rs140926439 — FN1

FN1 and Alzheimer's Disease — A Blood-Brain Barrier Protective Variant

Fibronectin 1 (FN1) encodes a large glycoprotein that forms the scaffold of the extracellular matrix around blood vessels. In the brain, fibronectin plays a structural role in the neurovascular unit¹¹ neurovascular unit
the interface between blood vessels and neurons, comprising endothelial cells, pericytes, astrocyte endfeet, and neurons, helping maintain blood-brain barrier integrity and coordinating the local response to injury. The rs140926439 variant introduces an amino acid substitution in fibronectin (p.Gly357Glu) that appears to reduce its pathological accumulation at the blood-brain barrier — and in APOE ε4 carriers, this reduction translates into substantially lower Alzheimer's disease risk.

The Mechanism

In APOE ε4 carriers, fibronectin abnormally accumulates in the walls of brain blood vessels²² In APOE ε4 carriers, fibronectin abnormally accumulates in the walls of brain blood vessels
This vascular fibronectin deposition correlates with impaired clearance of toxic amyloid-beta peptides and heightened neuroinflammation. The proposed pathway: APOE ε4 drives excess fibronectin deposition → thickened vascular walls impair gliovascular remodeling and microglial amyloid clearance → amyloid-beta accumulates → neurodegeneration accelerates.

The rs140926439 T allele encodes a Glu (glutamic acid) at position 357 instead of the common Gly (glycine). This Gly→Glu substitution introduces a charged residue into a region of fibronectin involved in matrix assembly, likely disrupting the protein's ability to multimerize or bind to vascular components in the same pathological configuration. Functional studies including zebrafish models support the idea that reduced FN1 function in the neurovascular unit enhances microglial responses and gliovascular remodeling³³ Functional studies including zebrafish models support the idea that reduced FN1 function in the neurovascular unit enhances microglial responses and gliovascular remodeling
Loss-of-function FN1 variants appear to be protective rather than harmful in the context of APOE ε4-driven amyloid accumulation.

The Evidence

The primary evidence comes from a multi-cohort whole-genome sequencing study combining NIA-AD FBS, WHICAP, and EFIGA cohorts, followed by validation in 7,185 APOE ε4 homozygous carriers⁴⁴ The primary evidence comes from a multi-cohort whole-genome sequencing study combining NIA-AD FBS, WHICAP, and EFIGA cohorts, followed by validation in 7,185 APOE ε4 homozygous carriers
Bhattarai et al. (Acta Neuropathologica, 2024) identified rs140926439 as a rare coding protective variant with OR = 0.29 (95% CI 0.11–0.78, p = 0.014), and found it delayed Alzheimer's disease onset by 3.37 years (95% CI 0.42–6.32, p = 0.025) in ε4 carriers. The pathway analysis showed significant enrichment in extracellular matrix processes among protective variants, consistent with FN1's role in vascular matrix biology.

An independent replication using UK Biobank data confirmed the protective signal⁵⁵ An independent replication using UK Biobank data confirmed the protective signal
Lehrer & Rheinstein (Alzheimer Disease and Associated Disorders, 2025) found AD prevalence of 0.43% among APOE ε4 carriers without the variant versus 0.10% among carriers with it — essentially eliminating the ε4 risk increment. The authors propose that fibronectin-driven pathological brain wound-healing may underlie a subset of Alzheimer's cases, and that variants impairing this cascade are protective.

Evidence is classified as emerging: the finding comes from two cohort studies (one discovery + functional validation, one replication), without clinical trial data. The T allele is rare (global frequency ~0.3%), limiting statistical power and making replication in larger datasets essential. The ClinVar classification is "Likely Benign" for the variant in isolation, consistent with its rarity and the context-dependent (APOE ε4-specific) protective effect.

Practical Actions

The T allele confers meaningful protection specifically in APOE ε4 carriers. For CT carriers who are also APOE ε4 positive, this variant provides a counterbalancing protective signal that significantly modifies their overall Alzheimer's risk trajectory. Since the variant is rare and evidence emerging, the actionable value is primarily informational — it informs how aggressively to pursue standard Alzheimer's prevention strategies rather than introducing new interventions specific to this variant alone.

Monitoring for early cognitive changes remains prudent for ε4 carriers regardless of FN1 status, as the T allele reduces but does not eliminate risk.

Interactions

The protective effect of rs140926439 is specifically documented in APOE ε4 carriers (rs429358 C allele + rs7412 C allele combination). Outside of the ε4 context, the variant has not shown significant effect on Alzheimer's risk — the interaction between FN1 and APOE at the neurovascular unit appears central to the mechanism.

Other Alzheimer's risk variants modulating blood-brain barrier function include rs3851179 (PICALM), which affects amyloid-beta transcytosis independently of APOE. An individual carrying both protective rs140926439 and the PICALM protective A allele would theoretically benefit from two complementary blood-brain barrier protection mechanisms, though no direct evidence for this combination exists.