPatchn = 153
V1/2
= 27.8 +/- 1.4 mV
Automated Electrophysiology of IKS (Transient transfection of CHO with KCNQ1/KCNE1)
Automated Electrophysiology of IKS (Transient transfection of CHO with KCNQ1/KCNE1)
Au
tom
ated
Man
ual
I132L
WT
C122Y
K196T
R174C
G179S
Automated vs Manual Electrophysiology
I132L
WT
C122Y
K196T
R174C
G179S
Au
tom
ated
Man
ual
Au
tom
ated
Man
ual
Criteria to Classify KCNQ1 Variant Function
Values Relative
to WT IKs
Normal Near
Normal
Mild
Loss of
Function
Severe
Loss of
Function
Severe
Gain of
Function Ipeak
@ +60 mV
90-110% 75-90 &
110-125% 25-75% <25% >150%
or
Activation V½ (mV)
<5 mV depolarization/
hyperpolarization
10-5 mV depolarization/
hyperpolarization
10-20 mV depolarization
>20 mV depolarization
>15 mV hyperpolarization
plus 120-150% Ipeak
Developed from a curated list of high-quality publications
on 107 variants with functional characterization
ClinVar Assertions for 36 KCNQ1 Variants
VUS Conflicting
Interpretation
Likely benign
Likely
Pathogenic
Pathogenic
No assertion
16/36 (44%) have clear classification
Original classifications
Reclassifiying 36 KCNQ1 Variants
Original Functional
Likely pathogenic Severe LOF
VUS Severe LOF
No assertion Severe LOF
No assertion Severe GOF
No assertion Normal
Likely benign Normal
Variant of Unknown
Significance Pathogenic
Likely
Pathogenic Benign
Likely
Benign
Reclassifiying 36 KCNQ1 Variants
Original Functional Reclassification
Likely pathogenic Severe LOF Pathogenic
VUS Severe LOF Likely pathogenic
No assertion Severe LOF Likely pathogenic
No assertion Severe GOF Likely pathogenic
No assertion Normal Likely benign
Likely benign Normal Benign
Variant of Unknown
Significance Pathogenic
Likely
Pathogenic Benign
Likely
Benign
ClinVar Assertions for 36 KCNQ1 Variants
VUS Conflicting
Interpretation
Likely benign
Likely
Pathogenic
Pathogenic
No assertion
16/36 (44%) have clear classification
VUS
Likely benign
Likely
Pathogenic
Pathogenic
30/36 (83%) have clear classification
Original classifications Revised classifications
Automated Electrophysiology of Human
Ion Channels
Sodium channels SCN1A Epilepsy, Dravet syndrome SCN2A Epilepsy, autism spectrum disorder SCN3A Epilepsy SCN5A Cardiac arrhythmia SCN8A* Epileptic encephalopathy SCN9A* Genetic pain disorders SCN10A* Genetic pain disorders SCN11A* Genetic pain disorders Potassium channels KCNE1 Cardiac arrhythmia KCNJ2 Anderson syndrome KCNB1 Epileptic encephalopathy KCNT1* Epileptic encephalopathy * Work in progress
epilepsy, familial migraine
epilepsy, autism
epilepsy
periodic paralysis, paramyotonia
epilepsy
painful neuropathy, congenital insensitivity to pain
LQTS, Brugada syndrome, cardiomyopathy
painful neuropathy, Brugada syndrome
painful neuropathy, congenital insensitivity to pain
>2,100 variants/mutations in HGMD
Sodium Channelopathies
SyncroPatch Recordings of NaV1.1 (stable expression in HEK293)
Holding potential -120 mV; single hole recordings
SyncroPatch Recordings of NaV1.1 (stable expression in HEK293)
SyncroPatch Recordings of NaV1.1 Persistent Current
NaV1.1-R1648H NaV1.1-WT
Pe
rsis
ten
t I N
a (
% P
ea
k)
NaV1.1-R1648H NaV1.1-WT NaV1.1-WT
NaV1.1-R1648H NaV1.1-WT
SyncroPatch Recordings of NaV1.1 Persistent Current
SCN1A Variants of Unknown Significance associated with Dravet syndrome
Transient expression in HEK293
Variants reported by Wu, et al. Pediatrics 2015
Automated Electrophysiology of NaV1.2*
Comparison of manual with automated patch clamp
*NaV1.2 is the sodium channel encoded by SCN2A
p.Glu430Ala
p.Tyr816Phe
p.Arg853Gln
p.Gly879Arg
p.Ala880Ser
p.Gly882Glu
p.Glu999Lys
p.Met1879Thr
p.Arg1882Gln
p.Arg188Trp p.Arg379His p.Arg937Cys p.Arg937His
p.Cys1386Arg p.Thr1420Met p.Lys1422Glu p.Glu1880Lys p.Arg1882Leu
Variants associated with epileptic encephalopathy or autism
Automated Electrophysiology of NaV1.2
Variants associated with epileptic encephalopathy or autism
Automated Electrophysiology of NaV1.2
Variants associated with epileptic encephalopathy or autism
Automated Electrophysiology of NaV1.2
Creation of Neuronal Cell Line (ND7/no-Nav) with suppressed endogenous Na current
Non-transfected
+NaV1.7 siRNA
+CRISPR_NaV1.7
5 ms
100 pA/pF
5 ms
100 pA/pF
ND7/23 ND7/no-Nav
Creation of Neuronal Cell Line (ND7/no-Nav) with suppressed endogenous Na current
Pharmacological Profiling of Sodium
Channel Mutations in Epilepsy
+
Acknowledgments
Northwestern Univesity
Carlos Vanoye, Ph.D.
Christoher Thompson, Ph.D.
Reshma Desai, M.S.
Katarina Fabre, M.S.
Jean-Marc DeKeyser, M.S.
Tatiana Abramova, M.S.
Vanderbilt University
Chuck Sanders, Ph.D.
Jens Meiler, Ph.D.
GeneDx, Inc.
Daniela Macaya, Ph.D.