Johanna Palmio, MD,
PhD
Sa u Sandell, MD, PhD
Michael G. Hanna, MD,
PhD
Roope Männikkö, PhD
Sini Pen ilä, MSc
Bja ne Udd, MD, PhD
Co espondence o
D . Palmio:
[email p o ec ed]i
Supplemen al da a
a Neu ology.o g
P edominan ly myalgic pheno ype caused
by he c.3466G.A p.A1156T mu a ion
in SCN4A gene
ABSTRACT
Objec i e: To cha ac e ize he clinical pheno ype in pa ien s wi h p.A1156T sodium channel
mu a ion.
Me hods: Twen y-nine Finnish pa ien s iden i ied wi h he c.3466G.A p.A1156T mu a ion in he
SCN4A gene we e ex ensi ely examined. In a subsequen s udy, 63 pa ien s wi h simila myalgic
pheno ype and wi h nega i e esul s in myo onic dys ophy ype 2 gene ic sc eening (DM2-neg
g oup) and 93 pa ien s diagnosed wi h ib omyalgia we e sc eened o he mu a ion. Func ional
consequences o he p.A1156T mu a ion we e s udied in HEK293 cells wi h whole-cell pa ch
clamp.
Resul s: The main clinical mani es a ion in p.A1156T pa ien s was no myo onia o pe iodic pa al-
ysis bu exe cise- and cold-induced muscle c amps, muscle s i ness, and myalgia. EMG myo onic
discha ges we e de ec ed in mos bu no all. Elec ophysiologic compound muscle ac ion po en-
ials exe cise es showed a iable esul s. The p.A1156T mu a ion was iden i ied in one pa ien
in he DM2-neg g oup bu no in he ib omyalgia g oup, making a o al o 30 pa ien s so a iden-
i ied. Func ional s udies o he p.A1156T mu a ion showed mild a enua ion o channel as
inac i a ion.
Conclusions: The unspeci ic symp oms o myalgia s i ness and exe cise in ole ance wi hou clin-
ical myo onia o pe iodic pa alysis in p.A1156T pa ien s make he diagnosis challenging. The
symp oms o milde SCN4A mu a ions may be con used wi h o he simila myalgic synd omes,
including ib omyalgia and myo onic dys ophy ype 2. Neu ology
®
2017;88:1520–1527
GLOSSARY
CK 5c ea ine kinase; CMAP 5compound muscle ac ion po en ials; DM2 5myo onic dys ophy ype 2; Hype PP 5hype -
kalemic pe iodic pa alysis; PMC 5pa amyo onia congeni a.
Mu a ions in he sodium channel gene SCN4A encoding he Na 1.4 ol age-ga ed sodium
channel a e well-known causes o he skele al muscle channelopa hies: pa amyo onia congeni a
(PMC), o he o ms o myo onia, and pe iodic pa alyses (hype kalemic pe iodic pa alysis [Hy-
pe PP], no mokalemic, and hypokalemic pe iodic pa alysis). To da e, .40 dominan mu a ions
in SCN4A ha e been epo ed o cause a iable pheno ypes, depending on he ype and loca ion
o he mu a ions.
1,2
In addi ion, a e ecessi e SCN4A mu a ions ha e been associa ed wi h
congeni al myas henia o congeni al myopa hy.
3,4
The ype o channel de ec and he deg ee o
depola iza ion accoun o clinical symp oms; memb ane hype exci abili y causes myo onia,
inc eased memb ane depola iza ion, and inexci abili y, leading o pa alysis,
5–7
while memb ane
hypoexci abili y unde lies myas henias and myopa hies.
The c.3466G.A p.A1156T mu a ion in he SCN4A gene was o iginally epo ed in a amily
o Finnish o igin wi h incomple e pene ance. The pheno ype a ied and consis ed o ea u es o
Hype PP, PMC, and myo onia.
8
We now epo clinical and elec ophysiologic indings in 30
F om he Neu omuscula Resea ch Cen e (J.P., S.P., B.U.), Depa men o Neu ology, Tampe e Uni e si y and Uni e si y Hospi al, Neu ology;
Seinäjoki Cen al Hospi al (S.S.), Depa men o Neu ology, Finland; MRC Cen e o Neu omuscula Disease (M.G.H., R.M.), UCL Ins i u e o
Neu ology, Queen Squa e, London, UK; Folkhälsan Ins i u e o Gene ics and he Depa men o Medical Gene ics (B.U.), Haa man Ins i u e,
Uni e si y o Helsinki; and Vaasa Cen al Hospi al (B.U.), Depa men o Neu ology, Finland.
Go o Neu ology.o g o ull disclosu es. Funding in o ma ion and disclosu es deemed ele an by he au ho s, i any, a e p o ided a he end o he a icle.
The A icle P ocessing Cha ge was paid by Medical Resea ch Council.
This is an open access a icle dis ibu ed unde he C ea i e Commons A ibu ion License 4.0 (CCBY), which pe mi s un es ic ed use,
dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed.
1520 Copy igh © 2017 The Au ho (s). Published by Wol e s Kluwe Heal h, Inc. on behal o he Ame ican Academy o Neu ology
Finnish pa ien s wi h he c.3466G.Ap.
A1156T mu a ion, including 1 pa ien ound
in he sc eening s udy. Thei unspeci ic, p e-
dominan ly myalgic pheno ype was e y con-
sis en in all pa ien s lacking mos o he o he
ypical ea u es o SCN4A channelopa hies.
The ela i ely modes gain in he unc ion o
p.A1156T channel may explain his unusual
clinical pheno ype o en lacking myo onic
indings.
METHODS p.A1156T pa ien s. We iden i ied 29 Finnish
pa ien s om 18 di e en amilies who had simila clinical phe-
no ype and a con i med c.3466G.A p.A1156T mu a ion in
SCN4A ( able e-1 a Neu ology.o g). The index amily (F1)
consis ed o 7 a ec ed membe s who had muscle s i ness and
myalgia ( igu e 1). Th ee o hem had an ea lie diagnosis o
congeni al myo onia based on myo onic discha ges obse ed on
EMG examina ion, bu when gene ic es ing became a ailable,
he gene ic de ec s in he chlo ide channel gene CLCN1 could be
uled ou . Fou amilies (F1–F4) had se e al a ec ed membe s in
2 o 3 gene a ions ( igu e 1); o he s we e single pa ien s, bu
many o hem had amily membe s wi h simila muscle symp-
oms. The pa ien s unde wen clinical examina ions by neu ol-
ogis s, including manual muscle s eng h e alua ion, assessmen
o clinical myo onia (g ip myo onia, pe cussion myo onia, and
eyelid myo onia) and pa amyo onia ( epea ed ac ion myo onia),
c ea ine kinase (CK) le els, and elec ophysiologic s udies.
Muscle biopsy was a ailable in 12 pa ien s. Th ee pa ien s we e
also examined by muscle MRI.
Sc eening s udy o myo onic dys ophy ype 2–nega i e
and ib omyalgia pa ien s. Six y- h ee pa ien s who we e ea -
lie suspec ed o ha e myo onic dys ophy ype 2 (DM2) on he
basis o exe cise-induced myalgia, s i ness, and EMG indings
and who had had nega i e esul s in he gene ic es ing o he
CNBP epea expansion mu a ion causing DM2 we e included in
he sc eening s udy (DM2-neg g oup). They had no mal o
Figu e 1 Pedig ee o he amilies
*DNA a ailable.
Neu ology 88 Ap il 18, 2017 1521
sligh ly ele a ed CK le els (up o 2 imes he uppe no mal limi ),
no e idence o ixed muscle weakness, and mino indings on
muscle biopsy, i.e., small 2A ibe s o inc eased amoun o
in e nal nuclei. On EMG, hey had ei he inc eased inse ional
ac i i y o some myo onic discha ges. In addi ion, 93 pa ien s
wi h a diagnosis o ib omyalgia bu no suspicion o myopa hy
we e sc eened o SCN4A exon 19 mu a ions.
Elec ophysiologic s udies. S anda d neu og aphy and EMG
in es iga ion was pe o med in 25 pa ien s wi h p.A1156T.
A minimum, 2 mo o and 2 senso y ne e conduc ion s udies
we e pe o med, and bo h p oximal and dis al muscle g oups
in es iga ed om a leas 1 uppe and 1 lowe limb. Compound
muscle ac ion po en ials (CMAP) exe cise es was ca ied ou in
14 pa ien s. A Fou nie p o ocol was used
9,10
consis ing o sho
(10–12 seconds) and long (5 minu es) exe cise es s. CMAP we e
e oked by sup amaximal ne e s imula ion. Tes ed muscles we e
he abduc o digi i minimi and ex enso digi o um b e is.
Muscle biopsy. Rou ine diagnos ic his ologic and his ochemical
s ainings we e pe o med in 12 pa ien s. In addi ion, chlo ide
channel CLC-1 immunohis ochemis y was pe o med in 4
samples wi h a published me hod.
11
Gene ic s udies. Genomic DNA was ex ac ed om leukocy es
by s anda d me hods. A he beginning o he in es iga ions, 13
pa ien s we e es ed o congeni al myo onia (CLCN1, a leas
he 3 mos common mu a ions in Finland), 14 o DM2 (CNBP
epea expansion mu a ion), and 2 o DM1 (DMPK epea
expansion mu a ion). The e was a he e ozygous known ecessi e
mu a ion in CLCN1 gene in 2 pa ien s, whe eas o he esul s
we e nega i e. Fo all pa ien s bu one (P13), he whole coding
sequence o exon 19 o SCN4A was sc eened. The egion s udied
was ampli ied by PCR and di ec ly sequenced wi h he Big-Dye
Te mina o e sion 3.1 ki on an ABI3130xl au oma ic DNA
sequence sys em (Applied Biosys ems, Fos e Ci y, CA). Se-
quences we e analyzed wi h Sequenche 5.1 so wa e (Gene
Codes Co po a ion, Ann A bo , MI). The p ime s used a e
a ailable on eques . Fo P13, he gene ic analysis was pe o med
using a ge ed nex -gene a ion sequencing as p e iously
desc ibed
12
wi h e sion 2 o he MYOcap gene panel ha is
a ge ed o he exons o 236 genes known o p edic ed o cause
muscula dys ophy o myopa hy.
Mu agenesis, in i o ansc ip ion, HEK293 cell
ans ec ion, and whole-cell pa ch clamp. The human
SCN4A exp ession clone was a gi om S.C. Cannon (Uni e si y
o Texas Sou hwes e n Medical Cen e , Dallas). Si e-di ec ed
mu agenesis was pe o med wi h he QuikChange ki (S a-
agene) and con i med by sequencing he en i e inse . HEK293
cells we e ans ec ed wi h 0.5 mg wild- ype o mu an plasmid
oge he wi h 50 ng plasmid coding o cop-GFP wi h Lip-
o ec amine 2000 (In i ogen, Wal ham, MA) in a 1.9-cm
2
well.
HEK293 cells wi h g een luo escence we e ol age clamped a
oom empe a u e 48 o 72 hou s a e ans ec ion wi h Ax-
opa ch 200B, Digida a 1440B, and pClamp so wa e (all Axon
Ins umen s, Sunny ale, CA). Ex acellula solu ion was (in
mmol/L): NaCl 145, KCl 4, MgCl2 1, CaCl2 2, and HEPES 10,
pH 7.4 (NaOH). Pa ch elec odes we e illed wi h pipe e solu-
ion (in mmol/L): NaCl 5, CsCl 145, EGTA 10, and HEPES 10,
pH 7.3 (CsOH). Liquid junc ion po en ial was es ima ed a
24.4 mV and no co ec ed o . Cu en s we e low-pass il e ed
a 5 kHz and sampled a 50 kHz. Se ies esis ance e o was kep
below 5 mV. Da a we e analyzed and illus a ed wi h pClamp,
O igin (O iginLab, No hamp on, MA), and Excel (Mic oso ,
Redmond, WA) so wa e. The ol age p o ocols a e desc ibed in
he igu e legends. The cu en - and conduc ance- ol age ela-
ionships we e i ed wi h he Bol zmann equa ion: G 5A1(B
2A)/{1 1exp[(V
1/2
2V)/V
slope
)]}, whe e A and B a e he
maximum and minimum ampli udes, V
1/2
is he ol age whe e
ampli ude is (A 2B)/2, and V
slope
is he slope ac o . The ime
cou se da a o eco e y om inac i a ion and o onse o as
inac i a ion we e i ed wi h single o double exponen ial unc-
ions, espec i ely. S a is ical compa isons we e pe o med wi h
he S uden es .
S anda d p o ocol app o als, egis a ions, and pa ien
consen s. The s udy was app o ed by he Ins i u ional Re iew
Boa d o Tampe e Uni e si y Hospi al. All pa icipan s p o ided
app op ia e consen , and he s udy was conduc ed acco ding o
he Helsinki Decla a ion.
RESULTS Clinical and gene ic indings o he pa ien s
wi h p.A1156T. In he pa ien coho , 17 we e emale
and 12 we e male. The age a onse o symp oms
anged om 4 o 53 yea s (mean 28.7 yea s). How-
e e , only 3 pa ien s epo ed an ea ly-childhood
onse o muscle symp oms; he mo e ypical age a
onse was be ween 20 and 40 yea s. The common
clinical ea u es we e exe cise and cold-induced
muscle c amps, muscle s i ness, and myalgia. Se -
e al pa ien s also expe ienced muscle weakness o
a igue du ing and sho ly a e exe cise, al hough
wi hou pa aly ic episodes (e.g., pa alysis a es a e
exe cise). Muscle s eng h was no mal in all bu 2
pa ien s on clinical examina ion. CK le el was sligh ly
inc eased in 1 pa ien (2.5 imes he uppe no mal).
Muscle biopsies showed mild abno mali ies, i.e., a e
highly a ophic ibe s o inc eased in e nal nuclei.
Howe e , he indings we e conside ed no mal in 7
pa ien s. Clinical myo onia (g ip, pe cussion, eyelid
myo onia) o ypical pa amyo onia ( epea ed ac ion
myo onia) was no e iden on clinical examina ion.
The diagnosis o ib omyalgia was e y common
among he amily membe s o he pa ien s. Clinical
de ails o each pa ien a e shown in able e-1.
All 29 pa ien s we e iden i ied wi h he he e ozy-
gous c.3466G.A p.A1156T mu a ion.
8
Two un e-
la ed pa ien s, P16 and P18, we e also ca ie s o he
ecessi e c.2680C.T p.R894X mu a ion in he
CLCN1 gene. Bo h had no mal CLC-1 p o ein
exp ession as judged by immunohis ochemical s ain-
ing, indica ing no addi ional de ec in he co espond-
ing gene.
11
Sc eening o he DM2-neg and ib omyalgia g oups. No
mu a ions we e obse ed in he ib omyalgia g oup.
In he DM2-neg g oup, one pa ien (P30) was
ound o ha bo he p.A1156T mu a ion. She had
exe cise-induced myalgia and c amps s a ing a age
28. Clinical examina ion was no mal, and he e was
no elec ophysiologic myo onia, bu EMG showed
inc eased inse ional ac i i y wi h uncha ac e is ic
epe i i e discha ges. She had occasional ele a ed CK
le els (2 imes he uppe no mal limi ), and he
1522 Neu ology 88 Ap il 18, 2017
muscle biopsy om gas ocnemius medialis muscle
was no mal.
Muscle MRI. Two siblings in amily 3 had mild degen-
e a i e changes in as us la e alis muscles, al hough
muscle s eng h was no mal on manual es ing. One
pa ien (P22) had mild degene a i e indings on
MRI in he le high muscles. She had had a ac u e
o he le emu 10 yea s ea lie due o ib ous dyspla-
sia o bone ha migh con ibu e o he imaging ind-
ings and o he clinical inding o muscle weakness on
he le lowe limb. The one pa ien om he DM2-
neg g oup who had he p.A1156T mu a ion in he
sc eening s udy showed no mal muscle MRI indings
a he age o 30.
Elec ophysiologic indings. EMG myo onic discha ges
we e de ec ed in mos bu no all pa ien s. EMG was
no s udied in 5 symp oma ic mu a ion ca ie amily
membe s o he p obands. Six een pa ien s had myo-
onia on EMG a some poin , bu in e es ingly, in 6
o hem, myo onic discha ges disappea ed on
epe i i e examina ions wi h mo e ad anced age
( able 1). The es o he pa ien s showed inc eased
inse ional ac i i y only. Thus, none o he ini ial
EMG s udies we e comple ely no mal. Fou een pa-
ien s unde wen CMAP exe cise es (Fou nie p o-
ocol) wi h a iable esul s. I was no mal in 6
pa ien s; 3 pa ien s showed a 35% o 55% dec ease
o CMAP ampli ude a e he sho exe cise; 4 pa-
ien s had a 28% o 60% dec ease a e he long
exe cise; and 2 pa ien s had a 30% o 44% dec ease
a e cooling.
Func ional s udies. When exp essed in HEK293 cells,
he p.A1156T channels ac i a ed no mally ( igu e 2,
A–C). Fas inac i a ion o p.A1156T channel was
mildly a enua ed compa ed o he wild- ype chan-
nel: V1/2 was shi ed ,3 mV o depola ized ol ages
(p,0.05) ( igu e 2D), and he eco e y om inac-
i a ion was accele a ed 1.7- old (p,0.001) ( igu e
2E). No signi ican changes in he a e ( igu e 2F) o
comple eness o he open-s a e inac i a ion we e de-
ec ed (no shown). The ol age dependence o slow
Table 1 Elec ophysiologic indings o pa ien s wi h p.A1156T
Pa ien Myo onia on EMG CMAP exe cise es
P1 (F1 II:2) No. Inc eased inse ion ac i i y, epe i i e discha ges No done
P3 (F1 II:4) Yes, a age 40; inc eased inse ion ac i i y only a age 55 No mal, sho exe cise induces myo onic po en ials
P4 (F1 III:3) Yes Long exe cise: 60% dec ease o CMAP ampli ude
P5 (F1 III:5) No. Inc eased inse ion ac i i y No mal
P6 (F1 III:6) Yes, a ages 16 and 20; inc eased inse ion ac i i y only a age 30 No mal (a age 30)
P8 (F2 II:5) Yes. Myo onia inc eased by cooling No done
P9 (F2 III:1) Yes, a i s s udy. Inc eased inse ion ac i i y only a age 49 Sho exe cise: 35% dec ease; cooling: 44% dec ease
P12 (F3 II:1) No. Inc eased inse ion ac i i y, epe i i e discha ges No mal
P13 (F3 II:2) No. Inc eased inse ion ac i i y, epe i i e discha ges Cooling: 30% dec ease o CMAP ampli ude
P14 (F4 II:2) Yes No done
P16 Yes No mal
P17 Yes No done
P18 Yes Sho exe cise: 35% dec ease o CMAP ampli ude
P19 No. Inc eased inse ion ac i i y No done
P20 Yes No mal
P21 Yes No done
P22 No. Inc eased inse ion ac i i y No done
P23 Yes Long exe cise: 31% dec ease o CMAP ampli ude
P24 No. Inc eased inse ion ac i i y, epe i i e discha ges No done
P25 Yes, a i s s udy. Inc eased inse ion ac i i y, epe i i e discha ges a age 58 Long exe cise: 28% dec ease o CMAP ampli ude
P26 Yes, a i s s udy. Inc eased inse ion ac i i y only a age 40 Sho exe cise: 55% dec ease o CMAP ampli ude
P27 No. Inc eased inse ion ac i i y No done
P28 Yes No done
P29 Yes, a i s s udy. No myo onia a age 18 Long exe cise: dec ease in du a ion o po en ials
P30 (DM2-neg) No. Inc eased inse ion ac i i y, epe i i e discha ges No done
Abb e ia ions: CMAP 5compound muscle ac ion po en ials; DM2 5myo onic dys ophy ype 2.
Neu ology 88 Ap il 18, 2017 1523
Figu e 2 Func ional cha ac e iza ion o p.A1156T channel
(A) Rep esen a i e cu en aces o wild- ype and p.A1156T channels in esponse o es ol ages anging om 260 o
50 mV. Scale ba s a e 1 millisecond (x-axis) and 20 pA/pF (y-axis). Dashed lines indica e 0 cu en le el. Vol age p o ocol o
ol age s eps anging om 2150 o 50 mV is shown in inse . (B) Peak cu en ampli ude in esponse o es ol ages anging
om 2150 o 50 mV in 10-mV inc emen s is plo ed agains he es ol age o wild- ype (solid ci cles) (n 518) and
p.A1156T (open squa es) (n 511) channels. (C) Vol age dependence o ac i a ion was es ima ed by plo ing he conduc ance
[peak cu en /( es ol age 2 e e sal ol age)] agains he es ol age o wild- ype (solid ci cles) (V
1/2
5220.3 60.7 mV, n 5
18) and p.A1156T (open squa es) (V
1/2
5220.4 60.7 mV, n 511) channels. Indi idual da a we e no malized o maximum
and minimum ampli ude o he Bol zmann i and a e aged. Solid lines ep esen he i o he Bol zmann equa ion o he mean
da a. (D) Vol age dependence o as inac i a ion was es ima ed by plo ing he peak ail cu en ampli ude a 210 mV a e
150-millisecond p epulse ol age s eps anging om 2150 o 0 mV in 10-mV inc emen s agains he p epulse ol age o wild
ype (solid ci cles) (V
1/2
5266.2 60.7 mV, n 518) and p.A1156T (open squa es) (V
1/2
5263.5 61.0 mV, n 511) channels.
Con inued
1524 Neu ology 88 Ap il 18, 2017

inac i a ion o p.A1156T channel was shi ed 7 mV
o he hype pola ized di ec ion compa ed o he wild-
ype channel ( igu e 2G) (p,0.001).
DISCUSSION Ou la ge coho o 30 pa ien s ca y-
ing he c.3466G.A p.A1156T mu a ion in he
SCN4A gene showed a consis en pheno ype o p e-
dominan myalgia, muscle s i ness, and exe cise
c amps wi hou signs o clinical myo onia, pa amyo-
onia, o pe iodic pa alyses. This also explains he
incomple e pene ance o myo onia epo ed in he
o iginal amily.
8
P edominan myalgic pheno ype ex-
pands he spec um o SCN4A channelopa hies and
in e e es wi h he e y la ge g oup o pa ien s wi h
myalgia. Rela i ely modes gain in he unc ion o
p.A1156T channel in whole-cell pa ch clamp s ud-
ies may explain he absence o clinical myo onia.
Mu a ions in SCN4A gene ypically p oduce se -
e al di e en sub ypes o skele al muscle diso de s con-
sis ing mainly o clinical and elec ophysiologic
myo onia o pe iodic pa alyses. Di e en mu a ions
usually accoun o di e en ion channel cha ac e is ics
and pheno ypes, al hough some o he mu a ions can
cause se e al pheno ypes.
2
The c.3466G.A p.A1156T
mu a ion has been epo ed wi h mos o he SCN4A
mani es a ions: PMC, Hype PP, and pu e myo o-
nia.
8,13
The a icles desc ibing hese pa ien s did no
epo myalgia as pa o he clinical mani es a ion.
The main symp om in he i s epo ed amily wi h
he p.A1156T mu a ion was Hype PP, which none o
ou pa ien sdemons a ed.
8
O 2 un ela ed Ko ean
pa ien s iden i ied wi h c.3466G.Ap.A1156T,1
pa ien s had myo onia wi h a wa m-up phenomenon
esembling chlo ide channel myo onia, and he o he
had Hype PP. Myo onic discha ges we e p esen on
EMG in bo h pa ien s.
13
The clinical p esen a ion in
ou la ge pa ien se ies was no consis en wi h pe i-
odic pa alysis o pa amyo onia, al hough many pa ien s
had expe ienced wo sening o he symp oms in cold
and some du ing epe i i e exe cise esembling pa a-
doxical myo onia. Two pa ien s wi h an addi ional he -
e ozygous CLCN1 mu a ion did no di e la gely om
he es o he pa ien s, al hough he e ozygous CLCN1
mu a ions ha e been ound o modula e pheno ype in
sodium channel myo onia.
14
The indings in elec ophysiologic s udies, bo h eg-
ula and CMAP exe cise es s, we e a iable and did
no di ec ly show ypical sodium channel pa e ns,
no we e hey clea ly compa ible wi h clinical symp-
oms as p e iously sugges ed.
9,10,15
Al hough mos o
he s udied pa ien s had myo onia on EMG a some
poin , hey did no show clinical myo onia, and u -
he mo e, myo onic discha ges could disappea la e in
li e, as was obse ed in 6 o ou pa ien s. The indings
on he CMAP exe cise es we e no mal in 43% o he
pa ien s s udied and inconsis en in he es .
Ou unc ional da a a e consis en wi h p e ious e-
po s showing a enua ed as inac i a ion o p.
A1156T channels.
16–18
The p.A1156T channels
showed a mild shi in he ol age dependence o as
inac i a ion and accele a ion o eco e y om as inac-
i a ion. Howe e , in con as o he p e ious epo s,
we did no ind de ec s in he ime cou se o open-s a e
as inac i a ion o signs o pe sis en la e cu en s.
While he unc ional exp ession epo s on A1156T
show some disc epancies, de ec i e as inac i a ion,
in pa icula accele a ed eco e y om inac i a ion, is
a consis en ea u e.
16–18
The gain o unc ion caused
by a enua ed inac i a ion is consis en wi h myo onic
pheno ype. The ela i ely modes shi in he ol age
dependence o as inac i a ion and accele a ion in he
a e o eco e y om inac i a ion and he absence o
de ec s in he open-s a e inac i a ion may unde lie he
milde clinical pheno ype in which clinical myo onia is
no he p edominan p esen a ion. In addi ion, i has
been sugges ed ha a enua ed slow inac i a ion is
common o SCN4A a ian s associa ed wi h pe iodic
pa alysis. The slow inac i a ion o p.A1156T channel
Figu e 2 legend, con inued:
The ol age p o ocol is shown on he igh . Indi idual da a we e no malized o maximum and minimum alues o he
Bol zmann equa ion and a e aged. Solid lines ep esen he i o he Bol zmann equa ion o he mean da a. (E) To es ima e
he ime cou se o eco e y om he inac i a ion, he cu en in esponse o a second ol age pulse o 0 mV (P2) di ided by
he cu en in esponse o he i s 10-millisecond pulse o 0 mV (P1) is plo ed agains he du a ion o he eco e y s ep
a 280 mV be ween he 2 pulses. Da a a e shown o wild- ype (solid ci cles) ( 56.5 60.4 milliseconds, n 516) and
p.A1156T (open squa es) ( 53.8 60.3 milliseconds, n 510) channels. The ol age p o ocol is shown in he inse . The solid
lines ep esen he i o exponen ial unc ion o he mean da a. (F) Time cou se o open-s a e inac i a ion was es ima ed by
i ing a double exponen ial unc ion o he cu en decay be ween 90% o peak cu en ampli ude and he cu en baseline.
Time cons an ( ) o inac i a ion a ol ages anging om 220 o 20 mV is shown o wild ype (solid ci cles) (n 518) and
p.A1156T (open squa es) (n 511) channels. Vol age p o ocol was as in panel (A). Only he ime cons an o he as
componen ha ca ies z95% o he ampli ude o he inac i a ing cu en is shown. (G) Vol age dependence o slow
inac i a ion was s udied by plo ing he peak ail cu en ampli ude a 210 mV a e a 10-second p epulse o ol age s eps
anging om 2130 o 50 mV in 10-mV inc emen s agains he p epulse ol age o wild- ype (solid ci cles) (V
1/2
5251.2 6
0.9 mV, n 512) and p.A1156T (open squa es) (V
1/2
5258.3 61.0 mV, n 57) channels. Be ween he p epulse and he es
pulse, he ol age was s epped o 2100 mV o 20 milliseconds o allow he channel o eco e om as inac i a ion. The
ol age p o ocol is shown in he inse . Indi idual da a we e no malized by di iding wi h maximum alue o he Bol zmann
equa ion and a e aged. Solid lines ep esen he i o he Bol zmann equa ion o he mean da a.
Neu ology 88 Ap il 18, 2017 1525
was enhanced compa ed o he wild- ype channel, con-
sis en wi h he absence o pe iodic pa alysis in ou
p.A1156T coho .
18
Myalgia is a e y common symp om in he gene al
popula ion, bu i is only a ely caused by an unde -
lying muscle disease. A p esen , no use ul guidelines
exis on how o cla i y he cause o myalgia.
19
Pain ul
c amps o myalgia wi h myo onia ha e been epo ed
in a ew amilies wi h o he SCN4A mu a ions,
20–23
bu o he bes o ou knowledge, myalgia wi hou
myo onic discha ges on EMG has no been epo ed
be o e. Besides p.A1156T SCN4A mu a ion, DM2 is
an impo an diso de o be conside ed in di e en ial
diagnos ics o myalgic synd omes. In he ea ly s ages
o DM2, he symp oms esemble hose o p.A1156T
because clinical o EMG myo onia also can be absen
in DM2.
24,25
The age a onse o muscle symp oms
was ea lie in ou coho han in ypical cases o DM2.
Clea ly, ele a ed CK le els, ixed muscle weakness, o
o he myopa hic signs on muscle his ology, imaging,
o EMG sugges DM2 a he han milde o m o
SCN4A disease. EMG seems o be a use ul ool
because he indings we e no comple ely no mal in
any o ou pa ien s. Howe e , he mildes change, i.e.,
inc eased inse ional ac i i y, can be easily o e looked
and no conside ed clinically ele an .
In he Exome Agg ega ion Conso ium da abase,
he mu a ion c.3466G.A p.A1156T has been iden-
i ied in 2 o 3,307 Finnish indi iduals. I his allele
equency o he mu a ion is ep esen a i e o he
whole popula ion, he e a e some 3,000 indi iduals
ca ying he mu a ion in Finland. E en wi h a lowe
equency and conside ing he usual adul onse o
symp oms, i can be es ima ed ha some 1,000 pa-
ien s in Finland may ha e myalgia, s i ness, and
c amps caused by his mu a ion.
Milde mu a ions in SCN4A gene may hus
unde lie myalgic synd omes, especially i he pa ien
has cold-induced wo sening o he symp oms and
inc eased inse ional ac i i y wi h o wi hou myo-
onic discha ges on EMG.
AUTHOR CONTRIBUTIONS
Johanna Palmio: s udy concep and design, d a ing o he manusc ip ,
analysis and in e p e a ion o da a, acquisi ion o da a. Sa u Sandell:
d a ing o he manusc ip , analysis and in e p e a ion o da a, acquisi ion
o da a. Michael G. Hanna: analysis and in e p e a ion o da a, acquisi-
ion o da a. Roope Männikkö and Sini Pen ilä: d a ing o he manu-
sc ip , analysis and in e p e a ion o da a, acquisi ion o da a. Bja ne
Udd: s udy concep and design, acquisi ion o da a, analysis and in e -
p e a ion o da a, c i ical e ision o manusc ip o in ellec ual con en ,
s udy supe ision.
STUDY FUNDING
Funded by he Mai e Taponen Founda ion (J.P.), Tampe e Uni e si y
Hospi al Resea ch Founda ion and Li & Hälsa Founda ion (B.U.),
and UK Medical Resea ch Council p ojec g an MR/M006948/1
(R.M.). M.G.H, and R.M. we e suppo ed by he Uni e si y College
London Hospi als Biomedical Resea ch Cen e.
DISCLOSURE
The au ho s epo no disclosu es ele an o he manusc ip . Go o
Neu ology.o g o ull disclosu es.
Recei ed Oc obe 10, 2016. Accep ed in inal o m Janua y 20, 2017.
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Neu ology 88 Ap il 18, 2017 1527
DOI 10.1212/WNL.0000000000003846
2017;88;1520-1527 Published Online be o e p in Ma ch 22, 2017Neu ology Johanna Palmio, Sa u Sandell, Michael G. Hanna, e al.
geneSCN4A
P edominan ly myalgic pheno ype caused by he c.3466G>A p.A1156T mu a ion in
This in o ma ion is cu en as o Ma ch 22, 2017
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