Mutation of GABRA1 in an autosomal dominant form of juvenile myoclonic epilepsy.

Although many genes that predispose for epilepsy in humans have been determined, those that underlie the classical syndromes of idiopathic generalized epilepsy (IGE) have yet to be identified. We report that an Ala322Asp mutation in GABRA1, encoding the alpha1 subunit of the gamma-aminobutyric acid receptor subtype A (GABA(A)), is found in affected individuals of a large French Canadian family with juvenile myoclonic epilepsy. Compared with wildtype receptors, GABA(A) receptors that contain the mutant subunit show a lesser amplitude of GABA-activated currents in vitro, indicating that seizures may result from loss of function of this inhibitory ligand-gated channel. Our results confirm that mutation of GABRA1 predisposes towards a common idiopathic generalized epilepsy syndrome in humans.

Chromosome 6q25 is linked to susceptibility to leprosy in a Vietnamese population.

Leprosy, a chronic infectious disease caused by Mycobacterium leprae, affects an estimated 700,000 persons each year. Clinically, leprosy can be categorized as paucibacillary or multibacillary disease. These clinical forms develop in persons that are intrinsically susceptible to leprosy per se, that is, leprosy independent of its specific clinical manifestation. We report here on a genome-wide search for loci controlling susceptibility to leprosy per se in a panel of 86 families including 205 siblings affected with leprosy from Southern Vietnam. Using model-free linkage analysis, we found significant evidence for a susceptibility gene on chromosome region 6q25 (maximum likelihood binomial (MLB) lod score 4.31; P = 5 x 10(-6)). We confirmed this by family-based association analysis in an independent panel of 208 Vietnamese leprosy simplex families. Of seven microsatellite markers underlying the linkage peak, alleles of two markers (D6S1035 and D6S305) showed strong evidence for association with leprosy (P = 6.7 x 10(-4) and P = 5.9 x 10(-5), respectively).

Susceptibility to leprosy is associated with PARK2 and PACRG.

Leprosy is caused by Mycobacterium leprae and affects about 700,000 individuals each year. It has long been thought that leprosy has a strong genetic component, and recently we mapped a leprosy susceptibility locus to chromosome 6 region q25-q26 (ref. 3). Here we investigate this region further by using a systematic association scan of the chromosomal interval most likely to harbour this leprosy susceptibility locus. In 197 Vietnamese families we found a significant association between leprosy and 17 markers located in a block of approx. 80 kilobases overlapping the 5' regulatory region shared by the Parkinson's disease gene PARK2 and the co-regulated gene PACRG. Possession of as few as two of the 17 risk alleles was highly predictive of leprosy. This was confirmed in a sample of 975 unrelated leprosy cases and controls from Brazil in whom the same alleles were strongly associated with leprosy. Variants in the regulatory region shared by PARK2 and PACRG therefore act as common risk factors for leprosy.

Identification of a chromosome 8p locus for early-onset coronary heart disease in a French Canadian population.

Susceptibility to coronary heart disease (CHD) has long been known to exhibit familial aggregation, with heritability estimated to be greater than 50%. The French Canadian population of the Saguenay-Lac Saint-Jean region of Quebec, Canada is descended from a founder population that settled this region 300-400 years ago and this may provide increased power to detect genes contributing to complex traits such as CHD. Probands with early-onset CHD, defined by angiographically determined coronary stenosis, and their relatives were recruited from this population (average sibship size of 6.4). Linkage analysis was performed following a genome-wide microsatellite marker scan on 42 families with 284 individuals. Nonparametric linkage (NPL) analysis provided suggestive evidence for a CHD susceptibility locus on chromosome 8 with an NPL score of 3.14 (P=0.001) at D8S1106. Linkage to this locus was verified by fine mapping in an enlarged sample of 50 families with 320 individuals. This analysis provided evidence of linkage at D8S552 (NPL score=3.53, P=0.0003), a marker that maps to the same location as D8S1106. Candidate genes in this region, including macrophage scavenger receptor 1, farnesyl-diphosphate farnesyltransferase 1, fibrinogen-like 1, and GATA-binding protein 4, were resequenced in all coding exons in both affected and unaffected individuals. Association studies with variants in these and five other genes did not identify a disease-associated mutation. In conclusion, a genome-wide scan and additional fine mapping provide evidence for a locus on chromosome 8 that contributes to CHD in a French Canadian population.

Spectrum of mutations in mut methylmalonic acidemia and identification of a common Hispanic mutation and haplotype.

Cobalamin nonresponsive methylmalonic acidemia (MMA, mut complementation class) results from mutations in the nuclear gene MUT, which codes for the mitochondrial enzyme methylmalonyl CoA mutase (MCM). To better elucidate the spectrum of mutations that cause MMA, the MUT gene was sequenced in 160 patients with mut MMA. Sequence analysis identified mutations in 96% of disease alleles. Mutations were found in all coding exons, but predominantly in exons 2, 3, 6, and 11. A total of 116 different mutations, 68 of which were novel, were identified. Of the 116 different mutations, 53% were missense mutations, 22% were deletions, duplications or insertions, 16% were nonsense mutations, and 9% were splice-site mutations. Sixty-one of the mutations have only been identified in one family. A novel mutation in exon 2, c.322C>T (p.R108C), was identified in 16 of 27 Hispanic patients. SNP genotyping data demonstrated that Hispanic patients with this mutation share a common haplotype. Three other mutations were seen exclusively in Hispanic patients: c.280G>A (p.G94R), c.1022dupA, and c.970G>A (p.A324T). Seven mutations were seen almost exclusively in black patients, including the previously reported c.2150G>T (p.G717V) mutation, which was identified in 12 of 29 black patients. Two mutations were seen only in Asian patients. Some frequently identified mutations were not population-specific and were identified in patients of various ethnic backgrounds. Some of these mutations were found in mutation clusters in exons 2, 3, 6, and 11, suggesting a recurrent mutation.

A survey of genetic and epigenetic variation affecting human gene expression.

The identification of human sequence polymorphisms that regulate gene expression is key to understanding human genetic diseases. We report a survey of human genes that demonstrate allelic differences in gene expression, reflecting the presence of putative allele-specific cis-acting factors of either genetic or epigenetic nature. The expression of allelic transcripts in heterozygous samples is assessed directly by relative quantitation of intragenic marker alleles in messenger or heteronuclear RNA derived from cells or tissues. This survey used 193 single-nucleotide polymorphisms (SNPs) from 129 genes expressed in lymphoblastoid cell lines, to identify 23 genes (18%) with common allele-specific transcripts whose expression deviated from the expected equimolar ratio. A subset of these deviations, or "allelic imbalances," can be observed in multiple samples derived from reference CEPH ("Centre d'Etude du Polymorphisme Humain") pedigrees and demonstrate a spectrum of patterns of transmission, including cosegregation of allelic skewing across generations compatible with Mendelian inheritance as well as random monoallelic expression for three genes (IL1A, HTR2A, and FGB). Additional studies for BTN3A2 provide evidence of SNPs and haplotypes in complete linkage disequilibrium with high- and low-expressing transcripts. The pipeline described herein offers tools for efficient identification and characterization of allelic expression allowing identification of regulatory sequence variants as well as epigenetic variation affecting human gene expression.

Significant linkage to airway responsiveness on chromosome 12q24 in families of children with asthma in Costa Rica.

Although asthma is a major public health problem in certain Hispanic subgroups in the United States and Latin America, only one genome scan for asthma has included Hispanic individuals. Because of small sample size, that study had limited statistical power to detect linkage to asthma and its intermediate phenotypes in Hispanic participants. To identify genomic regions that contain susceptibility genes for asthma and airway responsiveness in an isolated Hispanic population living in the Central Valley of Costa Rica, we conducted a genome-wide linkage analysis of asthma (n = 638) and airway responsiveness (n = 488) in members of eight large pedigrees of Costa Rican children with asthma. Nonparametric multipoint linkage analysis of asthma was conducted by the NPL-PAIR allele-sharing statistic, and variance component models were used for the multipoint linkage analysis of airway responsiveness as a quantitative phenotype. All linkage analyses were repeated after exclusion of the phenotypic data of former and current smokers. Chromosome 12q showed some evidence of linkage to asthma, particularly in nonsmokers (P < 0.01). Among nonsmokers, there was suggestive evidence of linkage to airway responsiveness on chromosome 12q24.31 (LOD = 2.33 at 146 cM). After genotyping 18 additional short-tandem repeat markers on chromosome 12q, there was significant evidence of linkage to airway responsiveness on chromosome 12q24.31 (LOD = 3.79 at 144 cM), with a relatively narrow 1.5-LOD unit support interval for the observed linkage peak (142-147 cM). Our results suggest that chromosome 12q24.31 contains a locus (or loci) that influence a critical intermediate phenotype of asthma (airway responsiveness) in Costa Ricans.

Sex-specific linkage to total serum immunoglobulin E in families of children with asthma in Costa Rica.

Serum total immunoglobulin E (IgE) is a critical intermediate phenotype of allergic diseases. Although total IgE exhibits sexual dimorphism in humans (with males demonstrating higher IgE than females), the molecular basis of this difference is unknown. A genome-wide scan of 380 short-tandem repeat (STR) markers was performed in eight extended pedigrees of asthmatic children (n=655) from the Central Valley of Costa Rica. Genome-wide linkage analysis of total IgE was performed by variance component models. Among all subjects, only one genomic region (chromosome 7p15) showed modest evidence of linkage to total IgE (LOD=1.60). In contrast, a sex-stratified analysis revealed distinct genetic architectures of total IgE in males and females and identified significant linkage to total IgE on a novel male-specific locus on chromosome 20p12 (LOD=3.63 at 36 cM). Genotyping of additional STRs on chromosome 20 resulted in improved evidence for linkage (LOD=3.75 at 33 cM) and a 1.5 LOD-unit support interval for the linkage peak between 26 and 38 cM. Three polymorphisms in two genes on chromosome 20p12 (JAG1 and ANKRD5) were then found to be associated with total IgE in 420 nuclear families of Costa Rican children with asthma. Two of these polymorphisms (in JAG1) were significantly associated with total IgE in families of boys (n=264) but not in families of girls (n=156) with asthma. JAG1 is a hematopoetic cell growth factor that may regulate normal B-cell development. This is the first demonstration of a possible genetic basis for differences in total IgE between sexes.

Genome-wide linkage analysis of pulmonary function in families of children with asthma in Costa Rica.

BACKGROUND: Although asthma is highly prevalent among certain Hispanic subgroups, genetic determinants of asthma and asthma-related traits have not been conclusively identified in Hispanic populations. A study was undertaken to identify genomic regions containing susceptibility loci for pulmonary function and bronchodilator responsiveness (BDR) in Costa Ricans. METHODS: Eight extended pedigrees were ascertained through schoolchildren with asthma in the Central Valley of Costa Rica. Short tandem repeat (STR) markers were genotyped throughout the genome at an average spacing of 8.2 cM. Multipoint variance component linkage analyses of forced expiratory volume in 1 second (FEV(1)) and FEV(1)/ forced vital capacity (FVC; both pre-bronchodilator and post-bronchodilator) and BDR were performed in these eight families (pre-bronchodilator spirometry, n = 640; post-bronchodilator spirometry and BDR, n = 624). Nine additional STR markers were genotyped on chromosome 7. Secondary analyses were repeated after stratification by cigarette smoking. RESULTS: Among all subjects, the highest logarithm of the odds of linkage (LOD) score for FEV(1) (post-bronchodilator) was found on chromosome 7q34-35 (LOD = 2.45, including the additional markers). The highest LOD scores for FEV(1)/FVC (pre-bronchodilator) and BDR were found on chromosomes 2q (LOD = 1.53) and 9p (LOD = 1.53), respectively. Among former and current smokers there was near-significant evidence of linkage to FEV(1)/FVC (post-bronchodilator) on chromosome 5p (LOD = 3.27) and suggestive evidence of linkage to FEV(1) on chromosomes 3q (pre-bronchodilator, LOD = 2.74) and 4q (post-bronchodilator, LOD = 2.66). CONCLUSIONS: In eight families of children with asthma in Costa Rica, there is suggestive evidence of linkage to FEV(1) on chromosome 7q34-35. In these families, FEV(1)/FVC may be influenced by an interaction between cigarette smoking and a locus (loci) on chromosome 5p.

Mutations in TMEM76* cause mucopolysaccharidosis IIIC (Sanfilippo C syndrome).

Mucopolysaccharidosis IIIC (MPS IIIC, or Sanfilippo C syndrome) is a lysosomal storage disorder caused by the inherited deficiency of the lysosomal membrane enzyme acetyl-coenzyme A: alpha -glucosaminide N-acetyltransferase (N-acetyltransferase), which leads to impaired degradation of heparan sulfate. We report the narrowing of the candidate region to a 2.6-cM interval between D8S1051 and D8S1831 and the identification of the transmembrane protein 76 gene (TMEM76), which encodes a 73-kDa protein with predicted multiple transmembrane domains and glycosylation sites, as the gene that causes MPS IIIC when it is mutated. Four nonsense mutations, 3 frameshift mutations due to deletions or a duplication, 6 splice-site mutations, and 14 missense mutations were identified among 30 probands with MPS IIIC. Functional expression of human TMEM76 and the mouse ortholog demonstrates that it is the gene that encodes the lysosomal N-acetyltransferase and suggests that this enzyme belongs to a new structural class of proteins that transport the activated acetyl residues across the cell membrane.

An atypical form of erythrokeratodermia variabilis maps to chromosome 7q22.

Erythrokeratodermia variabilis 3 (Kamouraska type) or EKV3 is a newly described autosomal recessive disorder observed in patients from the Bas St-Laurent region of Quebec. It has similar skin lesions as observed for EKV, including congenital hyperkeratosis and red patches of variable sizes, shapes, and duration. EKV3 is also characterized by ichthyosis, sensorineural hearing loss, peripheral neuropathy, psychomotor retardation, congenital chronic diarrhea, and an elevation of very long chain fatty acids (VLCFAs). To map the disease locus, we performed candidate gene analysis and a genomewide scan to identify a common homozygous region in affected individuals from three non-consanguineous families. Mutations in connexin 31 (GJB3) and connexin 30.3 (GJB4), implicated in previous reports of EKV, and connexin 26 (GJB2), implicated in palmoplantar keratoderma, were unlikely given the lack of shared homozygous haplotypes in the regions surrounding these genes. The most promising region of common homozygosity observed in a 4,600 single-nucleotide polymorphism genome scan was further characterized by using microsatellites. A 6.8-Mb region on chromosome 7 between D7S2539 and rs727708 was found to be homozygous for the same haplotype in all affected individuals but not in the parents or an unaffected sibling. This region contains connexin 31.3 (GJE1), and although no mutation have been observed in the coding region of this gene, further analyses are required in order to exclude it. Identification of the gene responsible for this disorder will provide insights into the etiology of this multisystemic disorder.

The c.419-420insA in the MTP gene is associated with abetalipoproteinemia among French-Canadians.

Abetalipoproteinemia (ABL) is a rare autosomal recessive disease characterised by the absence of apolipoprotein B (apoB) containing lipoproteins and, in consequence, very low triglyceride and cholesterol levels. Microsomal triglyceride transfer protein (MTP) has been associated with ABL. A search for sequence variants in the large subunit of MTP in a kindred of 10 individuals from Saguenay-Lac-St Jean area with a propositus exhibiting ABL as well as in four independent patients from the greater Quebec city area and exhibiting very low apoB and LDL-cholesterol levels identified 12 variations. Only one sequence variation, the c.419-420insA, was observed, in the homozygous form, in the abetalipoproteinemic patient. The -493G/-400A/-164T/282G/383T/419-420insA/453T/891C/969T/1151A/2884G haplotype carries the insertion and was found in all members of the family studied. In conclusion, the present study showed that the c.419-420insA alone, in the homozygous form, is a cause of classical recessive inherited ABL in the French-Canadian population.

Chromosome 6q25 is linked to susceptibility to leprosy in a Vietnamese population

Leprosy, a chronic infectious disease caused by Mycobacterium leprae, affects an estimated 700,000 persons each year. Clinically, leprosy can be categorized as paucibacillary or multibacillary disease. These clinical forms develop in persons that are intrinsically susceptible to leprosy per se, that is, leprosy independent of its specific clinical manifestation. We report here on a genome-wide search for loci controlling susceptibility to leprosy per se in a panel of 86 families including 205 siblings affected with leprosy from Southern Vietnam. Using model-free linkage analysis, we found significant evidence for a susceptibility gene on chromosome region 6q25 (maximum likelihood binomial (MLB) lod score 4.31; P = 5 x 10(-6)). We confirmed this by family-based association analysis in an independent panel of 208 Vietnamese leprosy simplex families. Of seven microsatellite markers underlying the linkage peak, alleles of two markers (D6S1035 and D6S305) showed strong evidence for association with leprosy (P = 6.7 x 10(-4) and P = 5.9 x 10(-5), respectively).