多巴胺D4受体第3外显子48 bp可变重复序列多态性与抽动障碍的传递不平衡检测

目的研究多巴胺D4受体第3外显子48 bp可变重复序列(DRD4 exonⅢ48bpVNTR)多态性是否与抽动障碍(tic disorder, Tic)存在关联.方法采用国际标准化的<Tourette综合征及其相关疾病遗传研究定式检查提纲>收集病史,运用核心家系传递不平衡分析方法(transmission disequilibrium test, TDT)对122个核心家系进行关联分析,根据是否合并注意缺陷多动障碍(attention deficit and hyperactivity disorder, ADHD),将122个核心家系分为合并ADHD的抽动障碍组[合并ADHD的Tourette综合征(Tourette syndrome, TS)和慢性抽动障碍(chronic tic, CT),共40例,TS&ADHD]和抽动障碍组[TS和CT,共82例,TS&CT]两组,采用聚合酶链反应、可变重复序列多态性分析等技术,进行抽动障碍与DRD4 exonⅢ48 bpVNTR多态性的TDT分析.结果在这一多态性位点存在5个等位基因,分别为DRD4 exonⅢ48 bp的2～6个重复等位基因.总体上没有发现抽动障碍与DRD4 exonⅢ48 bpVNTR多态性存在传递不平衡(χ2＝7.44,P＝0.12),进一步对不伴ADHD的抽动障碍组进行的TDT分析也没有发现存在这一位点的传递不平衡(χ2＝3.38,P＝0.50);而在合并ADHD的抽动障碍组中发现,合并ADHD的抽动障碍与DRD4 exonⅢ48 bpVNTR多态性在总体上存在传递不平衡(χ2＝11.74,P＝0.02),进一步对单个等位基因的TDT分析显示,合并ADHD的抽动障碍与DRD4 exonⅢ48 bp的5个重复和6个重复等位基因(长重复等位基因)存在传递不平衡(χ2＝10.57,P＝0.032,χ2＝6.13,P＝0.01).结论 DRD4 exonⅢ48 bpVNTR长重复等位基因与合并ADHD的抽动障碍存在关联,DRD4 exonⅢ48 bpVNTR长重复等位基因可能是中国人群合并ADHD的抽动障碍的遗传危险因素.     

Family-based association study between autism and glutamate receptor 6 gene in Chinese Han trios.

The glutamate pathways are involved in diverse processes such as learning and memory, epilepsy, and they play important roles in neural plasticity, neural development, and neurodegeneration. It has been proposed that autism could be a hypoglutamatergic disorder. Recently, Jamain et al. reported that the glutamate receptor 6 (GluR6 or GRIK2) is in linkage disequilibrium with autism. In the present study, the transmission disequilibrium test (TDT) and the haplotype transmission were performed to analyze the four SNPs (SNP1: rs995640; SNP2: rs2227281; SNP3: rs2227283; SNP4: rs2235076) of GluR6 in 174 Chinese Han parent-offspring trios. The TDT demonstrated that the two SNPs (SNP2 and SNP3) showed preferential transmission (TDT P = 0.032). The global chi(2) test for haplotype transmission also revealed an association between GluR6 and autism (chi(2) = 10.78, df = 3, P = 0.013). Our results suggested that GluR6 is in linkage disequilibrium with autism.     

Transmission disequilibrium tests confirm the link between DRD4 gene polymorphism and infant attachment.

Following up the results of a previous population association study (Lakatos et al. [2000: Mol Psychiatry 5:633-637; Lakatos et al. [2002: Mol Psychiatry 7:27-31]) by analyses based on parental genetic data confirmed the link between infant attachment and the dopamine D4 receptor (DRD4) gene. Extended transmission disequilibrium tests (ETDT) were performed to determine whether biased transmission of exon III 48 basepair repeat alleles occurred to infants displaying disorganized and secure attachment behavior with their mothers. The overall allele-wise TDTs were significant for both groups (P = 0.038 and 0.020, respectively): a trend for preferential transmission of the seven-repeat allele to disorganized infants was observed (TDT(chi)(2) = 3.27, df = 1, P = 0.071), and there was a significant non-transmission of the same allele to securely attached infants (TDT(chi)(2) = 6.00, df = 1, P = 0.014). Analysis of haplotypes of the exon III repeat and the -521 C/T promoter polymorphisms in family trios showed that the transmission bias in the larger secure group was due to the low-rate transmission of the T.7 haplotype containing both the seven-repeat and the -521 T alleles (TDT(chi)(2) = 4.46, df = 1, P = 0.035). This suggests that not carrying the T.7 haplotype of the DRD4 gene may act as a resilience factor in the optimal development of early attachment.     

5-HT 102T/C多态性与Tourette综合征的病例对照及核心家系关联分析

目的 研究5－羟色胺受体102T／C多态性是否与Tourette综合征（TS）相关联,方法对157个核心家系样本采用病例－对照关联分析,传递不平衡检验方法,聚合酶链反应及RFLP等技术,根据TS与强迫症（obsessive compulsive disorder,OCD)的同病现象,将TS划分亚组进行与5－羟色胺受体102T／C多态性的关联分析。结果 合并OCD的TS与该位点的基因型102C／C（X2＝8．38,P＝0．004）及等位基因102C／（X2＝4．84,P＝0．028）存在关联,进一步采用传递不平衡分析,发现合并（美国精神疾病诊断和统计手册IV》论断标准的OCD的TS与该位点存在关联或连锁不平衡（X2＝5．12,,P＝0．02）,而在TS总体样本及单纯TS样本中未发现与该位点的关联,结论 5－羟色胺受体102T／C多态性与中国人群合并OCD的TS存在关联,合并OCD的TS可能是TS中相对独立的一个亚型。     

Serotonin transporter (5-HTT) gene variants associated with autism?

An association study was performed to elucidate the role of the serotonin transporter (5-HTT) gene as a susceptibility factor for autism as treatment of patients with antidepressant drugs which selectively target 5-HTT reduced autistic or concomitant symptoms, such as repetitive behavior and aggression, and ameliorate language use. Using the transmission/disequilibrium test (TDT) an analysis was done for a common polymorphism in the upstream regulatory region (5-HTTLPR), a VNTR in intron 2 of the gene and a haplotype of both loci in 52 trios fulfilling stringent criteria for autism and an extended group of 65 trios including patients showing no language delay in their first 3 years of life. A higher frequency and preferential transmission of the long allele of the 5-HTTLPR was observed, but the TDT gave a statistically significant value ( P = 0. 032) only for the extended patient group. This result is in contrast to a recent study by a US group presenting preliminary evidence for preferential transmission of the short allele of 5-HTTLPR in 86 trios. Both studies failed to reveal significant linkage disequilibrium between the VNTR in intron 2 of the gene and autism. In our study haplotype analysis of the 5-HTTLPR and the VNTR in intron 2 supplied evidence for an association of 5-HTT and autism in the stringent ( P = 0.069) and extended patient group ( P = 0.049). Overall, we were not able to replicate the findings of the first study on 5-HTT and autism and instead observed a tendency for association of the opposite genetic variant of the gene with the disorder. The implications for genetic variants of the serotonin transporter in the etiology of autism and possible subgroups of patients, therefore, needs clarification in further studies with other and larger patient samples.      

The KPNA3 gene may be a susceptibility candidate for schizophrenia

The present study investigated the possible association of the KPNA3 locus in the 13q14 region with schizophrenia. We detected 7 single nucleotide polymorphisms (SNPs) on 13q14, one (rs6313) present at the HTR2A locus and the other 6 at the KPNA3 locus, among 124 British family trios consisting of mother, father and affected offspring with schizophrenia. The transmission disequilibrium test (TDT) showed allelic association for rs3736830 (chi(2)=8.66, P=0.003), rs2181185 (chi(2)=3.86, P=0.049) and rs626716 (chi(2)=5.82, P=0.016), but not for rs6313 (chi(2)=0.009, P=0.926). The global P-value was 0.029 for 1000 permutations with the TDT. The 2-SNP haplotype analysis showed a disease association for the rs2273816-rs3736830 haplotypes (chi(2)=7.63, d.f.=2, P=0.022), the rs3736830-rs2181185 haplotypes (chi(2)=10.30, d.f.=2, P=0.006) and the rs2181185-rs3782929 haplotypes (chi(2)=9.26, d.f.=2, P=0.01). The global P-value was 0.034 for 1000 permutations with the 2-SNP haplotype analysis. The 6-SNP haplotype system also showed a weak association with the illness (chi(2)=15.62, d.f.=8, P=0.048), although the 1-d.f. test did not show the association for nine individual haplotypes when a P-value was corrected by the Bonferroni corrections. The present study suggests that the KPNA3 may contribute genetically to schizophrenia in a small effect size.     

No association between 5HT-2A and bipolar disorder irrespective of genomic imprinting.

Recent evidence that 5HT-2A may be subjected to genomic imprinting prompted us to examine a collection of Irish family trios (an affected individual and both parents) for evidence of an association between 5HT-2A and bipolar disorder. Family trios offer an advantage over case control studies in regard to genomic imprinting since with family trios it is possible to trace the path of alleles from the parents to the offspring. Using haplotype-based haplotype relative risk (HHRR) and transmission/disequilibrium (TDT) analyses, no evidence was found for an association of 5HT-2A with bipolar affective disorder under the assumption of no imprinting and of imprinting.     

Association between 7q31 markers and Tourette syndrome

Tourette syndrome (TS) is a complex neuropychiatric disorder with a strong genetic basis. Although no specific susceptibility genes have been identified for TS, cytogenetic studies in selected cases suggest the existence of a predisposing gene located in the 7q31 chromosomal region. In order to test the hypothesis of a possible relationship between this region and TS at the population level, we undertook a family based association study in a sample of French Canadian patients from Quebec. For this purpose, markers D7S522, D7S523, and D7S1516 were tested using the extended transmission disequilibrium test (e-TDT). Marker D7S522 showed a biased transmission of alleles from heterozygote parents to their TS offsprings (allele-wise TDT chi(2) = 12.61, 4 df, P = 0.013, genotype-wise TDT chi(2) = 15.49, 7 df, P = 0.030). When the analysis was restricted to patients without ADHD or OCD comorbidity, similar results were observed both allele and genotype-wise (chi(2) = 10.68, 4 df, P = 0.03 and chi(2) = 12.55, 5 df, P = 0.028, respectively). In addition, marker D7S523 was also associated (allele-wise TDT chi(2) = 18.37, 7 df, P = 0.01 and genotype-wise TDT chi(2) = 46.26, 17 df, P = 0.00016), and showed a tendency for association in the comorbidity-free subgroup (genotype-wise TDT chi(2) = 18.7, 10 df, P = 0.044). Finally, marker D7S1516, contained in the inner mitochondrial membrane peptidase 2 like (IMMP2L) gene, also showed a tendency for association (genotype-wise TDT chi(2) = 32.87, 21 df, P = 0.048). These results may reflect the proximity of markers D7S522, D7S523, and possibly D7S1516 to a gene or regulatory region relevant to TS predisposition.     

Xp1区内定位注意缺损多动障碍易感位点的研究

目的探讨注意缺损多动障碍(attention deficit hyperactivity disorder, ADHD)与单胺氧化酶(monoamine oxidase, MAO)A型基因的遗传关系。方法采用基于单体型相对风险(haplotype-based haplotype relative risk,HHRR)和传递不平衡检验(transmission disequilibrium test, TDT)的方法,在60个ADHD儿童和双亲中进行了MAOCA微卫星多态性的关联和连锁分析。结果经HHRR和TDT分析,ADHD儿童与MAOCA位点114 bp相关联和连锁(χ2分别为4.90和4.84,P＜0.05)。结论 ADHD与MAO A型基因相关联和连锁,其易感位点可能定位于Xp1区。     

Lack of association between serotonin transporter gene promoter variants and autistic disorder in two ethnically distinct samples

Family-based studies performed to date provide conflicting evidence of linkage/association between autistic disorder and either the "short" [Cook et al., 1997: Mol Psychiatry 2:247-250] or the "long" [Klauck et al., 1997: Hum Mol Genet 6:2233-2238] allele of a polymorphic repeat located in the serotonin transporter (5-HTT) gene promoter region, affecting 5-HTT gene expression [Lesch et al., 1996: Science 274:1527-1531]. The present study was designed to assess linkage and linkage disequilibrium in two new ethnically distinct samples of families with primary autistic probands. The 5-HTT promoter repeat was genotyped in 54 singleton families collected in Italy and in 32 singleton and 5 multiplex families collected in the U.S.A., yielding a total sample of 98 trios. Linkage/association between 5-HTT gene promoter alleles and autistic disorder was assessed using the transmission/disequilibrium test (TDT) and the haplotype-based haplotype relative risk (HHRR). Both the Italian and the American samples, either singly or combined, displayed no evidence of linkage/association between 5-HTT gene promoter alleles and autistic disorder. Our findings do not support prominent contributions of 5-HTT gene variants to the pathogenesis of idiopathic infantile autism. Heterogeneity in pathogenetic mechanisms underlying the disease may require that linkage/association studies be targeted toward patient subgroups isolated on the basis of specific biochemical markers, such as serotonin (5-HT) blood levels. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:123-127, 2000.     

No association between 5HT‐2A and bipolar disorder irrespective of genomic imprinting

Recent evidence that 5HT‐2A may be subjected to genomic imprinting prompted us to examine a collection of Irish family trios (an affected individual and both parents) for evidence of an association between 5HT‐2A and bipolar disorder. Family trios offer an advantage over case control studies in regard to genomic imprinting since with family trios it is possible to trace the path of alleles from the parents to the offspring. Using haplotype‐based haplotype relative risk (HHRR) and transmission/disequilibrium (TDT) analyses, no evidence was found for an association of 5HT‐2A with bipolar affective disorder under the assumption of no imprinting and of imprinting. © 2001 Wiley‐Liss, Inc.     

Association of the oxytocin receptor gene (OXTR) in Caucasian children and adolescents with autism

The oxytocin receptor gene (OXTR) has been studied in autism because of the role of oxytocin (OT) in social cognition. Linkage has also been demonstrated to the region of OXTR in a large sample. Two single nucleotide polymorphisms (SNPs) and a haplotype constructed from them in OXTR have been associated with autism in the Chinese Han population. We tested whether these associations replicated in a Caucasian sample with strictly defined autistic disorder. We genotyped the two previously associated SNPs (rs2254298, rs53576) in 57 Caucasian autism trios. Probands met clinical, ADI-R, and ADOS criteria for autistic disorder. Significant association was detected at rs2254298 (p=0.03) but not rs53576. For rs2254298, overtransmission of the G allele to probands with autistic disorder was found which contrasts with the overtransmission of A previously reported in the Chinese Han sample. In both samples, G was more frequent than A. However, in our Caucasian autism trios and the CEU Caucasian HapMap samples the frequency of A was less than that reported in the Chinese Han and Chinese in Bejing HapMap samples. The haplotype test of association did not reveal excess transmission from parents to affected offspring. These findings provide support for association of OXTR with autism in a Caucasian population. Overtransmission of different alleles in different populations may be due to a different pattern of linkage disequilibrium between the marker rs2254298 and an as yet undetermined susceptibility variant in OXTR.     

Reelin gene polymorphisms in the Indian population: a possible paternal 5'UTR-CGG-repeat-allele effect on autism.

Autism is a neurodevelopmental disorder with high heritability factor and the reelin gene, which codes for an extracellular matrix protein involved with neuronal migration and lamination is being investigated as a positional and functional candidate gene for autism. It is located on chromosome 7q22 within the autism susceptible locus (AUTS1); identified in earlier genome scans and several investigations have been carried out on various ethnic groups to assess possible association and linkage of the gene with autism. However, the findings are still inconclusive. In the present study which represents the first report of such a study on the Indian population, genotyping analyses of CGG repeat polymorphism at 5'UTR, two single nucleotide polymorphisms (SNP) at exon 6 and exon 50 were performed in 73 autistic subjects, 129 parents, and 80 controls. The allelic distributions of the repeat polymorphism and exon 50 T/C SNP were quite different from earlier reports in other populations. Allelic and genotypic distribution of the markers did not show any differences between the cases and controls. While our preliminary data on family-based association studies on 58 trios showed no preferential transmission of any allele from the parents to the affected offspring, TDT and HHRR analyses revealed significant paternal transmission distortions for 10- and > or =11-repeat alleles of CGG repeat polymorphism. Thus, the present study suggests that 5'UTR of reelin gene may have a role in the susceptibility towards autism with the paternal transmission and non-transmission respectively of 10- and > or =11-repeat alleles, to the affected offspring.     

Investigation of two variants in the DOPA decarboxylase gene in patients with autism

Though genetic risk factors are important for the development of autism, no specific risk alleles have yet been identified. DOPA decarboxylase (DDC) is involved in both the catecholaminergic and serotonergic pathways and may be considered a functional candidate gene for autism. The present study is the first to test if two new variants of possible functional significance in the DDC gene increase the susceptibility to autism. A total of 90 parent-offspring trios recruited in Denmark and France were investigated using the transmission disequilibrium test (TDT). We found no evidence of linkage disequilibrium between autism and either of the two polymorphisms. Nor did we find linkage disequilibrium between autism and haplotypes of the two variants using a multiallelic TDT. These findings suggest that the DDC gene is unlikely to play a major role in the development of autism in our data set.     

精神分裂症与KCNN3基因1137～1140缺失移码突变传递不平衡研究

目的 探讨KCNN3基因1137～1140的4个碱基缺失所致移码突变与精神分裂症的关系。方法 95个核心家系共289名家庭成员，包含107例精神分裂症患者纳入本研究。精神分裂症采用CCMD-Ⅱ-R诊断标准。KCNN3基因1137～1140的4个碱基缺失基因型检测使用PCR技术和限制性内切酶DdeI消化方法。精神分裂症与KCNN3基因1137～1140缺失4个碱基的关联分析采用基于单倍型的单倍型相对风险率和传递／不平衡检验。结果 患者组与正常父母组比较，KCNN3基因1137～1140的4个碱基缺失的基因型频数和等位基因频率分布差异无统计学意义（χ^2=0．253，P〉0．05和χ^2＝0．010，P〉0．05）。基于单倍型的单倍型相对风险分析发现父母传递与不传递给患者的KCNN3基因等位基因之间差异无统计学意义（χ^2=0．042，P〉0．05）。传递不平衡检验结果发现KCNN3基因等位基因传递不存在连锁不平衡（χ^2=3．000，P＝0．0833）。结论 本组研究对象中，KCNN3基因第1外显子1137～1140的4个碱基缺失的发生率少；分析结果提示KCNN3基因第1外显子1137～1140的4个碱基缺失的等位基因与精神分裂症无关联。     

Serotonin 5-HT1B receptor gene and attention deficit hyperactivity disorder in Chinese Han subjects.

Serotonin is an endogenous neurotransmitter that regulates aggressive and impulsive behavior and may be involved in the development of attention deficit hyperactivity disorder (ADHD). 5-HT1B knockout mice display hyperactivity, increased exploratory activity and aggression, reduced anxiety, increased vulnerability to cocaine self-administration, and elevated alcohol consumption. Many of these same behaviors are seen in patients with ADHD. Prior studies reported excess transmission of the 861G allele of 5-HT1B to ADHD offspring. We used the transmission disequilibrium test (TDT) and haplotype analysis to investigate the A-161T and G861C polymorphisms in the 5-HT1B receptor gene in ADHD trios from the Chinese Han population. We found no association with ADHD but did find a tendency for excess transmission of the 861G allele (chi(2) = 3.766, P = 0.052) and the G/A haplotype (chi(2) = 2.925, df = 1, P = 0.087), and under-transmission of C/A haplotype (chi(2) = 3.707, df = 1, P = 0.054) to offspring with inattentive ADHD.     

Framingham Heart Study 100K project: genome-wide associations for cardiovascular disease outcomes

Background

The TPH2 gene encodes the enzyme responsible for serotonin (5-HT) synthesis in the Central Nervous System (CNS). Stereotypic and repetitive behaviors are influenced by 5-HT, and initial studies report an association of TPH2 alleles with childhood-onset obsessive-compulsive disorder (OCD) and with autism. GLO1 encodes glyoxalase I, the enzyme which detoxifies α-oxoaldehydes such as methylglyoxal in all living cells. The A111E GLO1 protein variant, encoded by SNP C419A, was identifed in autopsied autistic brains and proposed to act as an autism susceptibility factor. Hyperserotoninemia, macrocephaly, and peptiduria represent some of the best-characterized endophenotypes in autism research.

Methods

Family-based and case-control association studies were performed on clinical samples drawn from 312 simplex and 29 multiplex families including 371 non-syndromic autistic patients and 156 unaffected siblings, as well as on 171 controls. TPH2 SNPs rs4570625 and rs4565946 were genotyped using the TaqMan assay; GLO1 SNP C419A was genotyped by PCR and allele-specific restriction digest. Family-based association analyses were performed by TDT and FBAT, case-control by χ², endophenotypic analyses for 5-HT blood levels, cranial circumference and urinary peptide excretion rates by ANOVA and FBAT.

Results

TPH2 alleles and haplotypes are not significantly associated in our sample with autism (rs4570625: TDT P = 0.27, and FBAT P = 0.35; rs4565946: TDT P = 0.45, and FBAT P = 0.55; haplotype P = 0.84), with any endophenotype, or with the presence/absence of prominent repetitive and stereotyped behaviors (motor stereotypies: P = 0.81 and 0.84, verbal stereotypies: P = 0.38 and 0.73 for rs4570625 and rs4565946, respectively). Also GLO1 alleles display no association with autism (191 patients vs 171 controls, P = 0.36; TDT P = 0.79, and FBAT P = 0.37), but unaffected siblings seemingly carry a protective gene variant marked by the A419 allele (TDT P < 0.05; patients vs unaffected siblings TDT and FBAT P < 0.00001).

Conclusion

TPH2 gene variants are unlikely to contribute to autism or to the presence/absence of prominent repetitive behaviors in our sample, although an influence on the intensity of these behaviors in autism cannot be excluded. GLO1 gene variants do not confer autism vulnerability in this sample, but allele A419 apparently carries a protective effect, spurring interest into functional correlates of the C419A SNP.     

Association between the alpha-2C-adrenergic receptor gene and attention deficit hyperactivity disorder in a Korean sample

Findings from preclinical and clinical research support the involvement of central noradrenergic dysregulation in the etiology of attention deficit hyperactivity disorder (ADHD). Previous studies have suggested that the alpha-2C-adrenergic receptor gene (ADRA2C) is associated with ADHD. The aims of this study were to examine the association between the ADRA2C (GT)n repeat polymorphism (STR marker adra2c1) and ADHD in a Korean sample. In this case-control and family-based association study, we assessed 184 ADHD probands, 150 normal controls, and 98 trios. There were no significant differences in the allele frequencies of the ADRA2C polymorphism between the ADHD and control groups (p > 0.05). The overall allele-wise transmission disequilibrium test (TDT) analysis showed statistical significance (chi2 = 19.07, p = 0.025). We found a trend for preferential transmission of the 183-bp allele (chi2 = 3.72, p = 0.054), and a significantly lower-than-expected rate of transmission of the 187-bp allele (chi2 = 6.26, p = 0.012). With regard to the temperament profiles of the Junior Temperament and Character Inventory (JTCI), the ADHD subjects with the 183/183 genotype at the ADRA2C polymorphism showed a trend toward a lower score in the Novelty Seeking (p = 0.020) profile than did those with the other genotypes. Our findings provide important evidence that the ADRA2C polymorphism is involved in the etiology of ADHD in Korean subjects. In addition, our results provide evidence that the temperament of Novelty Seeking and ADHD might share molecular genetic characteristics related to the noradrenergic system.     

Adenosine deaminase alleles and autistic disorder: Case‐control and family‐based association studies

Adenosine deaminase (ADA) plays a relevant role in purine metabolism, immune responses, and peptidase activity, which may be altered in some autistic patients. Codominant ADA1 and ADA2 alleles code for ADA1 and ADA2 allozymes, the most frequent protein isoforms in the general population. Individuals carrying one copy of the ADA2 allele display 15 to 20% lower catalytic activity compared to ADA1 homozygotes. Recent preliminary data suggest that ADA2 alleles may be more frequent among autistic patients than healthy controls. The present study was undertaken to replicate these findings in a new case‐control study, to test for linkage/association using a family‐based design, and to characterize ADA2‐carrying patients by serotonin blood levels, peptiduria, and head circumference. ADA2 alleles were significantly more frequent in 91 Caucasian autistic patients of Italian descent than in 152 unaffected controls (17.6% vs. 7.9%, P = 0.018), as well as among their fathers. Family‐based tests involving these 91 singleton families, as well as 44 additional Caucasian‐American trios, did not support significant linkage/association. However, the observed preferential maternal transmission of ADA2 alleles, if replicated, may point toward linkage disequilibrium between the ADA2 polymorphism and an imprinted gene variant located in its vicinity. Racial and ethnic differences in ADA allelic distributions, together with the low frequency of the ADA2 allele, may pose methodological problems to future linkage/association studies. Direct assessments of ADA catalytic activity in autistic individuals and unaffected siblings carrying ADA1/ADA1 vs ADA1/ADA2 genotypes may provide stronger evidence of ADA2 contributions to autistic disorder. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:784–790, 2000. © 2000 Wiley‐Liss, Inc.