Sodium

channel gene defects in the periodic

paralyses

Robert 1. Barchi University

of Pennsylvania

Abnormal been

Na+

associated

hallmark various

of the point

School of Medicine,

currents with

periodic

mutations

voltage-dependent

Na+

are responsible

Current

that

the

produce

episodes paralyses.

in the gene channel

muscle

There

Recordings in isolated human fibers have shown that the membrane depolarization seen during paralysis in these diseases is due to an abnormal increase in sarcolemmal conductance to Na+ ions [2]. In several cases this abnormal conductance can be blocked by tetrodotoxin (TTX), suggesting the involvement of a voltage-dependent Na+ channel. The cloning and molecular characterization of Na+ channels from human muscle has recently provided the opportunity to test this association. Through analysis of restriction fragment length polymorphisms, several inherited forms of periodic paralysis have been linked to the region containing the adult skeletal muscle Na+ channel gene on chromosome 17. A number of specific mutations have now been defined, and a molecular definition for these fascinating diseases is rapidly emerging. This review will focus on current concepts of Na+ channel structure and on recent advances linking mutations in the structure of the skeletal muscle Na+ channel to the episodic weakness seen in periodic paralysis.

paralyses

Individuals with periodic paralysis have episodes of weakness that are often associated with characteristic changes in the serum K+ level, [K+ ] [ 1 ] The most common

abnormal

that

have are

evidence skeletal

USA

the that

muscle

currents,

and

of the disease phenotype.

in Neurobiology

The periodic paralyses are a group of rare muscle disorders that share the common phenotype of episodic skeletal muscle weakness or paralysis in the absence of abnormalities of the motor nerve, neuromuscular junction, or contractile proteins [ 11. These episodes of weakness often occur in individuals who appear clinically normal between attacks, although in some forms of periodic paralysis a persistent myopathy can eventually develop. The paralytic episodes are associated with acute depolarization of the muscle sarcolemma.

strong

the adult

these

Pennsylvania,

depolarization

weakness

is now

encoding

produce

Introduction

The periodic

membrane

of

for the expression

Opinion

Philadelphia,

1992,

2:631437

form of periodic weakness occurs in conjunction with transient hypokalemia. In other forms of periodic paraysis weakness develops concurrent with hyperkalemia or without significant change in serum [K+ 1. These shifts in serum [K+ ] have formed the basis for the traditional clinical classification of the disorders. The hereditary forms are classified as hypokalemic periodic paralysis (hypoPP), hyperkalemic periodic paralysis (hyperPP) either with or without myotonia, and paramyotonia congenita. Sporadic forms of periodic paralysis are known to exist, such as the periodic paralysis associated with hyperthyroidism, but most of the periodic paralyses are inherited in an autosomal dominant manner. In hypoPP, episodes of weakness often follow a heavy carbohydrate meal, or develop during the night [ 11. Weakness may persist for hours or occasionally as long as days. Although the degree of paralysis can be severe, the frequency of attacks tends to be lower than in the hyperkalemic form. Serum [K+ ] can dip as low as 1.5 mM to 2.OmM during an ictal period. Provocative testing with intravenous glucose and insulin to induce hypokalemia will often precipitate weakness or paralysis in these individuals. Patients with hyperPP may experience briefer, milder, but more frequent attacks of muscle weakness. Weakness often develops following a period of rest after exercise, and can be provoked clinically by oral K+ loading. During these attacks the serum [K+] may rise to 6&7.0mM. In some instances, these individuals may also exhibit signs of muscle membrane hyperexcitability, in the form of clinical and electrical myotonia. Paramyotonia congenita is an interesting variant of the periodic paralyses [ 31. In this disease, repetitive myotonic discharges occur in skeletal muscle that paradoxically worsen with exercise. These discharges interfere with normal muscle relaxation. Muscle stiffness, and occasionally paralysis, can be precipitated by exposure to cold, suggesting a pathophysiological relationship to the other periodic paralyses.

Abbreviations Hyper PP-hyperkalemic

periodic paralysis; @

Current

Hypo PP-hypokalemic Biology

periodic paralysis;

Ltd ISSN 09594388

TTX-tetrodotoxin.

631

632

Disease, transplantation and regeneration

Although it has been known for some time that all of these disorders are associated with depolarization of the muscle sarcolemma during the episodes of weakness [4], it is only within the past few years that the ionic mechanisms involved in this depolarization have become apparent. Riidel, LehmannHorn and their colleagues [ 5-71 have shown that, in each of these diseases, it is an increase in Na+ conductance that produces the membrane depolarization. For hyperPP and paramyotonia congenita, this pathological increase is triggered in vitro by increasing extracellular [K+ ] or by cooling the muscle fiber, respectively. The abnormal conductance can be specifically blocked by ‘ITX. As this small polar toxin is known to affect only voltage-dependent Na+ channels, the cumulative work of this group implicates this channel protein in the pathophysiology of these two diseases.

Voltage-dependent

Na+

channels

Voltage-dependent Na+ channels control the Na+ cur rents responsible for the upstroke of an action potential in nerve and muscle (for a review, see [8]). These than nels are selective for Na+ ions and have unitary conductances between 15-30 pS. They open briefly following a depolarization from the resting membrane potential, but then usually close to an inactivated state that persists until the membrane is repolarized [9]. Na+ channel proteins have been purified, characterized, and functionally reconstituted from a variety of nerve and muscle sources [8]. Each contains a single large a-subunit of -260kD. In some Na+ channels this is the sole component, while in others, such as mammalian nerve and muscle, one or two smaller p-subunits (3WiOkD) are also present in stoichiometric amounts [ lO,ll]. Na+ channel cl-subunit cDNA has been cloned and sequenced from mammalian brain and muscle as well as from eel and Drosophila [8]. All Na+ channels that have

cDNS/mRNA

Protein /

-8500

-2000

been characterized share a common molecular architecture. Within the 1800 to 2000 amino acids that comprise the primary sequence are four large regions of internal homology that seem to have arisen by duplication of a primordial channel-forming genetic element (Fig.1). Within each of these repeat domains are six putative transmembrane helices. Of these, the most highly conserved is the fourth helix (S4), which contains a unique motif of a positively charged Arg or Lys residue followed by two non-polar residues that repeat six to eight times to create an amphipathic helix. This structure is present in the same location in each of the repeat domains of all Na+ channels, as well as in comparable locations in both voltage-dependent Ca2 + [ 121 and K+ [ 131 channels, and is thought to function as the voltage-sensing element for these proteins. In their molecular architecture Na+ channels are closely related to voltage-dependent Ca2 + and K+ channels [ 141. The a-subunit of voltage-dependent Ca2+ channels is highly homologous to that of the Na+ channel, with the same four internal repeat domains, each containing six potential transmembrane helices [12]. The characteristic S4 amphipathic helix is present at the same location in each domain. The current concept of channel tertiaty structure is that these four domains in both channels are compactly organized within the plane of the membrane, arranged as ‘pseudo’ subunits around a central hydrophilic ion pore (Fig.2). K+ channel proteins are much smaller, but are found to be structurally related to a single Na+ or Ca2+ channel repeat domain [13]. K+ channels are probably formed as homotetromeric or beterotetrameric aggregates of these smaller proteins [ 151. Several other regions of the channel structure deserve note. The loops connecting the S5 and ~6 helices in each repeat domain contain conserved areas designated SSl and SS2 that have been shown to play a role in both ion selectivity and in the formation of the ion pore [ 161. These regions are also present in K+ channels, where the data implicating them in the formation of the ion pore is

base pairs

amino acids Dl

D2

NH..

D3

D4 COOH

Fig.1.Naf cipally

channels

from

designated encoded tains

Potential

the

formed

a-subunit.

by a mRNA

about

2000

present,

helices

helix in each

each

This protein, acid

repeat

residue

The

bears

at every

are

six potential

a-helices. domain

kb, conresidues.

domains

containing

transmembrane

prin-

polypeptide

of 8.5-9.0

amino

Four large internal

I transmembrane

charged

are

a single large

fourth

a positively

third

position.

Sodium

channel

gene defects in the periodic

paralyses

Barchi

Na+ channels

K+ channels

L

COOH

Ca+ channels

Channel structure top view

Fig.2. Na+ channels share a common structural motif with voltage-dependent Ca *+ channels. They both have a large a-subunit with four repeat domains that are thought to act as ‘pseudo’ subunits, coming together in the membrane to form a central polar ion pore. The much smaller subunits of a K+ channel resemble single repeat domains of the Naf or Ca*+ channels; four subunits form a functional ion channel.

particularly compelling [ 170,181. In Nat channels the SSl and SS2 regions also contain elements of the binding site for TTX [ 191. The highly conserved region of the primary sequence between domain 3 and domain 4 plays a major role in Na + channel inactivation [ 20,211. This segment of the protein lies exposed on the cytoplasmic surface of the membrane.

D17SS.

133

D17S2E

132

UYHZ , T?‘S3,

While all Na+ channels are closely related in both structure and function, isoforms with subtly different kinetics or pharmacological characteristics can be expressed in different tissues, or even in the same tissue under different physiological conditions [22]. For example, adult skeletal muscle normally expresses a Na+ channel (&Ml) that is blocked by nanomolar concentrations of TTX [ 23 1. During development or following denervation, however, it expresses a second isoform that is resistant to ‘ITX (SkM2) and is identical to that normally expressed in heart [24,25].

ErbAl CRYBI

RNAPGZA

E+B2, G.CSF D17S33

NGFR

I

i

MPG

T

SCN4A

1

lK.GH.Dl7S4

Recently, the primary sequence for both the human adult skeletal muscle and human heart Na+ channels have been reported [26**,27], and the a-subunits have been functionally expressed in oocytes using cRNA prepared from the cloned cDNA These channels are highly homologous to their counterparts in rat muscle. This homology is particularly striking in the sequence of the normally variable regions connecting Dl with D2, and D2 with

17

Fig.3. Location of the adult skeletal (SCN4A) on chromosome 17.

muscle

Na+

channel

gene

633

634

Disease,

transplantation

and regeneration

D3. It is clear from a comparison of these sequences that there is a far closer relationship between a single Na+ channel isoform in different species than there is between different isoforms of the channel in the same tissue of a single species.

Table 1. Muscle diseases linked to the adult skeletal muscle Na+ channel gene at chromosome 17q23.V25.3.

periodic

the Na+

channel

gene and

paralysis

HyperPP + myotonia

hNa2

4.00

1301 131.1

HyperPP + myotonia

PM~ hNa2, hCH

10.35 3.06

[321

HyperPP- myotonia

C6b, hNa2

4.09

[291

CHl

3.79

[291

PC

Once the cDNA encoding the human cardiac and skeletal muscle Na+ channel cc-subunits had been cloned [26**,27], localizing the genes encoding these proteins to a particular chromosome was readily accomplished. Using chimeric hamster-human cell lines, the cardiac channel gene was localized to the short arm of chromosome 3, while the adult skeletal muscle channel gene was mapped to a location between q23.3 and q25.1 on the long arm of chromosome 17 (Fig.3) [28*]. With this localization in hand, restriction fragment length polymor-

Reference

Probe

HyperPP + myotonia

Linkage between

LOD score

Disorder

PC

PM8

4.43

[341

MC + pain

PM8

4.19

[351

LOD scores are at zero recomblnatlon frequency. HyperPP, hyperkalemic periodic paralysis. MC, myotonia congenita. PC, paramyotonia congenita.

phisms were identified within these test linkage in families with periodic 1) [29]. Over a period of a few linkage with high LOD scores at

Channel

Codon

regions and used to paralysis (see Table months, reports of zero recombination

Residue

s4

Fig.4. Mutations

in several families with paramyotonia congenita affect a single codon that encodes an absolutely conserved Arg residue in helix 4 of domain 2. The resultant amino acid substitutions replace the positively charged Arg residue with uncharged His or Cys in this cntical voltage-sensing helix. Single letter codes for amino acids are used.

Sodium

frequency appeared for the adult skeletal muscle Na+ channel gene locus (designated SCN4A) and the phenotypic expression of hyperPP with myotonia [30,31*,32], hyperPP without myotonia [ 33.1, and paramyotonia congenita 1330,341. An atypical form of myotonia congenita with painful muscle contractions has also been mapped to the SCN4A locus [35]. Although linkage was tested for hypoPP, no evidence of association was found between that disease and the skeletal muscle Na+ channel.

Single channel studies in hyperkalemic periodic

paralysis

Electrophysiological evidence at the single channel level has been obtained by Cannon et al. [36-l that supports the presence of a specific defect in the Na+ channel in affected members of a family in which linkage to SCN4A had already been demonstrated. In myotubes grown from muscle biopsies of affected family members, Na+ channels showed normal activation and inactivation kinetics in the presence of 3.5 mM K+ ,but exhibited intermittent epochs of abnormal inactivation when the extracellular Kf was raised to 1OmM. During these periods, channels activated normally, but either failed to inactivate or showed abnormal return from the inactivated state during a single test depolarization. Failure of inactivation in this small percentage of abnormally functioning channels will give rise to a persistent non-inactivating current comparable with that previously described in intact muscle fibers from patients with this disease [6]. Such a persistent current can produce membrane depolarization, eventually inactivating normal channels, and leading to membrane inexcitability and muscle paralysis,

Paramyotonia R1448C

channel

Na+

gene defects in the periodic

channel mutations

in periodic

Within weeks a second report appeared describing another mutation that was present in other families with hyperPP with myotonia [38-1. These families differed from those reported by Ptacek [37-l in not having signs of an interictal myopathy. In these cases, a base change was identified that produced a substitution of a Val for a conserved Met (Met1592) in helix 6 of domain 4. In paramyotonia congenita, paralysis is typically associated with muscle cooling. McClatchey et al. [3W] have

congenita G1306V

Na+

riodic genita

known channel

paralysis

nel primary for amino

mutations

affecting

in hyperkalemic

and paramyotonia

are shown

tion of each paralysis

paralysis

Using this method, Ptacek et al. [37-l have reported the identification of a mutation in hyperPP that replaces a conserved Thr residue (Thr704) with a Met at the cytoplasmic end of the S5 helix in domain 2 of the adult skeletal muscle Na+ channel. This mutation cosegregated with the phenotype of the disease in three unrelated families, but was not found in any of 116 unrelated control individuals that were analyzed concurrently. These families exhibited a form of hyperPP associated with myotonia and persistent myopathic features.

Fig.5. The

periodic

Barchi

Using sequence information from the cloned cDNA, the genomic structure of the adult skeletal Na+ muscle than nel has been delined (A George et al, unpublished data). Once exon-intron boundaries had been identified, polymerase chain reaction primers were designed that allowed each exon to be amplified from a patient’s genomic DNA without the need to characterize the intervening intronic DNA In conjunction with gel analysis techniques, such as single strand conformational polymorphism and denaturing gradient gel electrophoresis, which can detect differences as small as a single base pair between DNA fragments hundreds of nucleotides long, this approach allows efficient screening of genomic DNA for defects in the coding region of the channel protein.

the

Hyperkalemic

paralyses

along

mutation structure.

with

within

the

peconloca-

the chan-

Single letter codes

acids are used.

635

636

Disease,

transplantation

and regeneration

analyzed several families with this phenotype and found two different mutations that produce amino acid substitutions several residues apart near the amino-terminal end of the highly conserved cytoplasmic loop connecting domains 3 and 4. As discussed above, this loop is known to play a major role in channel inactivation. In one case, a Val replaces an absolutely conserved Gly (Glyl306Val), while in the other family, a Thr three residues more distal in the sequence is replaced by Met (Thrl313Met). In another report, two mutations affecting the same codon were characterized in separate families with the same paramyotonia congenita phenotype [40-l. In these families, the base changes that were identified both produced amino acid substitutions for an absolutely conserved At-g residue near the amino-terminal end of the S4 helix in D2 (Fig.4). This residue, which is present in every Na+ channel cloned to date, forms a part of the putative voltage sensor for the protein. In one case, the positively charged Arg is replaced by Cys (Arg1448Cys), while in the other a His is introduced (Arg1448His). Both mutant residues are expected to be uncharged at physiological pH. Sitedirected mutagenesis of similarly-placed residues in K+ channels suggest that this substitution can have a profound effect on channel gating, and has the potential to alter both channel activation and inactivation [41].

Perspective It has rapidly become apparent that hyperPP and paramyotonia congenita are allelic disorders at the adult skeletal muscle Na+ channel gene locus. It appears that a number of different and apparently unrelated point mutations in the channel primary structure are capable of producing similar changes in Na+ channel kinetics and ultimately similar phenotypes at the tissue level. The membrane defect that produces hypoPP remains to be defined. Perhaps it will involve a channel P-subunit or a regulatory protein such as a kinase that modulates thannel function. Although mutations identified in hyperPP and paramyotonia congenita involve widely separated regions of the protein structure, each appears to affect the stability of the inactivated state. The conformational changes in protein tertiary structure associated with inactivation must be widespread to enable mutations at diverse locations to influence the equilibrium between active and inactivated states. Alternatively, the conformational energy of the two states may be very similar, so that minor alterations in structure, even at sites far removed from the ion pore, may influence the distribution of channels between these states. These disorders represent unique experiments of nature, site-directed mutagenesis on a grand scale in which the ‘transgenic preparations’ have already been made. On the one hand they represent a unique opportunity for us, as clinicians, to understand the molecular pathogenesis of this group of related diseases. On the other, they provide an invaluable window into the functional architecture of the normal Na+ channel.

References

and recommended

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: - _ . of special interest .. of outstanding interest 1.

RIGGSJE: The Periodic Paralyses. Neurological 6485-499.

2.

RUDELR, RICKERR: The Periodic Paralyses. Trends Neuraui

3.

BECKER PE: Paramyotonia Congenita (Eulenburg). In Fortscbritte der All,emeinen und Kliniscben Human-Genetik. Edited by Becker PE, Ianz W, Vogel I, Wendt GG. Stuttgart: Thieme; 1970, 3: 1-131.

Clinics 1988,

1985, 8:467470.

4.

HOFMANWW,

SMITH RA: HypokaIemic

Studied In Vitro. Brain

Periodic

Paralysis

1970, 93:445474.

5.

REPELR, LEHMANN-HORN F, RICKER K, KLITHER G: HypokaIemic Periodic Paralysis: In Vitro Investigation of Muscle Fiber Membrane Parameters. Muscle Nerue 1984, 7:11&120.

6.

LEHMANN-HORN F, KUTHERG, RICKERK, GRAFE P, BALL~NYI K, R~DEL R: Adynamia Episodica HereditarIa with Myoto-

nia: a Non-Inactivating Sodium Current and the Effect of ExtraceIIuIar pH. Muscle Nerue 1987, 10:363-374. 7.

LEHMANN-HORN F, RUDELR, RICKERK: Membrane Defects in Paramyotonia Congenita (Eulenburg). Muscle Nerzje 1987, 10:633-641.

8.

COHENSA, BARCHIRL: Voltage-Dependent Sodium Channels. In Kecqbtors, Transporters, and Membrane Proteins Academic Press; 1992, in press.

9.

KIRSCHGE, BROWNAM: Kinetic Properties of Single Sodium Channels in Rat Heart and Rat Brain. J Gen PLysiol 1989, 93:85-99.

10.

ROBERTS R, BARCHIR: The Voltage-Sensitive Sodium Channel from Rabbit Skeletal Muscle: Chemical Characterization of Subunits. J Biol Cbem 1987, 262:2298-2303.

Il.

ISOM LL, DE JONGH KS, PATI’ONDE, REBERBFX, OFFORDJ, CHARBONNEA~ H, WAISH K, G~XDINAL, CATI’ERUL WA: Primary

Structure and Functional Expression of the j3t Subunit of the Rat Brain Sodium Channel. Science 1992, 256:839-842. 12.

TANABER, TAKESHIMA H, MIKAMIA, FLOCKERZI V, TAKAHA~HI H, KANGAWA K, KOJIMAM, MATSUOH, HIROSET, NUMAS: Primary

Structure of the Receptor for Calcium Channel Blockers from Skeletal Muscle. Nature 1987, 328:313-318. 13.

TEMPELBL, PAPAZLAN DM, SCHWARZTL, JAN YN, JAN Ly:

Sequence of a Probable Potassium Channel Component Encoded at Shaker Locus of Drosophila. Science 1987, 237:77@775. It.

JAN LY, JAN YN: Voltage-Sensitive Ion Channels. Cell 1989, 56:13~25.

15.

MACKINNONR: Determination of the Subunit Stoichiometry of a Voltage-Activated Potassium Channel. Nature 1991, 350:232-235.

16.

HEINEMANN SH, TERUN H, STUHMERW, IMOTO K, NL’MA S:

Calcium Channel Characteristics Conferred on the Na+ Channel by Single Mutations. Nature 1992, 356:441443. S’I’UHMER W: Structure-Function Studies of Voltage-Gated Ion Channels. Annu Rezj Riopbys Biop& Cbem 1991) 20:65-78. An excellent review of recent structure-function studies involving sitedirected mutagenesis of cloned ion channels that are expressed in ooqtes or cell culture systems. 17. .

18.

KIRXH GE, DREWEJA, HARTMANN HA, TAGLLUTELAM, DE BL%%

M, BROW AM, JOHO RH: Differences Between the Deep Pores of K+ Channels Determined by an Interacting Pair of Nonpolar Amino Acids. Neuron 1992, 8:49+505.

Sodium

19.

TERLU H, HEINEMANNSH, ST~HMER W, PUSCH M, CONTI F, IMOTO K, NUMA S: Mapping the Site of Block by Tetrodotoxin and Saxitoxin of Sodium Channel II. FEBS Lett 1991, 29393-96.

20.

STUHMER

W, CONTI F, SUZUKI H, WANG X, NODA M, YAHAGI N, KUBO H, NUMA S: Structural Parts Involved in Activation and Inactivation of the Sodium Channel. Nature 1989, 339597-603.

21.

MOORMAN JR, KIRSCH GE, BROW AM, JOHO RH: Changes in Sodium Channel Gating Produced by Point Mutations in a Cytoplasmic Linker. Science 1990, 250:6a90.

22.

TRIMMERJS, AGNEWWS: Molecular Diversity of VoItage-Sensitive Na+ Channels. Annu Rev Pbysiol 1989, 51:401-418.

23.

TRIMMERJS, COOPERMANSS, TOMIKO S4, ZHOU J, CREAN SM, BOYLE MB, FALLEN RG, SHENG 2, BARCHI RL, SIGWORTH FJ, ef nl: primary Structure and Functional Expression of a Mammalian Skeletal Muscle Sodium Channel. Neuron 1989, 3:33-49.

24.

KWEN RG, SHENG 2, YANG J, CHEN L, ROGAFX R, BARCHI RL: primary Structure and Expression of a Sodium Channel Characteristic of Denervated and Immature Rat Skeletal Muscle. Neuron l‘%Q, 4:23>242.

25.

YANGJ, SLQKY JT, KALLENRG, BARCHIRL: ‘ITX-Sensitive and TI’X-Insensitive Sodium Channel mRNA Transcripts are Independently Regulated in Adult Skeletal Muscle after Denervation. Neuron 1991, 7:421-427.

26. ..

GEORGE & KOMISAROFJ, KALLENRG, BARCHI RL Primary Structure of the Adult Skeletal Muscle Voltage-Dependent Sodium Channel. Ann Neural 1992, 31:131-137. This paper presents the amino acid sequence of the human adult skeletal muscle Na+ channel and compares its structure with other Na+ channels that have been characterized previousiy. This sequence provides the normal background against which all potential mutations of the SCN4A gene are assigned. 27.

GELLENSME, GEORGE AL, CHEN L, CHAHINEM, HORN R, BARCHI

RL, KALL!zNRG: primary Structure and Functional Expression of the Human Cardiac lTX-Insensitive Voltage-Dependent Sodium Channel. Proc Nat1 Acad Sci US4 1992, 89:554558. GEORGEAF, LEDBETTER DH, KAIL!ZNRG, BARCHIRL: Assignment of a Human Skeletal Muscle Sodium Channel Alpha Subunit Gene (SCN4A) to 17q23.1-25.3. Genomicc 1991, 9:555-556. Primaty data are presented that define the localization of the skeletal muscle Na+ channel on chromosome 17 in man. This provides the basis for subsequent restriction fragment length polymorphism linkage studies.

28. .

29.

ERER~ GC, HUDSON AJ, GEORGE AL, BARCHI RL, KALL!ZNRG: RFLP for BgUII at the Human Skeletal Muscle Sodium Channel Locus. Nucleic A&i& Res 1991, 19:1166.

30.

FONTAINEB, KHURANATS, HOFFMAN EP, BRLINSGAP, HAINES JL, TROFA~R J4 HANXIN MP, RICH J, MCFARUNEH, YESEK DM, et al: Hyperkalemic Periodic Paralysis and the Adult Muscle Sodium Channel a-Subunit Gene. Science 1990, 250:100&1002.

PTACEKLJ, RYLERF, TRIMMERJS, AGNEW WS, LEPPERTM: Analysis in a Large Hyperkalemic Periodic Paralysis Pedigree Supports Tight Linkage to a Sodium Channel Locus. Am J Genet 1991, 49:37%382. A particularly large hyperPP pedigree is analyzed to demonstrate linkage to SCN4A 31. .

32.

M, GRIMMT, HOFFMANEP, RUDEL R, BENDER K, ZOLL B, HARPER P, LEHMANN-HORNF: Confirmation of Linkage of Hyperkalemic Periodic Paralysis to Chromosome 17. J Med Genet 1991, 28:583-586.

33.

EBER~ G, GEORGE AL, BARCHI RL, KALLEN RG, TING-PASSADOR SS,

.

LUHROP

KOCH MC, ROCKERK, Oreo

M,

BECKMANN J, HAHN AF,

BROWN WF,

CAMPBELL R,

channel

gene defects in the periodic

paralyses

Barchi

et al: Paramyotonia Congenita and Hyperkalemic Periodic Paralysis are Linked to the Adult Muscle Sodium Channel Gene. Ann Neural 1991, 30:81&816. One of several papers that extend linkage of the Na+ channel gene (SCN4A) to include the paramyotonia congenita phenotype. This and [34] provide evidence indicating that hyperPP and paramyotonia congenita are allelk disorders at the SCN4A locus. 34.

PTACEK

ROBERTS JW, PETAJAN JH, Paramyotonia Congenita and Hyperkalemic Periodic Paralysis Map to the Same Sodium Channel Gene Locus. Am J Hum Genet 1991, 49:851X354.

LJ,

TRIMMER JS, AGNEW WS,

LEPPERT M:

35.

PTACEK LJ, TAW~L R, GRIGGS RC, STORVICK D, LEPPERT MF: Linkage of Atypical Myotonia Congenita to a Sodium Channel Locus. Neurolom 1992, in press.

CANNON SC, BROWN RH, COREV DP: A Sodium Channel Defeet in Hyperkalemic Periodic Paralysis: Potassium-Induced Failure of Inactivation. Neuron 1991, 6:61#26. This is the most detailed study of Na + channel function at the single channel level in a family with hyperPP. The data show a K+ -dependent defect in channel inactivation that may be responsible for the pathos logical non-inactivating Na+ current seen in this disorder. 36. ..

37.

PTACEK LJ, GEORGE AL, GRIGGS RC, TA~IL R, KAUEN RG, BARCHI

. .

RL, ROBERTNN

M, LEPPERT MF: Identification of a Mutation in the Gene Causing Hyperkalemic Periodic Paralysis. Cell 1991, 67:1021-1027. In this report polymerase chain reaction techniques are used to amplify exons of the Nat channel for subsequent sequencing A specific point mutation is identified that cosegregates with the hyperPP phenotype and is not found in unaffected family members or in unrelated controls. This represents the first reported mutation in hyperPP.

ROJ~S CV, WANG J, SCHWARTZLS, HOFFMAN EP, POWEU BR, BROWN RH: A Met-to-W Mutation in the Skeletal Muscle Sodium Channel a-Subunit in Hyperkalemic Periodic Paralysis. Nature 1991, 354:387-389. Another report of a point mutation in the 5CN4A gene .alTecting several families with hyperPP. This mutation produces an amino acid substitution in the S6 helix of domain 4. These patiene differ from those analyzed by Ptacek et al. [37*-l in not having prominent myopathic features. 38. ..

39.

MCCLAIXZHEY AI, VAN DEN BERGH P, PEIUCAK-VANCE MA, R.&~KIND

VEREUIN C, MCKENNA-YASEK D, RAO K, HAINES JL, BIRI) T, BROW RH, et uI: Temperature-Sensitive Mutations in the III-IV Cytoplasmic Loop Region of the Skeletal Muscle Sodium Chan’nel Gene in Paramyotonia Congenita. Cell 1992, 68:76%774. Two mutations are described in families having the paramyotonia congenita phenotype. These mutations produce substitutions for highly consetved non-polar residues in intramembrane domains 3 and 4. These data confirm the allelic nature of paramyotonia congenita and hyperPP. . .

W,

40. ..

PTACEK

LJ, GEORGE

AL,

BARCHI

RL,

GRIGGS

‘il.

PAPA%IAN D, TIMPE LC, JAN YN, JAN LY: Akeration of VoltageDependence of Shaker Potassium Channel by Mutations in the S4 Sequence. Nature 1991, 349:305-310.

RC, RIGGS JE, in an S4 Segment of the Adult Skeletal Muscle Sodium Channel Cause Paramyotonia Congenita. Neuron 1992, in press. These authors present a vety interesting group of related mutations in families with paramyotonia congenita that affect the same codon, resulting in the replacement of an absolutely conserved Arg residue in an S4 helix with an uncharged amino acid, This helix has been implicated as the voltage sensor for this ion channel. ROHEKTSONM, LEPPERT MF: Mutations

RL Barchi, Mahoney Institute of Neurological Sciences, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.

637