Prediction of schizophrenia using MAOA-uVNTR polymorphism: A case-control study.

CONTEXT: Schizophrenia has been associated with disorder of the dopamine system, which is downregulated by projections of the serotonin pathway. Dopamine and serotonin levels are regulated by a system of transporters and enzymes. In this research, dopamine transporter polymorphism (DAT-VNTR), serotonin transporter polymorphism (5-HTTLPR), monoamine oxidase A (MAOA-uVNTR), and catechol-o-methyl transferase (COMT Val158Met) polymorphisms have been investigated. AIMS: The aim of this study was to asses frequencies of these polymorphisms in the healthy control group and patients and to asses association with schizophrenia. SETTINGS AND
DESIGN: Three hundred and fourteen healthy volunteers and 306 schizophrenia patients were included. Schizophrenia was diagnosed by Diagnostic and Statistical Manual-IV of the American Psychiatric Association, and mini international neuropsychiatric interview questionnaire was used for screening of healthy population.
MATERIALS AND METHODS: Genotyping was performed using polymerase chain reaction (PCR) reaction followed by gel electrophoresis and PCR-restriction fragment length polymorphism. STATISTICAL ANALYSIS: Categorical data were analyzed using the Chi-square test, age between subgroups was compared using the Mann-Whitney test, and all polymorphisms were tested for Hardy-Weinberg equilibrium. Logistic regression analysis was used to set the prediction model of schizophrenia.
RESULTS: Difference in genotype distribution was observed for COMT Val158Met in female and DAT-VNTR polymorphism in overall sample P = 0.021 and P = 0.028, respectively. Statistically significant association of MAOA-uVNTR and schizophrenia was observed after adjustment for anamnestic predictors of disease. P = 0.010, 80.45% participants were correctly classified.
CONCLUSION: Our results suggest an association of MAOA-uVNTR polymorphism with schizophrenia. The difference in the distribution of COMT Val158Met and DAT-VNTR polymorphism support the involvement of dopamine system components in the pathogenesis of schizophrenia. Copyright:

INTRODUCTION

Schizophrenia is a severe psychological disease that affects around 1% of the general population.[1] Its etiology is still unclear, but there are several hypotheses that are trying to explain the biological background of schizophrenia. The most accepted hypothesis involves neurotransmitter dopamine and dopamine pathway. It has been changed and upgraded through time, and the latest research in this field has associated schizophrenia with prefrontal hypodopaminergia and subcortical hyperdopaminergia.[2] The neurotransmitter dopamine is involved in the regulation of motoric functions acting through the nigrostriatal pathway. Cognition, motivation, and reward are regulated through the mesolimbic and mesocortical pathways.[3] Projections of serotonergic neurons start from dorsal raphe nuclei and spread to the cortex and striatal region. Overlapping of these systems allows their interaction and downregulation of dopamine by serotonin.[4]

Considering the function and interaction of serotonin and dopamine systems, many studies have tried to find an association of genes involved in the regulation of these two systems and psychiatric disorders. Dopamine transporter is responsible for dopamine reuptake regulating dopamine levels. Polymorphisms of dopamine transporter gene SLC6A3 have been associated with altered transcriptional activity of dopamine transporter and thus have been investigated and associated with attention deficit hyperactivity disorder, schizophrenia, and Parkinson's disease.[56]

Another regulator of dopamine levels is catechol-o-methyl transferase (COMT) which has been considered as a treatment target of cognitive symptoms in schizophrenia. Because of its function in the prefrontal cortex, COMT became a gene candidate in genetic association studies for schizophrenia.[7] Variants of COMT gene have been associated with cortical function including cognitive symptoms and psychosis.[8]

Serotonin regulates psychological functions such as anxiety, emotions, and aggression which is why regulation of this neurotransmitter became subject of interest in many psychiatric studies. Serotonin function is regulated by serotonin transporter which is responsible for serotonin reuptake from the synaptic cleft.[910] Serotonin transporter transcription is regulated by SLC6A4 gene, and extensively studied functional polymorphism (serotonin transporter-linked polymorphic region [5-HTTLPR]) of this gene has been associated with depression, anxiety traits, and poor response to serotonin-selective reuptake inhibitor therapy.[1112] However, some studies have failed to find any association of 5-HTTLPR polymorphism with depression.[9]

Monoamine oxidase A (MAOA) is an enzyme included in the degradation of biogenic amines such as serotonin, norepinephrine, and dopamine.[13] Its deficiency has been associated with aggressive behavior and mental retardation in males.[14] Furthermore, the association of MAOA gene with aggression and schizophrenia has been extensively investigated.[15]

Despite accepted dopamine hypothesis and interaction of serotonin and dopamine system, genetic association studies involving gene variants of these two systems have shown contradictory results.

In this research, we hypothesized that polymorphisms of dopamine transporter (DAT-VNTR), catechol-o-methyl transferase (COMT Val158Met), 5-HTTLPR, and monoamine oxidase-A (MAOA-uVNTR) might be associated with schizophrenia. The aim of this study was to assess the frequencies of these polymorphisms in healthy volunteers and patients with schizophrenia, to compare them, and to investigate their association with the disease.

MATERIALS AND METHODS

This study was conducted at the Department of clinical chemistry of the Sestre milosrdnice University Hospital Center in collaboration with the Clinic for psychiatry of the same institution. During a period of 6 years (June 2010–January 2017), 314 healthy volunteers and 306 patients with schizophrenia were included in this study. The sample size was estimated using available statistical tool for sample size estimation in genetic association studies by Gordon et al.[1617] The sample size was calculated for all tested polymorphisms, and the highest required number was obtained for 5-HTTLPR. For this polymorphism, expected frequencies for controls (wild type allele = 0.48; variant allele = 0.52) and cases (wild type allele = 0.40; variant allele = 0.60) were obtained from the literature. The calculated number was 302 cases and 302 controls. Schizophrenia was diagnosed based on the Diagnostic and Statistical Manual-IV of the American Psychiatric Association by professional personnel. To avoid therapy effect, only individuals with the first episode of schizophrenia or individuals who were not using therapy were included in this study.

Healthy control group consisted of volunteers and blood donors. On inclusion, their health condition was examined by a clinician. Individuals with mental illness in anamnestic data, drug or alcohol abuse, or the presence of other chronic diseases were excluded from the study. To exclude potential current psychological disorders, the mini international neuropsychiatric interview questionnaire was used. The questionnaire is accepted as a diagnostic screening tool for outpatient evaluation.[18] Other demographic data such as marital status, education, and employment were collected for patients and control groups during their initial assessment by an interview with the physician. Faculty degree was treated as higher level of education compared to high school level (or lower).

This study was approved by the institution's ethical committee. All participants gave their written informed consent. Blood was sampled into K3EDTA tubes. Deoxyribonucleic acid was isolated using high pure PCR template preparation kit (Roche, Basel, Switzerland) and stored at +4°C until analysis. DAT-VNTR, MAOA-uVNTR, and 5-HTTLPR polymorphisms were genotyped using PCR followed by agarose gel electrophoresis. COMT Val158Met polymorphism was detected by restriction fragment length polymorphism method after PCR amplification. Primer and reaction conditions are presented in Table 1.

Table 1

Polymerase chain reaction and electrophoresis conditions

Polymorphism	Primer	PCR reaction	Electrophoresis
5-HTTLPR	HTT419F: 5’-ATG CCA GCA CCT AAC CCC TAA TGT-3’HTT419R: 5’-GGA CCG CAA GGT GGG CGG GA-3’	Reaction mix: Total volume 20 µL: H2O, 10XPCR buffer, 25 mM Mg2+, 25 mM dNTP, 5 µM primers, 6 U/µL fast start high fidelity polymeraseInitial denaturation - 94°C/2 minExtension - 38 cycles: 94°C/45 s, 63°C/40 s, 72°C/60 sFinal elongation - 72°C/10 min	3.7% NuSieve 3:1 agarose gel containing 27 µL EtBr (Biozym Scientific GmbH, Hessisch Oldendorf, Germany)Electrophoresis conditions: 110 V/45 min
MAOA-uVNTR	MAOAF: 5’-ACA GCC TGA CCG TGG AGA AG-3’MAOAR: 5’-GAA CGG ACG CTC CAT TCG GA -3’	Reaction mix: Total volume 30 µL: H2O, 10XPCR buffer, 25 mM Mg2+, 25 mM dNTP, 5 µM primers, 5 U/µL Taq polymeraseInitial denaturation - 95°C/2 minExtension - 37 cycles: 95°C/45 s, 70°C/30 s, 72°C/40Final elongation - 72°C/10 min	1.8% NuSieve GP agarose gel containing 27 µL EtBr (Biozym Scientific GmbH, Hessisch Oldendorf, Germany)Electrophoresis conditions: 110 V/45 min
COMT Val158Met	COMTf: 5’-CTC ATC ACC ATC GAG ATC AA-3’COMTr: 5’-CAG TGA ACG TGG TGT GAA CAC-3’	Reaction mix: Total volume 30 µL: H2O, 10XPCR buffer with 15 mM Mg2+, 25 mM dNTP, 5 µM primers, 5 U/µL Taq polymeraseInitial denaturation - 94°C/2 minExtension - 35 cycles: 94°C/30 s, 60°C/30 s, 72°C/120 sFinal elongation - 72°C/10 minRFLP: Incubation with NlaIII restriction enzyme during night	PCR check: Commercial gel: PCR-check kit 110 V/15 minRFLP fragment detection: Spreadex EL 300 110 V/45 min. Gel (Elchrom Scientific AG, Cham, Switzerland). Staining with SYBR Gold (Invitrogen, Carslbad, CA USA)
DAT-VNTR	DAT1F: 5’-TGT GGT GTA GGG AAC GGC CT GAG-3’DAT1R: 5’-CTT CCT GGA GGT CAC GGC TCA AGG-3’	Reaction mix: Total volume 30 µL: H2O, 10XPCR buffer with 15 mM Mg2+, 25 mM dNTP, 5 µM primers, 5 U/µL Taq polymerase, DMSOInitial denaturation - 95°C/2 minExtension - 35 cycles :95°C/60 s, 68°C/60 s, 72°C/60 sFinal elongation - 72°C/10 min	1.8% NuSieve 3:1 agarose gel containing 27 µL EtBrElectrophoresis conditions: 110 V/45 min

5-HTTLPR – Serotonin-transporter-linked polymorphic region; MAOA-uVNTR – Monoamine oxidase A upstream variable number tandem repeats; COMT Val158Met – Catechol-o-methyl-transferase; DAT-VNTR – Dopamine transporter variable number tandem repeats; Mg2+ – Magnesium ions; dNTP – Deoxyribonucleotide triphosphate; DMSO – Dimethyl sulfoxide; EtBr – Ethidium bromide; RFLP – Restriction fragment length polymorphism; PCR – Polymerase chain reaction

Data are presented as numbers and percentages. Age is presented as median and range. All polymorphisms were tested for Hardy–Weinberg equilibrium. Genotype, allele frequencies, and other categorical data were compared using the Chi-square test. If the conditions for the Chi-square test were not met (small sample size), z-test (comparison of proportions) was performed. Age between subgroups was compared using the Mann–Whitney test. The level of significance was set at P < 0.05.

Anamnestic data in combination with genotypes were analyzed using logistic regression analysis for prediction of negative outcome – schizophrenia. Variables included in the logistic regression analysis were all four polymorphisms and variables statistically significant in previous data analysis: gender, marital status, education, and employment [Table 2]. Logistic regression analysis was performed in two steps. First, using a univariate model to avoid dropping out significant variables due to low statistical power caused by the inclusion of too many variables at the same time and for that reason, relaxed P value was used (P < 0.25) according to Sperandei.[19] Statistically significant variables from the univariate model (gender, marital status, education, employment, and MAO-uVNTR polymorphism) were included in multivariate analysis. The level of significance using a multivariate model was set at P < 0.05; odds ratio (OR) and 95% confidence intervals (95% CIs) were used as measures of association of tested variables and schizophrenia. Statistical analysis was performed using MedCalc statistical software (v12.7.2.0, Ostend, Belgium).

Table 2

Demographic characteristics of participants

	Healthy control (n=314)	Schizophrenia (n=306)	P
Age (years)	36 (18-68)	38 (18-84)	0.281*
Male gender, n (%)	164 (52.2)	222 (72.5)	<0.001**
Married, n (%)	135 (62.2)	81 (29.8)	<0.001**
Faculty degree, n (%)	138 (63.3)	52 (18.3)	<0.001**
Employed, n (%)	187 (86.2)	86 (31.7)	<0.001**

Age is presented as median (range). *Mann-Whitney test; **Chi-square test

RESULTS

Anamnestic data in schizophrenia patients and healthy controls are presented in Table 2. Schizophrenic patients were mostly males, not married, with a lower education degree, and mostly unemployed (P < 0.001).

All tested polymorphisms were in Hardy–Weinberg equilibrium: MAOA-uVNTR P = 0.986, DAT-VNTR P = 0.937, 5-HTTLPR P = 0.878, and COMT Val158Met P = 0.187.

Genotype and allele frequencies are presented in Table 3.

Table 3

Allele and genotype frequencies in a healthy control and schizophrenia patients

	Healthy control			Schizophrenia			P
	All, n (%)	Male, n (%)	Female, n (%)	All, n (%)	Male, n (%)	Female, n (%)	All	Male	Female
MAOA
High/high	178 (58.9)	114 (74.0)	64 (43.2)	166 (55.9)	131 (61.6)	35 (43.2)	0.010	0.010	0.242
High/low	58 (19.2)	/	58 (39.2)	38 (12.8)	/	38 (46.9)
Low/low	66 (21.9)	40 (26.0)	26 (17.6)	93 (31.3)	85 (39.4)	8 (9.9)
High	302 (67)	114 (74)	186 (63)	200 (53)	131 (61.6)	102 (67)	<0.001	0.010	0.941
Low	149 (33)	40 (26)	110 (37)	178 (47)	85 (39.4)	60 (33)
5-HTTLPR
LL	105 (33.4)	57 (35.4)	48 (32.0)	122 (39.9)	88 (40.0)	34 (39.5)	0.147	0.107	0.495
LS	152 (48.4)	81 (50.3)	71 (47.3)	125 (40.8)	88 (40.0)	37 (43.0)
SS	57 (18.2)	23 (14.4)	31 (20.7)	59 (19.3)	44 (20.0)	15 (17.4)
L	364 (58)	196 (61)	168 (56)	367 (60)	264 (60)	105 (61)	0.509	0.867	0.331
S	264 (42)	126 (39)	132 (44)	245 (40)	176 (40)	67 (39)
COMT
Val/Val	63 (20.0)	36 (22.4)	27 (18)	75 (24.8)	50 (22.7)	25 (29.1)	0.237	0.992	0.021
Val/Met	168 (53.5)	81 (50.3)	85 (56.7)	144 (47.1)	111 (50.5)	33 (38.4)
Met/Met	83 (26.4)	44 (27.3)	38 (25.3)	87 (28.4)	59 (26.8)	38 (44.2)
Val	295 (47)	155 (48)	144 (46)	294 (48)	211 (48)	83 (43)	0.750	0.981	0.345
Met	333 (53)	167 (52)	156 (53)	318 (52)	229 (52)	109 (57)
DAT
10/10	150 (47.8)	78 (48.4)	72 (48.0)	154 (50.3)	114 (51.8)	40 (46.5)	0.587	0.581	0.931
10/9	131 (41.7)	68 (42.2)	63 (42.0)	133 (43.5)	94 (42.7)	39 (45.3)	0.709	0.994	0.722
9/9	28 (8.9)	13 (8.1)	14 (9.3)	13 (4.2)	9 (4.1)	4 (4.8)	0.028	0.338	0.318
Other genotypes	5 (1.6)	2 (1.2)	1 (0.7)	6 (1.9)	3 (1.4)	3 (3.5)	0.983	0.775	0.284
10	431 (70)	224 (70)	207 (69)	441 (74)	322 (74)	119 (72)	0.163	0.290	0.692
9	187 (30)	94 (30)	91 (31)	159 (26)	112 (26)	47 (28)

COMT – Catechol-o-methyl-transferase; DAT – Dopamine transporter; VNTR – Variable number tandem repeats; 5-HTTLPR – Serotonin-transporter-linked polymorphic region; MAOA – Monoamine oxidase A

Significant differences in frequencies of MAOA-uVNTR polymorphism have been observed between healthy volunteers and patients with schizophrenia (P = 0.010), as well as male healthy volunteers and schizophrenic males (P = 0.010). Low activity alleles were more frequent in the schizophrenia subgroup. This difference was not observed in the female population investigated (P = 0.134). The Val158Met genotype of COMT polymorphism was found more frequent in females of the control group (P = 0.021). This difference was not observed in healthy males (P = 0.992). 9/9 genotype of DAT-VNTR polymorphism was more frequent in the healthy group than in patients with schizophrenia (P = 0.028), but no difference was found between groups when divided by gender. There was no statistically significant difference in the distribution of 5-HTTLPR polymorphism.

Results of univariate and multivariate logistic regression are presented in Table 4.

Table 4

Logistic regression analysis results

Variable	Univariate analysis		Multivariate analysis
	OR (95%CI)	P	OR (95%CI)	P
Gender (male)	2.38 (1.71-3.33)	<0.001	1.05 (0.64-1.72)	0.849
Marital status (not married)	3.88 (2.66-5.67)	<0.001	2.35 (1.48-3.73)	<0.001
Education (high school or lower)	7.19 (4.78-10.83)	<0.001	3.93 (2.45-6.30)	<0.001
Employment (unemployed)	13.27 (8.35-21.07)	<0.001	8.59 (5.17-14.29)	<0.001
COMT	0.96 (0.76-1.97)	0.699
DAT-VNTR	0.87 (0.68-1.12)	0.278
5-HTTLPR	0.92 (0.74-1.15)	0.469
MAOA-uVNTR	1.17 (0.98-1.41)	0.083	1.41 (1.08-1.84)	0.010

Correct classification of schizophrenia: 80.45%. Dependent variable: Schizophrenia; Independent variables: Gender, marital status, education, employment, COMT, DAT-VNTR, 5-HTTLPR, MAOA-uVNTR. Relaxed level of significance for univariate analysis was set at 0.25. Independent variables which were statistically significant (gender, marital status, education, employment, and MAOA-uVNTR) were included in multivariate analysis. COMT – Catechol-o-methyl-transferase; DAT-VNTR – Dopamine transporter variable number tandem repeats; 5-HTTLPR – Serotonin-transporter-linked polymorphic region; MAOA-uVNTR – Monoamine oxidase A upstream variable number tandem repeats; OR – Odds ratio; CI – Confidence interval

Statistically significant variables using the univariate model were gender, marital status, education, employment, and MAOA-uVNTR polymorphism. The univariate model revealed that the male gender, single marital status, high school level (or lower) of education, and unemployment were predictors for schizophrenia. 5-HTTLPR, COMT Val158Met, and DAT-VNTR ORs were not statistically significant in the univariant model and were not included in the multivariate model. However, MAOA-uVNTR polymorphism OR (95% CI) was 1.17 (0.98–1.41) and P = 0.083, which met criteria for inclusion in the multivariate model. The multivariate logistic regression showed that marital status, education, and employment status remained statistically significant, as well as MAOA-uVNTR (OR [95% CI] = 1.41 [1.08–1.84]; P = 0.01). Using this model, we managed to correctly classify 80.45% participants.

DISCUSSION

In this study, we found that low activity allele of MAOA-uVNTR polymorphism is significantly associated with schizophrenia even after adjusting for all significant anamnestic predictors of disease (male gender, marital status, education, and employment status). COMT Val158Met showed a difference in the genotype distribution of female participants between healthy control and schizophrenia patients. DAT-VNTR polymorphism showed a difference in genotype distribution between healthy and control; however, as for 5-HTTLPR polymorphism, overall association with schizophrenia was not observed. Using the proposed regression model, 80% participants could be correctly classified as schizophrenia patients.

Variable number tandem repeats (u-VNTR) polymorphism located upstream from the coding region of the MAOA gene was described. Its sequence comprises 30 base pairs (bp) repeated in 3, 3.5, 4, or 5 copies. The most frequent alleles found in Caucasians, according to Sabol et al., are the 4-repeat (64.8%) followed by a 3-repeat allele (33.1%).[13] Alleles with 3.5 or 5 copies are quite rare, comprising only 2.1%. The investigated allele frequencies in Croatian healthy population are in agreement with these findings. Namely, the frequencies of the 4-repeat, 3-repeat, and rare alleles in the healthy population investigated were 66%, 31%, and 3%, respectively. Furthermore, Sabol et al. assessed the allele transcriptional activity and discovered that 3.5- and 4-repeats are optimal allele lengths for transcriptional activity, while lower transcriptional activity was associated with 3- and 5-repeat alleles.[12] Conversely, Deckert et al. found that 5 and 4 repeats alleles of MAOA-uVNTR showed higher transcriptional activity, suggesting that longer alleles have higher transcriptional activity.[20] Since both studies agreed on the activity of 3- and 4-repeat alleles, we decided to include only participants with 3-(low activity) and 4-repeats (high activity) in our study. We found that the low activity allele (3-repeat) of MAOA-uVNTR was more frequent in male schizophrenic patients. Since MAOA gene is located on the short arm of chromosome X (Xp11.3) only female carriers can be heterozygotes. It is indicative that the presence of one high activity allele could have a protective effect which is possible only in females. Purves-Tyson et al. measured MAOA enzyme activity in postmortem brain tissue of schizophrenic patients and discovered higher MAOA activity. Their results could be explained by higher serotonin degradation with consequently reduced dopamine inhibition through 5-HT2 receptors in the substantia nigra.[21] Furthermore, higher MAOA activity in patients with schizophrenia could be explained by the possible effect of prolonged antipsychotic treatment. MAOA-uVNTR polymorphism has previously been investigated in schizophrenia research and was found related to schizophrenia. However, data are inconsistent, and a meta-analysis showed no association of this polymorphism with schizophrenia.[22]

Schizophrenia is a complex disorder characterized by a spectrum of symptoms mostly categorized as negative positive and general. It has been reported that negative symptoms have been associated with MAO-uVNTR polymorphism.[23] Furthermore, low allele activity was associated with the development of aggressive behavior in adults who were exposed to maltreatment during childhood, suggesting a modifying role of MAOA-uVNTR on brain development.[24] Further studies are needed to clarify whether the association of MAOA-uVNTR polymorphism is associated with schizophrenia or this association is merely a reflection of certain symptoms.

Dopamine projections are followed by projections of the serotonin pathway, which starts in the dorsal raphe nuclei and follows the dopamine pathway in substantia nigra, striatal, and cortical region where it inhibits dopamine action.[2526] The serotonin transporter is the main regulator of serotonin action, and its 5-HTTLPR polymorphism has been associated with mental disorders such as schizophrenia and depression.[272829] Serotonin transporter gene SLC6A4 is located on chromosome 17q11.2. A functional polymorphism of the 5-HTTLPR region consists of a 44 bp insertion/deletion sequence. Long variant (L) is associated with 2-fold higher expression compared to short allele (S).[30] In our study, no differences were found in frequencies of 5-HTTLPR genotype between schizophrenia patients and control.

Dopamine transporter gene, i.e. SLC6A3 is located on chromosome 5p15.33 with the VNTR polymorphism located in the untranslated region. This polymorphism consists of 40 bp sequence, repeated 3–11 times. Although the distribution of alleles varies between different populations, the most frequent alleles are the 10- and 9-repeats. 10 repeat allele is the most frequent in Caucasians (72%), followed by 9-repeat allele (28%), which is associated with lower transcriptional activity.[3132] Imaging brain studies have shown that the density of dopamine transporter is higher in the striatum, where it regulates dopamine action to the greatest extent. In contrast, its density is lower in the prefrontal cortex where dopamine action is mainly regulated by COMT. A nine-repeat allele of dopamine transporter has been associated with lower transcriptional activity and therefore lower dopamine reuptake.[3334] In our study, we found that the 9/9 genotype was less frequent in schizophrenia patients in comparison with healthy control (4.2 vs. 8.9%). Considering the dopamine hypothesis of schizophrenia which suggests subcortical hyperdopaminergia, we expected a higher frequency of 9/9 genotype in patients with schizophrenia. Our findings can be explained by conflicting results of studies on the transcriptional activity of DAT-VNTR polymorphism. For example, analysis on postmortem brain tissue has shown higher mRNA expression of 10/10 genotype in patients with schizophrenia but the lower expression in healthy controls. In this research, mRNA expression has been considered to correlate with gene expression; however, the density of dopamine transporter was not measured so the effect of external factors or drug abuse cannot be excluded. Research conducted on the cell culture model showed a higher expression of a 10-repeat allele.[35] In contrast, single-photon emission computed tomography (SPECT) analysis has shown that 10-repeat allele expression is lower compared to the 9-repeat allele. Limitation of SPECT analysis is low resolution because radioactive labeling does not specifically bind to dopamine transporter but also binds to serotonin transporter.[3637] Our study suggests an association of DAT-VNTR polymorphism with schizophrenia; however, the exact mechanism of action and the possible effect of other regulatory elements should be investigated.

Another regulator of dopamine concentration is COMT which has been associated with schizophrenia.[7] This enzyme is present in two isoforms, longer, membrane-bound (MB) and shorter, soluble, S-isoform. COMT gene is located on chromosome 22q11.21. G > A substitution at position 158 (Val158Met) (rs4680) results with reduced activity of MB COMT isoform.[3839] In our study, we found a statistically significant difference in COMT Val158Met polymorphism frequency between healthy and schizophrenic patients, but only in female participants. Dopamine function in the prefrontal cortex in association with COMT Val158Met seems to depend also on external factors. It has been investigated that optimal prefrontal cortex function is associated with a narrow dopamine level. Although COMT is important for regulation of dopamine level, the effect of Val158Met polymorphism is more complicated than high or low enzymatic activity. It has been suggested that a combination of environment and genetic factors like COMT Val158Met variant can contribute dopamine level.[7] Although promising, association studies of COMT Val158Met polymorphism and schizophrenia are inconsistent possibly due to the complex effect of COMT Val158Met polymorphism on dopamine levels. In our study, the association of COMT Val158Met polymorphism in a female population could be explained by molecular heterosis due to the relatively high frequency of heterozygous carriers.[40]

Distribution of gender in our study differed significantly between the schizophrenia group and healthy control which might affect our findings. Therefore, we performed logistic regression analysis to adjust for the influence of different variables on schizophrenia as an outcome. Participants who were not married, had a lower level of education, and were unemployed, had significantly higher odds to develop schizophrenia. It should be noted that our study is case–control by its design, and our results present participants in one moment of their lives. Abovementioned education level, marriage status, and unemployment are data collected at the time of participant recruitment and should be considered as indicators of poor cognitive abilities and poor establishing of interpersonal relationships and social functioning.

Limitation of our study is that we did not asses environment factors such as traumatic life events which might contribute to schizophrenia development.

In contrast to a traditional concept where one genetic variant is considered beneficial compared to another, the concept of genetic plasticity seems to be a more appropriate approach. In general, gene association studies are trying to find a direct association between certain genes and disease (in this case, schizophrenia). The direct association is difficult to find because psychiatric disorders depend also on environmental factors and some genes might be beneficial in combination to certain environmental factors.[41]

CONCLUSION

According to our study, MAOA-uVNTR seems to be a significant predictor of schizophrenia in combination with marriage status, education, and employment. Our findings support the complex and multifactorial nature of schizophrenia.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.