Abstract: Background: Schizophrenia (SCZ) is a complex disorder with high heritability (van Os and Kapur, 2009). Genome-wide association studies (GWAS) have provided compelling evidence of the polygenic architecture of this disorder (Ripke et al., 2014; Pardiñas et al., 2018). According to the results of these studies several thousands of genetic variants contribute to risk for schizophrenia. Polygenic risk scores (PRS) summarize the joint risk effect of such common risk variants. Since brain morphometry is found altered already at the onset of illness, in this study we aim to establish the link between the SCZ PRS and brain surface thickness in a sample of first episodes of psychosis. Methods: 153 psychosis first episode patients (FEP), selected through the PAFIP Program and 113 matched Healthy Controls (HC) were included in this study. After genotype quality control steps, these samples were imputed using the standard SHAPEIT2/IMPUTE2 pipeline. PLINK 1.90 was used for the calculation of PRS. These scores were calculated multiplying the imputation probability of each risk allele by the effect size of such genetic variant as reported in Ripke et al., 2014. The resulting values were summed up in an additive fashion obtaining an individual estimate of the genetic load in each subject. Three different P-value thresholds were used (5E-8, 0.05, 1). All images were taken in the same 3T Phillips scanner. The CAT 12 toolbox, which is implemented in SPM12 was used for SBM and VBM analysis of the data. A full factorial analysis was set with group (FEP/HC) as factor, PRS at the three different p-value thresholds as covariates of interest and sex, age, handedness, total intra-craneal volume and the first 4 principal components for ancestry as nuisance covariables. We applied threshold-free cluster enhancement (TFCE) with 5000 permutations and corrected for multiple comparisons (FWE) at p<0.05. Results: Our preliminary analyses show that even though there are differences in thickness and grey mater volume between patients and healthy controls, there were neither correlations nor interactions between PRS for SCZ and brain morphometry.

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