Molecular Genetics of Sodium Channel Myotonias

Michael Hanna

CINCH NDM Meeting Kansas 3 June 2007

MRC Centre for Neuromuscular Disease

Institute of Neurology

Queen SquareLondon

Overview

• Sodium channel gene

• Known phenotype-genotype links

• Review recent UK study of SCN4A gene and myotonia

α-subunit Complete Ion channel without accessory subunits

++

++

++

++

Nav1.4

Cav1.1x1

COOH

I

IIIII

IV

NHCOOH

Kir2.1 x4

x2CLC-1

C

D

B

E F G HI J

K

L M NO

PQ

R

A

NH

COOH

+++

-

--

NH

I II III IV

Skeletal muscle Sodium channel gene

Chromosome 17q24 exons36 point mutations all missenseMultiple foundersImpaired inactivation

++

++

++

++

Phenotype Common genotype

• Hypo PP R672H/G/S• Hyper PP T704M• K sensitive normoPP R675G/Q/W• HyperPP/PC R1448C/H• Cold induced HypoPP/myt P1158S• Paramyotonia congenita T1313A/M• Painful cong myotonia V445M• PAM-MF MP G1306A/V/E

Queen Square PP Database

Genetically defined June 2007:

• CACNA1S 26 R528H 34 R1239H

1 R1239G

• SCN4A 40 T704M13 M1592V 4 R675G 1 R672S

SCN4A genotype-phenotype

40 T704M HyperPP+/- myt

13 M1592V HyperPP+/- myt

4 R675G K sens-Normo

1 R672S HypoK

UK study Sodium channel Myotonia

• Aim –identify a sodium channel myotonia cohort

• Source-UK NSCAG Clinical database muscle channelopathies

• Inclusion criteria- “relaxed”Paramyotonia Prominent history cold exacerbation +/-weaknessEMG myotonia

• DM1 /DM2 excluded

• SCN4A automated DNA analysis and detailed clinical analysis of mutation positive vs negative cases

UK study Sodium channel Myotonia

• 39 cases identified

27 exon 22/exon 24 12 SCN4A neg

3/27 PAM mts 2/27 new mts

2/12 T704M 1/12 new mts

Mutation Reported Phenotypes Phenotypes in UK Cohort Frequency in UK cohort (patients/kindred)

Q270K**** PMC PMC 1/1

T704M HyperPPHyperPP/PMC

HyperPP/PMC 2/1

G1306A PAM (MF) PMC/PAM 2/1

G1306E PAM (MP) PMC/PAM 1/1

T1313M PMC PMC 10/6

R1448C PMCHyperPP/PMC

PMCHyperPP/PMC

4/2

R1448H PMCHyperPP/PMC

PMCHyperPP/PMC

2/2

R1448L**** - PMC 1/1

G1456E PMC PMC 3/1

F1473S PMC PMC 1/1 [3/1]

V1589M PAM PMC

PMC 2/1

L1436P**** - PMC 1/1

PAM mutations PMC clinical phenotype

• G1306E Myotonia Permanens

• G1306A Myotonia Fluctuans

• Clinical impression vs EMG criteria

Mutation Frequency in our cohort (patients/kindred)

***Q270K 1/1*** D1-S5

T704M 2/1

G1306A 2/1

G1306E 1/1

T1313M 10/6

R1448C 4/2

R1448H 2/2

***R1448L 1/1*** D4-S4

G1456E 3/1

F1473S 1/1

V1589M 2/1

***L1436P 1/1 **** D4-S4

{I693T 1/1}

R1448L

• Typical PMC

• Paramyotonia

• Cold exacerbation myotonia

• Cold induced weakness

• Additional features – sensorineural deafness, generalised chorea ?unrelated

I693T

L1436P

L1436P

• PMC phenotype

• Newcastle

• Myositis-immunosuppression

• Coincidence?/ other cases

I693T

L1436P

I693T

L1436P

Q270K

• PMC

• Marked pain

• Domain I mutations and pain?

• Treatment resistant

I693T

L1436P

Mutation “negative” cases

• Clear paramyotonia with cold exacerbation• Older age at onset• Prominent painful myotonia• Upper and lower limbs equally affected• Fixed weakness

? Additional SCN4A mutations, ?CLCN1? Non-genetic phenocopies

Summary

• Exon 22/24 hotspot for PMC mutations DNA screening strategy

• Extending SCN4A analysis 75% hit rate

• Clinical distinction between PAM-PMC not always easy ? Genetic overlap cf EMG

• ? Further heterogeneity associated with PMC phenotype

Ion Channel GroupQueen Square

Research NSCAG Service Doreen Fialho Veronica Tann Susie Tomlinson Martin KoltzenburgDimitri Kullmann Emma StanleyNick Wood Cath Woodward

Mary DavisSanj Rajakulendran Mary SweeneyEmma Matthews Dennis StevensStephanie Schorge Andrew HaworthTracey GravesHugh Bostock Funding Support

MRC, Wellcome Trust, ERF, BRT,ULCH-NHS Trust Special Trustees DoH NSCAG. Action Research, CINCH

Copyright ©2005 American Society for Clinical Investigation

Inherited mutations alter ion channel function and structure and cause human disease

Periodic Paralysis

Nondystrophic Myotonias

Hypo-kalaemic

PP

Hyper-kalaemic

PP

Myotonia congenita

Paramyotoniacongenita

AndersenTawil

Syndrome

VG skeletal muscle Na+ channel Nav1.4 (SCN4A)

VG Ca2+ channel (Dihydropyridine receptor) Cav1.1 (CACNA1S)

VG skeletal muscle Cl- channel CLC-1 (CLCN1)

Inward rectifying K+ channel Kir 2.1 (KCNJ2)

Clinical

Classification

Molecular

Classification

Skeletal muscle Na+ channel NaV1.4