Neuropeptide Y, stressful life events and personality trait conscientiousness: Preliminary associations from a Swedish longitudinal study

Psychiatry Research 263 (2018) 48–53

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Neuropeptide Y, stressful life events and personality trait conscientiousness: Preliminary associations from a Swedish longitudinal study

T

⁎

Philippe A. Melasa,b, , Peter Gubanc, Md Shafiqur Rahmand, Catharina Lavebrattb,e, Yvonne Forselld a

Department of Clinical Neuroscience, Karolinska Institutet, CMM L8:00, Karolinska University Hospital, 17176 Stockholm, Sweden Center for Molecular Medicine, Karolinska University Hospital, Stockholm, Sweden c Center for Epidemiology and Community Medicine, Stockholm County Council, Stockholm, Sweden d Department of Public Health Sciences, Karolinska Institutet, Stockholm, Sweden e Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden b

A R T I C L E I N F O

A B S T R A C T

Keywords: Neuropeptide Brain Personality Stress Personality genetics Personality biology Big Five

The heritability of the Five-Factor Model (FFM) of human personality is high, but few genes have been identified to underlie FFM traits. Neuropeptide Y (NPY) is a pleiotropic gene implicated in stress resilience that contains two well-studied functional SNPs: (1) rs16147, which lies in the NPY promoter and affects expression levels, and (2) rs16139, which lies in the coding sequence of NPY's precursor peptide, pre-pro NPY, and affects precursor processing. In the present study we examined whether these two polymorphisms are associated with FFM traits, using a Swedish cohort (rs16147, N = 2113; and rs16139, N = 1971), and found a significant association with rs16139. Specifically, the minor G-allele of the SNP, which encodes proline instead of leucine and leads to higher processing of pre-pro NPY into mature NPY, was associated with higher levels of conscientiousness. Next, we looked at exposure to life adversities, both in childhood and adulthood, and found that stressful life events were significantly associated with reduced levels of conscientiousness. These data provide insights into the neurobiology of human personality. However, given the difficulty in replicating genetic and environmental associations with behaviorally complex traits, these findings should be considered preliminary and warrant replication in additional cohorts.

1. Introduction The Five-Factor Model (FFM), also referred to as the Big Five, uses five major traits to measure and describe human personality: 1) openness (to experience), 2) conscientiousness, 3) extraversion, 4) agreeableness, and 5) neuroticism. All five FFM traits are known to be influenced by both genes and environment. Twin studies have shown that the heritability estimates of FFM range between 45% and 61% for openness, 38–53% for conscientiousness, 49–57% for extraversion, 33–52% for agreeableness, and 41–58% for neuroticism (Bouchard and McGue, 2003), whereas a recent family study found that the heritability estimates for openness, conscientiousness, extraversion, agreeableness and neuroticism were 49%, 30%, 32%, 18% and 25%, respectively (Bae et al., 2013). Genome-wide association (GWA) studies have been conducted in search of single-nucleotide polymorphisms (SNPs) that may underlie FFM traits. However, the reproducible identification of common genetic variants has largely remained elusive, suggesting that variants of small effect sizes or low frequency may contribute to FFM ⁎

traits (Power and Pluess, 2015). In addition, a recent GWA study for personality traits showed correlations with psychiatric disorders (Lo et al., 2017) suggesting that certain genetic variations may be shared between FFM traits and psychopathology. Neuropeptide Y (NPY) is one of the most abundant peptides expressed in numerous brain regions, as well as in the periphery. NPY, being a pleiotropic gene, is known to influence a number of phenotypic traits including vasoconstriction, immune functions, metabolism, feeding and reproductive behavior (Hirsch and Zukowska, 2012; Muroi and Ishii, 2016). In the neuropsychiatric field, NPY has been implicated in mood regulation, emotional processing and stress resilience. For example, low levels of NPY have been associated with psychiatric disorders including anxiety, depression and posttraumatic stress disorder (PTSD) in both preclinical and clinical settings (Cohen et al., 2012; Heilig et al., 2004; Melas et al., 2013, 2012; Mickey et al., 2011; Sah and Geracioti, 2013; Zhou et al., 2008). The human NPY gene contains two well-studied functional SNPs. The first of these SNPs, rs16147 (T/C), lies in the promoter region of the

Corresponding author at: Department of Clinical Neuroscience, Karolinska Institutet, CMM L8:00, Karolinska University Hospital, 17176 Stockholm, Sweden. E-mail address: [email protected] (P.A. Melas).

https://doi.org/10.1016/j.psychres.2018.02.041 Received 22 July 2017; Received in revised form 23 November 2017; Accepted 18 February 2018 Available online 21 February 2018 0165-1781/ © 2018 Elsevier B.V. All rights reserved.

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long periods with economic difficulties in the family, or (d) there were long periods of family friction at home. A summed up continuous score (i.e. 0 − 4) was used in the analyses, similar to previous genetic epidemiological studies (Liu et al., 2015; Rahman et al., 2017). Second, we scored adult adversities, i.e. SLEs that occurred within 12 months preceding the completion of the PART questionnaire at wave 1 and, separately, at wave 2. With regard to adult SLEs, information on 28 various stressful life events were considered, including separation with partner, conflict with family members/close relatives/friends, severe illness/death/accident of a spouse/child/relative/close friend, family member victimization, child having serious problem(s), serious problem/serious conflict at work, abortion, etc. Most of these questions have already been used in previous Swedish studies (Theorell et al., 1975; Theorell and Rahe, 1975), and the validity and reliability of these questionnaires have been tested (Rahe, 1972; Rahe et al., 1974). A summed up continuous score (i.e. 0 − 28) was used in the analyses. The Spearman's correlation coefficient for adulthood SLEs between wave 1 and wave 2 was 0.35 (P < 0.001). Finally, we also created a separate composite SLE variable by summing up childhood SLEs, and adulthood SLEs at wave 1 and at wave 2 (i.e. 0 − 60).

gene and the T-allele has been shown to account for an increase in NPY expression levels both in vitro and in vivo (Zhou et al., 2008). The second of these SNPs, rs16139, lies in the coding sequence of the NPY gene. Specifically, the coding sequence of NPY synthesizes a 97-aminoacid precursor peptide, pre-pro NPY, which needs to be cleaved and processed into the mature 36-amino-acid NPY peptide. The N-terminal region of pre-pro NPY contains a 28-amino-acid signaling peptide with the following sequence: MLGNKRLGLSGLTLALSLLVCLGALAEA. The leucine at position seven of the signaling peptide (Leu7; bold underlined L) can be substituted by a proline (Pro7) at the same position when rs16139, in the corresponding codon, changes from A to G. The minor G-allele of the SNP, encoding Pro7, has been shown to lead to a higher processing of pre-pro NPY into the mature NPY and has also been associated with higher NPY levels in blood plasma and cerebrospinal fluid, making it a functional genetic variant (Heilig et al., 2004; Kallio et al., 2001). In the present study, we first examined whether these functional SNPs in NPY are associated with FFM personality traits using genetic material from a Swedish cohort that is part of a longitudinal epidemiological study. In addition, since adverse life experiences have been associated with a) personality disorders (Battle et al., 2004; Pagano et al., 2004) and b) NPY-like immunoreactivity in the cerebrospinal fluid (Soleimani et al., 2014), we also examined putative interaction effects, between the NPY SNP showing significance (rs16139) and stressful life events, on personality trait scores.

2.4. DNA sampling and NPY genotyping During the period 2006–7, PART participants were requested to contribute DNA in the form of saliva using a whole-saliva collection device (Oragene•DNA sample collection kit; DNA Genotek Inc., Canada), as previously described (Melas et al., 2010; Sjoholm et al., 2009). The DNA participation rate was 54.6% (N = 3018). Genotyping of the rs16147 (− 399 T/C) SNP, in the NPY promoter region, was performed using a TaqMan SNP genotyping assay on an ABI 7900 HT instrument (Thermo Fisher Scientific, Waltham, MA, USA), and genotyping of the rs16139 SNP (+ 1128 A/G), encoding Leu7Pro of pre-pro NPY, was performed on a PSQ96™ instrument using the PSQ96™ SNP reagent kit (Qiagen, Hilden, Germany), as previously described (Sjoholm et al., 2009). Out of 2917 assayed samples, 2777 and 2568 participants with valid genotyped data on rs16147 and rs16139, respectively, were included in the study. The TT, CT and CC frequencies of rs16147 were 23.7% (N = 659), 50.5% (N = 1402) and 25.8% (N = 716), respectively. The AA (Leu7Leu), AG (Leu7Pro) and GG (Pro7Pro) frequencies of rs16139 were 93.8% (N = 2408), 6.2% (N = 158) and 1% (N = 2), respectively.

2. Methods 2.1. Participants Individuals used for the present genetic study were participants of the PART study. PART is a longitudinal epidemiological study of mental health, work and relations in adults (20 − 64 years) in Stockholm County, Sweden (Hällström et al., 2002). PART included three waves (wave I: 1998–2000, wave II: 2001–2003, and wave III: 2010–2011) and has investigated mental health and wellbeing on the basis of extensive self-reported questionnaires, registry data and subset psychiatric interviews. The response rates of wave I, wave II and wave III were 53% (N = 10,443), 84% (N = 8613) and 66% (N = 5650), respectively. The PART study has been approved by the ethical review board at the Karolinska Institutet and informed consent has been obtained from all participants. The study is in compliance with the Code of Ethics of the World Medical Association's (WMA) Declaration of Helsinki. 2.2. FFM assessment

2.5. Statistical analyses For FFM personality assessment, a Swedish translation of Schafer's FFM rating scales was used (Hochwalder, 2006; Schafer, 1999), which was part of PART's questionnaire in wave III only. Briefly, conscientiousness was characterized by orderliness, responsibility and dependability; openness (to experience) by originality, curiosity and ingenuity; extraversion by talkativeness, assertiveness and energy; agreeableness by good-naturedness, co-cooperativeness and trust; and neuroticism by emotional instability and being easily upset. The Schafer FFM rating scale has previously been validated against the 44 item-Big Five Inventory in a Swedish setting and was found to have satisfactory and stable psychometric properties (Hochwalder, 2006). FFM traits were scored on a continuous scale from 1 (low) to 9 (high), except for neuroticism that was scored from 1 (high) to 9 (low).

Differences in the sample were examined using Chi-squared tests for categorical variables and t-tests for continuous variables. Of the successfully genotyped participants for rs16147 and rs16139, 76.1% and 76.7%, respectively, had participated in PART wave III and had, thus, contributed with FFM trait data. The associations between NPY SNPs and FFM traits were assessed using Analyses of Variance (ANOVA). To study the effect of putative predictors on the FFM trait that turned out to be significant in the ANOVA test, we used a Linear regression model. The effects of (a) the NPY SNP that passed the P threshold of 0.05, (b) childhood SLEs, and (c) adulthood (wave 1 and wave 2) SLEs, were tested in relation to the conscientiousness trait controlling for age, gender and ethnicity. Both crude and adjusted models are reported. To assess cumulative effect of SLEs on conscientiousness levels, a composite SLE variable was used in the separate linear regression model including NPY SNP, and the model was adjusted for age, gender and ethnicity. The interaction effect of gene by SLEs was assessed entering the product term “NPY SNP*Composite Stressful Life Events” into the model. All statistical analyses were performed in SPSS version 22.0. Statistical tests were two-tailed and Alpha (α) was set at < 0.05.

2.3. Stressful life events (SLEs) All SLE data was collected using PART's self-reported questionnaires. First, we scored childhood adversities, i.e. SLEs that occurred before the age of 18. With regard to childhood SLEs, the following stressful incidents were considered: (a) the participant lost one or both parents, (b) the participant's parents divorced, (c) there were 49

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Table 1 Participant characteristics by NPY rs16147 (T/C) and rs16139 (A/G) genotypes. NPY rs16147 Characteristics Age; N (%) < 30 years 30 – 40 years 41 – 50 years > 50 years Gender; N (%) Male Female Education; N (%) < 12 years > 12 years Ethnicity; N (%) Born in Sweden Born outside of Sweden Childhood SLE; Mean (SD) SLE in wave 1; Mean (SD) SLE in wave 2; Mean (SD)

NPY rs16139

TT N = 659

CT N = 1402

CC N = 716

P

AA N = 2408

AG + GG N = 160

P

93 (14.1) 136 (20.6) 174 (26.4) 256 (38.8)

250 259 351 542

136 156 143 280

0.018

428 481 588 910

20 42 35 63

(12.5) (26.3) (21.9) (39.4)

0.12

266 (40.4) 393 (59.6)

561 (40) 841 (60.0)

316 (44.1) 400 (55.9)

0.17

966 (40.1) 1442 (59.9)

71 (44.4) 89 (55.6)

0.28

305 (46.3) 354 (53.7)

680 (48.6) 719 (51.4)

313 (43.7) 403 (56.3)

0.057

850 (43.8) 1091 (56.2)

61 (48.4) 65 (51.6)

0.31

601 (91.3) 57 (8.7) 0.26 (0.54) 1.96 (2.16) 1.65 (1.74)

1282 (91.5) 119 (8.5) 0.23 (0.52) 1.68 (1.72) 1.53 (1.59)

658 (91.9) 58 (8.1) 0.23 (0.53) 1.74 (1.91) 1.51 (1.55)

0.92

2194 (91.2) 212 (8.8) 0.24 (0.54) 1.80 (1.92) 1.59 (1.63)

148 (92.5) 12 (7.5) 0.27 (0.59) 1.57 (1.63) 1.48 (1.67)

0.56

(17.8) (18.5) (25.0) (38.7)

(19.0) (21.8) (20.0) (39.2)

0.41 0.006 0.29

(17.8) (20) (24.4) (37.8)

0.57 0.14 0.43

rs16139 AA: Leu7Leu; AG + GG: Leu7Pro + Pro7Pro. SLE: Stressful Life Events, SD: Standard Deviation, P: p-value.

3. Results

N

Mean

SD

P

482 1080 551

7.36 7.32 7.22

1.10 1.16 1.17

0.062b 0.12b

In the crude model, rs16139 and adulthood SLEs (wave 1 and wave 2) were found to be significantly associated with conscientiousness, while childhood SLEs showed a borderline association with conscientiousness. Next, we also adjusted for age, gender and ethnicity because (a) age is known to affect FFM traits (Srivastava et al., 2003), (b) gender had an effect on conscientiousness, as shown in Table 2, and (c) country of origin/ethnicity can have small effects on FFM traits (Kajonius and Mac Giolla, 2017). In the adjusted model, the effect of genotype was similar to the crude model's, with AA homozygosity being again significantly associated with lower conscientiousness levels (unstandardized B: − 0.24, 95% CI: − 0.46, − 0.02) compared to G allele carriers. However, none of the SLE scores had an effect on conscientiousness in the adjusted model. Nonetheless, we found that age and gender also had significant effects, with older and female individuals having higher levels of conscientiousness compared to younger and male individuals, respectively. Table 4 shows the adjusted association between NPY's rs16139 (Leu7Pro), composite SLE and conscientiousness. Rs16139 was again significantly associated with conscientiousness and the composite measure of SLE was associated with a reduction in conscientiousness, when controlling for other covariates. However, no gene by environment effect was detected when the interaction term of NPY rs16139 *Composite Stressful Life Events was entered into the model, as specified in Table 4. To ensure that an association between rs16147 and conscientiousness was not hidden by age and SLE (SNP-dependencies shown in Table 1), rs16147 was also assessed together with composite SLE in the adjusted regression model. Rs16147 had no significant effect in this model (data not shown), but composite SLE, females and older groups were again significantly associated with conscientiousness, similar to rs16139.

1853 118

7.27 7.51

1.17 1.03

0.016

4. Discussion

729 50

7.08 7.43

1.25 1.29

0.024

1124 68

7.40 7.58

1.10 1.04

0.20

Table 1 shows the characteristics of the participants by NPY rs16147 and rs16139 genotype. For rs16147, age and stressful life events (in wave 1) differed between genotypes. For rs16139, none of the characteristics differed statistically between AA- and G-allele carriers. Rs16147, which lies in NPY's promoter region, showed no statistically significant associations with any of the FFM traits. However, there was a tendency for association with higher levels of conscientiousness in TT (vs. CC) carriers (P = 0.062; Table 2). Rs16139, which encodes the Leu7Pro polymorphism of pre-pro NPY, showed a significant association with conscientiousness (Table 2) but not with any other of the remaining FFM traits. Specifically, conscientiousness was found to be significantly higher in individuals with the G (Pro7)-allele (Table 2). Stratification by gender showed that male individuals carrying the Gallele had higher levels of conscientiousness compared to homozygote AA-carriers. However, no such difference was found for female participants. Table 3 shows both crude and adjusted associations between NPY's rs16139 (Leu7Pro), stressful life events and the conscientiousness trait.

Table 2 Mean value of conscientiousness by NPY rs16147 (T/C) and rs16139 (A/G) genotypes. Conscientiousnessa

NPY rs16147 TT CT CC NPY rs16139 AA AG + GG Males AA AG + GG Females AA AG + GG

In the present study we examined whether two functional variants in the NPY gene, one in the promoter region (rs16147) and one in the coding region of NPY's precursor peptide, pre-pro NPY (rs16139; Leu7Pro) associated with FFM traits in a Swedish cohort. We found that only conscientiousness was significantly associated with the rs16139 (Leu7Pro) polymorphism. Specifically, conscientiousness was found to be significantly higher in individuals with the Pro7 (G) allele. In addition, we found that an accumulation of stressful life events was associated with lower levels of conscientiousness, although there was no

SD: Standard Deviation, P: p-value. AA: Leu7Leu; AG + GG: Leu7Pro + Pro7Pro. a Conscientiousness scoring range: 1 (min.; low levels) − 9 (max.; high levels). b Compared to the CC genotype.

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Table 3 Crude and adjusted effects of NPY rs16139 (A/G) and stressful life events (SLE) on conscientiousness. Conscientiousness Adjusted modelb

Crude estimate

NPY rs16139 AA AG + GG Childhood SLE SLE in wave 1 SLE in wave 2 Age > 50 41 − 50 30 − 40 < 30 Gender Female Male Ethnicity Born in Sweden Born outside of Sweden

B (95% CI)

P

Ba (95% CI)

P

−0.24 Ref −0.05 −0.03 −0.02

(−0.46 to −0.02)

0.03

0.03

(−0.12 to 0.001) (−0.05 to 0.01) (−0.04 to −0.004)

0.05 0.001 0.021

−0.24 (−0.46 to −0.02) Ref 0.01 (−0.09 to 0.11) −0.02 (−0.05 to 0.01) −0.04 (−0.07 to 0.00)

0.85 0.15 0.05

0.10 (0.02–0.19) 0.14 (0.04–0.23) 0.09 (−0.007 to 0.18) Ref

0.018 0.004 0.69

0.18 (0.03–0.33) 0.29 (0.13–0.45) 0.11 (−0.06 to 0.28) Ref

0.02 0.000 0.22

0.28 (0.22–0.34) Ref

< 0.001

0.34 (0.23–0.44) Ref

< 0.001

−0.01 (−0.12 to 0.10) Ref

0.86

0.04 (−0.16 to 0.23) Ref

0.72

AA: Leu7Leu; AG + GG: Leu7Pro + Pro7Pro. SLE: Stressful Life Events, Ref: Reference category, CI: Confidence Interval, P: p-value. a Unstandardized beta-coefficient. b Adjusted for age, gender and ethnicity.

Kallio et al., 2001). Thus, in theory, the Pro7 allele among individuals with high conscientiousness would lead to more NPY which, in turn, has been suggested to confer protection against anxiety and depressive disorders (Cohen et al., 2012; Heilig et al., 2004; Melas et al., 2013, 2012; Mickey et al., 2011; Sah and Geracioti, 2013; Sjoholm et al., 2009; Zhou et al., 2008). This is in accord with a meta-analysis of associations between FFM traits and psychiatric disorders (Kotov et al., 2010), which found that almost all examined disorders (including anxiety and depression) were defined by low conscientiousness (and high neuroticism). It is also noteworthy that even if our second examined NPY SNP (rs16147), which lies in NPY's promoter region, showed no statistically significant associations with FFM traits, there was still a tendency for association with higher levels of conscientiousness in TT carriers. The T-allele is known to account for an increase in NPY expression levels both in vitro and in vivo (Zhou et al., 2008), further supporting a positive correlation between NPY and conscientiousness levels. Older age and female gender were also associated with higher levels of conscientiousness in our study. With regard to age, our findings are consistent with other studies showing that conscientiousness and agreeableness increase throughout early and middle adulthood, whereas neuroticism declines among women but not among men, supporting the fact that FFM traits are not purely genetic but also subject to developmental influences (Srivastava et al., 2003). With regard to gender, female individuals are cross-culturally known to score higher on extraversion, agreeableness and neuroticism, and - in line with our findings - in most cultures the same gender difference also holds true for conscientiousness (Costa et al., 2001; Feingold, 1994; Schmitt et al., 2008; Weisberg et al., 2011). Information about other genes, which may underlie conscientiousness, comes from GWA studies on FFM traits. More specifically, a study conducted in Italy (Terracciano et al., 2010), found that DYRK1A and SMOC1 were associated with conscientiousness. DYRK1A (Dual Specificity Tyrosine-Phosphorylation-Regulated Kinase 1A) is a strong candidate gene for learning defects associated with Down syndrome (Park et al., 2009) that has also been associated with autism spectrum disorder (O'Roak et al., 2012), whereas SMOC1 (SPARC Related Modular Calcium Binding 1) has been found to be essential for ocular and limb development (Okada et al., 2011). A study of families in the United States and Denmark (Bae et al., 2013), found a suggestion for

moderating effect of cumulative stressful life events on the association between NPY genotype and conscientiousness. Conscientiousness has been described as a "spectrum of constructs that describe individual differences in the propensity to be self-controlled, responsible to others, hardworking, orderly, and rule abiding" and is considered "a core determinant of health, positive aging, and human capital." (Roberts et al., 2014) Within neuropsychiatry, conscientiousness has been correlated negatively with depressive, anxiety and substance use disorders (Kotov et al., 2010), and positively with volumes in the lateral prefrontal cortex (DeYoung et al., 2010); a brain region involved in planning and the voluntary control of behavior. The presence of Pro7 has been shown to lead to a higher processing of prepro NPY into the mature NPY, and has also been associated with higher NPY levels in blood plasma and cerebrospinal fluid (Heilig et al., 2004;

Table 4 Adjusted effect of NPY rs16139 (A/G) and composite stressful life events on conscientiousness. Conscientiousness

NPY rs16139 AA AG + GG Composite SLEb Gender Female Male Age > 50 41 − 50 30 − 40 < 30 Ethnicity Born in Sweden Born outside of Sweden

Ba (95% CI)

P

−0.24 (−0.45 to −0.02) Ref −0.03 (−0.04 to −0.01)

0.03 0.003

0.34 (0.23–0.44) Ref

< 0.001

0.19 (0.04–0.34) 0.30 (0.13–0.46) 0.11(−0.06 to 0.28) Ref

0.01 < 0.001 0.20

0.05 (−0.14 to 0.24) Ref

0.60

AA: Leu7Leu; AG + GG: Leu7Pro + Pro7Pro. SLE: Stressful life events, Ref: Reference category, CI: Confidence Interval, P: p-value. a Unstandardized beta-coefficient. b Composite SLE is the sum of childhood SLEs and SLEs reported in wave 1 and 2.

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References

association between IGDCC3 and conscientiousness. Little is known about IGDCC3 (Immunoglobulin Superfamily DCC Subclass Member 3) but its alternative name (PUNC; Putative Neuronal Cell Adhesion Molecule) provides some clues about its putative function. Another study conducted in Korea (Kim et al., 2013), found that variants in IGF2BP3, BTAF1 and CPEB3 associated with conscientiousness. IGF2BP3 (Insulinlike Growth Factor 2 mRNA-Binding Protein 3) is a glioblastoma-specific marker that activates the Mitogen-Activated Protein Kinase (MAPK) pathway (Suvasini et al., 2011), BTAF1 (B-TFIID TATA-Box Binding Protein Associated Factor 1) is a general transcription factor that binds directly to the TATA-binding protein forming the B-TFIID complex (Pereira et al., 2003), and CPEB3 (Cytoplasmic Polyadenylation Element Binding Protein 3) is an RNA-binding protein that has been implicated in memory formation (Fioriti et al., 2015). Finally, a meta-analysis of GWA studies with participants of European ancestry (de Moor et al., 2012), found that the autism-related gene KATNAL2 (Katanin Catalytic Subunit A1 Like 2; (Neale et al., 2012)) was associated with conscientiousness. Whereas none of these studies reported an association between NPY and conscientiousness, it is worth mentioning that the Leu7Pro polymorphism (rs16139) has a rather low minor allele frequency (2–4% reported in the literature; e.g. (Ding, 2003; Zhang et al., 2012)), which may result in the polymorphism not always passing screening for inclusion in GWA studies (e.g. personal correspondence with Dr. Antonio Terracciano, who did not have association data for rs16139 in his datasets). Nonetheless, there is one study, to our knowledge, that has genotyped rs16139 but found no statistically significant association with conscientiousness (Lo et al., 2017), thus warranting the need for additional studies in the future that examine the NPY Leu7Pro polymorphism. In addition, since NPY's functions are mediated through its G-protein-coupled receptors (e.g., Y1, Y2, Y4, Y5) (Pedragosa-Badia et al., 2013), examining NPY's role in the neurobiology of human personality will also require an inclusion of polymorphisms in the genes encoding NPY receptors.

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5. Conclusions To our knowledge, this is the first study to find a link between a genetic variant in the coding region of NPY (the Leu7Pro polymorphism of pre-pro NPY) and an FFM personality trait: conscientiousness. Since Pro7 has been linked to higher NPY levels in blood plasma and cerebrospinal fluid, these data may provide insights into the neurobiological basis of human personality. In addition, since stressful life events were found to be associated with reduced levels of conscientiousness, this highlights the importance of studying life adversities and their influence on FFM traits. However, given the difficulty in replicating both genetic and environmental associations with complex behavioral traits, these findings should be considered preliminary and warrant replication in additional cohorts. Acknowledgements and funding We wish to thank the participants of the PART study and the funding sources that have made this research possible. This work was supported by the Swedish Research Council (www.vr.se; 2009–5546 YF, 2010–3631 CL, 2014–10171), the Karolinska Institutet's Faculty Funds (https://fonder.ki.se), the regional agreement on medical training and clinical research between Stockholm County Council and Karolinska Institutet (www.forskningsstod.sll.se; 20090281 YF, SLL20110560 CL, SLL20140484 CL) and the Swedish Brain Foundation. The funding sources had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Disclosure of interest The authors declare no conflict of interest with this study. 52

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