Tandem Repeats In Exon Research Articles

Kennedy's Disease, a Mimic of Amyotrophic Lateral Sclerosis: A Case Report

Kennedy’s disease is an adult-onset, X-linked recessive trinucleotide, polyglutamine disorder, caused by expansion of a polymorphic CAG tandem-repeat in exon 1 of the androgen-receptor (AR) gene on chromosome Xq11-12. We report a case of 60 year old non-diabetic, normotensive, non-smoker, non-alcoholic male presenting with gradual onset and progressive difficulty in swallowing, nasal intonation of speech, generalized wasting and weakness with cramps and fasciculation without any sensory symptoms for around one year. Examination revealed gross emaciation, bilateral gynaecomastia, nasal speech with absent gag reflex, wasted and fasciculating tongue, wasted limb muscles with widespread fasciculation, bilateral postural tremor without any cognitive and sensory impairment. Investigation revealed elevated serum CPK. Electro-diagnostic (EDX) features are consistent with a slowly progressive and very chronic degeneration of the anterior horn cells and dorsal root ganglia (absent SNAP). MRI of the brain and cervical spine revealed no abnormality. Ultimately the genetic analysis confirmed the case as Kennedy’s disease.

Kennedy's disease and partial androgen insensitivity syndrome. Report of 4 cases and literature review

Kennedy's disease, also known as bulbospinal muscular atrophy, is a rare, X-linked recessive neurodegenerative disorder affecting adult males. It is caused by expansion of an unstable cytosine-adenine-guanine tandem-repeat in exon 1 of the androgen-receptor gene on chromosome Xq11-12, and is characterized by spinal motor neuron progressive degeneration. Endocrinologically, these patients often have the features of hypogonadism associated to the androgen insensitivity syndrome, particularly its partial forms. We report 4 cases with the typical neurological presentation, consisting of slowly progressing generalized muscle weakness with atrophy and bulbar muscle involvement; these patients also had several endocrine manifestations; the most common non-neurological manifestation was gynecomastia. In all cases reported, molecular analysis showed an abnormal cytosine-adenine-guanine triplet repeat expansion in the androgen receptor gene.

Enfermedad de Kennedy y resistencia parcial androgénica. Descripción de 4 casos y revisión de la literatura

Kennedy's disease, also known as bulbospinal muscular atrophy, is a rare, X-linked recessive neurodegenerative disorder affecting adult males. It is caused by expansion of an unstable cytosine-adenine-guanine tandem-repeat in exon 1 of the androgen-receptor gene on chromosome Xq11-12, and is characterized by spinal motor neuron progressive degeneration. Endocrinologically, these patients often have the features of hypogonadism associated to the androgen insensitivity syndrome, particularly its partial forms.We report 4 cases with the typical neurological presentation, consisting of slowly progressing generalized muscle weakness with atrophy and bulbar muscle involvement; these patients also had several endocrine manifestations; the most common non-neurological manifestation was gynecomastia. In all cases reported, molecular analysis showed an abnormal cytosine-adenine-guanine triplet repeat expansion in the androgen receptor gene.

Dopamine receptor D4 (DRD4) gene polymorphism among Gaddi tribe of Himachal Pradesh

Dopamine is one of the most important neurotransmitters in the brain. D4 receptor of dopamine shows the largest number of polymorphisms among all the dopamine receptors and is one of the most polymorphic of all the genes studied (Chang et al. 1996). DRD4 gene is reported to be associated with personality associated traits like novelty-seeking. The polymorphism in DRD4 is revealed in the variable number of imperfect 48 bp tandem repeats in exon 3, ranging from two to 11 repeat units, with over 67 different haplotype variants (Ding et al. 2002; Grady et al. 2003). In most geographical locations, the 4R allele is the most common, whereas 2R and 7R allele frequencies vary widely (Chang et al. 1996; Ding et al. 2002). The 4R and 7R repeats are the most common worldwide (Chang et al. 1996). 4R and 2R alleles are common in Indian subcontinent (Chen et al.1999). 7R is absent among most of the Indian tribes and rare among caste groups of India. (Saraswathy et al. 2013). This study is an attempt to find out the distribution of DRD4 variants among Gaddis, a transhumant population of northern India. Transhumance is seasonal and short term (for 2–4 months) micro-migration with livestock due to unfavourable change in environment.

Association study of ERβ, AR, and CYP19A1 genes and MtF transsexualism.

The etiology of male-to-female (MtF) transsexualism is unknown. Both genetic and neurological factors may play an important role. To investigate the possible influence of the genetic factor on the etiology of MtF transsexualism. We carried out a cytogenetic and molecular analysis in 442 MtFs and 473 healthy, age- and geographical origin-matched XY control males. The karyotype was investigated by G-banding and by high-density array in the transsexual group. The molecular analysis involved three tandem variable regions of genes estrogen receptor β (ERβ) (CA tandem repeats in intron 5), androgen receptor (AR) (CAG tandem repeats in exon 1), and CYP19A1 (TTTA tandem repeats in intron 4). The allele and genotype frequencies, after division into short and long alleles, were obtained. We investigated the association between genotype and transsexualism by performing a molecular analysis of three variable regions of genes ERβ, AR, and CYP19A1 in 915 individuals (442 MtFs and 473 control males). Most MtFs showed an unremarkable 46,XY karyotype (97.96%). No specific chromosome aberration was associated with MtF transsexualism, and prevalence of aneuploidy (2.04%) was slightly higher than in the general population. Molecular analyses showed no significant difference in allelic or genotypic distribution of the genes examined between MtFs and controls. Moreover, molecular findings presented no evidence of an association between the sex hormone-related genes (ERβ, AR, and CYP19A1) and MtF transsexualism. The study suggests that the analysis of karyotype provides limited information in these subjects. Variable regions analyzed from ERβ, AR, and CYP19A1 are not associated with MtF transsexualism. Nevertheless, this does not exclude other polymorphic regions not analyzed.

X-Chromosome Inactivation Pattern Analysis for the Assessment of Cell Clonality in Cats

X-chromosome inactivation pattern (XCIP) analysis has been widely used to assess cell clonality in various types of neoplasms in humans. In the present study, a polymerase chain reaction-based feline XCIP analysis using the feline androgen receptor gene was developed. To construct the system of the analysis, polymorphism in CAG tandem repeats within the feline androgen receptor gene was explored using somatic DNAs from 50 male and 103 female cats. CAG tandem repeats in exon 1 of the feline androgen receptor gene were found to be polymorphic, containing 15 to 22 CAG repeats. Of the 103 female cats, 70 (68%) were heterozygous for the number of CAG repeats, indicating the possible usefulness of XCIP analysis in cats. Application of the feline XCIP analysis to 3 feline mammary gland adenocarcinoma cell lines revealed distinctly skewed XCIPs in these cell lines, indicating their clonal origins. Twelve (80%) of the 15 primary tissue/cell samples obtained from cats with various neoplastic diseases showed skewed XCIPs. Moreover, bone marrow samples from 3 cats with myelodysplastic syndrome were also found to have skewed XCIPs. The polymerase chain reaction-based XCIP analysis developed in this study can provide information on cell clonality in female cats, potentially facilitating the differential diagnosis of various disorders in cats.

Dopamine D4 receptor, but not the ADHD-associated D4.7 variant, forms functional heteromers with the dopamine D2S receptor in the brain

Polymorphic variants of the dopamine D4 receptor have been consistently associated with attention-deficit hyperactivity disorder (ADHD). However the functional significance of the risk polymorphism (variable number of tandem repeats in exon 3) is still unclear. Here we show that whereas the most frequent 4-repeat (D4.4) and the 2-repeat (D4.2) variants form functional heteromers with the short isoform of the dopamine D2 receptor (D2S), the 7-repeat risk allele (D4.7) does not. D2 receptor activation in the D2S-D4 receptor heteromer potentiates D4 receptor-mediated MAPK signaling in transfected cells and in the striatum, which did not occur in cells expressing D4.7 or in the striatum of knock-in mutant mice carrying the 7 repeats of the human D4.7 in the third intracellular loop of the D4 receptor. In the striatum D4 receptors are localized in cortico-striatal glutamatergic terminals, where they selectively modulate glutamatergic neurotransmission by interacting with D2S receptors. This interaction shows the same qualitative characteristics than the D2S-D4 receptor heteromer-mediated MAPK signaling and D2S receptor activation potentiates D4 receptor-mediated inibition of striatal glutamate release. It is therefore postulated that dysfunctional D2S-D4.7 heteromers may impair presynaptic dopaminergic control of corticostriatal glutamatergic neurotransmission and explain functional deficits associated with ADHD.

P.1.021 Different influence of dopaminergic system genes on working memory in healthy subjects

D2-like receptors are key targets for methamphetamine in the CNS, and their activation is an initial and indispensable effect in the induction of dependence and psychosis. It is possible that genetic variants of D2-like receptors may affect individual susceptibility to methamphetamine dependence and psychosis. To test this hypothesis, 6 putatively functional polymorphisms of D2-like receptors, −141C Ins/Del, Ser311Cys and TaqIA of the DRD2 gene, Ser9Gly of the DRD3 gene, and −521C>T and a variable number of tandem repeats in exon 3 of the DRD4 gene, were analyzed in 202 patients with methamphetamine dependence and/or psychosis and 243 healthy controls in a Japanese population. No polymorphism examined showed significant association with methamphetamine dependence, but two polymorphisms of DRD2 were associated with the clinical course and prognosis of methamphetamine psychosis. The A1/A1 homozygote of DRD2 was a negative risk factor for a poorer prognosis of psychosis that continues for more than 1 month after the discontinuance of methamphetamine abuse and the beginning of treatment with neuroleptics (p = 0.04, odds ratio (OR) = 0.42, 95% CI; 0.27–0.65) and the complication of spontaneous relapse of methamphetamine psychosis after remission (p = 0.014, OR = 0.34, 95% CI; 0.22–0.54). The genotype of −141C Del positive (Del/Del and Del/Ins) was at risk for rapid onset of methamphetamine psychosis that develops into a psychotic state within 3 years after initiation of methamphetamine abuse (p = 0.00037, OR = 3.62, 95% CI 2.48–5.28). These findings revealed that genetic variants of DRD2, but not DRD3 or DRD4, confer individual risks for rapid onset, prolonged duration, and spontaneous relapse of methamphetamine psychosis.

A Variable-Number-of-Tandem-Repeats Polymorphism in the Dopamine D4 Receptor Gene Affects Social Adaptation of Alcohol Use

Research suggests that people adapt their own drinking behavior to that of other people. According to a genetic-differences approach, some individuals may be more inclined than others to adapt their alcohol consumption level to that of other people. Using a 3 (drinking condition) × 2 (genotype) experimental design (N = 113), we tested whether susceptibility to alcohol-related cues (i.e., seeing someone drink) was related to the variable number of tandem repeats in exon 3 of the D4 dopamine receptor gene. A strong gene-environment interaction showed that participants carrying at least one copy of the 7-repeat allele consumed substantially more alcohol in the presence of a heavy-drinking individual than did participants without this allele. This study highlights that individual variability in sensitivity to other people’s drinking behavior may be attributable to genetic differences. Carrying the 7-repeat allele may increase the risk for heavy alcohol use or abuse in the company of heavy-drinking peers.

Variability of CAG tandem repeats in exon 1 of the androgen receptor gene is not related with dog intersexuality

Numerous mutations of the human androgen receptor ( AR) gene cause an intersexual phenotype, called the androgen insensitivity syndrome. The intersexual phenotype is also quite often diagnosed in dogs. The aim of this study was to conduct a comparative analysis of the entire coding sequence (eight exons) of the AR gene in healthy and four intersex dogs, as well as in three other canids (the red fox, arctic fox and Chinese raccoon dog). The coding sequence of the studied species appeared to be conserved (similarity above 97%) and polymorphism was found in exon 1 only. Altogether, 2 SNPs were identified in healthy dogs, 14 in red foxes, 16 in arctic foxes and 6 were found in Chinese raccoon dogs, respectively. Moreover, a variable number of tandem repeats (CAG and CAA), encoding an array of glutamines, was also observed in this exon. The CAA codon numbers were invariable within species, but the CAG repeats were polymorphic. The highest number of the CAG and CAA repeats was found in dogs (from 40 to 42) and the observed variability was similar in intersex and healthy dogs. In the other canids the variability fell within the following ranges: 29–37 (red fox), 37–39 (arctic fox) and 29–32 (Chinese raccoon dog). In addition, a polymorphic microsatellite marker in intron 2 was found in the dog, red fox and Chinese raccoon dog. It was concluded that the polymorphism level of the AR gene in the dog was lower than in the other canids and none of the detected polymorphisms, including variability of the CAG tandem repeats, could be related with the intersexual phenotype of the studied dogs.

Bulbar and spinal muscular atrophy (Kennedy’s disease): a review

Bulbar and spinal muscular atrophy (BSMA) is an adult-onset, X-linked recessive trinucleotide, polyglutamine disorder, caused by expansion of a polymorphic CAG tandem-repeat in exon 1 of the androgen-receptor (AR) gene on chromosome Xq11-12. Pathogenetically, mutated AR accumulates in nuclei and cytoplasm of motor neurons, resulting in their degeneration and loss. Phenotypically, patients present with amyotrophic, proximal or distal weakness and wasting of the facial, bulbar and limb muscles, occasionally sensory disturbances, and endocrinologic disturbances, such as androgen resistance, gynecomastia, elevated testosterone or progesterone, and reduced fertility. There may be mild hyper-CK-emia, abnormal motor and sensory nerve conduction studies, and neuropathic and myopathic alterations on muscle biopsy. The golden standard for diagnosing BSMA is genetic analysis, demonstrating a CAG-repeat number >40. No causal therapy is available, but symptomatic therapy should be provided for tremor, endocrinologic abnormalities, sensory disturbances, or muscle cramps. The course is slowly progressive, the ability to walk lost only late in life, only few patients require ventilatory support, and life expectancy only slightly reduced.

Genetic variants of D2 but not D3 or D4 dopamine receptor gene are associated with rapid onset and poor prognosis of methamphetamine psychosis

D2-like receptors are key targets for methamphetamine in the CNS, and their activation is an initial and indispensable effect in the induction of dependence and psychosis. It is possible that genetic variants of D2-like receptors may affect individual susceptibility to methamphetamine dependence and psychosis. To test this hypothesis, 6 putatively functional polymorphisms of D2-like receptors, −141C Ins/Del, Ser311Cys and TaqIA of the DRD2 gene, Ser9Gly of the DRD3 gene, and −521C>T and a variable number of tandem repeats in exon 3 of the DRD4 gene, were analyzed in 202 patients with methamphetamine dependence and/or psychosis and 243 healthy controls in a Japanese population. No polymorphism examined showed significant association with methamphetamine dependence, but two polymorphisms of DRD2 were associated with the clinical course and prognosis of methamphetamine psychosis. The A1/A1 homozygote of DRD2 was a negative risk factor for a poorer prognosis of psychosis that continues for more than 1 month after the discontinuance of methamphetamine abuse and the beginning of treatment with neuroleptics ( p = 0.04, odds ratio (OR) = 0.42, 95% CI; 0.27–0.65) and the complication of spontaneous relapse of methamphetamine psychosis after remission ( p = 0.014, OR = 0.34, 95% CI; 0.22–0.54). The genotype of −141C Del positive (Del/Del and Del/Ins) was at risk for rapid onset of methamphetamine psychosis that develops into a psychotic state within 3 years after initiation of methamphetamine abuse ( p = 0.00037, OR = 3.62, 95% CI 2.48–5.28). These findings revealed that genetic variants of DRD2, but not DRD3 or DRD4, confer individual risks for rapid onset, prolonged duration, and spontaneous relapse of methamphetamine psychosis.

A Rearrangement Resulting in Small Tandem Repeats in the F3′5′H Gene of White Flower Genotypes Is Associated with the Soybean W1 Locus

Soybean [Glycine max (L.) Merr.] flower pigmentation is controlled by six loci, W1, W2, W3, W4, Wm, and Wp, of which the functions of only three are known. None of the recessive alleles at those three loci completely abolishes flower pigmentation. On the other hand, the W1 locus is required for flower pigmentation. Based on genetic and biochemical data, the trihydroxylation reaction to form delphinidin‐3‐glucoside has been postulated as a likely function of the W1 locus. We report the cloning and sequencing of a flavonoid 3′5′‐hydroxylase (F3′5′H) gene from the purple flower soybean line L79‐908 (W1) and the recessive allele from the white flower isoline ‘Williams’ (w1). The mutation is a rearrangement leading to a small (65 bp) insertion of tandem repeats in exon 3 that truncates the translation product prematurely. The presence of this insertion in all‐white flower soybean lines examined and in those of a segregating population resulting from a cross between purple and white flower lines presents compelling evidence that the F3′5′H gene isolated is likely encoded by the W1 locus of Glycine max. GmF3′5′H is a single‐copy gene, expressed at very low levels in all tissues examined, including flower and seed coats, but sufficient to account for the contribution of the delphinidin‐based anthocyanins and/or proanthocyanins in these tissues.

MICA‐STR, HLA‐B haplotypic diversity and linkage disequilibrium in the Hunan Han population of southern China

Major histocompatibility complex (MHC) class I chain-related gene A (MICA) is located 46 kb centromeric to HLA-B and encodes a stress-inducible protein. MICA allelic variation is thought to be associated with disease susceptibility and immune response to transplants. This study was aimed to investigate the haplotypic diversity and linkage disequilibrium between human leukocyte antigen (HLA)-B and (GCT)(n) short tandem repeat in exon 5 of MICA gene (MICA-STR) in a southern Chinese Han population. Fifty-eight randomly selected nuclear families with 183 members including 85 unrelated parental samples were collected in Hunan province, southern China. HLA-B generic typing was performed by polymerase chain reaction-sequence-specific priming (PCR-SSP), and samples showing novel HLA-B-MICA-STR linkage were further typed for HLA-B allelic variation by high-resolution PCR-SSP. MICA-STR allelic variation and MICA gene deletion (MICA*Del) were detected by fluorescent PCR-size sequencing and PCR-SSP. Haplotype was determined through family segregation analysis. Statistical analysis was applied to the data of the 85 unrelated parental samples. Nineteen HLA-B specificities and seven MICA-STR allelic variants were observed in 85 unrelated parental samples, the most predominant of which were HLA-B*46, -B60, -B*13, and -B*15, and MICA*A5, MICA*A5.1 and MICA*A4, respectively. Genotype distributions of HLA-B, MICA-STR loci were consistent with Hardy-Weinberg proportions. The HLA-B-MICA-STR haplotypic phases of all 85 unrelated parental samples were unambiguously assigned, which contained 30 kinds of HLA-B, MICA-STR haplotypic combinations, nine of them have not been reported in the literature. Significant positive linkage disequilibria between certain HLA-B and MICA-STR alleles, including HLA-B*13 and MICA*A4, HLA-B*38 and MICA*A9, HLA-B*58 and MICA*A9, HLA-B*46 and MICA*A5, HLA-B*51 and MICA*A6, HLA-B*52 and MICA*A6, and HLA-B60 and MICA*A5.1, were observed. HLA-B*48 was linked to MICA*A5, MICA*A5.1 and MICA*Del. HLA-B*5801-MICA*A10 linkage was found in a family. Our data indicated a high degree of haplotypic diversity and strong linkage disequilibrium between MICA-STR and HLA-B in a southern Chinese Han population, the data will inform future studies on anthropology, donor-recipient HLA matching in clinical transplantation and HLA-linked disease association.

Human Arsenic Methyltransferase (AS3MT) Pharmacogenetics: GENE RESEQUENCING AND FUNCTIONAL GENOMICS STUDIES

Arsenic contaminates ground water worldwide. Methylation is an important reaction in the biotransformation of arsenic. We set out to study the pharmacogenetics of human arsenic methyltransferase (AS3MT, previously CYT19). After cloning the human AS3MT cDNA, we annotated the human gene and resequenced its 5'-flanking region, exons, and splice junctions using 60 DNA samples from African-American (AA) and 60 samples from Caucasian-American (CA) subjects. We observed 26 single nucleotide polymorphisms (SNPs), including 3 non-synonymous cSNPs, as well as a variable number of tandem repeats in exon 1 within an area encoding the cDNA 5'-untranslated region. The nonsynonymous cSNPs included T860C (M287T) with frequencies of 10.8 and 10% in AA and CA subjects, respectively, as well as C517T (A173W) in one AA and C917T (T306I) in one CA sample. Haplotype analysis showed that Ile(306) was linked to Thr(287), so this double variant allozyme was also studied functionally. After expression in COS-1 cells and correction for transfection efficiency, the Trp(173) allozyme displayed 31%, Thr(287) 350%, Ile(306) 4.8%, and Thr(287)/Ile(306) 6.2% of the activity of the wild type (WT) allozyme, with 20, 190, 4.4, and 7.9% of the level of WT immunoreactive protein, respectively. Apparent K(m) values for S-adenosyl-l-methionine were 4.6, 3.1, and 11 mum for WT, Trp(173), and Thr(287) allozymes, with K(m) values for sodium arsenite with the same allozymes of 11.8, 8.9, and 4.5mum. The Ile(306) and Thr(287)/Ile(306) allozymes expressed too little activity for inclusion in the substrate kinetic studies. Expression of reporter gene constructs for the 5'-flanking region and the variable number of tandem repeats in the 5'-untranslated region demonstrated cell line-dependent variation in reporter gene expression, with shorter repeats associated with increased transcription in HepG2 cells. These results raise the possibility that inherited variation in AS3MT may contribute to variation in arsenic metabolism and, perhaps, arsenic-dependent carcinogenesis in humans.

Identification and Characterization of a Tandem Repeat in Exon III of the Dopamine Receptor D4 (DRD4) Gene in Cetaceans

A large number of mammalian species harbor a tandem repeat in exon III of the gene encoding dopamine receptor D4 (DRD4), a receptor associated with cognitive functions. In this study, a DRD4 gene exon III tandem repeat from the order Cetacea was identified and characterized. Included in our study were samples from 10 white-beaked dolphins (Lagenorhynchus albirostris), 10 harbor porpoises (Phocoena phocoena), eight sperm whales (Physeter macrocephalus), and five minke whales (Balaenoptera acutorostrata). Using enzymatic amplification followed by sequencing of amplified fragments, a tandem repeat composed of 18-bp basic units was detected in all of these species. The tandem repeats in white-beaked dolphin and harbor porpoise were both monomorphic and consisted of 11 and 12 basic units, respectively. In contrast, the sperm whale harbored a polymorphic tandem repeat with size variants composed of three, four, and five basic units. Also the tandem repeat in minke whale was polymorphic; size variants composed of 6 or 11 basic units were found in this species. The consensus sequences of the basic units were identical in the closely related white-beaked dolphin and harbor porpoise, and these sequences differed by a maximum of two changes when compared to the remaining species. There was a high degree of similarity between the cetacean basic unit consensus sequences and those from members of the horse family and domestic cow, which also harbor a tandem repeat composed of 18-bp basic units in exon III of their DRD4 gene. Consequently, the 18-bp tandem repeat appears to have originated prior to the differentiation of hoofed mammals into odd-toed and even-toed ungulates. The composition of the tandem repeat in cetaceans differed markedly from that in primates, which is composed of 48-bp repeat basic units.

Identification and Characterization of Tandem Repeats in Exon III of Dopamine Receptor D4 (DRD4) Genes from Different Mammalian Species

In this study we have identified and characterized dopamine receptor D4 (DRD4) exon III tandem repeats in 33 public available nucleotide sequences from different mammalian species. We found that the tandem repeat in canids could be described in a novel and simple way, namely, as a structure composed of 15- and 12- bp modules. Tandem repeats composed of 18-bp modules were found in sequences from the horse, zebra, onager, and donkey, Asiatic bear, polar bear, common raccoon, dolphin, harbor porpoise, and domestic cat. Several of these sequences have been analyzed previously without a tandem repeat being found. In the domestic cow and gray seal we identified tandem repeats composed of 36-bp modules, each consisting of two closely related 18-bp basic units. A tandem repeat consisting of 9-bp modules was identified in sequences from mink and ferret. In the European otter we detected an 18-bp tandem repeat, while a tandem repeat consisting of 27-bp modules was identified in a sequence from European badger. Both these tandem repeats were composed of 9-bp basic units, which were closely related with the 9-bp repeat modules identified in the mink and ferret. Tandem repeats could not be identified in sequences from rodents. All tandem repeats possessed a high GC content with a strong bias for C. On phylogenetic analysis of the tandem repeats evolutionary related species were clustered into the same groups. The degree of conservation of the tandem repeats varied significantly between species. The deduced amino acid sequences of most of the tandem repeats exhibited a high propensity for disorder. This was also the case with an amino acid sequence of the human DRD4 exon III tandem repeat, which was included in the study for comparative purposes. We identified proline-containing motifs for SH3 and WW domain binding proteins, potential phosphorylation sites, PDZ domain binding motifs, and FHA domain binding motifs in the amino acid sequences of the tandem repeats. The numbers of potential functional sites varied pronouncedly between species. Our observations provide a platform for future studies of the architecture and evolution of the DRD4 exon III tandem repeat, and they suggest that differences in the structure of this tandem repeat contribute to specialization and generation of diversity in receptor function.

Indian ethnic populations characterized by dopamine (D4) receptor VNTR polymorphism

The human dopamine receptor D4 gene (DRD4) contains a 48-bp tandem repeat in exon 3 and shows alleles varying between repeats 2 and 11. The gene shows a high level of expression in the prefrontal cortex of the brain and association of particular alleles of this locus with various neuropsychiatric and personality disorders have been reported. The present study reports allele frequency distribution at the DRD4 variable number tandem repeat (VNTR) locus among five ethnic populations of India. This background information is fundamental to the field of pharmacogenetics for disease susceptibility and association studies. Three hundred and thirty two healthy unrelated adult individuals belonging to five ethnic groups: Konkanastha Brahmins, Marathas, Ezhavas, Nairs and Muslims, have been typed. Genomic DNA, extracted from peripheral blood, was PCR amplified using a two-enzyme system. The use of ALF Express DNA sequencer was found to be helpful for large-scale population genotyping. Statistical analysis was performed using the POPGENE and DISPAN programs. A total of eight alleles ranging from repeat 2 to repeat 9 were observed. Allele 4 was the predominant allele among all the five populations, consistent with the data on other world populations. A rare allele 9 was detected exclusively among Marathas. The observed heterozygosity was low, ranging from 0.38 to 0.54 while other parameters like Polymorphism Information Content (PIC) and Power of Discrimination (PD) showed moderate values. The populations were in genetic equilibrium when tested under Hardy-Weinberg expectations. The allele frequency estimates for DRD4 provided here will contribute towards developing an informative database for this functionally relevant locus. This will prove useful when studying the association between genetic factors and pathogenesis of disease in Indian populations and will address the concern of biased results of association due to population admixtures.

A dyslexia susceptibility locus (DYX7) linked to dopamine D4 receptor (DRD4) region on chromosome 11p15.5

Dyslexia is a disability in acquiring reading and spelling skills that is independent of general intelligence and educational opportunity, and is highly heritable. It is known that dyslexia often co-occurs with attention deficit hyperactivity disorder (ADHD), and the 7-repeat allele of the 48-bp tandem repeat in exon 3 of the dopamine D4 receptor (DRD4) has been implicated in ADHD. We, therefore, investigated DRD4 as a candidate gene for dyslexia by testing for linkage and association with 14 markers at and around the DRD4 locus on chromosome 11p15.5. Using 100 families having at least two siblings affected with dyslexia, model-free linkage analysis revealed evidence for linkage to the DRD4-exon 3 repeat (two-point MFLOD = 2.27, P = 0.001) and to HRAS located just proximal to DRD4 (two-point MFLOD = 2.68, P = 0.0004). Evidence for linkage was maximal between DRD4 and HRAS (three-point MFLOD = 3.57, P = 0.00005). However, linkage disequilibrium analysis showed no significant evidence for association between dyslexia and DRD4 or HRAS. In particular, dyslexic subjects showed no significant increase of the DRD4 7-repeat allele associated with ADHD. It is possible that other DRD4 variants, not in strong linkage disequilibrium with the exon 3 repeat polymorphism, or alternatively, another gene very closely linked to DRD4, may influence susceptibility to dyslexia.

Genes and attention-deficit hyperactivity disorder

In a collaborative research program on attention-deficit hyperactivity disorder (ADHD) initiated 20 years ago at UC Irvine, we adopted Cantwell's (1994) approach to define a refined phenotype for use in studies of the biological bases of this disorder. We have used this refined phenotype (ADHD-Combined Type without internalizing comorbidities) in our molecular genetic studies of ADHD, which have paralleled the emerging literature in this new field. In our research program, we used the candidate gene approach, with hypotheses derived from the dopamine theory of ADHD and Posner and Raichle's (1994) theory of attention. We proposed a candidate dopamine gene (DRD4) and discovered an association with ADHD due to an increase prevalence of the ‘7-repeat’ allele defined by a 48-base-pair variable number of tandem repeats in exon III. The DRD4–ADHD association has now been confirmed by multiple groups around the world. In the next steps of our research program, we are evaluating the impact of a putative DRD4 risk allele on cognition, initiating an investigation of DNA sequence variation in DRD4 alleles, and investigating the association of ADHD with other candidate genes. Using our collaborative research program as an example, we will review the history and current status of molecular genetic studies of ADHD.