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Genetic factors play a major role in the etiology of idiopathic generalized epilepsy. However, in most syndromes, especially the common ones, multiple genetic factors seem to be involved. Mutations in K(+) channel genes have previously found to be associated with epilepsy both in humans and in mice. The weaver mice phenotype, characterized by ataxia, tremor, male infertility, and tonic-clonic seizures, is caused by a point mutation in the inwardly rectifier K(+) channel gene KCNJ6 (GIRK2). A knockout mouse model deprived of functional KCNJ6 protein is susceptible to spontaneous and provoked seizures without showing the histological signs of neuronal cell death found in the weaver mouse. Thus, the KCNJ6 gene seems to play an important role in seizure control. We therefore performed a mutation analysis of KCNJ6 and the related KCNJ3 gene in 38 patients with juvenile myoclonic epilepsy (JME). Two novel same-sense nucleotide exchanges were identified, but none of these changed the coding sequence. These results do not support a major role for the KCNJ6/KCNJ3 heteromeric receptor in the etiology of JME. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:8-11, 2000, (Copyright 2000 Wiley-Liss, Inc.)

Evidence for genetic influences in epilepsy is strong, but reports identifying specific chromosomal origins of those influences conflict. One early study reported that human leukocyte antigen (HLA) markers were genetically linked to juvenile myoclonic epilepsy (JME); this was confirmed in a later study. Other reports did not find linkage to HLA markers. One found evidence of linkage to markers on chromosome 15, another to markers on chromosome 6, centromeric to HLA. We identified families through a patient with JME and genotyped markers throughout chromosome 6. Linkage analysis assuming equal male-female recombination probabilities showed evidence for linkage (LOD score 2.5), but at a high recombination fraction (theta), suggesting heterogeneity. When linkage analysis was redone to allow independent male-female thetas, the LOD score was significantly higher (4.2) at a male-female theta of.5,.01. Although the overall pattern of LOD scores with respect to male-female theta could not be explained solely by heterogeneity, the presence of heterogeneity and predominantly maternal inheritance of JME might explain it. By analyzing loci between HLA-DP and HLA-DR and stratifying the families on the basis of evidence for or against linkage, we were able to show evidence of heterogeneity within JME and to propose a marker associated with the linked form. These data also suggest that JME may be predominantly maternally inherited and that the HLA-linked form is more likely to occur in families of European origin.

Several loci and candidate genes for epilepsies or epileptic syndromes map or have been suggested to map to chromosome 8. We investigated families with adolescent-onset idiopathic generalized epilepsy (IGE), for linkage to markers spanning chromosome 8. The IGEs that we studied included juvenile myoclonic epilepsy (JME), epilepsy with only generalized tonic-clonic seizures occurring either randomly during the day (random grand mal) or on awakening (awakening grand mal), and juvenile absence epilepsy (JAE). We looked for a gene common to all these IGEs, but we also investigated linkage to specific subforms of IGE. We found evidence for linkage to chromosome 8 in adolescent-onset IGE families in which JME was not present. The maximum multipoint LOD score was 3.24 when family members with IGE or generalized spike-and-waves (SW) were considered affected. The LOD score remained very similar (3.18) when clinically normal family members with SW were not considered to be affected. Families with either pure grand mal epilepsy or absence epilepsy contributed equally to the positive LOD score. The area where the LOD score reaches the maximum encompasses the location of the gene for the beta3-subunit of the nicotinic acetylcholine receptor (CHRNB3), thus making this gene a possible candidate for these specific forms of adolescent-onset IGE. The data excluded linkage of JME to this region. These results indicate genetic heterogeneity within IGE and provide no evidence, on chromosome 8, for a gene common to all IGEs.

In genetic analysis of diseases in which the underlying model is unknown, "model free" methods-such as affected sib pair (ASP) tests-are often preferred over LOD-score methods, although LOD-score methods under the correct or even approximately correct model are more powerful than ASP tests. However, there might be circumstances in which nonparametric methods will outperform LOD-score methods. Recently, Dizier et al. reported that, in some complex two-locus (2L) models, LOD-score methods with segregation analysis-derived parameters had less power to detect linkage than ASP tests. We investigated whether these particular models, in fact, represent a situation that ASP tests are more powerful than LOD scores. We simulated data according to the parameters specified by Dizier et al. and analyzed the data by using a (a) single locus (SL) LOD-score analysis performed twice, under a simple dominant and a recessive mode of inheritance (MOI), (b) ASP methods, and (c) nonparametric linkage (NPL) analysis. We show that SL analysis performed twice and corrected for the type I-error increase due to multiple testing yields almost as much linkage information as does an analysis under the correct 2L model and is more powerful than either the ASP method or the NPL method. We demonstrate that, even for complex genetic models, the most important condition for linkage analysis is that the assumed MOI at the disease locus being tested is approximately correct, not that the inheritance of the disease per se is correctly specified. In the analysis by Dizier et al., segregation analysis led to estimates of dominance parameters that were grossly misspecified for the locus tested in those models in which ASP tests appeared to be more powerful than LOD-score analyses.

The importance of inflammatory/immune mechanisms in Alzheimer's disease is supported by evidence that the human leukocyte antigen (HLA)-DR genotype influences risk of the disease, with a protective effect associated with the HLA-DR4 allele. We investigated the influence of the HLA-DR4 allele on glial activity, assessed by quantification of glial fibrillary acidic protein (GFAP), in hippocampal tissue from subjects with Alzheimer's disease. The mean GFAP level was significantly higher in Alzheimer's disease hippocampal specimens lacking the HLA-DR4 allele compared to specimens with similar neuropathological findings that were HLA-DR4 positive. Apolipoprotein E genotype had no influence on GFAP levels. These results indicate that HLA-DR4 may exert a protective influence on Alzheimer's disease via modulation of glial activity.

The linkage reports of bipolar disease and chromosome 18 markers are controversial. We used the GAW10 data sets to further explore several observations: 1) a possible parent-of-origin effect with only 'paternal' pedigrees showing linkage; 2) the preponderance of women affected with bipolar disease, and 3) the possible existence of phenocopies in the bipolar data sets. We performed linkage analysis allowing for independent male/female recombination fractions. Our hypothesis was that if there is linkage only in 'paternal' pedigrees, then the lod score would maximize at low male and high female recombination fractions. We did not find such an effect in the combined data set. There was no consistent effect on the difference of male and female recombination fractions, suggesting that an effect is not detectable in this data set with this method. In addition, there is interesting evidence for a recessively inherited, highly penetrant gene in a subset of families. Allowing for higher penetrances for bipolar disease in women than in men had no effect on the lod scores. There was also not much difference in the lod scores calculated under the assumption of a phenocopy rate versus no phenocopies. From simulation studies, we would have expected some effect if there were linkage and phenocopies were present.

Reports have suggested an association of juvenile myoclonic epilepsy (JME) with an HLA-DR allele. We examined the HLA-DR and DQ frequencies in two populations of epilepsy patients: (1) JME patients and (2) patients with other forms of adolescent-onset, idiopathic generalized epilepsy (IGE). We did DNA-based HLA typing on 24 JME patients and 24 patients with non-JME forms of adolescent-onset IGE, forms that are clinically similar to JME. In typing the HLA region, we paid particular attention to the alleles contributing to the HLA-DR13 type and also to the DQB1 locus alleles that are in linkage disequilibrium with the alleles that comprise the DR13 type. We also examined the HLA-AP locus, which is centromeric to the DR locus. The frequency of DR13 was significantly higher in JME compared with the non-JME patients. Nine JME patients, compared with two non-JME patients, carried that type (chi 2 = 5.78 [p < 0.017, 1 df]). The odds ratio was 6.6. Furthermore, the DQB1 alleles in linkage disequilibrium with the alleles contributing to the DR13 type were also more frequent in JME than in non-JME epilepsy patients. The chi 2 is highly significant (8.1, p < 0.005) with an odds ratio of 13.8. These results confirm that JME is an HLA-associated form of epilepsy. They also show that the JME locus probably lies within the HLA region, most likely between the HLA-DP and HLA-B loci. The association studies also confirm linkage results showing that JME is genetically different from some other IGEs and emphasize that careful diagnosis is critical to genetic studies of the epilepsies.

In this study we explore whether a phenocopy frequency (defined as a "penetrance' for nondisease genotypes) can approximate or model genetic heterogeneity in a single-locus analysis. We simulated two types of heterogeneity situations: "sporadic models', where there are two forms of a disease, one genetic and linked to a marker and the other purely random, and "genetic heterogeneity models', where the disease is caused by either of two different loci, one linked to the marker and the other unlinked. We analyzed simulated data sets for linkage, assuming a single-locus analysis with varying phenocopy frequency, in analogy with earlier work on epistatic two-locus models. We found that in the presence of purely random sporadics, there was a difference between assuming any nonzero phenocopy frequency and a zero frequency, but that the actual value of the assumed phenocopy frequency had little effect on the maximum lod score. In contrast, when both forms of disease are genetic, and are generated under similar genetic parameters, assuming a positive phenocopy frequency will not, in general, compensate for the presence of the unlinked form. However, when the modes of inheritance of the two forms differ, the assumption of a nonzero phenocopy frequency does have an effect, either to increase or decrease the maximum lod score, depending on the modes of inheritance of the two disease forms. We conclude with practical recommendations for investigators, based on these results.

Hereditary factors play a major role in the etiology of idiopathic generalized epilepsies (IGEs). A trait locus (EBN1) for a rare subtype of IGEs, the benign neonatal familial convulsions, and a susceptibility gene (EEGV1) for the common human low-voltage electroencephalogram have been mapped close together with D20S19 to the chromosomal region 20q13.2. Both loci are potential candidates for the susceptibility to IGE spectra with age-related onset beyond the neonatal period. The present study tested the hypothesis that a putative susceptibility locus linked to D20S19 predisposes to spectra of IGEs with age-related onset from childhood to adolescence. Linkage analyses were conducted in 60 families ascertained through IGE patients with juvenile myoclonic epilepsy, juvenile absence epilepsy or childhood absence epilepsy. Our results provide evidence against linkage of a putative susceptibility gene for four hierarchically broadened IGE spectra with D20S19 assuming tentative single-locus genetic models. The extent of an "exclusion region" (lod scores below-2) varied from 0.5 cM up to 22 cM on either side of D20S19 depending on the trait assumed. These results are contrary to the expectation that a susceptibility gene in vicinity to D20S19 confers a common major gene effect to the expression of IGE spectra with age-related onset from childhood to adolescence.

Both linkage and association studies provide strong evidence that a gene locus on chromosome 6 is involved in the expression of juvenile myoclonic epilepsy (JME), an adolescent-onset form of primary idiopathic generalized epilepsy (IGE). This epilepsy-related gene locus, designated EJM-1, may also influence the expression of other forms of IGE. We report here evidence that at least one form of epilepsy that is similar to JME--pure, adolescent-onset grand mal epilepsy in which the seizures occur at any time during waking--is not linked to the EJM-1 locus. However, we also have evidence that another form of pure, adolescent-onset grand mal that occurs on awakening is linked to the EJM-1 locus and may be genetically the same as JME. This work suggests that clinically similar epileptic syndromes may have different genetic bases and underscores the critical importance of careful clinical observations in studying the genetics of the epilepsies.

We applied linkage analysis with a sib-pair method, which also takes into account information on unaffected siblings, and family-based methods of association analysis to determine the disease affecting loci in Problem 1. Whereas the first two disease loci were correctly identified by association analysis, the sib-pair linkage method failed to detect the disease loci 3 and 4. We therefore determined the data structure and sample size necessary for demonstrating linkage to these loci.

Heterogeneity, both inter- and intrafamilial, represents a serious problem in linkage studies of common complex diseases. In this study we simulated different scenarios with families who phenotypically have identical diseases but who genotypically have two different forms of the disease (both forms genetic). We examined the proportion of families displaying intrafamilial heterogeneity, as a function of mode of inheritance, gene frequency, penetrance, and sampling strategies. Furthermore, we compared two different ways of analyzing linkage in these data sets: a two-locus (2L) analysis versus a one-locus (SL) analysis combined with an admixture test. Data were simulated with tight linkage between one disease locus and a marker locus; the other disease locus was not linked to a marker. Our findings are as follows: (1) In contrast to what has been proposed elsewhere to minimize heterogeneity, sampling only "high-density" pedigrees will increase the proportion of families with intrafamilial heterogeneity, especially when the two forms are relatively close in frequency. (2) When one form is dominant and one is recessive, this sampling strategy will greatly decrease the proportions of families with a recessive form and may therefore make it more difficult to detect linkage to the recessive form. (3) An SL analysis combined with an admixture test achieves about the same lod scores and estimate of the recombination fraction as does a 2L analysis. Also, a 2L analysis of a sample of families with intrafamilial heterogeneity does not perform significantly better than an SL analysis. (4) Bilineal pedigrees have little effect on the mean maximum lod score and mean maximum recombination fraction, and therefore there is little danger that including these families will lead to a false exclusion of linkage.

Juvenile myoclonic epilepsy (JME) is a clearly defined subform of idiopathic generalized epilepsy with a high aggregation of epilepsy in family members. With the HLA-system used as a genetic marker, a linkage between JME and the HLA region was demonstrated. Linkage with the HLA region suggests that JME may be associated with an HLA-antigen. An association could indicate that the gene lies in the HLA region and is in linkage disequilibrium with one of the HLA-antigens. Eighty-eight unrelated patients with JME were typed for the HLA-A and HLA-B locus, 77 were typed for the HLA-C locus, and 76 were typed for the DR locus. The antigen frequency was compared with those of healthy blood donors. The highest difference was noted in the frequency of DRw6 (39.5% in patients vs. 22.1% in controls). This weak association is open to question because DRw6 is known to split into DRw13 and DRw14.

Children of epileptic mothers have a greater risk for congenital malformations than is seen in the general population. This risk has been attributed mostly to teratogenic effects of antiepileptic drugs, but other risk factors have been suggested, such as epilepsy, per se, or some underlying genetic defects associated with epilepsy. Previous studies do not answer the question of whether genetic factors contribute to the high risk of malformations in children of epileptic parents. Genetic studies in families of patients with neural-tube defects and cleft lip (CL), with and without cleft palate (CP), as well as genetic studies in families of patients with epilepsy, show evidence for the possible existence of genes on the short arm of chromosome 6. The suspected gene for CL and CP is linked to factor XIIIa and is neither identical with or linked to a gene for idiopathic generalized epilepsy, which is close to the HLA region. The short arm of chromosome 6 also contains a human homologue of the mouse t-complex. Alterations of the mouse t-complex are involved in defects of neural-crest development in mice. Relationships between a human homologue of the mouse t-complex, epilepsy, and birth defects have yet to be proven.

Our knowledge of genetic factors influencing expression of epilepsy has increased enormously in the last 10 years. In this article, we review the advantages and problems of population genetics studies, twin studies, and linkage analysis as applied to the study of epilepsy. Population genetics, twin studies, and linkage analysis have placed the evidence for the genetic basis of the generalized epilepsies on a firm foundation. The identification and confirmation of a gene locus involved in the expression of juvenile myoclonic epilepsy and other forms of generalized epilepsy is proof of at least one genetic influence. We also review the evidence that other, still-undiscovered genetic factors might influence the expression of other forms of generalized epilepsy.

Heterogeneity is a major factor in many common, complex diseases and can confound linkage analysis. Using computer-simulated heterogeneous data we tested what effect unlinked families have on a linkage analysis when heterogeneity is not taken into account. We created 60 data sets of 40 nuclear families each with different proportions of linked and unlinked families and with different modes of inheritance. The ascertainment probability was 0.05, the disease had a penetrance of 0.6, and the recombination fraction for the linked families was zero. For the analysis we used a variety of assumed modes of inheritance and penetrances. Under these conditions we looked at the effect of the unlinked families on the lod score, the evaluation of the mode of inheritance, and the estimate of penetrance and of the recombination fraction in the linked families. 1. When the analysis was done under the correct mode of inheritance for the linked families, we found that the mode of inheritance of the unlinked families had minimal influence on the highest maximum lod score (MMLS) (i.e., we maximized the maximum lod score with respect to penetrance). Adding sporadic families decreased the MMLS less than adding recessive or dominant unlinked families. 2. The mixtures of dominant linked families with unlinked families always led to a higher MMLS when analyzed under the correct (dominant) mode of inheritance than when analyzed under the incorrect mode of inheritance. In the mixtures with recessive linked families, assuming the correct mode of inheritance generally led to a higher MMLS, but we observed broad variation.(ABSTRACT TRUNCATED AT 250 WORDS)

Juvenile myoclonic epilepsy (JME) is a distinct subform of idiopathic generalized epilepsy of adolescence. Linkage studies with Bf and serologic HLA markers in families of JME patients have shown a tight linkage on chromosome 6. We present a linkage analysis with HLA-DQ restriction fragment length polymorphisms on more extended families, paying particular attention to the epilepsy type of the affected family members. We studied 21 families of JME patients with a total of 143 family members and obtained a highest logarithm of the odds (lod) score of 3.9 (theta m = 0.01, theta f = 0.01) assuming a dominant mode of inheritance and 70% penetrance when family members with JME, absence epilepsy, or epilepsy with generalized tonic-clonic seizures (GTCS) were considered as affected. When we also classified clinically normal family members with generalized spike-wave discharges in the EEG as "affected," the maximum lod score was 4.1 (theta m = 0.01, theta f = 0.3) under a dominant mode of inheritance and 90% penetrance. These findings support the conclusion that a gene locus for a group of idiopathic generalized epilepsies (JME, epilepsy with absences, and epilepsy with GTCS) maps to chromosome 6p.

Juvenile myoclonic epilepsy (JME) is a generalized, non-progressive epilepsy characterized by an adolescent onset of sudden, involuntary myoclonic jerks. Greenberg et al. (American Journal of Medical Genetics 31:185-192, 1988b; Cytogenetics and Cell Genetics 51:1008, 1989b) reported tight linkage of a JME locus to the HLA region of chromosome 6p. We confirm this linkage assignment, although at a larger recombination fraction than previously reported. Twenty-three, mostly nuclear, families were ascertained through a JME proband. The affected status of relatives of the probands was assigned by 4 different clinical criteria, and separate analyses were done assuming an autosomal dominant model with 90% penetrance and an autosomal recessive model with full penetrance. A linear age-of-onset correction with maximum penetrance at age 20 years was incorporated into the analyses. The maximum lod score obtained was 3.11 at (-)m = 0.001, (-)f = 0.20, assuming autosomal dominant inheritance and using the second definition of the disease phenotype. There was strong support for linkage using the other phenotype definitions and the autosomal dominant model, although the lod scores did not exceed 3.0. There was also support for linkage of a JME locus to this region under the autosomal recessive model, although the results varied depending upon the definition of the disease phenotype. There was no significant evidence for linkage heterogeneity.