B Chioza

Association between the α1a calcium channel gene CACNA1A and idiopathic generalized epilepsy

Idiopathic generalized epilepsy (IGE) is a common disorder with a strong genetic component. For common forms of IGE, the pattern of inheritance is complex, probably resulting from the action of a few or many genes of small to moderate effect. In this study we used an association strategy to investigate whether genetic variation of the α1A subunit of the voltage-gated calcium channel gene ( CACNA1A ) influences risk for IGE. The α1A subunit is the pore-forming channel in P/Q-type channels that plays an important role in neurotransmitter release.1 Mutations in the mouse homologue cause seizures and ataxia in tottering and leaner mice.2 In humans, mutations in CACNA1A result in episodic ataxia type 2, spinocerebellar ataxia type 6, and familial hemiplegic migraine.3 In an earlier study of only 55 probands, no association was reported between a CAG repeat polymorphism in exon 47 and the common subtypes of IGE.4nnAlthough IGE includes many syndromes, their phenotypes overlap and more than one subtype is often observed within …

Genome-wide High-Density SNP-Based Linkage Analysis of Infantile Hypertrophic Pyloric Stenosis Identifies Loci on Chromosomes 11q14-q22 and Xq23

Infantile hypertrophic pyloric stenosis (IHPS) has an incidence of 1-8 per 1000 live births and is inherited as a complex sex-modified multifactorial trait with a striking male preponderance. Syndromic and monogenic forms exist, and two loci have been identified. Infants present with vomiting due to gastric-outlet obstruction caused by hypertrophy of the smooth muscle of the pylorus. A genome-wide SNP-based high-density linkage scan was carried out on 81 IHPS pedigrees. Nonparametric and parametric linkage analysis identified loci on chromosomes 11q14-q22 (Z(max) = 3.9, p < 0.0001; HLOD(max) = 3.4, alpha = 0.34) and Xq23 (Z(max) = 4.3, p < 0.00001; HLOD(max) = 4.8, alpha = 0.56). The two linked chromosomal regions each harbor functional candidate genes that are members of the canonical transient receptor potential (TRPC) family of ion channels and have a potential role in smooth-muscle control and hypertrophy.

Linkage and mutational analysis of CLCN2 in childhood absence epilepsy

In order to assess the chloride channel gene CLCN2 as a candidate susceptibility gene for childhood absence epilepsy, parametric and non-parametric linkage analysis was performed in 65 nuclear pedigrees. This provided suggestive evidence for linkage with heterogeneity: NPL score=2.3, p<0.009; HLOD=1.5, alpha=0.44. Mutational analysis of the entire genomic sequence of CLCN2 was performed in 24 unrelated patients from pedigrees consistent with linkage, identifying 45 sequence variants including the known non-synonymous polymorphism rs2228292 (G2154C, Glu718Asp) and a novel variant IVS4+12G>A. Intra-familial association analysis using the pedigrees and a further 308 parent-child trios showed suggestive evidence for transmission disequilibrium of the G2154C minor allele: AVE-PDT chi(1)2 = 5.17, p<0.03. Case-control analysis provided evidence for a protective effect of the IVS4+12G>A minor allele: chi(1)2 = 7.27, p<0.008. The 65 nuclear pedigrees were screened for three previously identified mutations shown to segregate with a variety of idiopathic generalised epilepsy phenotypes (597insG, IVS2-14del11 and G2144A) but none were found. We conclude that CLCN2 may be a susceptibility locus in a subset of cases of childhood absence epilepsy.

Evaluation of CACNA1H in European patients with childhood absence epilepsy

CACNA1H was evaluated in a resource of Caucasian European patients with childhood absence epilepsy by linkage analysis and typing of sequence variants previously identified in Chinese patients. Linkage analysis of 44 pedigrees provided no evidence for a locus in the CACNA1H region and none of the Chinese variants were found in 220 unrelated patients.

Suggestive evidence for association of two potassium channel genes with different idiopathic generalised epilepsy syndromes

Several potassium channel genes have been implicated in epilepsy. We have investigated three such genes, KCNJ3, KCNJ6 and KCNQ2, by association studies using a broad sample of idiopathic generalised epilepsy (IGE) unselected by syndrome. One of the two single nucleotide polymorphisms (SNPs) examined in one of the inward rectifying potassium channel genes, KCNJ3, was associated with IGE by genotype (P=0.0097), while its association by allele was of borderline significance (P=0.051). Analysis of the different clinical subgroups within the IGE sample showed more significant association with the presence of absence seizures (P=0.0041) and which is still significant after correction for multiple testing. Neither SNP in the other rectifying potassium channel gene, KCNJ6, was associated with IGE or any subgroup. None of the three SNPs in the voltage-gated potassium channel gene, KCNQ2, was associated with IGE. However, one SNP was associated with epilepsy with generalised tonic clonic seizures only (P=0.016), as was an SNP approximately 56 kb distant in the closely linked nicotinic acetylcholine gene CHRNA4 (P=0.014). These two SNPs were not in linkage disequilibrium with each other, suggesting that if they are not true associations they have independently occurred by chance. Neither association remains significant after correcting for multiple testing.

Linkage and association analysis of CACNG3 in childhood absence epilepsy.

Childhood absence epilepsy (CAE) is an idiopathic generalised epilepsy characterised by absence seizures manifested by transitory loss of awareness with 2.5–4u2009Hz spike–wave complexes on ictal EEG. A genetic component to aetiology is established but the mechanism of inheritance and the genes involved are not fully defined. Available evidence suggests that genes encoding brain expressed voltage-gated calcium channels, including CACNG3 on chromosome 16p12–p13.1, may represent susceptibility loci for CAE. The aim of this work was to further evaluate CACNG3 as a susceptibility locus by linkage and association analysis. Assuming locus heterogeneity, a significant HLOD score (HLOD=3.54, α=0.62) was obtained for markers encompassing CACNG3 in 65 nuclear families with a proband with CAE. The maximum non-parametric linkage score was 2.87 (P<0.002). Re-sequencing of the coding exons in 59 patients did not identify any putative causal variants. A linkage disequilibrium (LD) map of CACNG3 was constructed using 23 single nucleotide polymorphisms (SNPs). Transmission disequilibrium was sought using individual SNPs and SNP-based haplotypes with the pedigree disequilibrium test in 217 CAE trios and the 65 nuclear pedigrees. Evidence for transmission disequilibrium (P≤0.01) was found for SNPs within a ∼35u2009kb region of high LD encompassing the 5’UTR, exon 1 and part of intron 1 of CACNG3. Re-sequencing of this interval was undertaken in 24 affected individuals. Seventy-two variants were identified: 45 upstream; two 5’UTR; and 25 intronic SNPs. No coding sequence variants were identified, although four variants are predicted to affect exonic splicing. This evidence supports CACNG3 as a susceptibility locus in a subset of CAE patients.

Genome wide high density SNP-based linkage analysis of childhood absence epilepsy identifies a susceptibility locus on chromosome 3p23-p14

Summary Childhood absence epilepsy (CAE) is an idiopathic generalised epilepsy (IGE) characterised by typical absence seizures manifested by transitory loss of awareness with 2.5–4 Hz spike-wave complexes on ictal EEG. A genetic component to the aetiology is well recognised but the mechanism of inheritance and the genes involved are yet to be fully established. A genome wide single nucleotide polymorphism (SNP)-based high density linkage scan was carried out using 41 nuclear pedigrees with at least two affected members. Multipoint parametric and non-parametric linkage analyses were performed using MERLIN 1.1.1 and a susceptibility locus was identified on chromosome 3p23-p14 (Zmean = 3.9, p < 0.0001; HLOD = 3.3, α = 0.7). The linked region harbours the functional candidate genes TRAK1 and CACNA2D2. Fine-mapping using a tagSNP approach demonstrated disease association with variants in TRAK1.

Haplotype and linkage disequilibrium analysis to characterise a region in the calcium channel gene CACNA1A associated with idiopathic generalised epilepsy

Idiopathic generalised epilepsy (IGE) is a common form of epilepsy, including several defined and overlapping syndromes, and likely to be due to the combined actions of mutations in several genes. In a recent study we investigated the calcium channel gene CACNA1A for involvement in IGE, unselected for syndrome, by means of association studies using several polymorphisms within the gene. We reported a highly significant case/control association with a silent single nucleotide polymorphism (SNP) in exon 8 that we confirmed by within-family analyses. In this present study we screened the gene for novel SNPs within 25u2009kb of exon 8, which have enabled us to define the critical region of CACNA1A in predisposing to IGE. Several intronic SNPs were identified and three, within 1.5u2009kb of exon 8 and in strong linkage disequilibrium with each other and with the original SNP, were significantly associated with IGE (P=0.00029, P=0.0015 and P=0.010). The associations were not limited to an IGE syndrome or other subgroup. Another SNP, 25u2009kb away, in intron 6 was also significantly associated with IGE (P=0.0057) but is not in linkage disequilibrium with the SNPs around exon 8. Haplotype predictions revealed even more significant associations (3-marker haplotype: P<10−6). Logistic regression showed that all the data can be explained by two of the SNPs, which is consistent with two functionally significant variants being responsible for all five associations, although a single variant cannot be excluded. The functionally significant variant(s) are unlikely to be exonic and suggests an effect on expression or alternative splicing.

Linkage of monogenic infantile hypertrophic pyloric stenosis to chromosome 16q24

Infantile hypertrophic pyloric stenosis (IHPS) is the most common inherited form of gastrointestinal obstruction in infancy. The disease is considered a paradigm for the sex-modified model of multifactorial inheritance and affects males four times more frequently than females. However, extended pedigrees consistent with autosomal dominant inheritance have been documented. We have analysed data from an extended IHPS family including eight affected individuals (five males and three females) and mapped the disease locus to chromosome 16q24 (LOD score=3.7) through an SNP-based genome wide scan. Fourteen additional multiplex pedigrees did not show evidence of linkage to this region, indicating locus heterogeneity.

Infantile hypertrophic pyloric stenosis: evaluation of three positional candidate genes, TRPC1, TRPC5 and TRPC6, by association analysis and re-sequencing.

Infantile hypertrophic pyloric stenosis (IHPS) is the most common inherited form of gastrointestinal obstruction in infancy with a striking male preponderance. Infants present with vomiting due to gastric outlet obstruction caused by hypertrophy of the smooth muscle of the pylorus. Two loci specific to extended pedigrees displaying autosomal dominant inheritance have been identified. A genome scan identified loci on chromosomes 11q14–q22 and Xq23–q24 which are predicted to be responsible for a subset of smaller families with IHPS demonstrating non-Mendelian inheritance. The two linked chromosomal regions both harbour functional candidate genes which are members of the canonical transient receptor potential (TRPC) family of ion channels. Both TRPC5 (Xq23–q24) and TRPC6 (11q14–q22) have a potential role in smooth muscle control and hypertrophy. Here, we report suggestive evidence for a third locus on chromosome 3q12–q25 (Zmaxxa0=xa02.7, pxa0<xa00.004), a region which harbours a third TRPC gene, TRPC1. Fine mapping of all three genes using a tagSNP approach and re-sequencing identified a SNP in the promoter region of TRPC6 and a missense variant in exon 4 of TRPC6 which may be putative causal variants.