Background
Schizophrenia (SCZ) is a chronic, hereditary and disabling neuropsychiatric disorder with a worldwide prevalence of approximately 1% [
1,
2]. In the etiology of SCZ, genetic factors are thought to play an important role, the heritability currently ranges from 64 to 81%; although, genetic mechanisms remain unclear [
2‐
4]. The mainstay of schizophrenia treatment has been antipsychotic drugs over the past 60 years; however, clinical response differs significantly between patients, with an overall response rate of 50–70% [
1,
5‐
7]. Many of patients with schizophrenia discontinue or switch drug regimens due to lack of treatment efficacy and/or drugs adverse side effects. In this view, there is a great need for the identification of predictive clinical and biological markers of treatment consequence [
8,
9]. Pharmacogenetic biomarkers focus to predict which patients could improve with specified drugs according to genetic variants. Thus, genotype-based customized drug treatments may allow optimizing the antipsychotic treatment, while helping to minimize drugs side effects [
10,
11].
To date, pharmacogenomics studies of response to treatment in schizophrenia, have typically focused on genes encoding for drug targets, called pharmacodynamics related genes. Many of the research investigating the association of pharmacodynamic genes with antipsychotic treatment response have concentrated on dopaminergic pathways, one of the primary mechanisms of function of antipsychotics, especially the gene coding for the dopamine D2 receptor(
DRD2 gene), which is a binding object for all available antipsychotic drugs. Dopaminergic gene SNPs are strongly related to drug sensitivity of antipsychotics; for example, several studies have indicated positive associations between
DRD2 gene and antipsychotic response [
10‐
14].
Lingyue Ma and et al. by a systematic review and meta-analysis indicated that for Asian patients, at rs1799978(A241G) in
DRD2 gene AA genotype had a significantly greater improvement after risperidone therapy [
15]. Three studies analyzed the association between rs1801028 in
DRD2 gene and antipsychotic or risperidone treatment in China and Slovenia; findings were inconsistent between the two studies [
15‐
17].
Dopamine receptor D4(
DRD4) is one of the main targets of antipsychotic drugs; furthermore,
DRD4 polymorphisms were associated with antipsychotic treatment response. A 120-bp duplication in the promoter region of
DRD4 is hypothesized to influence the clinical response to antipsychotic drugs [
18‐
20].
Catechol-O-methyltransferase (COMT) is a major enzyme that inactivates dopamine in the prefrontal cortex by enzymatic degradation. The efficacy of antipsychotic drugs in patients with schizophrenia has been indicated to be related to rs4680 SNP in
COMT gene; associated with a decreased therapeutic effect, leading to pharmacotherapy resistance. In addition, the
COMT interacts with several genes to produce various phenotypes [
20‐
24].
Based on our search in Pubmed and Google Scholar, this is the first study examining the frequencies of
DRD2 rs1799978,
DRD2 rs1801028 and
DRD4 120-bp duplication in Iranian population. Although, the frequency of rs4680 in
COMT gene is assessed in several studies with Iranian ethnicity. According to the mentioned studies, the overall frequency of G allele of rs4680 in a total sample size of n = 2200 is 51.57% in Iranian population [
25‐
30].
Consequently, there is insufficient evidence of associations between polymorphisms in dopaminergic genes and antipsychotic treatment, particularly in Iran; thus, we carried out an association study between clinical factors and four polymorphisms in DRD2, DRD4 and COMT genes, and antipsychotic treatment response in Iranian population. To best of our knowledge this is the first study in Iran that is investigating the effects of genetic factors and antipsychotic treatment response.
Discussion
The key findings of the present study were as follows. First, our genetic analysis for
DRD2 A-241G(rs1799978) polymorphism detected a significantly higher frequency of G allele in resistant to treatment patients in comparison with responders. A possible explanation for this association could be that since DRD2 binds to dopamine and is a G-protein coupled receptor, A-241G polymorphism is considered to be related to DRD2 density and affinity [
37]. Furthermore, regarding DRD2, it is recorded that this receptor lonely could adjust effects of atypical antipsychotics; suggesting that DRD2 plays a substantial role in patients response to atypical antipsychotics [
15]. In addition, Mingzhe Zhao et al. carried out a genome-wide and whole exome sequencing joint study analyzing both the common and rare genetic variants associated with risperidone treatment response in patients with schizophrenia. Significantly, after combining the findings of GWAS and WES they identified signaling pathways which were all associated with DRD2 and HTR2A, targets of risperidone [
38]. Interestingly, consistent with our result, Escamilla R et al. in Mexico reported a significantly higher frequency of G allele in resistant to antipsychotic treatment patients [
10]. Also, a meta-analysis reported a significant association between AA genotype and greater improvement after risperidone therapy in Asian patients [
15]. However, inconsistently to our result, Yan P et al. studying 267 Han Chinese patients, documented a significantly superior response to olanzapine in G allele carriers comparing to wild AA patients with schizophrenia [
37]. There are reasonable arguments to explain the disagreement observed in this study. First, diverse ethnicities as genetic background plays an important role in antipsychotic treatment response. Second, response phenotype is a complicated trait, differences observed in different studies may be related to heterogeneity of definitions or criteria of response phenotype [
15,
39]. Third, different sample sizes may result in various outcomes.
Second, in the case of the Ser311Cys (rs1801028) polymorphism in
DRD2 gene, Cys allele displayed a very low frequency in Iranian population, therefore we did not found any type of association related to this polymorphism. Consistent to our result, Terzić T et al. did not observe any association between
DRD2 Ser311Cys polymorphism and antipsychotic treatment response in 138 Slovenian patients with schizophrenia [
17]. Although, a study which was carried out in 690 Han Chinese patients, revealed a significant association between
DRD2 Ser311Cys polymorphism and PANSS total improvement rates [
16]. Since the genetic background is completely different in Han Chinese ethnicity compared to Iranian ethnicity and the sample size of the last study was larger than our study, the differences observed in these two studies could be explained.
Our third result, regarding
DRD4 120-bp duplication polymorphism, documented a significant association between 240/240 genotype with resistant to treatment comparing to 120/240 and 120/120 genotypes. To the best of our knowledge this is the first time that this association is reported. The 120-bp duplication region which is located in the upstream of the first codon, includes common sequences for various transcriptional factors and the longer allele decreases transcription of
DRD4 gene [
20,
40]. Limited studies exist around the association of
DRD4 120-bp duplication polymorphism with antipsychotic response; and a review study mentioned this polymorphism as a newly discovered marker of antipsychotic treatment response [
41]. Rudi Hwang et al. reported a significant association between 120 bp allele and non-responder group in African Americans, although they did not observe this association in Caucasians [
42]. Again the variety in ethnicities results in different outcomes.
Fourth, in case of
COMT Val158Met (rs4680) polymorphism, we did not observe genotypic and allelic association with antipsychotic treatment response. Consistent to our result, Hajj A et al. studying 100 Lebanese patients, did not detect association between
COMT Val158Met and antipsychotic treatment response in their whole sample. Although, they noted a gender-related difference for
COMT SNP. In 27 men involved in their study, they reported that Met allele carriers were more prone to be resistant to treatment in comparison with men with Val/Val genotype [
32].
Fifth, our results revealed three two-way and one three-way gene-gene interaction in
DRD2 A-241G,
COMT rs4680 and
DRD4 120-bp duplication polymorphisms with antipsychotic treatment response. In case of these four gene-gene interactions,
COMT Val158Met polymorphism was involved in three of them. First, we observed that in the
COMT Val/Val subset, patients with
DRD4 240/240 genotype had a significantly higher risk for resistant to treatment. To our knowledge this is the first time that this interaction is reported. Second, in the
DRD2 A-241G AA homozygotes,
COMT Met allele carriers demonstrated a significantly better response to antipsychotics. Third, our three way gene-gene interaction analysis indicated that
DRD4 120-bp allele carriers and
COMT Met allele carriers among
DRD2 AA homozygotes had a significantly superior response to antipsychotic treatment. Interestingly, Rajagopal VM et al. observed that patients with
COMT Val/Met or Met/Met genotype who also had
DRD4 120-bp allele revealed a significantly better response to clozapine [
20]. Additionally, a meta-analysis reported a significantly improved response in Met-allele carrier Asian and Caucasian patients [
39]. COMT is a catabolic enzyme with functional connection to dopaminergic transmission, the first target of most antipsychotics [
11,
39]. Protein function and structure studies indicate that COMT Val-Met substitution causes a disruption in enzyme stability and decreases enzymatic activity [
11,
39]. Importantly, tonic-phasic dopamine hypothesis supports our findings; according to this hypothesis, the lower activity form of COMT enzyme encoded by Met-allele may increase tonic dopamine and reduce phasic dopamine subcortically. Thus, this hypothesis suggest that the Met allele would predict a greater response to atypical antipsychotics, as they are considered to increase tonic dopamine transmission [
7,
43]. Finally, the fourth gene-gene interaction found in this study was between
DRD2 A-241G and DR4 120-bp duplication polymorphisms, where the combination of
DRD2 AA homozygotes and 120-bp allele carriers was found to be associated with an improved response to antipsychotics. To the best of our knowledge this is the first study analyzing the three way gene-gene interaction between
DRD2 A-241G,
DRD4 120-bp duplication and
COMT Val158Met polymorphisms together. Also, the first to analyze the interaction between
DRD2 A-241G with
DRD4 120-bp duplication, and
DRD2 A-241G with
COMT Val158Met.
Sixth, regarding our clinical and demographic results, we demonstrated that marriage and smoking were significantly associated with better response to antipsychotics. Furthermore, duration of hospitalization, total PANSS score and chlorpromazine equivalent daily dose were significantly higher in resistant to treatment patients. Interestingly, Escamilla R et al. in Mexico reported significantly more weeks of disease evolution in ultra-resistant to treatment patients. Also, they observed the most prolonged durations of untreated psychosis and an earlier age of schizophrenia onset in ultra-resistant to treatment group. Besides, they observed more married patients in the responder group, however it did not reach the significance level [
10]. Furthermore, consistent to our result, Aline Hajj et al. in Lebanon which studied 100 patients with schizophrenia, revealed a significantly higher chlorpromazine equivalent daily dose in resistant to treatment patients; either when the resistant to treatment was according to BRPS scale or when the PANSS scale was applied as the criteria of treatment resistant [
32].Different ethnicities and sample sizes may be a reasonable explanation for diverse results obtained in these studies.
However, considerable work has been carried out in order to recognize valid biomarkers to predict antipsychotic treatment response in patients with schizophrenia, data acquired is still inadequate and with the exclusion of singular samples, no general guidelines regarding possible utilization of pharmacogenetics in antipsychotic consumption in daily clinical practice, exist. The exceptions are related to CYP2D6 and CYP3A4 enzymes. Clinical Pharmacogenetics Implementation Consortium and the Dutch Pharmacogenetics Working Group published guidelines recommending CYP2D6 genotyping for patients under treatment with risperidone; DPWG recommends a decreased dose of risperidone in patients with the poor metabolizer phenotype of CYP2D6 and altering treatment to another drug or titration of the dose in patients who are classified as CYP2D6 ultrarapid metabolizers. Again DPWG together with the FDA, in treatment with aripiprazole, recommends a reduced dose for patients who are classified as CYP2D6 poor metabolizers. Furthermore, in cases where aripiprazole and CYP3A4 inhibitors are taken simultaneously, the FDA suggests a decrease in aripiprazole dose, with no determination of the patient’s phenotype. Nevertheless, these guidelines are advantageous, they are in no way adequate, therefore, more and more emphasizing the need for further research in this field [
44].
Our sample size was relatively small especially the subgroups in gene-gene interactions were small and a few genotypes had limited carriers(GG genotype of DRD2 A-241G polymorphism and GC of DRD2 rs1801028); also CC genotype of DRD2 rs1801028 was not observed in our sample. Furthermore, no multiple testing correction was used for P-values in gene-gene interaction analyses and we should mention the type I error possibility as a limitation. Besides, the studied population was only from one single ethnicity. Consequently, further investigations with larger sample sizes and meta-analyses, from various ethnicities analyzing several polymorphisms involved in pathways related to antipsychotics actions are warranted; in order to move toward personalized medicine in schizophrenia.
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