The Neuritin-1 gene (NRN1), involved in neurodevelopment and synaptic plasticity, is associated with schizophrenia (SZ) and related clinical, cognitive, and neuroimaging phenotypes. Additionally, it is one of the most differentially methylated genes in the prefrontal cortex (PFC) in SZ and is responsive to neurotherapeutic agents. We aimed to investigate NRN1's molecular mechanisms in SZ by analyzing its expression, methylation, and genotypic profiles in PFC and hippocampus (HIPP) post-mortem samples from 30 control (CTL) subjects and 20 individuals with SZ (10 treated with clozapine, SZ-Clz, and 10 without antipsychotic drugs at death, SZ-ApFree). We compared the NRN1 mRNA expression between groups, measured by qPCR, and methylation levels across three CpG islands, assessed through EpiTYPER. Sparse Partial Least Square Discriminant Analysis identified key CpG units contributing to group differences. We then explored the relationship between NRN1 methylation and expression, considering the influence of 11 polymorphisms genotyped by qPCR. We found that SZ-Clz had lower NRN1 mRNA levels in the PFC than SZ-ApFree and CTL. SZ-Clz presented distinct methylation patterns across multiple CpG units in both brain regions compared to CTL. In the PFC, the methylation of the CpG units differentiating SZ-Clz from CTL correlated to NRN1 expression, and the NRN1-rs12333117 and NRN1-rs2208870 polymorphisms influenced this effect. These findings reveal distinct correlations between NRN1 epigenetic expression in SZ-Clz and CTL, shaped by genotypic variability. They emphasize region-specific alterations in SZ and underscore the importance of integrative approaches for a better understanding of the role of candidate genes in SZ etiology.

The authors have declared no competing interest.

This work was supported by: i) Fundacion Alicia Koplowitz, ii) Academia de les Ciencies Mediques i de la Salut de Catalunya i de Balears, iii) Fondo de Investigaciones Sanitarias (FIS), Ministerio de Economia y Competitividad, Instituto de Salud Carlos III (PI17/00872; Fondo Europeo de Desarrollo Regional, Union Europea, "Una manera de hacer Europa"). This study also received funding provided by: i) the Instituto de Salud Carlos III through a Miguel Servet contract to MF-V (CP20/00072), co-funded by European Regional Development Fund (ERDF)/European Social Fund "Investing in your future"; (i) the Academia de les Ciencies Mediques i de la Salut de Catalunya i de Balears through a contract to CA-P; iii) the Comissionat per a Universitats i Recerca del DIUE of the Generalitat de Catalunya, Agencia de Gestio d'Ajuts Universitaris i de Recerca (AGAUR: 2021SGR1475); iv) the Basque Government (IT1512/22).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study adhered to legal and ethical standards, with all samples collected at the Basque Institute of Legal Medicine (Bilbao, Spain) in accordance with Spanish national research policies and ethical guidelines for post-mortem brain studies in effect at the time of collection (Law 14/2007). Additional approval was obtained from the Research Ethics Committees of the Universidad del Pais Vasco/Euskal Herriko Unibertsitatea (UPV/EHU) in Pais Vasco, Spain (RD 1716/2011), and Germanes Hospitalaries in Catalunya, Spain (PR-2016-07).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors