Correspondence: In-Jin Jang, Department of Pharmacology, College of Medicine, Seoul National University, 28 Yongon-dong, Chongno-gu, Seoul 110-799, Korea
Tel: +82-2-740-8290, Fax: +82-2-745-7996, E-mail: ijjang@snu.ac.kr

Introduction

Obsessive-compulsive disorder (OCD) is a common and severe psychiatric illness whose estimated lifetime prevalence rate is 1 to 3% of the population.¹ Patients afflicted with OCD experience intrusive, disturbing, repetitive thoughts (obsessions) and an uncontrollable urge to repeatedly enact stereotypic rituals (compulsions).² Twin and family studies have suggested the existence of a genetic component in the etiology of OCD, although the mode of inheritance is unknown.³
It has been theorized that serotonin plays a role in OCD because of the remarkable efficacy of selective serotonin reuptake inhibitors (SSRIs), including clomipramine, fluvoxamine, fluoxetine, sertraline and paroxetine, in treating this disorder.⁴ This has led to the hypothesis that the pathophysiology of OCD may be associated with the dysregulation of serotonergic neurotransmission.⁵ Based on these findings, it was sug-gested that serotonin (5-HT) related genes may be involved in the pathogenesis of OCD.⁶
By determining the magnitude and duration of the 5- HT synaptic signal, it was found that the 5-HT transporter (5-HTT) plays a key role in the regulation of serotonergic neurotransmission,⁷ and is therefore considered to be an interesting candidate in neuropsychiatric association studies. 5-HTT is encoded by a single copy gene located on chromosome 17q12.⁸ Two well-known polymorphic regions have been identified in the 5-HTT gene: a 44 base pair insertion/deletion in the promoter region (5-HTT gene-linked polymorphic region, HTTLPR)⁹ and a 17 bp variable number of tandem repeats in the second intron (VNTR).¹⁰
In vitro transfection studies have demonstrated that the long (L) and short (S) variants of the promoter polymorphism differentially modulate the transcription of the 5-HTT gene, with the S variant being less efficient.⁹ These findings were confirmed in peripheral native-expressing cells, which showed that lymphoblasts of L/L homozygotes have a higher rate of 5-HTT mRNA transcription, 5-HTT ligand binding and 5- HT uptake than those containing at least one copy of the S allele.¹¹ HTTLPR alleles were shown to affect platelet 5-HT uptake,¹² binding¹³ and content¹⁴ in the same manner. Further studies demonstrated that HTTL-PR had analogous functional effects on 5-HTT expression in brain cells and tissues, affecting the transcription rate,¹⁵ abundance,¹⁶ and function¹⁷ of neuronal 5- HTT protein, although in some of these studies no evidence was found for the allele-specific functional differences.^18,19,20
The allele-dependent differential enhancer activity of the polymorphic region in intron 2 was demonstrated as different levels of reporter gene (luciferase) expression in embryonic stem cells²¹ and in mouse embryos.²² It has been suggested that the VNTR region may act as a transcriptional regulator of the 5-HTT gene, with the 12-repeat allele having stronger enhancer-like properties than the 10-repeat allele. Individual repeat elements within the VNTR domain were later shown to differ in their enhancer activity in the embryonic stem cell model, indicating that not only the number of repeats but also the primary structure of VNTR could affect the transcription of the gene.²³ Only two studies have been conducted to investigate the functional consequences of VNTR polymorphisms in native-expressing cells, and these found no significant effect of the genotype on either the platelet 5-HT uptake²⁴ or 5- hydroxyindoleacetic acid level in the cerebrospinal fluid.²⁵ However, the latter paper reported a significantly higher level of norepinephrine metabolite in 12/12 homozygotes.
In recent years, evidence has accumulated that, in addition to the serotonergic system, the dopaminergic system might be involved in OCD.²⁶ The role of dopamine in the pathophysiology of OCD is supported by preclinical and clinical evidence. Preclinical evidence includes the induction of stereotypies in experimental animal models through increased dopaminergic transmission.²⁷ In fact, the putative animal models for OCD depend primarily on changes in the dopaminergic system; the DRD2 being of primary interest, because rats treated chronically with the selective DRD2 agonist, quinpirole, develop compulsive checking behavior.²⁸ Clinical evidence includes the observation that insults to basal ganglia structures, which are intimately linked to rich dopaminergic innervations, are associated with the emergence of obsessive-compulsive behavior.²⁹ Furthermore, pharmacologic agents influencing the dopaminergic system, such as methylphenidate, cocaine, and bromocriptine, have been shown to induce obsessive-compulsive symptoms.³⁰ Finally, imaging studies of the neurobiological processes in OCD have pointed consistently to abnormalities in the cortico-triatal- halamo-ortical circuits (especially the caudate nucleus).³¹ There is a considerable amount of experimental evidence supporting the hypothesis that the dopaminergic system plays a pivotal role in the function of these cortico-triatal-halamo-ortical circuits, by fine-tuning the patterns of activity in the direct and indirect pathways.³²
Therefore, this study aimed to identify new single nucleotide polymorphisms in the 5-HTT, 5-HT_2A, and DRD2 genes in the general Korean population, and to investigate the possible association between the candidate SNPs in the 5-HTT and 5-HT_2A genes and the susceptibility to and clinical features of OCD.

Methods and Materials

1. SNP scanning
A total of 148 healthy unrelated Korean volunteers living in the Seoul metropolitan area (107 males and 41 females, 21.6 1.6 years old) were scanned for the 5-HTT (SLC6A4), 5-HT_2A (HTR_2A) and DRD2 genes. All of the volunteers gave their written informed consent to participate in this study, which was approved by the Institutional Review Board of Seoul National University Hospital.
The mutations or polymorphisms of the 5-HTT, 5- HT2A and DRD2 genes were identified by Two-Dimensional Gene Scanning (TDGS) analysis. The GenBank accession numbers used as a reference sequence in this study for the 5-HTT, 5-HT_2A and DRD2 genes were NM_001045, NM_000621, and NM_000795, respectively. The PCR primer sets used for the TDGS analysis of the above genes were designed using similar algorithms to those used in a previous report.³³ In brief, the entire coding regions and exon-intron junctions were amplified from genomic DNA in a 7-plex long distance PCR. Individual exons or parts of exons were amplified in four multiplex groups of eight, fourteen and ten fragments in the 5-HT_2A, 5-HTT and DRD2 genes, respectively, using the long distance 7-plex PCR products as a template. The products of the four multiplex groups were combined, mixed with sample buffer, and loaded directly into the slot of a 2-dimensional gel. Electrophoresis was performed in an automated 2-dimensional electrophoresis system and the gels were stained with ethidium bromide. The spot patterns were interpreted visually for the appearance of four spots rather than one, indicating the presence of a heterozygous mutation or polymorphism (Figure 1A and B). Each sample was analyzed only once, under the same conditions, and those fragments that were absent or faint were repeated by one dimensional gradient gel electrophoresis. Those fragments that showed a four spot pattern that could be recognized as a previously detected polymorphism on the basis of their characteristic configurations were assigned as such. New variants were subjected to sequence analysis (Figure 1C). The sequence analysis was either carried out by ourselves on a Beckman CEQ2000 sequencer (75% of fragments) or contracted out to Davis Sequencing (Davis, CA, USA) (25% of fragments).

2. SNP genotyping
One hundred and fifty seven healthy unrelated Korean volunteers participated in this study as a control group. We also studied one hundred and three patients who met the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) criteria for OCD on the Structured Clinical Interview for Axis I Disorders (SCID-I).³⁴ The controls did not undergo diagnostic interviews, and were considered as being representative of the local general population. The subjects were enrolled in the study only after providing written consent, and the study protocol was approved by the Institutional Review Board of Seoul National University Hospital.
Whole blood (8 mL) was obtained from each subject, and genomic DNA was extracted from peripheral lymphocytes using a QIAamp DNA Blood Mini Kit (QIAGEN, Hilden, Germany). Genotypes for the promoter HTTLPR, intron 2 VNTR, exon 5 878C>T, exon 12 1815A>C in the 5-HTT gene and exon 1 102T>C polymorphism in the 5-HT_2A gene were analyzed by PCR, PCR-RFLP, or the SNaPshot method in both the control and OCD patients groups. Briefly, approximately 100 ng of genomic DNA in a total volume of 20 L, were added to PCR mixtures consisting of 0.25 to 0.5 M of each specific primer pair, 10 PCR buffer with 1.5 mM MgCl₂, 0.2 mM of deoxyribonucleotide triphosphates (dNTPs), and 0.5 U of recombinant Taq DNA polymerase (Takara, Shiga, Japan). After initial denaturation at 95 for 5 min, the DNA was amplified using³⁵ cycles (denaturation 95 for 1 min, annealing for 1 min, and extension at 72 for 1.5 min) and then extended at 72 for 5 min. PCR was carried out in a GeneAmp PCR System 2400 (Perkin Elmer, Boston, USA). The details of the primer sequences, annealing temperatures, and RFLP conditions used for genotyping are summarized in (Table 1). The oligonucleotides used for PCR were commercially synthesized at Bioneer Co. Ltd. (Daejeon, Korea).

3. Chart reviews
Specific demographic data, including current age, age at onset of OCD and gender, was obtained from the OCD patients. In addition, the Yale-rown Obsessive-ompulsive Checklist and Severity Scale (Y-BOCS) was implemented in order to assess the typology and severity of the OCD symptoms.³⁵ However, this information was not able to be collected for the majority of the patients.

4. Statistical analysis
The deviation of the allele and genotype frequencies for the various SNPs from Hardy-Weinberg equilibrium was assessed using Fisher's exact test. The differences in the genotype frequencies between the healthy volunteers and patients were determined using the chi-square test. The variations in the genotypes with gender were analyzed by Kruskal-Wallis ANOVA. The differences between the ages of onset according to gender were determined with the student t test. A P value of < .05 was considered statistically significant. Values are presented as arithmetic mean SD or as median and range. All statistical analyses were performed with SAS statistical software (version 8.0; SAS Institute Inc. Cary, NC, USA).

Results

1. SNP scanning
We identified a total of eight SNPs in the coding regions; there were two polymorphisms in the 5-HT_2A gene, and three polymorphisms in both the 5-HTT and DRD2 genes. There were three polymorphisms in the 5-HTT gene which were 684T>C in exon 3, 878C>T in exon 5 and 1815A>C in exon 12. Among these polymorphisms, 684T>C and 878T>C were not previously reported. The allele frequencies of these SNPs were 0.34% (684T>C), 1.01% (878C>T) and 3.38% (1815A>C) (Table 2). The 5-HT_2A gene polymorphisms identified were 102T>C in exon 1 and 744C>T in exon 3. For the new polymorphism, 744T>C, only one of the one hundred and forty eight volunteers was heterozygote. These SNPs were all synonymous variations. The other gene, DRD2, had three SNPs; 932C>G, 939T>C and C957T in exon 6 (Table 2). C957T was a polymorphism which was newly detected, and the allele frequency of this SNP was 5.07%.

2. Candidate polymorphism selection and association analysis
Among the 103 patients, 69 were male and 34 were female. Their age was 27.0±10.2 (years, mean SD) for the males, and 31.9±8.2 for the females. Among the 157 control subjects, 83 were male and 74 were female. The age of the males and females was almost the same (24.1±2.0, overall).
Among the screened SNPs, we chose three candidate SNPs which were expected to be related to obsessive-compulsive disorder. In addition, we searched previous reports to study the relationship between the genetic variations and psychiatric disorders such as depression, schizophrenia, etc. We selected two additional polymorphisms in the 5-HTT gene for genotyping; one was a 44bp insertion or deletion in the promoter region (HTTLPR) and the other was various tandem nucleotide repeats (VNTR) in intron 2. Therefore, we studied HTTLPR, VNTR, C878T and A1815C in 5- HTT, and T102C in 5-HT_2A. The other polymorphisms identified in this study (T684C for 5-HTT, C744T for 5-HT_2A, and C932G, T939C and C957T for DRD2) were not analyzed in the patients, due to either their low frequency or a limitation of resources. To identify the SNPs, we used PCR or PCR-RFLP as appropriate. The observed genotype frequency distribution did not show a significant deviation from Hardy-Weinberg equilibrium (P>.05)
We compared the genotype frequencies between the OCD patients and controls by additive, recessive and dominant methods. Genotype frequencies did not reach statistical significance in any of the statistical methods that we employed (Table 3). The differences in the allele frequencies between the groups were not significant either (data not shown).
We evaluated the genotype distributions and age of onset according to gender in the patients group. Although we could not find any differences in the genotype distribution, there was a significant difference in the age of onset according to gender (17.5±6.4 vs. 23.1±11.2 for males and females, respectively; P<0.01). The difference in the age of onset between the genotypes was not remarkably different (Table 4).

Discussion

We found the unreported polymorphisms, 684T>C and C878T, in the 5-HTT gene, 744C>T in the 5-HT_2A gene and 957C>T in the DRD2 gene. Among these SNPs, 878C>T and 1815A>C in the 5-HTT gene had the amino acid substitutions 293Ser>Phe and 605Lys>Asn, respectively.
This investigation failed to obtain evidence that the 5-HTT and 5-HT_2A polymorphisms influence the OCD risk factors in Koreans. The association between HTTLPR polymorphisms and OCD has been studied by several research groups. Billett et al. did not find any association in 72 OCD patients compared to 72 controls.³⁶ However, a relative increase (not statistically significant, P=0.07) in the prevalence of homozygous L allele variants was observed in the OCD group. We also found that the L allele was more frequent in the OCD patients than in the control group, but this difference did not reach statistical significance. The frequencies of the L allele were 20.7% and 26.2% in the control group and OCD patients, respectively (Table 3).

   The possible correlation of the 102T>C polymorphism with various psychiatric diseases has been extensively studied, but many of these studies failed to obtain evidence of any such association. Frisch et al. reported no differences with respect to the genotypic and allelic distribution of the 5-HT_2A receptor gene in 75 unrelated OCD patients compared to controls.³⁷ Similarly, Nicolini et al. found no association between the 5-HT_2A receptor gene T102C polymorphism and OCD.³⁸ These findings have only partial significance as related to OCD. A recent study by Tot. et al. found that the TT genotype of the T102C polymorphism and the AA genotype of the -1438 G/A polymorphism were observed at a higher rate in patients with severe OCD, as compared to those with moderate or moderate-severe OCD (P = 0.027 and P = 0.03, respectively).³⁹ Since we could not obtain the Y-BOCS score in most of these OCD patients, we could not compare the two groups according to the severity level. It might be important to subdivide the patient group according to the severity of their symptoms, because the patient group may have included patients who were not really OCD patients.
We found that compared to the females, the males with OCD had an earlier age of onset. Our finding of a roughly equal distribution of males and females with OCD is consistent with a number of previous clinical research studies.⁴⁰ The finding of an earlier age of OCD onset in males is also consistent with previous reports.⁴¹ Nestadt et al. suggested that the genetic transmission of OCD differs in males and females.⁴² Gender differences in clinical manifestations have been described for several psychiatric disorders, including OCD.⁴³ These differences have been attributed to epigenetic hormonal influences that affect disease processes or the efficacy of pharmacotherapy. The results obtained from the present study, as well as those of earlier studies ^44,45 extend these observations and suggest the possibility that more profound gender differences in genetic susceptibilities may exist for OCD.
Our results do not suggest that OCD is associated with the HTTLPR, VNTR, 878C>T and 1815A>C polymorphisms in the 5-HTT gene and the 102T>C polymorphism in the 5-HT_2A receptor gene. The main limitation of this study is that it is of a retrospective design in which the population could be biased in some way. Also, population stratification might have helped to increase the statistical power of the analysis conducted to detect possible associations, as previously discussed in relation to the disease severity. The sample size may also have been too small. It is highly possible that multiple interactions of several different neurotransmitter systems and signal transduction pathways are involved in OCD. Complex psychiatric disorders like OCD are likely to be associated with common variations in the function of several genes. The inheritance of OCD is not simple and most likely involves a number of susceptibility genes and environmental influences.

1. Weissman MM, Bland RC, Canino GJ. The cross national epidemiology of obsessive compulsive disorder. J Clin Psychiatry 1994; 55: 5-10.

2. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders (4th ed.), Washington, DC, American Psychiatric Press, 1994.

3. Pauls DL, Alsobrook JP, Phil M. A family study of obsessive-compulsive disorder. Am J Psychiatry 1995; 152: 76-84.

4. Barr LC, Goodman WK, Price LH. 1993. The serotonin hypothesis of obsessive compulsive disorder. Int Clin Psychopharmacol 1993; 2: 79-82.

5. Murphy DL, Greenberg B, Altemus M. The neuropharmacology and neurobiology of obsessive compulsive disorder: An update on the serotonin hypothesis. In: Advance in the Neurobiology of Anxiety Disorders. Ed. by Westenberg HGM, Murphy DL, and Boer JAD, 1995, pp278-298.

6. McDougle CJ, Epperson CN, Price LH, Gelernter J. Evidence for linkage disequilibrium between serotonin transporter protein gene (SLC6A4) and obsessive-ompulsive disorder. Mol Psychiatry 1998; 3: 270-273.

7. Lesch KP, Mossner R. Genetically driven variation in serotonin uptake: Is there a link to affective spectrum, neurodevelopmental, and neurodegenerative disorders? Biol Psychiatry 1998; 44: 179-192.

8. Rammamoorthy S, Bauman AL, Moore KR, Han H, Yang-Feng T, Chang AS. Antidepressant- and cocaine-sensitive human serotonin transporter: Molecular cloning, expression, and chromosomal localization. Proc Natl Acad Sci USA 1993; 90: 2542-2546.

9. Heils A, Teufel A, Petri S, Stober G, Riederer P, Bengel D, Lesch KP. Allelic variation of human serotonin transporter gene expression. J Neurochem 1996; 66: 1-4.

10. Lesch KP, Balling U, Gross J, Strauss K, Wolozin BL, Murphy DL, Riederer P. Organization of the human serotonin transporter gene. J Neural Trans Gen Sect 1994; 95: 157-162.

11. Lesch KP, Bengel D, Heils A, Sabol SZ, Greenberg BD, Petri S. Association of anxiety-related traits with a polymorphism in the serotonin transporter gene regulatory region. Science 1996; 274: 1527-1531.

12. Nobile M, Begni B, Giorda R, Frigerio A, Marino C, Molteni M. Effects of serotonin transporter promoter genotype on platelet serotonin transporter functionality in depressed children and adolescents. J Am Acad Child Adolesc Psychiatry 1999; 38: 1396-1402.

13. Stoltenberg SF, Twitchell GR, Hanna GL, Cook EH, Fitzgerald HE, Zucker RA, Little KY. Serotonin transporter promoter polymorphism, peripheral indexes of serotonin function, and personality measures in families with alcoholism. Am J Med Genet 2002; 114: 230-234.

14. Hanna GL, Himle JA, Curtis GC, Koram DQ, Weele JVV, Leventhal BL, Cook EH. Serotonin transporter and seasonal variation in blood serotonin in families with obsessive-compulsive disorder. Neuropsychopharmacol 1998; 18: 102-111.

15. Mortensen OV, Thomassen M, Larsen MB, Whittemore SR, Wiborg O. Functional analysis of a novel human serotonin transporter gene promoter in immortalized raphe cells. Brain Res Mol Brain Res 1999; 68: 141-148.

16. Heinz A, Jones DW, Mazzanti C, Goldman D, Ragan P, Hommer D. A relationship between serotonin transporter genotype and in vivo protein expression and alcohol neurotoxicity. Biol Psychiatry 2000; 47: 643-649.

17. Hariri AR, Mattay VS, Tessitore A, Kolachana B, Fera F, Goldman D. Serotonin transporter genetic variation and the response of the human amygdala. Science 2002; 297: 400-403.

18. Willeit M, Stastny J, Pirker W, Praschak-Rieder N, Neumeister A, Asenbaum S. No evidence for in vivo regulation of midbrain serotonin transporter availability by serotonin transporter promoter gene polymorphism. Biol Psychiatry 2001; 50: 8-12.

19. Sakai K, Nakamura M, Ueno S, Sano A, Sakai N, Shirai Y, Saito N. The silencer activity of the novel human serotonin transporter linked polymorphic regions. Neurosci Lett 2002; 327: 13-16.

20. Shioe K, Nakamura M, Ueno S, Sano A, Sakai N, Shirai Y, Saito N. No association between genotype of the promoter region of serotonin transporter gene and serotonin transporter binding in human brain measured by PET. Synapse 2003; 48: 184-188.

21. Fiskerstrand CE, Lovejoy EA, Quinn JP. An intronic polymorphic domain often associated with susceptibility to affective disorders has allele dependent differential enhancer activity in embryonic stem cells. FEBS Lett 1999; 458: 171-174.

22. MacKenzie A, Quinn J. A serotonin transporter gene intron 2 polymorphic region, correlated with affective disorders, has allele-dependent differential enhancer-like properties in the mouse embryo. Proc Natl Acad Sci USA 1999; 96: 15251-15255.

23. Lovejoy EA, Scott AC, Fiskerstrand CE, Bubb VJ, Quinn JP. The serotonin transporter intronic VNTR enhancer correlated with a predisposition to affective disorders has distinct regulatory elements within the domain based on the primary DNA sequence of the repeat unit. Eur J Neuroscience 2003; 17: 417-420.

24. Kaiser R, Muller-Oerlinghausen B, Filler D, Tremblay PB, Berghofer A, Roots I, Brockmoller J. Correlation between serotonin uptake in human blood platelets with the 44-bp polymorphism and the 17-bp variable number of tandem repeat of the serotonin transporter. Am J Med Genet 2002; 114: 323-328.

25. Jonsson EG, Nothen MM, Gustavsson JP, Neidt H, Bunzel R, Propping P, Sedvall GC. Polymorphisms in the dopamine, serotonin, and norepinephrine transporter genes and their relationships to monoamine metabolite concentrations in CSF of healthy volunteers. Psychiatry Res 1998; 79: 1-9.

26. Goodman WK, McDougle CJ, Price LH, Riddle MA, Pauls DL. Leckman JF. Beyond the serotonin hypothesis: A role for dopamine in some forms of obsessive compulsive disorder? J Clin Psychiatry 1990; 51: 36-43.

27. Randrup A, Munkvad I. Stereotyped activities produced by amphetamine in several animal species and man. Psychopharmacologia 1967; 11: 300-310.

28. Tizabi Y, Louis VA, Taylor CT, Waxman D, Culver KE, Szechtman H. Effect of nicotine on quinpirole-induced checking behavior in rats: Implications for obsessive-compulsive disorder. Biol Psychiatry 2002; 51: 164-171.

29. Carmin CN, Wiegartz PS, Yunus U, Gillock KL. Treatment of late-onset OCD following basal ganglia infarct. Depress Anxiety 2002; 15: 87-90.

30. Jenike MA, Baer L, Summergrad P, Minichiello WE, Holland A, Seymour R. Sertraline in obsessive-compulsive disorder: A double-blind comparison with placebo. Am J Psychiatry 1990; 147: 923-928.

31. Saxena S, Brody AL, Schwartz JM, Baxter LR. Neuroimaging and frontal-subcortical circuitry in obsessive- compulsive disorder. Br J Psychiatry 1998; 35: 26-37.

32. Alexander GE, Crutcher MD. Functional architecture of basal ganglia circuits: Neural substrates of parallel processing. Trends Neurosci 1990; 13: 266-271.

33. Lee JH, Choi JH, Namkung W, Hanrahan JW, Chang J, Song SY, Park SW, Kim DS, Yoon JH, Suh Y, Jang IJ, Nam JH, Kim SJ, Cho MO, Lee JE, Kim KH, Lee MG. A haplotype-based molecular analysis of CFTR mutations associated with respiratory and pancreatic diseases. Hum Mol Genet 2003; 12: 2321-2332.

34. Han HS, Hong JP. Korean Version of Statistical Clinical Interview Schedule for DSM-IV Axis I, Seoul, Hana Medical Publishing, 2000.

35. Goodman WK. Pharmacotherapy of obsessive-compulsive disorder. In: Zwangsstorungen. Duphar Medical Communication, Band 5, Ed. by Hand I, Goodman WK, and Evers U, Berlin, Springer-Verlag 1992, pp141-151.

36. Billett EA, Richter MA, King N, Heils A, Lesch KP, Kennedy JL. Obsessive compulsive disorder, response to serotonin reuptake inhibitors and the serotonin transporter gene. Mol Psychiatry 1997; 2: 403-406.

37. Frisch A, Michaelovsky E, Rockah R, Amir I, Hermesh H, Laor N, Fuchs C, Zohar J, Lerer B, Buniak SF, Landa S, Poyurovsky M, Shapira B, Weizman R. Association between obsessive-compulsive disorder and polymorphisms of genes encoding components of the serotonergic and dopaminergic pathways. Eur Neuropsychopharmacol 2000; 10 (3): 205-209.

38. Nicolini H, Cruz C, Camarena B, Orozco B, Kennedy JL, King N, Weissbecker K, de la Fuente JR, Sidenberg D. DRD2, DRD3 and 5HT2A receptor genes polymorphisms in obsessive-compulsive disorder. Mol Psychiatry 1996; 1 (6): 461-465.

39. Tot S, Erdal ME, Yazici K, Yazici AE, Metin O. T102C and -1438 G/A polymorphisms of the 5-HT_2A receptor gene in Turkish patients with obsessive-compulsive disorder. Eur Psychiatry 2003; 18 (5): 249-254.

40. Bebbington PE. Epidemiology of obsessive-compulsive disorder. Br J Psychiatry (Suppl) 1998; (35): 2-6.

41. Zohar AH. The epidemiology of obsessive-compulsive disorder in children and adolescents. Child Adolesc Psychiatr Clin N Am 1999; 8 (3): 445-460.

42. Nestadt G, Lan T, Samuels J, Riddle M, Bienvenu OJ 3rd, Liang KY, Hoehn-Saric R, Cullen B, Grados M, Beaty TH, Shugart YY. Complex segregation analysis provides compelling evidence for a major gene underlying obsessive-compulsive disorder and for heterogeneity by sex. Am J Hum Genet 2000; 67 (6): 1611-1616.

43. Castle DJ, Deale A, Marks IM. Gender differences in obsessive compulsive disorder. Austr N Z J Psychiatry 1995; 29: 114-117.

44. Rubinsztein DC, Leggo J, Goodburn S. Genetic association between monoamine oxidase A microsatellite and RFLP alleles and bipolar affective disorder: analysis and meta-analysis. Hum Mol Genet 1996; 5: 779-782.

45. Karayiorgou M, Altemus M, Galke BL. Genotype determining low catechol-O-methyltransferase activity as a risk factor for obsessive-compulsive disorder. Proc Natl Acad Sci USA 1997; 94: 4572-4575.