Genome-Wide Association Study in a Rat Model of Temperament Identifies Multiple Loci for Exploratory Locomotion and Anxiety-Like Traits

Abstract

Common genetic factors likely contribute to multiple psychiatric diseases including mood and substance use disorders. Certain stable, heritable traits reflecting temperament, termed externalizing or internalizing, play a large role in modulating vulnerability to these disorders. To model these heritable tendencies, we selectively bred rats for high and low exploration in a novel environment (bred High Responders (bHR) vs. Low Responders (bLR)). To identify genes underlying the response to selection, we phenotyped and genotyped 558 rats from an F2 cross between bHR and bLR. Several behavioral traits show high heritability, including the selection trait: exploratory locomotion (EL) in a novel environment. There were significant phenotypic and genetic correlations between tests that capture facets of EL and anxiety. There were also correlations with Pavlovian conditioned approach (PavCA) behavior despite the lower heritability of that trait. Ten significant and conditionally independent loci for six behavioral traits were identified. Five of the six traits reflect different facets of EL that were captured by three behavioral tests. Distance traveled measures from the open field and the elevated plus maze map onto different loci, thus may represent different aspects of novelty-induced locomotor activity. The sixth behavioral trait, number of fecal boli, is the only anxiety-related trait mapping to a significant locus on chromosome 18 within which the Pik3c3 gene is located. There were no significant loci for PavCA. We identified a missense variant in the Plekhf1 gene on the chromosome 1:95 Mb QTL and Fancf and Gas2 as potential candidate genes that may drive the chromosome 1:107 Mb QTL for EL traits. The identification of a locomotor activity-related QTL on chromosome 7 encompassing the Pkhd1l1 and Trhr genes is consistent with our previous finding of these genes being differentially expressed in the hippocampus of bHR vs. bLR rats. The strong heritability coupled with identification of several loci associated with exploratory locomotion and emotionality provide compelling support for this selectively bred rat model in discovering relatively large effect causal variants tied to elements of internalizing and externalizing behaviors inherent to psychiatric and substance use disorders.

Bioenergetic-Related Gene Expression in the Hippocampus Predicts Internalizing vs. Externalizing Behavior in a F2Cross of Selectively-Bred Rats

“…Thus, a higher percent of females than males were classified as ST versus GT (Fig Notably, although many of these behaviors were influenced by sex (discussed above), including sex in the model did not appear to modulate the relationships between behavioral variables (all sex interaction terms p>0.164) or fully explain those relationships (all behavioral relationships still had p<0.0284). The re-creation of correlations between bHR/bLR phenotypical behaviors in a subset of the F2 generation imply a shared etiology for these behaviors, including environmental and genetic influences, and is consistent with the phenotypical and genetic correlations of the whole F2 cohort from our GWAS findings (Chitre et al, 2022). shown previously to have bHR/bLR differences in (Flagel et al, 2010)).…”

“…Both genes have also been found to be differentially expressed in the hippocampus of other rat models with resilience to internalizing-like tendencies (Blaveri et al, 2010;Meckes et al, 2018). Mfge8 is located close to behavioral QTLs in both our F2 GWAS and exome sequencing study (Chitre et al, 2022;Z. Zhou et al, 2019), whereas Fcrl2 is not found in a region implicated by our genetic studies.…”

“…We have identified Fzd6 and Ucp2 as genes that are likely to mediate the effects of selective breeding on novelty-induced locomotor activity by broadly affecting mood and exploration-related behaviors and stress responses. In addition to exhibiting differential expression based on bHR/bLR lineage and in association with exploratory locomotion in the F2s, these genes are also located near QTLs identified in our GWAS (Chitre et al, 2022) on chromosomes 1 (Ucp2) and 7 (Fzd6). Fzd6, part of the WNT signaling pathway that is involved in neurogenesis and synaptogenesis (Yli-Karjanmaa et al, 2019), has been linked to anxiety and depressive-like behavior.…”

“…To create the summary: The x-axis indicates the chromosomal location for the starting coordinates for each gene included in our analysis (genes found within both our F0 and F2 datasets). The y-axis represents the maximum -log10(p-value) (LOD) for the association between SNVs identified within each 1 MB bin in the F2 dataset by a GWAS and bHR/bLR-related behaviors (i.e., Total Locoscore, Lateral Locoscore, Rearing Locoscore, Distance Traveled in the EPM, % Time in the Open Arms, # of Boli on the EPM, (Chitre et al, 2022)). Chromosomes are indicated with an alternating black and grey pattern.…”

“…The copyright holder for this preprint (which this version posted July 15, 2022. ; https://doi.org/10.1101/2022.07.14.500129 doi: bioRxiv preprint phenotypical behaviors, including locomotor response to novelty and anxiety-like behaviors (Chitre et al, 2022).…”