NRN1 epistasis with BDNF and CACNA1C: mediation effects on symptom severity through neuroanatomical changes in schizophrenia

Abstract

The expression of Neuritin-1 (NRN1), a neurotrophic factor crucial for neurodevelopment and synaptic plasticity, is enhanced by the Brain Derived Neurotrophic Factor (BDNF). Although the receptor of NRN1 remains unclear, it is suggested that NRN1's activation of the insulin receptor (IR) pathway promotes the transcription of the calcium voltage-gated channel subunit alpha1 C (CACNA1C). These three genes have been independently associated with schizophrenia (SZ) risk, symptomatology, and brain differences. However, research on how they synergistically modulate these phenotypes is scarce. We aimed to study whether the genetic epistasis between these genes affects the risk and clinical presentation of the disorder via its effect on brain structure. First, we tested the epistatic effect of NRN1 and BDNF or CACNA1C on (i) the risk for SZ, (ii) clinical symptoms severity and functionality (onset, PANSS, CGI and GAF), and (iii) brain cortical structure (thickness, surface area and volume measures estimated using FreeSurfer) in a sample of 86 SZ patients and 89 healthy subjects. Second, we explored whether those brain clusters influenced by epistatic effects mediate the clinical profiles. Although we did not find a direct epistatic impact on the risk, our data unveiled significant effects on the disorder's clinical presentation. Specifically, the NRN1-rs10484320 x BDNF-rs6265 interplay influenced PANSS general psychopathology, and the NRN1-rs4960155 x CACNA1C-rs1006737 interaction affected GAF scores. Moreover, several interactions between NRN1 SNPs and BDNF-rs6265 significantly influenced the surface area and cortical volume of the frontal, parietal, and temporal brain regions within patients. The NRN1-rs10484320 x BDNF-rs6265 epistasis in the left lateral orbitofrontal cortex fully mediated the effect on PANSS general psychopathology. Our study not only adds clinical significance to the well-described molecular relationship between NRN1 and BDNF but also underscores the utility of deconstructing SZ into biologically validated brain-imaging markers to explore their mediation role in the path from genetics to complex clinical manifestation.

Keywords: Genetic epistasis; Mediation; Neuroimaging; Schizophrenia; Symptoms.

Fig. 1

Schematic representation of the different moderated mediation models used. (A) Conceptual diagrams representing the used model. The grey arrows indicate the indirect effect of X on Y through Me and modulated by Mo. (B) Statistical diagram depicting the different equations used in the moderated mediation model. Two equations are used to estimate the index of moderation mediation (IMM): first, the effect of X (a1) and Mo (a2) variables and their interaction (a3) on Me (model 1: Me = β0 + β1X + β2Mo + β3XMo + ε); second, the effect of X (c’1), Mo (c’2), their interaction (c’3) and Me (b1) on Y (model 2: Y = β0 + β1Me + β2X + β3Mo + β4XMo + ε). The different letters represent the coefficients of each one of these effects. Then, the index of moderated mediation was calculated as the product of the coefficients a3*b1

Fig. 2

Bar plots showing the significant epistatic effects detected on clinical measures within SZ subjects. Each bar represents the scores’ marginal mean (± 2 standard error) for each epistatic group. A) NRN1-rs10484320 x BDNF-rs6265 interaction on Positive and Negative Symptoms Scale (PANSS) general psychopathology. B) NRN1-rs4960155 x CACNA1C-rs1006737 interaction on Global Assessment of Functioning (GAF)

Fig. 3

Brain regions where significant epistatic effects between NRN1 and BDNF were detected in the whole-brain FreeSurfer analyses. We represented significant gene-gene interactions related to cortical surface area (CSA) (A) and cortical volume (CV) (B) on the pial surface, where a specific colour distinguishes each cluster, and both lateral and medial views are presented for each hemisphere (LH: left hemisphere; RH: right hemisphere). The bar plots depict the marginal mean scores (± 2 standard error) for each epistatic group

Fig. 4

Brain regions where significant epistatic effects between NRN1 and CACNA1C were detected in the whole-brain FreeSurfer analyses. We represented significant gene-gene interactions related to cortical surface area (CSA) (A) and cortical volume (CV) (B) on the pial surface, where each cluster is distinguished by a specific colour, and both lateral and medial views are presented for each hemisphere (LH: left hemisphere; RH: right hemisphere). The bar plots depict the marginal mean scores (± 2 standard error) for each epistatic group

Fig. 5

Scatter plots showing the significant correlations between brain clusters and clinical measures within subjects with SZ. For both clinical and morphometric measures, the reported values signify estimated marginal means. These means have been adjusted for potential confounding factors, including age and sex. In the case of brain measures, the adjustments also encompass intracranial volume. PANSS: positive and negative syndrome scale; GP: general psychopathology; GAF: Global Assessment Functioning; CV: cortical volume; CSA: cortical surface area; L-LOFC: left lateral orbitofrontal cortex

Fig. 6

Conceptual diagram showing the significant moderated mediation model. Epistatic effects between NRN1-rs10484320 x BDNF-rs6265 on the left lateral orbitofrontal cortex (L-LOFC) volume mediates the epistatic effects on Positive and Negative Symptoms Scale (PANSS) General Psychopathology