Torsion dystonia genes in two populations confined to a small region on chromosome 9q32-34.

Idiopathic torsion dystonia (ITD) is characterized by sustained, involuntary muscle contractions, frequently causing twisting and repetitive movements or abnormal postures. Most familial forms of ITD display autosomal dominant inheritance with reduced penetrance. Linkage analysis has been previously used to localize a dystonia gene to the 9q32-34 region in a large non-Jewish family and in a group of Ashkenazi Jewish families. Utilizing GT repeat polymorphisms from this region, here we demonstrate that the gene causing dystonia in Ashkenazi Jews can be localized to the 11-cM interval between AK1 and D9S10. Linkage analysis in the non-Jewish family is also consistent with occurrence of the gene in this region, although positive lod scores extend over a greater than 20-cM interval in that family. These results set the stage for positional cloning of the dystonia gene. Currently there are no known candidate genes in this region.     

Strong allelic association between the torsion dystonia gene (DYT1) and loci on chromosome 9q34 in Ashkenazi Jews

The DYT1 gene responsible for early-onset, idiopathic torsion dystonia (ITD) in the Ashkenazi Jewish population, as well as in one large non-Jewish family, has been mapped to chromosome 9q32-34. Using (GT)n and RFLP markers in this region, we have identified obligate recombination events in some of these Jewish families, which further delineate the area containing the DYT1 gene to a 6-cM region bounded by loci AK1 and ASS. In 52 unrelated, affected Ashkenazi Jewish individuals, we have found highly significant linkage disequilibrium between a particular extended haplotype at the ABL-ASS loci and the DYT1 gene. The 4/A12 haplotype for ABL-ASS is present on 69% of the disease-bearing chromosomes among affected Jewish individuals and on only 1% of control Jewish chromosomes (chi 2 = 91.07, P much less than .001). The allelic association between this extended haplotype and DYT1 predicts that these three genes lie within 1-2 cM of each other; on the basis of obligate recombination events, the DYT1 gene is centromeric to ASS. Furthermore, this allelic association supports the idea that a single mutation event is responsible for most hereditary cases of dystonia in the Jewish population. Of the 53 definitely affected typed, 13 appear to be sporadic, with no family history of dystonia. However, the proportion of sporadic cases which potentially carry the A12 haplotype at ASS (8/13 [62%]) is similar to the proportion of familial cases with A12 (28/40 [70%]). This suggests that many sporadic cases are hereditary, that the disease gene frequency is greater than 1/15,000, and that the penetrance is lower than 30%, as previously estimated in this population. Most affected individuals were heterozygous for the ABL-ASS haplotype, a finding supporting autosomal dominant inheritance of the DYT1 gene. The ABL-ASS extended-haplotype status will provide predictive value for carrier status in Jewish individuals. This information can be used for molecular diagnosis, evaluation of subclinical expression of the disease, and elucidation of environmental factors which may modify clinical symptoms.     

The DYT1 Gene on 9q34 Is Responsible for Most Cases of Early Limb-Onset Idiopathic Torsion Dystonia in Non-Jews

Idiopathic torsion dystonia (ITD) is characterized by involuntary twisting movements and postures. A gene for this disorder, DYT1, was mapped to chromosome 9q34 in 12 Ashkenazi Jewish (AJ) families and one large non-Jewish kindred. In the AJ population, strong linkage disequilibrium exists between DYT1 and adjacent markers within a 2-cM region. The associated haplotype occurs in >90% of early limb-onset AJ cases. We examined seven non-Jewish ITD families of northern European and French Canadian descent to determine the extent to which early-onset ITD in non-Jews maps to DYT1. Results are consistent with linkage to the DYT1 region. Affected individuals in these families are clinically similar to the AJ cases; i.e., the site of onset is predominantly in the limbs and at least one individual in each pedigree had onset before age 12 years. None carries the AJ haplotype; therefore, they probably represent different mutations in the DYT1 gene. The two French Canadian families, however, display the same haplotype. Estimates of penetrance in non-Jewish families range from .40 to .75. We identified disease gene carriers and, with adjustments for age at onset, obtained a direct estimate of penetrance of .46. This is consistent with estimates of 30%–40% in the AJ population. Two other non-Jewish families with atypical ITD (later onset and/or cranial or cervical involvement) are not linked to DYT1, which indicates involvement of other genes in dystonia.     

Search for a founder mutation in idiopathic focal dystonia from Northern Germany.

Both the discovery of the DYT1 gene on chromosome 9q34 in autosomal dominant early-onset torsion dystonia and the detection of linkage for one form of adult-onset focal dystonia to chromosome 18p (DYT7) in a family from northern Germany provide the opportunity to further investigate genetic factors in the focal dystonias. Additionally, reports of linkage disequilibrium between several chromosome 18 markers and focal dystonia, both in sporadic patients from northern Germany and in members of affected families from central Europe suggest the existence of a founder mutation underlying focal dystonia in this population. To evaluate the role of these loci in focal dystonia, we tested 85 patients from northern Germany who had primary focal dystonia, both for the GAG deletion in the DYT1 gene on chromosome 9q34 and for linkage disequilibrium at the chromosome 18p markers D18S1105, D18S1098, D18S481, and D18S54. None of these patients had the GAG deletion in the DYT1 gene. Furthermore, Hardy-Weinberg analysis of markers on 18p in our patient population and in 85 control subjects from the same region did not support linkage disequilibrium. Taken together, these results suggest that most cases of focal dystonia in patients of northern German or central European origin are due neither to the GAG deletion in DYT1 nor to a proposed founder mutation on chromosome 18p but must be caused by other genetic or environmental factors.     

The TOR1A (DYT1) gene family and its role in early onset torsion dystonia

Most cases of early onset torsion dystonia are caused by a 3-bp deletion (GAG) in the coding region of the TOR1A gene (alias DYT1, DQ2), resulting in loss of a glutamic acid in the carboxy terminal of the encoded protein, torsin A. TOR1A and its homologue TOR1B (alias DQ1) are located adjacent to each other on human chromosome 9q34. Both genes comprise five similar exons; each gene spans a 10-kb region. Mutational analysis of most of the coding region and splice junctions of TOR1A and TOR1B did not reveal additional mutations in typical early onset cases lacking the GAG deletion (N = 17), in dystonic individuals with apparent homozygosity in the 9q34 chromosomal region (N = 5), or in a representative Ashkenazic Jewish individual with late onset dystonia, who shared a common haplotype in the 9q34 region with other late onset individuals in this ethnic group. A database search revealed a family of nine related genes (50-70% similarity) and their orthologues in species including human, mouse, rat, pig, zebrafish, fruitfly, and nematode. At least four of these genes occur in the human genome. Proteins encoded by this gene family share functional domains with the AAA/HSP/Clp-ATPase superfamily of chaperone-like proteins, but appear to represent a distinct evolutionary branch.     

Identification of a highly polymorphic microsatellite VNTR within the argininosuccinate synthetase locus: exclusion of the dystonia gene on 9q32-34 as the cause of dopa-responsive dystonia in a large kindred

Dopa-responsive dystonia is a clinical variant of idiopathic torsion dystonia that is distinguished from other forms of dystonia by the frequent occurrence of parkinsonism, diurnal fluctuation of symptoms, and its dramatic therapeutic response to L-dopa. Linkage of a gene causing classic dystonia in a large non-Jewish kindred (DYT1) and in a group of Ashkenazi Jewish families, to the gelsolin (GSN) and arginino-succinate synthetase (ASS) loci on chromosome 9q32-34, respectively, was recently determined. Here we report the discovery of a highly informative (GT)n repeat VNTR polymorphism within the ASS locus. Analysis of a large kindred with dopa-responsive dystonia, using this new polymorphism and conventional RFLPs for the 9q32-34 region, excludes loci in this region as a cause of this form of dystonia. This provides proof of genetic heterogeneity between classic idiopathic torsion dystonia and dopa-responsive dystonia.     

A radiation-reduced hybrid cell line containing 5 Mb/17 cM of human DNA from 9q34.

Disease gene loci for tuberous sclerosis (TSC1), idiopathic torsion dystonia (DYT1), and nail-patella syndrome (NPS1) have been mapped by genetic linkage analysis to human chromosome 9q band 34. To create a resource for physical mapping and manipulation of this region of the genome, we have created a radiation-reduced hybrid cell line containing DNA from human 9q34 as its only human component. This cell line, E6B, has been characterized by Southern blot and PCR analysis using a panel of 9q markers and fluorescent in situ hybridization. We estimate that it contains 5 Mb of human DNA, equal to 17 cM of genetic distance, extending from AK1 to ABO on 9q34.     

Molecular genetics of dopa-responsive dystonia

The causative genes of two types of hereditary dopa-responsive dystonia (DRD) due to dopamine (DA) deficiency in the nigrostriatum DA neurons have been elucidated. Autosomal dominant DRD (AD-DRD) was originally described by Segawa as hereditary progressive dystonia with marked diurnal fluctuation (HPD). We cloned the human GTP cyclohydrolase I (GCH1) gene, and mapped the gene to chromosome 14q22.1-q22.2 within the HPD/DRD locus, which had been identified by linkage analysis. GCH1 isthe rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin (BH4), the cofactor for tyrosine hydroxylase (TH), which is the first and rate-limiting enzyme of DA synthesis. We proved that the GCH1 gene is the causative gene for HPD/DRD based on the identification of mutations of the gene in the patients and decreases in the enzyme activity expressed in mononuclear blood cells to 2-20% of the normal value. About 60 different mutations (missense, nonsense, and frameshift mutations) in the coding region or in the exon-intron junctions of the GCH1 gene have been reported in patients with AD-DRD all over the world. Recent findings indicate that the decreased GCH1 activity in AD-DRD may be caused by the negative interaction of the mutated subunit with the wild-type one, i.e., a dominant negative effect, and/or by decreases in the levels of GCH1 mRNA and protein caused by inactivation of one allele of the GCH1 gene. Autosomal recessive DRD (AR-DRD) with Segawa's syndrome was discovered in Germany. The AR-DRD locus was mapped to chromosome 11p15.5 in the chromosomal site of the TH gene. In the AR-DRD with Segawa's syndrome, a point mutation in TH (Gln381Lys) resulted in a pronounced decrease in TH activity to about 15% of that of the wild type. Several missense mutations in the TH gene have been found in AR-DRD in Europe. The phenotype of AR-DRD with the Leu205Pro mutation in the TH gene, which produces a severe decrease in TH activity to 1.5% of that of the wild type, was severe, not dystonia/Segawa's syndrome, but early-onset parkinsonism. However, a marked improvement of all clinical symptoms with a low dose of L-dopa was reported in AR-DRD/parkinsonism patients. These findings on DRD indicate that the nigrostriatal DA neurons may be most susceptible to the decreases in GCH1 activity, BH4 level, TH activity, and DA level, and that DRD is the DA deficiency without neuronal death in contrast to juvenile parkinsonism or Parkinson's disease with DA cell death.     

Alpha-catulin maps to the familial dysautonomia region on 9q31.

Familial dysautonomia is a severe autosomal-recessive neurodegenerative disease that primarily affects the Ashkenazi Jewish population. We present the mapping of alpha-catulin and show that it maps precisely to the familial dysautonomia candidate region on 9q31. Patient sequence analysis identified two new sequence variants, which show linkage disequilibrium with this disease. A G to A transition at nucleotide 423 in exon 3 is a silent base change that does not alter the Val residue at position 141. A G to C transversion at nucleotide 1579 changes the Glu at postion 527 to Gln. These base changes were analyzed in several patients, unaffected Ashkenazi Jewish controls, and non-Jewish controls. Because of the presence of these sequence variants in several unaffected individuals, alpha-catulin is unlikely to be the causative gene in this disease.     

The genetic history of Cochin Jews from India

Cochin Jews form a small and unique community on the Malabar coast in southwest India. While the arrival time of any putative Jewish ancestors of the community has been speculated to have taken place as far back as biblical times (King Solomon’s era), a Jewish community in the Malabar coast has been documented only since the 9th century CE. Here, we explore the genetic history of Cochin Jews by collecting and genotyping 21 community members and combining the data with that of 707 individuals from 72 other Indian, Jewish, and Pakistani populations, together with additional individuals from worldwide populations. We applied comprehensive genome-wide analyses based on principal component analysis, F _ST, ADMIXTURE, identity-by-descent sharing, admixture linkage disequilibrium decay, haplotype sharing, allele sharing autocorrelation decay and contrasting the X chromosome with the autosomes. We find that, as reported by several previous studies, the genetics of Cochin Jews resembles that of local Indian populations. However, we also identify considerable Jewish genetic ancestry that is not present in any other Indian or Pakistani populations (with the exception of the Jewish Bene Israel, which we characterized previously). Combined, Cochin Jews have both Jewish and Indian ancestry. Specifically, we detect a significant recent Jewish gene flow into this community 13–22 generations (~470–730 years) ago, with contributions from Yemenite, Sephardi, and Middle-Eastern Jews, in accordance with historical records. Genetic analyses also point to high endogamy and a recent population bottleneck in this population, which might explain the increased prevalence of some recessive diseases in Cochin Jews.     

Molecular genetic analysis of essential tremor

Essential tremor (ET) is the most common extrapyramidal disorder of the central nervous system with autosomal dominant transmission in the majority of cases and age-dependent penetrance of the mutant gene. In a number of cases, it shares some phenotypic features with autosomal dominant idiopathic torsion dystonia (locus DYT1 on chromosome 9q32-34) and is genetically heterogeneous: distinct variants of ET were mapped to chromosomes 3q13 (ETM1) and 2p22-25 (ETM2). We performed studies of candidate loci in a group of Slavonic (11 patients) and Tajik (19 patients) families with ET. Mutational analysis of the DYT gene in probands did not reveal the major deletion 946-948delGAG characteristic of idiopathic torsion dystonia, which allows one to genetically distinguish the studied hereditary forms of ET and torsion dystonia. Based on analysis of genetic linkage in informative Tajik pedigrees with ET, linkage to locus ETM1 on chromosome 3q13 was established in four families. Maximum pairwise Lod score was 2.46 at recombination fraction of theta = 0.00; maximum combined multipoint Lod score was 3.35 for marker D3S3720 and a common "mutant" haplotype for markers D3S3620, D3S3576, and D3S3720 allowed us to locate a mutant gene in a relatively narrow chromosome region spanning 2 cM. In one informative pedigree with ET, both candidate loci ETM1 and ETM2 were definitely excluded on the basis of negative Lod scores obtained by linkage estimations, which testifies to the existence of another distinct gene for autosomal dominant ET.     

Homozygosity mapping in families with Joubert syndrome identifies a locus on chromosome 9q34.3 and evidence for genetic heterogeneity

Joubert syndrome is a rare developmental defect of the cerebellar vermis, with autosomal recessive inheritance. The phenotype is highly variable and may include episodic hyperpnea, abnormal eye movements, hypotonia, ataxia, developmental delay, and mental retardation. Even within sibships the phenotype may vary, making it difficult to establish the exact clinical diagnostic boundaries of Joubert syndrome. To genetically localize the gene region, we have performed a whole-genome scan in two consanguineous families of Arabian/Iranian origins, with multiple affected probands. In one family, we detected linkage to the telomeric region of chromosome 9q, close to the marker D9S158, with a multipoint LOD score of Z=+3.7. The second family did not show linkage to this region, giving a first indication of genetic heterogeneity underlying Joubert syndrome. These findings were supported by subsequent analysis of two smaller families-one compatible with linkage to 9q; the other, unlinked. We conclude that Joubert syndrome is clinically and genetically heterogeneous and that one locus maps to chromosome 9q.     

Chromosome 6-linked autosomal recessive early-onset Parkinsonism: linkage in European and Algerian families, extension of the clinical spectrum, and evidence of a small homozygous deletion in one family. The French Parkinson's Disease Genetics Study Group, and the European Consortium on Genetic Susceptibility in Parkinson's Disease.

The gene for autosomal recessive juvenile Parkinsonism (AR-JP) recently has been mapped to chromosome 6q25.2-27 in Japanese families. We have tested one Algerian and 10 European multiplex families with early-onset Parkinson disease for linkage to this locus, with marker D6S305. Homogeneity analysis provided a conditional probability in favor of linkage of >.9 in eight families, which were analyzed further with eight microsatellite markers spanning the 17-cM AR-JP region. Haplotype reconstruction for eight families and determination of the smallest region of homozygosity in two consanguineous families reduced the candidate interval to 11.3 cM. If the deletion of two microsatellite markers (D6S411 and D6S1550) that colocalize on the genetic map and that segregate with the disease in the Algerian family is taken into account, the candidate region would be reduced to <1 cM. These findings should facilitate identification of the corresponding gene. We have confirmed linkage of AR-JP, in European families and in an Algerian family, to the PARK2 locus. PARK2 appears to be an important locus for AR-JP in European patients. The clinical spectrum of the disease in our families, with age at onset <=58 years and the presence of painful dystonia in some patients, is broader than that reported previously.     

Genetic homogeneity of autoimmune polyglandular disease type I.

Autoimmune polyglandular disease type I (APECED) is an autosomal recessive autoimmune disease (MIM 240300) characterized by hypoparathyroidism, primary adrenocortical failure, and chronic mucocutaneous candidiasis. The disease is highly prevalent in two isolated populations, the Finnish population and the Iranian Jewish one. Sporadic cases have been identified in many other countries, including almost all European countries. The APECED locus has previously been assigned to chromosome 21q22.3 by linkage analyses in 14 Finnish families. Locus heterogeneity is a highly relevant question in this disease affecting multiple tissues and with great phenotypic diversity. To solve this matter, we performed linkage and haplotype analyses on APECED families rising from different populations. Six microsatellite markers on the critical chromosomal region of 2.6 cM on 21q22.3 were analyzed. Pairwise linkage analyses revealed significant LOD scores for all these markers, maximum LOD score being 10.23. The obtained haplotype data and the geographic distribution of the great-grandparents of the Finnish APECED patients suggest the presence of one major, relatively old mutation responsible for approximately 90% of the Finnish cases. Similar evidence for one founder mutation was also found in analyses of Iranian Jewish APECED haplotypes. These haplotypes, however, differed totally from the Finnish ones. The linkage analyses in 21 non-Finnish APECED families originating from several European countries provided independent evidence for linkage to the same chromosomal region on 21q22.3 and revealed no evidence for locus heterogeneity. The haplotype analyses of APECED chromosomes suggest that in different populations APECED is due to a spectrum of mutations in a still unknown gene on chromosome 21.     

Transmission test for linkage disequilibrium: the insulin gene region and insulin-dependent diabetes mellitus (IDDM).

A population association has consistently been observed between insulin-dependent diabetes mellitus (IDDM) and the "class 1" alleles of the region of tandem-repeat DNA (5'' flanking polymorphism [5''FP]) adjacent to the insulin gene on chromosome 11p. This finding suggests that the insulin gene region contains a gene or genes contributing to IDDM susceptibility. However, several studies that have sought to show linkage with IDDM by testing for cosegregation in affected sib pairs have failed to find evidence for linkage. As means for identifying genes for complex diseases, both the association and the affected-sib-pairs approaches have limitations. It is well known that population association between a disease and a genetic marker can arise as an artifact of population structure, even in the absence of linkage. On the other hand, linkage studies with modest numbers of affected sib pairs may fail to detect linkage, especially if there is linkage heterogeneity. We consider an alternative method to test for linkage with a genetic marker when population association has been found. Using data from families with at least one affected child, we evaluate the transmission of the associated marker allele from a heterozygous parent to an affected offspring. This approach has been used by several investigators, but the statistical properties of the method as a test for linkage have not been investigated. In the present paper we describe the statistical basis for this "transmission test for linkage disequilibrium" (transmission/disequilibrium test [TDT]). We then show the relationship of this test to tests of cosegregation that are based on the proportion of haplotypes or genes identical by descent in affected sibs. The TDT provides strong evidence for linkage between the 5''FP and susceptibility to IDDM. The conclusions from this analysis apply in general to the study of disease associations, where genetic markers are usually closely linked to candidate genes. When a disease is found to be associated with such a marker, the TDT may detect linkage even when haplotype-sharing tests do not.     

Genetic testing for early-onset torsion dystonia (DYT1): introduction of a simple screening method, experiences from testing of a large patient cohort, and ethical aspects

Early-onset, generalized primary torsion dystonia (PTD) is an autosomal dominantly inherited disorder, characterized by involuntary movements and abnormal postures. The majority of cases are caused by a 3-bp deletion in the DYT1 gene on chromosome 9q34 that allows for specific genetic testing. We developed a simple, reliable, and cost-effective, PCR-based screening method for this mutation. Testing results from a cohort of 550 cases, including patients with different forms of dystonia and unclassified movement disorders, revealed that 72.2% of the patients with typical early-onset generalized PTD carried the GAG deletion in the DYT1 gene. Among 300 cases with late-onset focal/segmental dystonia, only 3 patients tested positive for the GAG deletion whereas 12.8% of the patients with an unclassified movement disorder were GAG positive. Our results confirm a genotype/phenotype correlation in early-onset PTD and show that application of strict clinical criteria leads to accurate prediction of carrier status in more than two-thirds of patients with this type of dystonia. Currently, we suggest that testing be recommended in individuals with age of onset of dystonia below 30 years and/or a positive family history of early-onset PTD. Testing is not recommended in patients with onset of symptoms after 30 years or in asymptomatic individuals under the age of 18.     

Molecular Genetic Analysis of Essential Tremor

Essential tremor (ET) is the most common extrapyramidal disorder of the central nervous system with autosomal dominant transmission in the majority of cases and age-dependent penetrance of the mutant gene. In a number of cases, it shares some phenotypic features with autosomal dominant idiopathic torsion dystonia (locusDYT1on chromosome 9q32–34) and is genetically heterogeneous: distinct variants of ET were mapped to chromosomes 3q13 (ETM1) and 2p22–25 (ETM2). We performed studies of candidate loci in a group of Slavonic (11 patients) and Tajik (19 patients) families with ET. Mutational analysis of the DYT1 gene in probands did not reveal the major deletion 946–948delGAG characteristic of idiopathic torsion dystonia, which allows one to genetically distinguish the studied hereditary forms of ET and torsion dystonia. Based on analysis of genetic linkage in informative Tajik pedigrees with ET, linkage to locus ETM1 on chromosome 3q13 was established in four families. Maximum pairwise Lod score was 2.46 at recombination fraction of = 0.00; maximum combined multipoint Lod score was 3.35 for marker D3S3515 and a common mutant haplotype for markers D3S3620, D3S3576, and D3S3720 allowed us to locate a mutant gene in a relatively narrow chromosome region spanning 2 cM. In one informative pedigree with ET, both candidate loci ETM1 and ETM2 were definitely excluded on the basis of negative Lod scores obtained by linkage estimations, which testifies to the existence of another distinct gene for autosomal dominant ET.     

Paying the price at the pump: dystonia from mutations in a Na+/K+ -ATPase

Dystonia is a disorder of involuntary sustained muscle contraction, which usually affects a focal region of the body but may be generalized and results in twisting contorted movements or abnormal postures. Several clinical subtypes of dystonia have been delineated and many have a strong inherited basis. In this issue of Neuron, de Carvalho Aguiar and colleagues report the identification of missense mutations in the gene for the Na+/K+ -ATPase alpha3 subunit (ATP1A3) as a cause of rapid-onset dystonia-parkinsonism (RDP, DYT12).     

Genetic heterogeneity in X-linked amelogenesis imperfecta.

The AMELX gene located at Xp22.1-p22.3 encodes for the enamel protein amelogenin and has been implicated as the gene responsible for the inherited dental abnormality X-linked amelogenesis imperfecta (XAI). Three families with XAI have been investigated using polymorphic DNA markers flanking the position of AMELX. Using two-point linkage analysis, linkage was established between XAI and several of these markers in two families, with a combined lod score of 6.05 for DXS16 at theta = 0.04. This supports the involvement of AMELX, located close to DXS16, in the XAI disease process (AIH1) in those families. Using multipoint linkage analysis, the combined maximum lod score for these two families was 7.30 for a location of AIH1 at 2 cM distal to DXS16. The support interval around this location extended about 8 cM proximal to DXS92, and the AIH1 location could not be precisely defined by multipoint mapping. Study of recombination events indicated that AIH1 lies in the interval between DXS143 and DXS85. There was significant evidence against linkage to this region in the third family, indicating locus heterogeneity in XAI. Further analysis with markers on the long arm of the X chromosome showed evidence of linkage to DXS144E and F9 with no recombination with either of these markers. Two-point analysis gave a peak lod score at DXS144E with a maximum lod score of 2.83 at theta = 0, with a peak lod score in multipoint linkage analysis of 2.84 at theta = 0. The support interval extended 9 cM proximal to DXS144E and 14 cM distal to F9.(ABSTRACT TRUNCATED AT 250 WORDS)