rs4702 — FURIN proBDNF Processing Variant

The Hidden Switch in Your Brain's Nerve Growth Factory

Every neuron in your brain depends on a steady supply of mature brain-derived neurotrophic factor¹¹ brain-derived neurotrophic factor
BDNF is a protein that acts like fertilizer for neurons — it promotes their survival, growth, and connection-forming (synaptogenesis). Low BDNF is consistently found in depression, anxiety, and neurodegenerative diseases (BDNF) to survive, form memories, and regulate mood. But BDNF doesn't start out active — it's made as a larger, inactive precursor called proBDNF, which must be cleaved by the enzyme FURIN before it becomes the mature, beneficial form. A variant in the FURIN gene's regulatory region, rs4702, controls how much of this essential cleavage enzyme is produced — and the G allele reduces FURIN levels enough to shift the BDNF system toward its pro-apoptotic, rather than pro-survival, mode.

The Mechanism

The FURIN gene's 3' untranslated region (3'UTR) is a stretch of RNA that doesn't code for protein but instead controls how much FURIN protein is made. The rs4702 variant sits within this regulatory sequence. When the G allele is present, it creates a binding site for a small RNA molecule called miR-338-3p²² miR-338-3p
microRNAs are short RNA sequences that bind to messenger RNA and suppress translation. miR-338-3p is expressed in the brain and is involved in neuronal differentiation and axon growth.

Hou and colleagues³³ Hou and colleagues
Hou Y, Liang W, Zhang J et al. Schizophrenia-associated rs4702 G allele-specific downregulation of FURIN expression by miR-338-3p reduces BDNF production. Schizophr Res, 2018 demonstrated this mechanism in 2018 using luciferase reporter assays: the G allele allows miR-338-3p to dock on FURIN's 3'UTR and suppress its expression, while the A allele disrupts the binding site and preserves FURIN levels. This allele-specific FURIN suppression was then shown to reduce production of mature BDNF in human cells.

The consequences are significant. When FURIN levels are reduced, more proBDNF accumulates instead of being converted to mature BDNF. These two forms of BDNF have opposite effects: mature BDNF binds TrkB receptors⁴⁴ mature BDNF binds TrkB receptors
TrkB (tropomyosin receptor kinase B) is the high-affinity receptor for mature BDNF. Activation promotes neuronal survival, LTP, and memory consolidation and promotes neuronal survival and synaptic plasticity, while proBDNF binds p75NTR receptors⁵⁵ proBDNF binds p75NTR receptors
p75NTR is a low-affinity neurotrophin receptor that, when activated by proBDNF, triggers apoptosis (programmed cell death) and long-term depression of synaptic strength and can trigger apoptosis and synaptic weakening.

A 2021 review⁶⁶ 2021 review
Wang M, Xie Y, Qin D. Proteolytic cleavage of proBDNF to mBDNF in neuropsychiatric and neurodegenerative diseases. Brain Res Bull, 2021 concluded that "insufficient proBDNF transformation into mature BDNF is potentially critical to disease pathogenesis" across depression, anxiety, schizophrenia, Parkinson's disease, and Alzheimer's disease.

The Evidence

Schizophrenia association. The rs4702 G allele emerged as a genome-wide significant locus from large schizophrenia GWAS studies. The Hou et al. 2018 paper was specifically motivated by the 108 confirmed schizophrenia loci from the Psychiatric Genomics Consortium GWAS and identified rs4702 as a functional 3'UTR variant among these loci. A 2019 Nature Genetics study⁷⁷ 2019 Nature Genetics study
Schrode N, Ho SM, Yamamuro K et al. Synergistic effects of common schizophrenia risk variants. Nat Genet, 2019 using CRISPR-edited iPSCs demonstrated that the rs4702 G allele affects neuronal morphology and electrophysiological properties in excitatory neurons, with synergistic effects when combined with other schizophrenia eQTL variants.

FURIN as a cis-eQTL. An eQTL study⁸⁸ eQTL study
Turpeinen H, Seppälä I, Lyytikäinen LP et al. A genome-wide expression quantitative trait loci analysis of proprotein convertase subtilisin/kexin enzymes identifies a novel regulatory gene variant for FURIN expression and blood pressure. Hum Genet, 2015 of over 1,400 human blood samples confirmed rs4702 as a significant cis-eQTL for FURIN expression and found nominal associations with blood pressure phenotypes (diastolic p=0.012, peripheral vascular resistance p=0.003), suggesting the variant's regulatory effects extend beyond the brain.

Causal evidence from iPSCs. The Dobrindt et al. 2021⁹⁹ Dobrindt et al. 2021
Dobrindt K, Hoagland DA, Seah C et al. Common genetic variation in humans impacts in vitro susceptibility to SARS-CoV-2 infection. Stem Cell Reports, 2021 study used CRISPR-based allelic conversion to establish causality: GG iPSC-derived neurons expressed ~0.74-fold less FURIN than AA counterparts, confirming that the genotype drives FURIN expression differences, not just associations.

PTSD and trauma. A 7-year prospective cohort study¹⁰¹⁰ 7-year prospective cohort study
Tamman AJF, Wendt FR, Pathak GA et al. Attachment style moderates polygenic risk for incident PTSD in US military veterans. Biol Psychiatry, 2022 of 1,083 trauma-exposed veterans found that rs4702 showed the strongest gene×environment interaction with cumulative lifetime trauma burden for incident PTSD, suggesting the FURIN-BDNF axis is particularly relevant to stress-related psychiatric outcomes.

Neuroprotection after brain injury. In a clinical study of 106 glioma patients receiving radiotherapy, Yang and colleagues¹¹¹¹ Yang and colleagues
Yang S, Fu ZZ, Zhang YQ et al. The G to A transformation of rs4702 polymorphism in 3'UTR of FURIN reduced the risk of radiotherapy-induced cognitive impairment in glioma patients. J Cell Mol Med, 2022 found that rs4702-A carriers maintained higher FURIN and BDNF expression post-treatment and experienced less cognitive impairment, providing clinical evidence that the A allele's preservation of FURIN activity matters under neurotoxic stress.

Practical Actions

For GG carriers, the priority is supporting the pathways that compensate for reduced FURIN-mediated proBDNF cleavage. Aerobic exercise is the most robustly evidence-backed intervention for raising mature BDNF levels directly, bypassing the FURIN bottleneck by stimulating BDNF gene transcription via FNDC5/irisin and PGC-1α pathways. Omega-3 fatty acids (EPA/DHA) support neuronal membrane health and have been shown to raise BDNF expression in depressed and healthy populations. Ensuring adequate zinc and magnesium intake matters because both cofactors support metalloprotease activity, including enzymes in the proBDNF processing cascade.

Stress management deserves particular attention for G allele carriers given the documented PTSD interaction: chronic stress suppresses BDNF expression independently of genotype, compounding the FURIN-related reduction. The rs4702 × trauma interaction data suggest that carriers under high cumulative stress load face disproportionately elevated psychiatric risk.

Interactions

The most clinically meaningful interaction is with rs6265 (BDNF Val66Met) in the BDNF gene itself. Both variants converge on reduced mature BDNF availability through different mechanisms: rs4702 G reduces FURIN expression, limiting proBDNF cleavage upstream; rs6265 T (the Met allele) impairs activity-dependent secretion of BDNF downstream. Individuals carrying both the rs4702 G allele and the rs6265 T allele face compound reduction in mature BDNF signaling — reduced production from proBDNF and reduced regulated release of whatever mature BDNF is produced. This combination is a candidate for a compound action targeting both upstream (FURIN support) and downstream (BDNF secretion) components of the neurotrophin pathway.