ORIGINAL INVESTIGATION

Association between the brain-derived neurotrophic factorVal66Met polymorphism and therapeutic response to olanzapinein schizophrenia patients

Matea Nikolac Perkovic & Gordana Nedic Erjavec & Maja Zivkovic & Marina Sagud &

Suzana Uzun & Alma Mihaljevic-Peles & Oliver Kozumplik & Dorotea Muck-Seler &

Nela Pivac

Received: 27 December 2013 /Accepted: 15 February 2014# Springer-Verlag Berlin Heidelberg 2014

AbstractRationale Brain-derived neurotrophic factor (BDNF) is aneurotrophin that plays a major role in neurogenesis andneuroplasticity, and in the modulation of several neurotrans-mitter systems including the dopaminergic system. There aremixed reports about the association between the BDNFVal66Met polymorphism, schizophrenia, and treatment re-sponse to antipsychotic drugs.Objectives The present study evaluated the association of theBDNF Val66Met polymorphism with treatment response toatypical antipsychotic olanzapine in schizophrenia and thepossible predictive value of the BDNF Val66Met genotypestatus in treatment response to antipsychotic medication.Methods The study included 590 ethnically homogenousCaucasian patients with schizophrenia (diagnosed using theSCID), 40.2±12.0 years old, treated with olanzapine mono-therapy (10–20 mg/day), or with other antipsychotics such asrisperidone (3–6 mg/day), clozapine (100–500 mg/day), hal-operidol (3–115 mg/day), fluphenazine (4–25 mg/day), and

quetiapine (50–800 mg/day). Patients were subdivided intoresponders and non-responders according to a 50 % reductionin the Positive and Negative Syndrome Scale (PANSS) totaland subscale scores after 8 weeks of treatment.Results The results, corrected for possible effects of genderand age, showed a significant association between the BDNFVal66Met polymorphism and treatment response toolanzapine in patients. The Val/Val genotype was observedmore frequently in treatment responders to olanzapine, andthis genotype was associated with an improvement in clinicalsymptoms.Conclusions Our results suggest that BDNF Val66Met vari-ants might influence the response to 8 weeks of monotherapywith olanzapine, in a relatively large sample of patients withschizophrenia.

Keywords Antipsychotics . Olanzapine . Risperidone .

Clozapine . Haloperidol . Fluphenazine . Quetiapine . BDNFVal66Met polymorphism . Schizophrenia . Treatmentresponse

Introduction

Brain-derived neurotrophic factor (BDNF) is a neurotrophinthat plays a major role in neurogenesis, neuroplasticity,neurodevelopment (Binder and Scharfman 2004; Chao et al.2006; Hong et al. 2011), cognition (Russo et al. 2009), andmodulation of major neurotransmitter systems including thedopaminergic, glutamatergic, and serotonergic systems(Gratacos et al. 2007; Russo-Neustadt 2003; Tyler et al.2002). Therefore, BDNF has been proposed as a candidatefor various personality traits (Terracciano et al. 2010), cogni-tive function, and psychiatric disorders including

Electronic supplementary material The online version of this article(doi:10.1007/s00213-014-3515-4) contains supplementary material,which is available to authorized users.

M. Nikolac Perkovic :G. Nedic Erjavec :D. Muck-Seler :N. Pivac (*)Division of Molecular Medicine, Rudjer Boskovic Institute,Bijenicka 54, 10000 Zagreb, Croatiae-mail: npivac@irb.hr

M. Zivkovic : S. Uzun :O. KozumplikDepartment of General Psychiatry, Clinics for Psychiatry Vrapce,Bolnicka cesta 32, 10000 Zagreb, Croatia

M. Sagud :A. Mihaljevic-PelesUniversity Hospital Center Zagreb, University of Zagreb School ofMedicine, Salata 3, 10000 Zagreb, Croatia

PsychopharmacologyDOI 10.1007/s00213-014-3515-4

schizophrenia (Angelucci et al. 2005; Gratacos et al. 2007;Hong et al. 2011; Nieto et al. 2013; Nurjono et al. 2012;Russo-Neustadt 2003). In drug-naïve first-episode schizo-phrenia patients, reduced BDNF levels were reported (Jindalet al. 2010), while antipsychotics seem to increase BDNFlevels in neuroblastoma cells (Lee et al. 2010). AbnormalBDNF expression induces dysfunctional neural development,resulting in disturbed neural cytoarchitecture and alteredneuroplasticity, leading to development of schizophrenia(Thome et al. 1998; Angelucci et al. 2005; Hong et al. 2011;Nieto et al. 2013), a chronic psychiatric neuroprogressive andneurodevelopmental illness (Dodd et al. 2013) characterizedby altered BDNF levels in various brain regions (Hong et al.2011) and frequently associated with treatment resistance(Jindal et al. 2010; McIlwain et al. 2011). Pharmacogeneticstudies might help in selecting optimal treatment approachesfor individual patients (Zhang and Malhotra 2011).

The single nucleotide polymorphism BDNF Val66Met(rs 6265), a valine (Val) to methionine (Met) substitution,affects the production of mature protein (Chen et al.2004). The Met allele is associated with reducedactivity-dependent BDNF secretion from cultured hippo-campal neurons and poorer hippocampus dependentmemory (Egan et al. 2003). There are mixed results onthe association of the BDNF Val66Met polymorphismwith schizophrenia (Gratacos et al. 2007; Hong et al.2011; Kawashima et al. 2009) or with therapeutic re-sponse to different antipsychotics (Hong et al. 2003;Xu et al. 2010; Zai et al. 2012; Zhang et al. 2013).Numerous studies investigated the association betweenthe BDNF Val66Met polymorphism and treatment re-sponse to clozapine (Hong et al. 2003; Xu et al. 2010;Zai et al. 2012; Zhang et al. 2013). However, due to itsside effect profile, other antipsychotics such asolanzapine and risperidone are frequently prescribed inschizophrenia, but their association with BDNFVal66Met is poorly investigated.

Based on our preliminary data (Nikolac et al. 2012) andother studies in the literature (Jindal et al. 2010; Lee et al.2010; Zai et al. 2012; Zhang et al. 2013), the hypothesis of thisstudy was that the Met allele will be associated with poorerresponse to olanzapine treatment. In order to test this hypoth-esis, we evaluated the potential predictive value of the BDNFVal66Met genotype status in treatment response to 8 weeks ofmonotherapy with olanzapine compared to other frequentlyprescribed antipsychotics. The aim of the study was to deter-mine, based on the BDNF Val66Met genotype, whether apatient with the Val/Val genotype has a higher probabilitybeing a responder to olanzapine, and whether this genotypemight be able to predict which of the antipsychotics mightresult in the best treatment response, in ethnically homoge-nous Caucasians with schizophrenia, sampled from the sameregion.

Experimental procedures

Participants

The present study included 590 patients with schizophrenia,diagnosed using a structured clinical interview for DSM-IV(First et al. 1995), and 40.2±12.0 years old, range 18–82 years.All participants completed 8 weeks of pharmacotherapy.Inclusion criteria were as follows: schizophrenia diagnosis;monotherapy with olanzapine; treatment with risperidone,clozapine, haloperidol, fluphenazine, and quetiapine; benzo-diazepines when needed; given informed consent; and age≥18 years. Exclusion criteria were the use of antidepressants.Schizophrenic patients were evaluated with the Positive andNegative Syndrome Scale (PANSS) during the first few daysof admission and after 8 weeks of treatment. Raters did notknow the BDNF Val66Met genotypes of patients, and inter-rater reliability was 97 %. Patients were ethnically homoge-nous unrelated Caucasians of European ancestry (of Croatianorigin), sampled from the Zagreb county (University HospitalCenter Zagreb and Clinics for Psychiatry Vrapce). Of the 590patients, 214 patients (71.2 % men) received olanzapine (10–20 mg/day) monotherapy, while 376 patients were included inthe comparison group referred as “other antipsychotics”: 111patients (63.1 % men) received risperidone (3–6 mg/day) and120 patients (74.2 % men) received clozapine (100–500 mg/day) monotherapy; 145 patients (55.9 % men) were treatedwith haloperidol (n=22; 3–15 mg/day), fluphenazine (n=21;4–15 mg/day), and quetiapine (n=39; 400–800 mg/day) asmonotherapy, or combinations of quetiapine + fluphenazine(n=40) or quetiapine + haloperidol (n=23). All patients weredrug-naïve before the inclusion in the protocol.

Treatment response was defined as an a priori chosen cutoffpoint of 50% reduction in the initial PANSS total and subscalescores (Leucht et al. 2007; Suzuki et al. 2012) after 8 weeks oftreatment. As previously suggested (Leucht et al. 2007;Obermeier et al. 2011), the scoring system of the PANSSwas corrected from values of 1–7 to values of 0–6.

Written informed consent was obtained from all partici-pants, after explaining the aims and procedures of the study,under guidelines approved by the local Ethics committees. Allhuman studies were performed with the full cooperation andunderstanding of the participants, and have therefore beenperformed in accordance with the ethical standards laid downin the 1964 Declaration of Helsinki.

Blood collection and genotyping

Blood samples were taken from patients during routine labo-ratory visits. Genomic DNA was extracted from peripheralblood using a salting out method (Miller et al. 1988). TheBDNF Val66Met (rs6265) polymorphism was genotyped in atotal volume of 25 μL with 20 ng of DNA using ABI

Psychopharmacology

Prism7300 Real-Time PCR System apparatus (ABI, FosterCity, USA). The primers and probes were purchased fromApplied Biosystems as TaqMan® SNP Genotyping Assay(assay ID: C_11592758_10), and genotyping was carried outaccording to the manufacturer’s instructions. The possibilityof genotyping errors was excluded by randomly choosing 5 %of samples for repeated genotyping with 100 % concordance.

Statistical evaluation

The results were evaluated with Sigma Stat 3.5 (JandelScientific Corp. San Rafael, CA, USA) and Microsoft Excel.Age and PANSS score data deviated from a normal distribu-tion and were analyzed with Kruskal–Wallis ANOVA onranks, Dunn’s method, and Mann–Whitney tests. Genotypefrequencies and Hardy–Weinberg equilibrium (HWE) wereevaluated using Pearson’s χ2 test with a Yates correction forcontinuity. To confirm the association of the BDNF genotypewith treatment response and to correct for the possible effectof gender and age on the reduction in PANSS total andsubscale scores, logistic regression and multiple linear regres-sion analyses were used. Since the frequency of the homozy-gous Met/Met genotype is low (3–4 %) in the Croatian pop-ulation (Nedic et al. 2013; Pivac et al. 2009; Pivac et al. 2011),the combined Met/Val and Met/Met genotypes (Met carriers)were compared to the homozygous Val/Val genotype. Effectsize for significant χ2 test (Φc) results was evaluated usingCramer’s Vor C statistic (Daniel 1990; Spatz 2011). For dataanalyzed by Mann–Whitney tests, the Z value was used tocalculate effect size (r) (Fritz et al. 2012). To avoid issues withmultiple testing (Leucht et al. 2007) and correction for the

multiple testing, statistical analyses applied a cutoff point thatwas chosen a priori (a 50 % reduction in PANSS scorescompared to baseline). All tests were two-tailed and α wasset at 0.05.

Results

The demographic data are shown in Table 1. Patients treatedwith other antipsychotics were significantly (P=0.001) olderand had significantly (P=0.030) higher PANSS negativescores at baseline compared to patients treated witholanzapine (Table 1).

Table 2 presents PANSS-derived responder rates (Leuchtet al. 2007) in increments of 25 %. The distribution of patientsresponding to olanzapine or other antipsychotics, subdividedaccording to PANSS-derived responder rates, did not signifi-cantly differ across groups (χ2=2.271; df=3; P=0.518). Inboth treatment groups, 46.0–48.1 % patients were in the 50–74 % symptom reduction category, confirming the selectedcutoff point of 50 % reduction in PANSS scores to showclinical improvement.

The frequency of the BDNF Val66Met genotypes inschizophrenic patients (χ2=3.444; P=0.064) did not deviatefromHWE. Since the frequency of the Met/Met genotype wasrare (2.5 %) in our sample, in all analyses, Met carriers (thecombined Met/Met and Val/Met genotype) were compared toVal/Val homozygotes.

The BDNF genotype frequency was similar (χ2=3.103;df=2; P=0.212) between patients treated with olanzapine orother antipsychotics. To evaluate if the rs6265 genotype

Table 1 Age of the patients and total and subscale scores in Positive and Negative Syndrome Scale (PANSS0−6) in 590 schizophrenic patients treatedwith olanzapine or other antipsychotics at baseline (week 0) and after 8 weeks

Population characteristics OlanzapineN=214(76.2 % males)

Other antipsychoticsN=376(65.7 % males)

Kruskal–Wallis test

Median Percentiles Median Percentiles (df=1)

25th 75th 25th 75th H P

Age (years) 37 29 46 41 32 50 11.960 <0.001

Total PANSS0–6 scores (week 0) 86 73 100 88 73 103 0.487 0.485

Total PANSS0–6 scores (week 8) 40 28 52 42 24 62 1.401 0.237

PANSS0–6 positive (week 0) 24 18 29 24 19 29 0.065 0.800

PANSS0–6 positive (week 8) 9 5 13 9 5 14 2.536 0.111

PANSS0–6 negative (week 0) 19 15 26 20 17 27 4.727 0.030

PANSS0–6 negative (week 8) 12 8 14 12 7 18 0.324 0.569

PANSS0–6 general psychopathology (week 0) 41 35 46 42 33 49 0.122 0.726

PANSS0–6 general psychopathology (week 8) 18 14 26 20 13 29 1.031 0.310

Values are given as median and percentiles (25th and 75th)

N number of subjects, PANSS Positive and Negative Syndrome Scale

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affects the treatment response in the whole sample, patientswere classified into responders and non-responders. Therewere no significant (χ2 test) differences in the frequency ofthe Met carriers versus the Val/Val genotype between re-sponders and non-responders subdivided according to thePANSS total (χ2=1.386; df=1; P=0.239), negative (χ2=0.227; df=1; P=0.634), and general psychopathology (χ2=0.349; df=1; P=0.555) scores. In the whole sample, thedistribution of the BDNF genotypes differed significantlyonly in patients subdivided according to the PANSS positivescores (χ2=4.879; df=1; P=0.027).

Table 3 shows a significantly different (χ2 test) distributionof the BDNF genotypes in olanzapine-treated patientssubdivided according to the reduction in total (P=0.046;Φc=0.27) and positive (P=0.026; Φc=0.34) PANSS scores.BDNF genotype frequency did not differ significantly in otherpatients treated with other antipsychotics subdivided accord-ing to treatment response (Table 3).

Although the frequency of the Met carriers and Val/Valhomozygotes was not significantly different between male

and female patients treated with olanzapine (χ2=0.953; P=0.329) or other antipsychotics (χ2=1.757; P=0.415), an ad-ditional logistic regression analysis was performed to controlfor potential effects of gender and age on the significantassociation between BDNF genotype and treatment responseto olanzapine. The results of logistic regression analyses, withgender, age, and BDNF genotype used as independent vari-ables and treatment response as the dependent variable, con-firmed an association between BDNF genotype and treatmentresponse in olanzapine-treated patients, subdivided accordingto the reduction in total (OR=1.90; 95 % CI=1.06–3.41; P=0.032) and positive (OR=1.98; 95 % CI=1.05–3.73; P=0.034) PANSS scores.

To confirm this significant association between the Val/Valgenotype and treatment response to olanzapine, the reductionin PANSS total and subscale scores after 8 weeks of treatmentin all groups, subdivided into Met carriers and Val/Val homo-zygotes, was compared with Mann–Whitney tests (Table 4).PANSS total (P=0.039; r=0.14) and positive (P=0.008, r=0.18) score reductions differed significantly in olanzapine-

Table 2 PANSS-derived response rates in schizophrenic patients after 8 weeks of treatment with olanzapine or other antipsychotics

Total N <25 % PANSS reduction 25–49 % PANSS reduction 50–74 % PANSS reduction 75–100 % PANSS reductionN (%) N (%) N (%) N (%)

Olanzapine 214 19 (8.9) 71 (33.2) 103 (48.1) 21 (9.8)

Other antipsychotics 376 38 (10.1) 114 (30.3) 173 (46. 0) 51 (13.6)

N number of subjects, PANSS Positive and Negative Syndrome Scale

Table 3 The BDNF genotype count and frequencies in schizophrenic patients treated with olanzapine or other antipsychotics, subdivided intoresponders and non-responders according to a 50 % reduction in the PANSS total and subscale scores

OlanzapineN=214

Other antipsychoticsN=376

Met carriers Val/Val Met carriers Val/Val

Total PANSS0–6 scores reduction at week 8 NR 42 (46.7) 48 (53.4) 61 (34.9) 114 (65.1)

R 40 (32.2) 84 (67.7) 70 (34.8) 131 (65.2)

χ2=3.991; P=0.046 χ2=0.010; P=0.919

PANSS0–6 positive symptom scores reduction at week 8 NR 31 (50.8) 30 (49.2) 49 (38.9) 77 (61.1)

R 51 (33.3) 102 (66.7) 82 (32.8) 168 (67.2)

χ2=4.927; P=0.026 χ2=1.113; P=0.291

PANSS0–6 negative symptom scores reduction at week 8 NR 56 (39.2) 87 (60.8) 89 (35.6) 161 (64.4)

R 26 (36.6) 45 (63.4) 42 (33.3) 84 (66.7)

χ2=0.044; P=0.833 χ2=0.103; P=0.748

PANSS0–6 general psychopathology scores reduction at week 8 NR 44 (45.8) 52 (54.2) 64 (33.0) 130 (67.0)

R 38 (32.2) 80 (67.8) 67 (36.8) 115 (63.2)

χ2=3.604; P=0.058 χ2=0.448; P=0.503

Frequencies (%) are shown in parenthesis. df=1 (for all χ2 tests); results were evaluated with χ2 tests; statistically significant results (P values) areshown in italics

N number of subjects, PANSS Positive and Negative Syndrome Scale, Met methionine, Val valine, Met carriers the combined Met/Met and Val/Metgenotypes, NR non-responders, R responders

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treated patients, but not in patients treated with other antipsy-chotics, since Val/Val homozygotes had a greater reduction intotal and positive PANSS scores than Met carriers (Table 4).

To control for the influence of gender and age on thereported association between BDNF genotype and theolanzapine-induced PANSS total and subscale score reduc-tions, a multiple linear regression analysis was used withgender, age, and BDNF genotype as independent variables,and the reduction in PANSS total or subscale scores as thedependent variable. The model was significant (R2

adj=0.055;F=4.857; P=0.003) and confirmed the association betweenthe reduction in PANSS positive subscale scores and BDNFgenotype (β=0.177; P=0.012), while a significant modelexplaining PANSS total score reduction (R2

adj=0.037; F=3.536; P=0.016) was marginally influenced by BDNF geno-type (β=0.139; P=0.051).

To ascertain whether the effect of rs6265 genotype isspecifically related to the response to olanzapine, the BDNFgene variants were also evaluated in the comparison group inpatients who received only monotherapy with risperidone orclozapine. These data are presented in SupplementaryTables S1, S2, and S3. The PANSS-derived response ratesconfirmed the 50 % chosen cutoff point (SupplementaryTable S1); distribution of the BDNF genotypes was similarin responders and non-responders to risperidone, but wassignificantly (P=0.030; Φc = 0.43) different only inclozapine-treated patients classified according to a reductionin the PANSS genera l psychopatho logy scores(Supplementary Table S2). This significance was not con-firmed since the PANSS general psychopathology score re-duction was similar in clozapine-treated Met carriers com-pared to Val/Val genotype carriers (SupplementaryTable S3). These results confirmed that BDNF Val66Metvariants were significantly associated with the response to8 weeks of monotherapy with olanzapine, but not with otherinvestigated antipsychotic drugs.

Discussion

Our data revealed that BDNFVal66Met variants might predictthe response to 8 weeks of monotherapy with olanzapine, butnot to other antipsychotics (clozapine, risperidone, quetiapine,haloperidol, or fluphenazine), in a relatively large sample (N=590) of ethnically homogenous unrelated Caucasian patientswith schizophrenia. The distribution of the BDNF Val66Metgenotypes significantly differed between responders and non-responders to olanzapine treatment, classified according toreduct ions in total and posi t ive PANSS scores .Schizophrenic patients, subdivided into Met carriers and Val/Val homozygotes, treated for 8 weeks with olanzapine, hadsignificantly different treatment response outcomes, and theseresults were confirmed after controlling for the effects ofgender and age. Specifically, the Val/Val genotype was ob-served more frequently in treatment responders; additionally,this genotype was associated with an improvement in symp-toms evaluated using the PANSS, especially the PANSS pos-itive symptoms. These results suggest an over-representationof Val alleles in responders to olanzapine in schizophrenia, assuggested earlier (Zai et al. 2012). Even though there aredifferences between this and our study in the research design,criteria for defining treatment response, treatment duration,inclusion criteria, and antipsychotics used, these data suggestthat a good therapeutic response to antipsychotics could beassociated with the Val/Val genotype or Val allele of theBDNF Val66Met in schizophrenic patients of European an-cestry (present study; Zai et al. 2012). In support to our results,treatment resistance to antipsychotics was significantly

Table 4 Reduction in the PANSS0–6 total and subscale scores after8 weeks of treatment with olanzapine or other antipsychotics in schizo-phrenic patients subdivided according to the BDNF genotypes

OlanzapineN=214

Other antipsychoticsN=376

Median Percentiles Median Percentiles

25th 75th 25th 75th

Total PANSS0–6 score reduction after 8 weeks of treatment

BDNF genotype

Met carriers 41 30 51 44 33 54

Val/Val 47 33 60 42 32 54

U=6319.0; P=0.039 U=15157.0; P=0.375

PANSS0–6 positive symptom score reduction after 8 weeks of treatment

BDNF genotype

Met carriers 13 9 16 13 10 17

Val/Val 15 11 21 13 10 17

U=6579.0; P=0.008 U=16252.0; P=0.839

PANSS0–6 negative symptom score reduction after 8 weeks of treatment

BDNF genotype

Met carriers 7 3 12 9 5 13

Val/Val 7 3 12 8 4 12

U=5313.0; P=0.823 U=15257.0; P=0.431

PANSS0–6 general psychopathology score reduction after 8 weeks oftreatment

BDNF genotype

Met carriers 20 13 24 18 13 24

Val/Val 22 14 28 19 14 25

U=6187.0; P=0.078 U=14542.0; P=0.134

Values are given as median and percentiles (25th and 75th); results wereevaluated with Mann–Whitney tests. Statistically significant results(P values) are shown in italics

N number of subjects, PANSS Positive andNegative Syndrome Scale,Metmethionine, Val valine,Met carriers the combined Met/Met and Val/Metgenotypes

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associated with minor alleles of the several BDNF SNPs,including the Met allele of the BDNF Val66Met (Zhanget al. 2013).

Consistent with studies that have found no significantassociation of the Val66Met variants with treatment responseto risperidone (Xu et al. 2010), clozapine (Hong et al. 2003;Xu et al. 2010), chlorpromazine (Xu et al. 2008), and typicalantipsychotics (Anttila et al. 2005), our study also showed alack of a significant association between BDNFVal66Met andresponse to risperidone, clozapine, haloperidol, fluphenazine,or quetiapine. In contrast to our clozapine data, BDNF geno-type was associated with clozapine treatment response (Zaiet al. 2012; Zhang et al. 2013). To elucidate the differencesacross studies, we determined the distribution of the BDNFgenotypes in responders and non-responders to clozapine orrisperidone monotherapy, and found the significant differencein the BDNF genotype frequency only in clozapine-treatedpatients classified according to the PANSS general psychopa-thology score reduction. However, this result was not con-firmed as there was no difference in the PANSS generalpsychopathology scores between clozapine-treated Met car-riers compared to Val/Val genotype carriers. These differencesin pharmacogenetic findings could be due to different meth-odological designs (inclusion/exclusion criteria, outcomemeasures), population stratification, and other non-geneticfactors that can cause difficulties in detecting associationsbetween a genotype and neurobiological phenotypes (Walkeret al. 2008). The underlying mechanism responsible for thepharmacogenetic effect of the Val/Val genotype is still un-known. This genotype is associated with increased synapticplasticity, better hippocampus dependent memory and cogni-tive performances, and increased activity-dependent BDNFrelease (Egan et al. 2003). At present, the differences in BDNFVal66Met between the response to olanzapine and other anti-psychotic agents are not clear: they might be due to thedifferent age between patients treated with olanzapine com-pared to other antipsychotics, as olanzapine-treated patientswere younger than other groups, suggesting some possiblevariations in the stage and duration of illness. The differencesmight also be due to the smaller number of patients treatedwith other antipsychotics compared to olanzapine, or to dif-ferent doses of antipsychotics used, since the effects of anti-psychotics on BDNF are dose-related, or the variations in thepatient profile (treatment responsive or refractory), asdiscussed before (Buckley et al. 2007). Our data might sug-gest that olanzapinemore strongly affected positive symptomsin Val/Val homozygous group (i.e., in subjects with increasedBDNF expression) compared to the Met carriers, and there-fore, the Met allele might be responsible for more frequentnon-response to olanzapine or to other antipsychotics (presentstudy; Zhang et al. 2013).

Although we did not measure plasma BDNF levels in ourstudy (assumed to reflect the brain changes in BDNF levels), it

might be speculated that non-response to olanzapine wasassociated with reduced BDNF secretion in Met allelecarriers, and that treatment response to olanzapine wasassociated with higher plasma BDNF levels in Val/Valhomozygous genotype, as reviewed recently (Nieto et al.2013). However, in the further studies, measurements ofthe serum or cerebrospinal BDNF levels might allow tomonitor brain tissue BDNF alterations (Sartorius et al.2009) and some aspects of neuronal plasticity (Langet al. 2007).

A limitation of the study was that only the BDNFVal66Met was evaluated. However, this is a functional poly-morphism that influences the production of the mature BDNFprotein (Chen et al. 2004). Besides BDNF Val66Met, othervariants of the BDNF gene were also studied for the associa-tion with antipsychotic treatment response. However, eitherno significant association between C270T (Anttila et al. 2005)or rs2030324, rs7103873, rs10835210, and rs11030101 (Paeet al. 2012), or significant association between rs11030104(Zai et al. 2012) or (GT)n dinucleotide repeat polymorphism(Xu et al. 2010) and response to antipsychotic drugs wasreported. Although some BDNF haplotype combinations ap-peared to be significantly associated with antipsychotic treat-ment response (Xu et al. 2010; Xu et al. 2008; Zai et al. 2012),these studies included a much smaller number of schizophre-nia patients. Divergent pharmacogenetic results might alsobe explained by the ethnic differences in BDNF Val66Metgenotype distribution (Gratacos et al. 2007; Petryshenet al. 2010; Pivac et al. 2009) and gender related effects(Hong et al. 2011) that were controlled in the presentstudy. Strengths of the present study include monotherapywith olanzapine, use of a comparison antipsychotic group(Leucht et al. 2007), sufficiently long duration of the trial(e.g., 8 weeks) (Suzuki et al. 2012), detailed evaluation ofpatients, inclusion of ethnically homogenous Caucasianmen and women with schizophrenia, and in the definitionof the response as an a priori cutoff point of a 50 %reduction in total and subscale PANSS scores relative tobaseline (Leucht et al. 2007).

As previously highlighted, the individual BDNF gene var-iants are thought to have only a weak or modest role in thetreatment response to specific antipsychotic drugs (Xu et al.2010), in line with small to moderate effect sizes of othergenetic variants on clinical outcome to antipsychotics (Zhangand Malhotra 2011). Regardless, our results have shown thatthe Val/Val genotype was foundmore frequently in respondersto olanzapine treatment, determined in patients who showedclinical improvement (Leucht et al. 2007). These findingssuggest a putative predictive value of the BDNF Val66Metgenotype status on olanzapine treatment response. Namely, incase of patients with schizophrenia carrying the Val/Val geno-type, there might be a higher probability of having a positivetreatment outcome. Therefore, these results should be

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replicated in the larger cohorts since, if confirmed, they offer apossibility of a personalized approach to schizophreniatherapy.

Acknowledgments The study was supported by the Croatian Ministryof Science, Education and Sport, grants numbers 098-0982522-2455,098-0982522-2457, 108-1083509-3511, and 108-1083509-3513. Thefunding source had no further role in the study design; in the collection,analysis, and interpretation of data; in the writing of the report; and in thedecision to submit the paper for publication.

Conflict of interest All authors declare that they have no conflicts ofinterest.

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