No Evidence for a Major Susceptibility Locus forJuvenile Myoclonic Epilepsy on Chromosome 15q

Martina Durner,1 Shlomo Shinnar,2 Stanley R. Resor,3 Solomon L. Moshe,2 David Rosenbaum,4Jeffrey Cohen,5 Cynthia Harden,6 Harriet Kang,2 Sharon Hertz,7 Sibylle Wallace,8 Daniel Luciano,9Karen Ballaban-Gil,2 and David A. Greenberg1,10*1Department of Psychiatry and Department of Biostatistics, Mount Sinai Medical Center, New York, New York2Department of Neurology and Pediatrics, Comprehensive Epilepsy Management Center, Montefiore Medical Center,Albert Einstein College of Medicine, Bronx, New York

3Columbia Presbyterian Medical Center, New York, New York4Department of Neurology, Mount Sinai Medical Center, New York, New York5Beth Israel Medical Center, New York, New York6New York Hospital, Cornell University, New York, New York7Sinai Hospital, Baltimore, Maryland8Division of Neuropediatrics, Mount Sinai Medical Center, New York, New York9Hospital for Joint Diseases, New York University, New York, New York10Department of Biostatistics, Mount Sinai Medical Center, New York, New York

Juvenile myoclonic epilepsy (JME) is a dis-tinct epileptic syndrome with a complexmode of inheritance. Several studies foundevidence for a locus involved in JME onchromosome 6 near the HLA region. Re-cently, Elmslie et al. [1997] reported evi-dence of linkage in JME to chromosome15q14 assuming a recessive mode of inheri-tance with 50% penetrance and 65% linkedfamilies. The area on chromosome 15q14 en-compasses the location of the gene for thea-7 subunit of the nicotinic acetylcholine re-ceptor. This could fit the hypothesis thatthere are two interacting loci, one on chro-mosome 6 and on chromosome 15 or thatthere is genetic heterogeneity in JME. In anindependent dataset of JME families, wetested for linkage to chromosome 15 butfound little evidence for linkage. Moreover,families with more than one family memberaffected with JME provide a lodscore of 3.4for the HLA-DR/DQ haplotype on chromo-some 6. The lodscore for these same familieson chromosome 15q14 is <−2 assuming ho-mogeneity and the maximum lodscore is 0.2assuming a = .25. Only one of these families

has a negative lodscore on chromosome 6and a positive lodscore of 0.5 on chromo-some 15q14. Our results indicate that thispossible gene on chromosome 15 plays atmost a minor role in our JME families. Am.J. Med. Genet. (Neuropsychiatr. Genet.)96:49–52, 2000. © 2000 Wiley-Liss, Inc.

KEY WORDS: Juvenile Myoclonic Epilepsy;chromosome 15; linkageanalysis

INTRODUCTION

Juvenile myoclonic epilepsy (JME) is a clinically dis-tinct form of idiopathic generalized epilepsy (IGE)characterized by bilateral myoclonic jerks experiencedwith full consciousness. In addition to the jerks, mostpatients also have generalized tonic clonic seizures(GTCS) and approximately one third have absence sei-zures [Janz, 1989]. The underlying pathology is un-known. Family and twin studies indicate an almostexclusive genetic basis of JME [Berkovic et al., 1994;Gedda and Tatarelli, 1971; Greenberg et al., 1992]. Themode of inheritance is complex and the involvement ofmore than one gene is suggested from segregationanalysis [Greenberg et al., 1988a].

Recently, Steinlein et al. [1995] identified a mutationin the a-4 subunit of the neuronal nicotinic acetylcho-line receptor (nAChR) for a rare form of idiopathic focalepilepsy with a clear autosomal dominant inheritance(ADNFLE 4 autosomal dominant nocturnal frontallobe epilepsy). Genes coding for subunits of the nAChRwere thought to be possible candidate genes for otheridiopathic epilepsies. Elmslie et al. [1997] tested this

Contract grant sponsor: NIH; Contract grant numbers:NS27941, DK31775, MH48858; Contract grant sponsor: EpilepsyFoundation of America.

*Correspondence to: David A. Greenberg, PhD, Department ofPsychiatry, Mount Sinai Medical Center, Box 1229, One GustaveLevy Place, New York, NY 10029.E-mail: dag@shallot.salad.mssm.edu

Received 10 December 1998; Accepted 18 June 1999

American Journal of Medical Genetics (Neuropsychiatric Genetics) 96:49–52 (2000)

© 2000 Wiley-Liss, Inc.

notion in families with JME and found evidence forlinkage to chromosome 15q14, which harbors the a-7subunit of the nAChR [Chini et al., 1994]. The hetero-geneity lodscore was 4.4 at a 4 0.65 assuming an au-tosomal recessive inheritance with 50% penetrance.

We tried to replicate their finding in an independentdataset of JME families.

FAMILIES AND METHODSOur sample consisted of 53 families ascertained

through a JME proband. Twenty families had morethan one family member affected with idiopathic gen-eralized epilepsy. Ten family members of these familiesother than the proband had JME, 5 had absence epi-lepsy, and 12 had epilepsy with GTCS. Six family mem-bers gave no history of seizures but had generalizedspike-and-waves (sw) in the EEG. A careful medicaland family history was taken from all participatingfamily members by the principal investigator and di-agnoses were made according to the revised classifica-tion of epilepsy and epileptic syndromes of the Inter-national League Against Epilepsy [1989].

DNA was obtained from peripheral blood from 290family members. These family members were geno-typed for microsatellite markers located on chromo-some 15q14 in the area where linkage has been re-ported by Elmslie et al. [1997] (D15S165–D15S214). Inaddition, we typed markers spanning the length ofchromosome 15 in approximately 5 to 10 cM intervals.The following markers were tested: 15ptel—D15S128-13cM- D15S165 -7cM- D15S144 -7cM- D15S971 -6cM-D15S214 -6cM- D15S126 -5cM- D15S117 -11cM-D15S153 -9cM- D15S131 -8cM- D15S205 -20cM-D15S130 -13cM- D15S120 -15qtel. Primers were fluo-rescent labeled and the microsatellite polymorphismswere amplified by polymerase chain reaction (PCR) us-ing standard protocols [Ziegle et al., 1992]. Allelecounts were detected with an ABI310 genetic analyzer.Genotypes were determined manually without knowl-edge of the clinical affectedness status.

We performed a multipoint linkage analysis withGENEHUNTER [Kruglyak et al., 1996] assuming a re-cessive mode of inheritance with 50% penetrance, aphenocopy risk of 0.001, and several affectedness mod-els where the family members with the following wereconsidered as affected: (a) JME, (b) JME or spike-and-wave, (c) any IGE (i.e., JME or epilepsy with GTCS orabsence epilepsy), and (d) any IGE or spike-and-wave.Affectedness Model 1 corresponds to the disease modelElmslie et al. used. However, because the seizure typesof myoclonic jerks, absence seizures, and GTCS occurin patients with JME as well as in family members ofpatients with JME, this might indicate that this trio ofseizures shares a common genetic basis. The rationalefor also including spike-and-waves as affected is thatspike-and-waves are, like seizures, signs of neuronalhyperexitability, albeit on a subclinical level.

RESULTSUnder the assumption of homogeneity and a reces-

sive mode of inheritance with 50% penetrance, the datashow no evidence for linkage to chromosome 15q14 forall affectedness models (Fig. 1). Homogeneity lodscores

for Models 2, 3, and 4 were <−2, and the lodscore was−1 for Model 1 in this area. At a 4 .65, the heteroge-neity lodscores were less negative for Models 2, 3, and4 but reached positive values of only 0.5 for Model 1.When the lodscores were maximized over a, there wasa positive peak between D15S114 and D15S214 on15q14 for all affectedness models. The highest lodscoreof 0.7 was found under Model 1 (Only family memberswith JME were considered as affected) at a 4 .45.

However, telomeric of D15S128, a maximum lodscoreof 1.6 was calculated under Model 1 assuming almost100% linked families. Under the broader affectednessmodels the heterogeneity lodscores were also positive,but to a lesser extent. Under affectedness Model 4 (IGE+ SW) the lodscore was 0.4 at a 4 .35.

Telomeric of 15q14 up to 15qtel, no further evidencefor linkage was found in JME families under all theaffectedness models.

DISCUSSIONIn this independent dataset of JME families there is

little evidence for linkage to chromosome 15q14, thearea which harbors the a-7 subunit of the nicotinic ace-tylcholine receptor and where Elmslie et al. [1997] re-ported evidence for linkage. When the lodscore wasmaximized over affectedness models, the maximumlodscore in this area was only 0.7 at a 4 .45 underaffectedness Model 1 (JME). However it is noteworthythat this is the affectedness model also used in theanalysis by Elmslie et al. [1997]. It is therefore possiblethat the proportion and information content of linkedfamilies in our dataset is too small to detect a majoreffect of this possible gene on chromosome 15q14.

The JME families of Elmslie et al. [1997] had mostlymore than 1 family member affected with JME. There-fore we also looked for linkage to chromosome 15q14 inour 10 multiplex families for JME separately. The lod-score maximized at 0.2 at a 4 .25. Linkage in JME hasalso been reported in three independent datasets tochromosome 6p near the HLA region [Durner et al.,1991; Greenberg et al., 1988; Greenberg et al., 1997;Sander et al., 1995; Weissbecker et al., 1991]. Those 10multiplex families, in contrast, provide a lodscore of 3.4for the HLA-DR/DQ haplotype assuming a dominantmode of inheritance and 70% penetrance. Heterogene-ity within JME is very likely [Liu et al., 1996] and it isconceivable that some forms of JME are linked to chro-mosome 6 and others are linked to another locus, pos-sibly on chromosome 15. However, when we looked atthose multiplex families individually, we observed thatonly 1 family had a negative lodscore on chromosome 6and a positive lodscore of 0.5 on chromosome 15. Thus,from our results we have little support for a locus forJME on chromosome 15.

We also obtained a positive linkage signal centro-meric of 15q14 in an area where the three GABA-Areceptor subunits a3, b3, and g3 cluster [Greger et al.,1995]. GABA-A receptors are inhibitory in neurotrans-mission and their possible role in epileptogenesis hasbeen studied intensively [Olsen and Avoli, 1997]. An-tiepileptic drugs acting on the GABA system have beendeveloped and applied successfully [Meldrum, 1996].In addition, Sander et al. [1997] found evidence for

50 Durner et al.

linkage to this region on chromosome 15 containing theGABA gene cluster in JME families assuming a reces-sive mode of inheritance with 70% penetrance whenfamily members with IGE and spike-and-waves wereconsidered as affected. The maximum lodscore was 1.4at a 4 .37. This result taken alone is not very strong,but given that we also found positive lodscores in thisregion might indicate that this area may harbor a genewhich is involved in the epileptogenesis of a few fami-lies.

ACKNOWLEDGMENTSWe thank the families who made this work possible.

This work was supported by NIH Grants NS27941,DK31775, MH48858 (D. A. G.) and in part by a grantfrom the Epilepsy Foundation of America (M. D.).

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