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Original article

The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study P. Ibarra a,1, S. Alemany a,b,1, M. Fatjo´-Vilas a,b, A. Co´rdova-Palomera a,b, X. Goldberg a,b, ˜ ana´s a,b,* B. Arias a,b, I. Gonza´lez-Ortega b,c, A. Gonza´lez-Pinto b,c, I. Nenadic d, L. Fan a

Anthropology Unit, Department of Animal Biology, Faculty of Biology, University of Barcelona and Biomedicine Institute of the University of Barcelona (IBUB), Avenue Diagonal, 643, 08028 Barcelona, Spain Centre for Biomedical Research Network on Mental Health (CIBERSAM), Doctor Esquerdo, 46, 28007 Madrid, Spain c Department of Psychiatry, Alava University Hospital (Santiago), EHU/UPV, Kronikgune, Olaguibel 29, Vitoria, Spain d Department of Psychiatry and Psychotherapy, Jena University Hospital, 07743 Jena, Germany b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 December 2013 Received in revised form 14 March 2014 Accepted 16 March 2014 Available online xxx

Purpose: To test whether firstly, different parental rearing components were associated with different dimensions of psychiatric symptoms in adulthood, secondly BDNF-Val66Met polymorphism moderated this association and thirdly, this association was due to genetic confounding. Method: Perceived parental rearing according to Parental Bonding Instrument (PBI), psychiatric symptoms evaluated with the Brief Symptom Inventory (BSI) and the BDNF-Val66Met polymorphism were analyzed in a sample of 232 adult twins from the general population. Results: In the whole sample, paternal care was negatively associated with depression. Maternal overprotection was positively associated with paranoid ideation, obsession-compulsion and somatization. Gene-environment interaction effects were detected between the BDNF-Val66Met polymorphism and maternal care on phobic anxiety, paternal care on hostility, maternal overprotection on somatization and paternal overprotection also in somatization. In the subsample of MZ twins, intrapair differences in maternal care were associated with anxiety, paranoid ideation and somatization. Conclusions: Met carriers were, in general, more sensitive to the effects of parental rearing compared to Val/Val carriers in relation to anxiety and somatization. Contra-intuitively, our findings suggest that high rates of maternal care might be of risk for Met carriers regarding anxiety. Results from analyses controlling for genetic confounding were in line with this finding. ß 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Parental rearing Psychiatric symptoms BDNF-Val66Met polymorphism Twins General population Gene-environment interaction

1. Introduction Converging research claims that in the absence of supportive care, stressors experienced during sensitive developmental periods such as childhood can leave permanent imprints in the neural substrate of emotional and cognitive processes [26]. Furthermore, inadequate parenting can become a major risk for developing physical and psychological health problems. Numerous

* Corresponding author. Unitat d’Antropologia, Dep. Biologia Animal, Facultat Biologia, Universitat de Barcelona, Avenue Diagonal 643, 08028 Barcelona, Spain. Tel.: +34 93 402 1461; fax: +34 93 403 5740. ˜ ana´s). E-mail address: [email protected] (L. Fan 1 These authors contributed equally to this work.

studies have linked parental rearing, personality traits and psychiatric symptoms in adulthood [9,23,31,33]. However, the mechanisms underlying how early experiences, including interaction with caregivers, may have long-term consequences on adult mental health remain largely unclear [33]. Why some parental rearing styles can have noxious effects in children behavior and mental health while other may have favourable effects is still not fully understood. It is plausible that individual genetic variation may be related to differential response to the same environmental factor such as parenting [44]. In this regard, a genetic variant likely to play a role in individual variation in response to parental rearing is the BDNF-Val66Met polymorphism. The Brain-Derived Neurotrophic Factor (BDNF) is a neurotrophin that promotes the growth and differentiation of developing neurons in central and peripheral nervous systems.

http://dx.doi.org/10.1016/j.eurpsy.2014.03.001 0924-9338/ß 2014 Elsevier Masson SAS. All rights reserved.

Please cite this article in press as: Ibarra P, et al. The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study. European Psychiatry (2014), http://dx.doi.org/10.1016/j.eurpsy.2014.03.001

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BDNF is also implicated in the survival of neuronal cells in response to stress [14]. It has been shown that early stress can influence BDNF expression and produce long-lasting effects on neurotrophic processes, thereby impacting on neuronal maturation and plasticity in later life [18,49]. In humans, the BDNF gene, located at chromosome 11p14, contains a functional polymorphism which consists of a methionine (Met) substitution for valine (Val) at codon 66 (Val66Met) [14]. The Val variant is associated with higher neuronal BDNF secretory activity than is the Met variant. Additionally, the co-expression of Val and Met alleles in heterozygotes results in less efficient intracellular trafficking and processing, leading to decreased BDNF secretion [15]. Previous research indicates that Met carriers who have experienced childhood adversity could also be more genetically vulnerable to the development of affective symptoms and psychotic experiences, in comparison to Val homozygotes [1,2]. Furthermore, recent research indicates that the BDNF-Val66Met polymorphism moderates the effects of parental rearing on adult personality dimensions [47,48]. However, up to our knowledge, whether this polymorphism may modulate the influence of parental rearing on the development of psychiatric symptoms in adulthood has not been examined yet. Also, although inadequate parenting constitutes a recognized environmental risk factor for psychopathology, whether the association between parenting quality and negative outcomes is likely causal or merely reflects gene-environment correlation or interaction remains to be examined. For this reason, it is of great interest to clarify whether parental rearing may have an environmental impact per se on the expression of adult psychiatric symptoms, controlling for genetic confounding. In this context, the monozygotic (MZ) twin differences approach has been referred to as a strong test of the unique environmental experiences that make family members different from each other independently of genetic components [11,39,50]. Considering MZ twins are, nearly always, identical at the DNA sequence level [7], phenotypic differences observed between MZ twins must be explained by differential exposure to environmental factors. Thus, if differences in the expression of subclinical psychiatric symptoms in MZ twins were associated with differences regarding parental rearing, this approach would indicate that this association is not solely due to genetic confounding. The present study aimed to examine:  whether different parental rearing components were associated with different dimensions of psychiatric symptoms in adulthood;  whether the BDNF-Val66Met polymorphism moderated this association;  whether this association was due to genetic confounding.

2. Subjects and methods

interviewed face-to-face for personal medical records (S.A. and X.G.). All subjects were from Caucasian origin. Written informed consent was obtained from all participants after a detailed description of the study aims and design, approved by the local Ethics Committee. Further details of the sample and recruitment can be found elsewhere [3]. All procedures were carried out in accordance with the declaration of Helsinki. 2.2. Measures 2.2.1. Parental rearing Parental rearing was assessed by means of the Parental Bonding Instrument (PBI) [25,37]. This widely used self-reported questionnaire constituted by 25 items was designed to assess the perceived parental rearing attitudes during the first 16 years. The PBI includes two factors, care and overprotection. Care dimension is composed by 12 items assessing positive feelings such as love, care and empathy (e.g., ‘‘My mother spoke to me in a warm and gentle voice’’). The overprotection factor consists of 13 items referring to restraint and controlling behaviors (e.g., ‘‘My father tried to control everything I did’’). Participants were requested to answer each item on a Likert scale of 4 points from ‘‘very often’’ to ‘‘almost never’’. The participants in this study had to respond two questionnaires, one regarding the relationship with the mother, and the other one, regarding the relationship with the father. Four scores are obtained from each subject, maternal and paternal care (ranging from 0–39) and maternal and paternal overprotection (ranging from 0–36). The present study used a continuous score for each parental rearing component. The PBI has been shown to have a high reliability and validity [35,36,45]. Furthermore, long-term stability of the PBI has been also reported [34,51]. Of note, the cut-offs to classify ‘‘high’’ and ‘‘low’’ parental measures are reported to facilitate interpretation of the data reported. The established cut-offs according to Parker et al. [37] are: 27 and 24 for maternal and paternal care respectively and 13.5 and 12.5 for maternal and paternal overprotection respectively. 2.2.2. Psychiatric symptoms assessment Psychiatric symptoms were assessed using the Brief Symptom Inventory (BSI) [17,43], which focuses in the experience of different types of psychiatric symptoms in the last 30 days. This self-administered dimensional scale is a screening instrument designed to identify common psychiatric symptoms. In the present study, we used the Spanish validated version of the BSI, which includes 46 items grouped into six subscales:      

depression; anxiety; paranoid ideation; obsession-compulsion; somatization; hostility [43].

2.1. Sample The sample consisted of 232 Spanish adult twins from the general population who gave permission to be contacted for research purposes (92 MZ twin pairs and 24 DZ twin pairs). Identified twin pairs were first contacted by telephone and invited to participate. Exclusion criteria applied included age under 17 and over 65, a medical history of neurological disturbance, presence of sensory or motor alterations and current substance misuse or dependence. A battery of psychological and neurocognitive tests were administered to the twins by trained psychologists (S.A. and X.G.). Likewise, they were

The questionnaire was conceived to measure psychiatric symptoms from a dimensional perspective and designed to be used both in clinical and non-clinical population. Each item of the BSI is rated on a 5-point scale of distress ranging from ‘‘not at all’’ to ‘‘extremely’’, according to autoperception of symptoms through items like ‘‘Never feeling close to another person’’ or ‘‘Suddenly scared for no reason’’ among others. 2.2.3. DNA analysis Genomic DNA was extracted from peripheral blood cells using the Real Extraction DNA Kit (Durviz S.L.U., Valencia, Spain), or from

Please cite this article in press as: Ibarra P, et al. The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study. European Psychiatry (2014), http://dx.doi.org/10.1016/j.eurpsy.2014.03.001

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buccal mucosa on a cotton swab using the BuccalAmp DNA Extraction Kit (Epicentre1 Biotechnologies, Madison, WI). Zygosity was determined using a standard zygosity questionnaire (administered to the mothers), which has been shown to have 93% accuracy [42]. Furthermore, zygosity was confirmed for most of the sample using DNA markers. Molecular zygosity was established genotyping 16 loci: 15 short tandem repeat (STR) loci and amelogenin, the gender determining marker. The PowerPlex1 16 System (Promega Corporation) allowed the co-amplification and three-color detection of 16 loci. Twins with only one divergent allele were genotyped a second time to limit the scope for genotyping error. Identity on all the markers can be used to assign zygosity with greater than 99% accuracy [40]. The Single Nucleotide Polymorphism rs6265 (Val66Met) of the BDNF gene was determined using an Applied Biosystems Taqman 50 -exonuclease assay. The final volume of the PCR reaction was 5 mL, which contained 10 ng of genomic DNA, 2.5 mL of TaqMan Master Mix, and 0.125 mL of 40  genotyping assay. The cycling parameters were as follows: 95 8C for 10 min followed by 40 cycles of denaturation at 92 8C for 15 s and annealing/extension at 60 8C for 1 min. Polymerase chain reaction plates were read on an ABI PRISM 7900HT instrument with SDS v2.1 software (Applied Biosystems). Due to the low frequency of Met/Met genotype carriers, BDNF genotype was included as a binary variable in the analyses [Met allele carriers (Met/Met + Val/Met) vs. Val homozygotes (Val/Val)]. 2.3. Statistical analysis For the first and second objectives, testing the association between parental rearing and psychiatric symptoms dimensions and a possible gene-environment interaction with the BDNFVal66Met polymorphism, multiple regression models were conducted using the whole sample. Psychiatric symptoms dimensions (i.e. depression, anxiety, paranoid ideation, obsession-compulsion, somatization and hostility) were tested in separated models and constituted the outcomes of the study. The independent variables for main and interactions effects models were: maternal care, maternal overprotection, paternal care, paternal overprotection and BDNF genotypes (i.e. Met carriers and Val/Val). In the interaction effects models, four two-way interaction terms were also included: maternal care*BDNF genotypes, maternal overprotection*BDNF genotypes, paternal care*BDNF genotypes and paternal overprotection*BDNF genotypes. Sex and age were included as covariates in all analyses. If a gene-environment interaction was detected, the effect size was calculated using eta squared (h2). This parameter can be used to estimate the proportion of variance in the outcome that is accounted for by the predictor. The log-likelihood ratio test was used to assess the difference in model fit between main effects and interaction effects models. In this case, if a significant interaction effect was detected, the log-likelihood ratio test was used to examine whether the addition of the interaction term significantly improved the model fit compared to the main effects model. The non-independence of clustered twin data was corrected for by using tests based on the sandwich or Huber/White variance estimator [52]. Of note, in these analyses, the individual was the unit of analysis. The third objective, testing whether the association between parental rearing components and psychiatric symptoms is due to genetic confounding, was tested in a subsample of MZ twins. Of note, only twins with confirmed molecular zygosity were included in these analyses leaving a final subsample of 86 MZ twin pairs. As a preliminary step, MZ intrapair correlations were calculated for parental rearing and psychiatric symptoms. These analyses were necessary in order to confirm that MZ twins differed in their

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exposure and perception of parental rearing (despite being reared together) and their scores for psychiatric symptoms. Intrapair correlations different from 1 indicate that twins are not identical for the variables studied. Specifically, the proportion of the variance of the phenotype which can be directly attributable to unique environment (i.e. environmental influences nonshared by twins that contribute to make them different) can be obtained through this formula: 1  rMZ, where r represents the within-pair correlation [41]. Finally, associations between intrapair differences in parental bonding and psychiatric symptoms were analyzed by linear regression analyses. Intrapair scores were calculated for parental bonding subscales and psychiatric symptoms dimensions by subtracting the score of the Twin 2 from the score of Twin 1 (Twin 1  Twin 2). Twins were randomly assigned to be 1 or 2. Because intrapair analyses in MZ twins fully control for genetic influences, any association between the abovementioned variables would be only attributable to environmental factors [39], and thus, would allow rejecting the hypothesis that the association is due to genetic confounding. In the last two analyses each MZ twin pair was the unit of analysis. Statistical analyses were carried out using STATA 10.0 [46] following the procedures described in Carlinet al. [10]. In addition to the standard statistical significance threshold of P < 0.05, since the current study included multiple outcomes and, consequently, multiple tests needed to be conducted, we also consider correcting for multiple comparisons by calculating a more stringent P-value. Six regressions, one for each outcome (i.e. depression, anxiety, paranoid ideation, obsession-compulsion, somatization and hostility), were conducted to test each hypothesis, thus the P-value based on Bonferroni correction is P = 0.05/ 6 = 0.0083.

3. Results 3.1. Descriptive statistics The whole sample (n = 232 subjects) was mainly composed by females (67%), and the average age was 33.8 years (SD = 13.2). Around 58% of the subjects had completed university educational level, 25.7% had completed high school, and the rest had completed elementary school. Regarding to birthplace, 58.9% were born in rural areas and 41.2% were from urban origin. Most of the sample (65.3%) had an average socioeconomic status. Means and standard deviations for the PBI subscales and psychiatric dimensions can be found in Table 1 for the whole sample and the MZ twin pairs subsample. As expected, these descriptive results were very similar among the whole sample and the MZ subsample (Table 1). One subject was removed due to outlier scores and two subjects have missing data for the variables of interest leaving a final sample of 229 twins. Of note, as above mentioned, the individual was the unit of analysis to test the first aim so that it was possible to carry out the analyses despite the fact one member of a twin pair was excluded. The genotype frequencies for the BDNF-Val66Met polymorphism in the final sample were: Val/Val: 58.1% (n = 133); Val/Met: 40.2% (n = 92); and Met/Met: 1.7% (n = 4). From these 229 individuals, 183 were MZ (103 Val/Val, 76 Val/Met and 4 Met/Met) and 46 were DZ (30 Val/Val and 16 Val/Met) with 19 concordant pairs and 4 discordant pairs among the DZ twins. These frequencies are similar to others described in previous studies based on Caucasian samples with analogous characteristics [2,18]. One subject from each twin pair was randomly selected to estimate Hardy-Weinberg equilibrium, which was verified for the present sample (x2 = 2.45, df = 2, P = 0.29).

Please cite this article in press as: Ibarra P, et al. The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study. European Psychiatry (2014), http://dx.doi.org/10.1016/j.eurpsy.2014.03.001

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Table 1 Sample characteristics for demographic, parental rearing components (PBI) and psychiatric symptoms dimensions (BSI) at the whole sample and at the MZ twins sample. Whole sample (MZ + DZ pairs)

MZ twins subsample (pairs)

Subjects Gender distribution (M/F) Age in years, mean (SD)

232 (92 + 24) 76/156 33.8 (13.2)

184 (92) 62/122 34.3 (13.1)

Parental rearing (PBI), mean (SD) Maternal care Paternal care Maternal overprotection Paternal overprotection

27.2 25.3 12.6 12.3

27.1 25.0 12.5 12.2

Psychiatric symptoms (BSI), Mean (SD) Depression Phobic anxiety Paranoid Ideation Obsession-compulsion Somatization Hostility/aggressivity

4.2 1.3 4.2 5.0 3.4 1.0

(7.1) (6.2) (6.0) (7.1)

(4.4) (2.4) (4.0) (5.0) (3.9) (1.6)

4.6 1.4 4.5 5.3 3.6 1.0

(7.3) (6.5) (6.0) (7.2)

(4.6) (2.6) (4.1) (5.2) (4.1) (1.7)

M: male; F: female; SD: standard deviation.

3.2. Parental rearing and adult psychiatric dimensions Paternal care was negatively associated with depression (b = .17; SE = .08; P = .040) while maternal overprotection was positively associated with paranoid ideation (b = .14; SE = .05; P = .010), somatization (b = .14; SE = .05; P = .010) and hostility symptoms (b = .04; SE = .02; P = .007). Only the last association remains significant after correcting for multiple comparisons. There were no other significant associations between parental components and psychiatric symptoms dimensions and there was no significant main effect of the BDNF-Val66Met polymorphism (Table 2). 3.3. Gene-environment interaction effects between BDNF-Val66Met polymorphism and parental rearing components Four significant gene-environment interaction effects were detected (Table 2). First, the BDNF-Val66Met polymorphism showed to moderate the association between maternal care and anxiety (b = .16; SE = .06; P = .013). Maternal care significantly affects the expression of anxiety in Met carriers. That is, at high rates of maternal care, the Met carriers present significantly more anxiety compared to Val/Val carriers; however, at low rates of maternal care, the anxiety scores of Met carriers were significantly lower compared to Val/Val carriers (Fig. 1a). This interaction effect accounted for 0.1% (h2 = .001) of anxiety variation and did not

result in a significant improvement of the model fit (x2 = 8.6; df = 4; P = .072). Second, an interaction was found between the BDNF-Val66Met polymorphism and maternal overprotection on somatization (b = .21; SE = .09; P = .032). Met carriers presented significantly more somatization when reporting high rates of maternal overprotection compared to Val/Val genotype carriers (Fig. 1b). This interaction accounted for 1.1% (h2 = .011) of the variance of somatization. Third, the BDNF-Val66Met polymorphism significantly interact with paternal overprotection in the development of somatization symptoms (b = .21; SE = .07; P = .004). This interaction indicated that at high rates of paternal overprotection Met carriers presented more somatization compared to Val homozygotes. This interaction effect remains significant after correcting for multiple comparisons. This effect accounted for 6.7% (h2 = .067) of somatization variance. The interaction effects significantly improved the model fit for somatization (x2 = 16.2; df = 4; P = .003). Finally, an interaction was found between the polymorphism analyzed and paternal care on hostility symptoms (b = .09; SE = .04; P = .023). Paternal care has opposite effects on the presence of hostility symptoms depending on the BDNF-Val66Met polymorphism genotypes. Met carriers reporting high rates of paternal care presented significantly lower hostility symptoms, while Val/Val carriers presented an opposite pattern. Only in this interaction the scores for psychiatric symptoms of Val homozygotes vary as a function of the parental measure. This interaction accounted for 4.4%

Table 2 Analysis of associations between parental bonding components and the BDNF genotype on dimensions of psychiatric symptoms in the whole sample (n = 229) showing the main effects and the interaction effects. Phobic anxiety

Depression

Paranoid ideation

Obsessioncompulsion

Somatization

Hostility

b

SE

P

b

SE

P

b

SE

P

b

SE

P

b

SE

P

b

SE

P

Main effects Care (M) Care (F) Overprotection (M) Overprotection (F) BDNF genotypes

.01 .17 .06 .02 .08

.05 .08 .07 .04 .57

.731 .040 .369 .635 .879

.03 .08 .03 .02 .19

.03 .04 .03 .02 .34

.351 .084 .255 .384 .566

.01 .11 .14 .00 .26

.05 .08 .05 .05 .54

.731 .164 .010 .872 .634

.11 .12 .09 .02 .37

.06 .08 .07 .05 .67

.870 .134 .172 .643 .575

.01 .07 .14 .02 .06

.06 .08 .05 .04 .54

.785 .358 .010 .562 .909

.00 .01 .05 .00 .06

.01 .02 .02 .01 .18

.612 .621 .007* .802 .708

Interaction effects Care (M)* BDNF Care (F)* BDNF Overprotection (M)* BDNF Overprotection (F)* BDNF

.07 .03 .13 .15

.10 .15 .13 .09

.503 .818 .340 .113

.16 .16 .06 .03

.06 .08 .06 .05

.013 .059 .326 .474

.12 .06 .02 .12

.09 .12 .10 .08

.219 .622 .794 .148

.07 .07 .09 .19

.12 .14 .13 .10

.531 .591 .477 .060

.10 .16 .21 .21

.11 .14 .09 .07

.392 .238 .032 .004*

.05 .09 .01 .01

.03 .04 .04 .02

.158 .023 .748 .592

Significant associations are indicated in bold. Significant associations that remained significant after correcting for multiple testing are indicated with an asterisk. M: mother; F: father; b: unstandardized coefficient; SE: standard error.

Please cite this article in press as: Ibarra P, et al. The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study. European Psychiatry (2014), http://dx.doi.org/10.1016/j.eurpsy.2014.03.001

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Fig. 1. Graphical representations of the observed gene-environment interaction effects found. The x-axis indicates variation in the environmental factor (parental rearing dimensions) from lower to higher; the y-axis indicates the outcome (psychiatric symptoms dimensions) from negative to positive. Genotypic frequencies for the BDNFVal66met polymorphism were 133 subjects Val/Val and 96 Met carriers (n = 229). Upper graphs (a and b) represent the interaction between the BDNF-Val66Met genotypes and maternal care and maternal overprotection on phobic anxiety and somatization symptoms respectively. Lower graphs (c and d) represent the interaction between the BDNF-Val66Met genotypes and paternal overprotection and paternal care on somatization and hostility symptoms respectively. Of note, only interaction (c) remained significant after correcting for multiple testing.

(h2 = .044) of the hostility variance and resulted in a significant improvement of the model fit (x2 = 10.3; df = 4; P = .035). 3.4. A MZ twin differences approach: analyzing the unique environmental effects As above mentioned, in order to confirm that MZ twin pairs differ for the variables of interest, within-pair correlations at the MZ subsample (n = 86 MZ twin pairs) were conducted to index the similarity for the variables of interest. The within-pair correlations for the subscales of the BSI were: depression (r = .45; P < 0.001), anxiety (r = .39; P < 0.001), paranoid Ideation (r = .42; P < 0.001), obsession-compulsion (r = .44; P < 0.001), somatization (r = .48; P < 0.001) and hostility (r = .25; P = .018). In regard to parental rearing variables, the within-pair correlations for maternal and paternal care were r = .60 (P < .001) and r = .61 (P < .001) respectively, and the within-pair correlations for maternal and paternal control were r = .52 (P < .001) and r = .54 (P < .001) respectively. Thus, the intrapair correlations confirmed that twins were not identical for any of the measures of the study

since around 52–75% of the variance of BSI dimensions and around 39–48% of the variance of PBI components can be attributable to unique environmental effects. Tests for associations between intrapair differences for BSI and PBI measures revealed that MZ differential exposure to some components of parental rearing was associated with phenotypic differences in the expression of several adult psychiatric symptoms. Specifically, intrapair differences in maternal care were positively associated with differences in anxiety (b = 0.13; SE = 0.06; P = 0.031), paranoid ideation (b = 0.20; SE = 0.09; P = 0.028) and somatization (b = 0.18; SE = 0.09; P = 0.037) within MZ twin pairs. These results indicate that the twin exposed to higher rates of maternal care reported higher scores for the indicated psychiatric symptoms dimension. Also, intrapair differences for paternal care were negatively associated with intrapair differences in paranoid ideation (b = 0.21; SE = 0.10; P = 0.030), indicating that the twin exposed to higher rates of paternal care reported lower scores for paranoid ideation compared to his/her co-twin (Table 3). However, none of these results survive the Bonferroni multiple comparisons P-value adjustment.

Please cite this article in press as: Ibarra P, et al. The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study. European Psychiatry (2014), http://dx.doi.org/10.1016/j.eurpsy.2014.03.001

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Table 3 Associations between intrapair scores (twin 1–twin 2) for parental bonding components of care and overprotection and intrapair scores for the different dimensions of psychiatric symptoms in a subsample of MZ twin pairs (n = 86 pairs). Intrapair depression

Intrapair Intrapair Intrapair Intrapair

care (M) care (F) overprotection (M) overprotection (F)

Intrapair s omatization

Intrapair obsessioncompulsion

Intrapair paranoid ideation

Intrapair phobic anxiety

Intrapair hostility

b

SE

P

b

SE

P

b

SE

P

b

SE

P

b

SE

P

b

SE

P

.14 .07 .06 .00

.11 .12 .13 .10

.189 .556 .651 .994

.13 .09 .08 .08

.06 .07 .07 .06

.031 .176 .278 .167

.20 .21 .10 .16

.09 .10 .10 .08

.028 .030 .319 .066

.18 .16 .09 .08

.12 .13 .14 .11

.138 .228 .498 .486

.18 .12 .17 .10

.09 .09 .10 .08

.037 .216 .089 .218

.07 .02 .07 .02

.04 .05 .05 .04

.110 .609 .166 .555

Significant associations are indicated in bold. M: mother; F: father; b: unstandardized coefficient; SE: standard error.

4. Discussion Largely in agreement with existent evidence examining the consequences of childrearing practices, we found associations between different parental rearing components and adult psychiatric symptoms [20,23,28,31]. According to previous findings, we showed that the parent-child relationships could be a context, which can ultimately contribute to prevent or promote the development of psychiatric symptoms in adulthood [31,33]. Paternal care was associated with lower scores of depression symptoms, showing a protective effect for these symptoms. This is in line with previous research that indicates that high quality paternal care may be a protective factor for healthy males [47,48], thus, our findings contribute to extend this protective role of paternal care to females. Furthermore, differential effects of care depending on the gender of the parent have been previously suggested [29]. In this regard, care coming from the mother is usually promoted by cultural stereotypes; while the role of the father is frequently characterized by other sort of qualities related with control and security [9] and fathers are expected to spend less time with the child than do mothers [8]. Hence, it is possible to suggest that when the father is capable of delivering overtly care, empathy and closeness (qualities usually attributed to the mother [6]) above the average or the traditional cultural standards, this affection may be perceived by the child as a supplementary doses of care, thus receiving unexpected benefits in terms of self-esteem and confidence, which has been considered protective features by previous studies [16]. Concerning to the associations found in the present study between maternal overprotection and paranoid ideation, somatization and hostility symptoms, it should be noted that overprotective behavior, characterized by excessive contact, childishness and prevention of independent behavior, is vital to guarantee infant’s survival in early stages of their development. However, when caregivers fail to adequately adapt throughout years to changes in child capabilities and increase of autonomy, parental overprotective behaviors become noxious and ultimately may contribute to the development of psychopathological symptoms [28,31]. We also found evidence indicating gene-environment interaction effects. In general, Met carriers of the BDNF-Val66Met polymorphism showed to be more sensitive to parental rearing compared to Val/Val homozygotes. As abovementioned, Met allele carriers present less efficient intracellular trafficking and processing, leading to decreased BDNF secretion, compared to Val homozygotes [18] and several studies indicate early stress like childhood adversity impacts more strongly on adult depressive symptomatology in Met carriers compared to Val homozygotes [1,21]. Our results are in line with these findings but we also observed that Met carriers were, in general, more sensitive to the effects of parental rearing than Val homozygotes. This is in keeping with Suzuki et al. results on personality traits [48] and also with

the emerging idea of plasticity genes, which would lead some individuals to be more genetically susceptible than others to both negative and positive environmental influences [4]. This process has also been reported for other genes such as the catechol-Omethyltransferase (COMT) gene in previous studies examining the relationship between child maltreatment and cognition in this same sample [24]. However, in the case of the relationship between paternal care and hostility symptoms, Val/Val homozygotes were also sensitive to the effects of paternal care. Indeed, the effects of paternal care on hostility were in opposite directions for Met carriers and Val homozygotes. Val/Val carriers presented higher scores on hostility at high rates of paternal care. This was an unexpected result since in the other three interactions found the Val/Val genotype group did not present variations from low to high parental care and overprotection. Previous studies have shown that Val/Val carriers can be vulnerable to detrimental effects of environmental factors like childhood maltreatment [38] and life events [13]. Also, it has been pointed out that Met carriers functioned better, at an emotional level, than Val/Val homozygotes under any or low early adversity conditions [27]. Thus, whether high rates of paternal care may have negative consequences for the development of hostility symptoms in Val homozygotes requires further investigation. Regarding to the effects of high maternal care exposure, Met carriers showed higher scores in anxiety symptoms compared to Val/Val homozygotes, suggesting that high maternal care may have negative effects on anxiety symptoms in Met carriers. This is in contrast to previous research [48] although this inconsistency can be explained, at least partially, by the fact that we examined a different outcome. Interestingly, a recent study in mice suggests that maternal care may not always be beneficial [30]. In the study, high rates of maternal care were associated with low levels of neurogenesis. Authors suggest that high maternal care might be beneficial only when necessary, but extra care might be inadequate or have undesirable consequences on offspring development and behavior since it would not help to improve how offspring adapt to environment [30]. Finally, our results based in a MZ twin differences approach suggest that both maternal and paternal care may have a genuine environmental influence on the development of anxiety, paranoid ideation and somatization symptoms. Up to our knowledge, only Cecil and colleagues [12] have explored the effect of parenting on children self-control using a MZ twin differences approach applied to a longitudinal sample. Authors concluded that early parenting was an important nonshared environmental influence in the development of self-control differences in children [12]. Our study also provides support for a unique environmental effect of maternal and paternal care independent of genetic confounding. Specifically, we found that high levels of maternal care were associated with anxiety, paranoid ideation and somatization symptoms while high levels of paternal care were negatively

Please cite this article in press as: Ibarra P, et al. The BDNF-Val66Met polymorphism modulates parental rearing effects on adult psychiatric symptoms: A community twin-based study. European Psychiatry (2014), http://dx.doi.org/10.1016/j.eurpsy.2014.03.001

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associated with paranoid ideation within MZ twin pairs, thus, ruling out genetic effects. Thus, paternal care remained showing protective qualities on the likelihood of developing psychiatric symptoms in adulthood. However, the twin reporting higher rates of maternal care, also scored higher in anxiety, paranoid ideation and somatization symptoms compared to his/her co-twin. Thus, it seems that, independently of genetic confounding, high levels of maternal care can be a risk factor for these symptoms. As abovementioned, this putative risk linked to maternal care is counterintuitive and contradicts previous studies that have showed detrimental consequences on child mental health when maternal care is insufficient or inconsistent and, conversely, beneficial effects when the mother is sensitive and affectionate [31,33]. Therefore, these findings need further research and replication. The current study has several limitations. First, the sample size was modest. Second, if stringent Bonferroni P-value adjustments for multiple testing were applied, only one association and one geneenvironment interaction effect would remain significant while none of the associations found when correcting for genetic confounding would survive the multiple comparisons test. This indicates that the current findings need replication and should be interpreted considering the limitations of the study. That said, although Bonferroni is the classical method of adjusting P-value, it is often considered to be overly conservative, especially in analyses correcting for clustered data such as those from the first two aims of this study [5,19,22]. Furthermore, other aspects of the results such as the methodology applied and the effect sizes should be considered regardless of the P-value adjustments [19]. Third, although correcting for education level, urbanicity and SES would have been appropriate due to the potential effect of these factors on both psychiatric symptoms and parental style, the valid sample drastically decreased when including such covariates. Therefore, results were only corrected by sex and age. Third, several considerations need to be mentioned regarding the PBI measures. The retrospective measure of parental style may constitute an inherent source of bias. Also, responses are subject to the individual perception and interpretation of the individuals which may not fully reflect the actual parental style received [32]. Furthermore, characteristics of the children such as personality traits can elicite certain parenting styles, thus, reverse causality cannot be totally discarded. Nevertheless, as abovementioned the validity, reliability and stability of the PBI have been demonstrated [34,35,45,51]. Fourth, the outcome investigated, adult psychiatric symptoms, was assessed using only one instrument. Fifth, our results must be interpreted with caution, because environmental influence, excluding the genes, cannot be exclusively arrogated to the parental influence, thus many others possibilities like particular experiences, sibling interactions or extrafamilial influences among others such as epigenetic mechanisms should be target of further studies.

5. Conclusion This study highlights the protective effect of paternal care regarding the development of adult psychiatric symptoms and suggests that high levels of maternal care may not be protective under certain circumstances. Furthermore, the BDNF-Val66Met polymorphism showed to have a moderating role in the abovementioned association being, in general, Met carriers more sensitive to the effects of parental rearing styles compared to homozygotes of the Val allele. Interestingly, using a MZ twin differences methodology, an environmental effect from maternal and paternal care was detected suggesting that parental rearing may have an effect on adult psychopathology independently of the individual genetic background.

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