FADS1

FADS1 rs174546 — 3'UTR Desaturase Control Switch

Most genetic variants in the FADS1 gene cluster affect expression through intronic
regulatory elements, but rs174546 operates through a distinct mechanism: it sits
in the 3' untranslated region | 3'UTR — the section of an mRNA transcript downstream
of the protein-coding sequence, critical for mRNA stability, translation efficiency,
and microRNA-mediated regulation of the
FADS1 transcript, where it alters a binding site for the microRNA miR-149-5p. When
the T allele is present, the miRNA binds more effectively and suppresses FADS1
translation — reducing the amount of delta-5 desaturase | FADS1 — the enzyme
responsible for the final step converting DGLA to arachidonic acid in the omega-6
pathway and ETA to EPA in the omega-3 pathway
protein that reaches the cell. The functional and clinical consequences mirror the
broader FADS1 impairment seen across the haplotype: elevated precursor fatty acids,
lower long-chain PUFA products, and measurably higher serum triglycerides.

The Mechanism

rs174546 creates a quantifiable drop in FADS1 mRNA output through a two-layer
miRNA mechanism. In a luciferase reporter study | Hermant et al. Identification
of a functional FADS1 3'UTR variant associated with erythrocyte n-6 polyunsaturated
fatty acids levels. J Clin Lipidol,
2018 of 540 subjects, the T allele
haplotype reduced reporter gene activity by 30% at baseline. When miR-149-5p
was co-expressed in the same system, the suppression deepened to 60% — and this
amplified suppression was partially reversed when an miR-149-5p inhibitor was added,
confirming that the miRNA is directly responsible for the allele-dependent effect.

Separately, the T allele interacts with miR-6728-3p in an in vivo Korean cohort |
Lee et al. Functional Impact of the FADS1 rs174546 Single Nucleotide Polymorphism
on Serum Lipid Levels. Mol Nutr Food Res,
2024 of 8,842 adults, confirming that
the 3'UTR functional effect is not limited to a single miRNA species or a single
cell line model. The downstream result of reduced FADS1 protein — regardless of
which miRNA mediates it — is the same: the delta-5 desaturation step slows, DGLA
accumulates in the omega-6 arm, and ETA-to-EPA conversion in the omega-3 arm is
rate-limited.

What distinguishes rs174546 from other FADS1 variants on the platform is the
mechanistic specificity: we know exactly which part of the gene is disrupted,
which miRNA binds the disrupted site, and by how much transcription falls. This makes
it the most directly characterized FADS1 3'UTR variant studied to date.

The Evidence

The triglyceride signal from rs174546 is independently documented in 8,842 Korean
participants. Lee et al. 2024 found
that each T allele increases fasting serum triglycerides by 6.48 ± 1.84 mg/dL
— an additive effect consistent with reduced FADS1-mediated LC-PUFA production
altering VLDL assembly and postprandial triglyceride clearance. The effect is
detectable on a standard fasting lipid panel, not just on specialized fatty acid
measurements, placing rs174546 in the same clinical-consequence tier as the
more-studied rs174547.

The FADS1 locus-wide evidence anchors the population context. The landmark
InCHIANTI GWAS | Tanaka et al. Genome-wide association study of plasma
polyunsaturated fatty acids. PLoS Genet,
2009 demonstrated that the FADS1
haplotype (of which rs174546 is a member) accounts for 18.6% of all additive
variance in circulating arachidonic acid — the largest explained variance for any
common variant in PUFA metabolism. The CHARGE Consortium meta-analysis | Lemaitre
et al. Genetic loci associated with plasma phospholipid n-3 fatty acids. PLoS Genet,
2011 across 8,866 participants confirmed
that FADS1 cluster minor alleles predict lower circulating EPA (p=5×10⁻⁵⁸) and
higher plant-sourced ALA (p=3×10⁻⁶⁴), validated across European, African, Chinese,
and Hispanic ancestry groups.

Practical Actions

The 3'UTR mechanism does not change the practical implication of carrying the T
allele: FADS1 produces less delta-5 desaturase protein, and the conversion of ALA
to EPA slows accordingly. For CT heterozygotes, 1–2 g preformed EPA+DHA daily
from marine or algae-based sources supplements the partially impaired conversion
step. For TT homozygotes, where both alleles carry the T variant, the suppression
is more complete — 2–4 g daily becomes appropriate, and relying solely on plant-
sourced ALA (flaxseed, chia, walnuts) is insufficient because that ALA requires
the impaired FADS1 step to reach EPA.

The triglyceride association adds a monitoring dimension. Because each T allele
raises fasting triglycerides by approximately 6.5 mg/dL, TT homozygotes may carry
a baseline elevation of ~13 mg/dL from this variant alone — detectable on a
standard lipid panel when combined with dietary and metabolic factors.

Interactions

rs174546 is in high linkage disequilibrium with the established FADS1 haplotype
that includes rs174541, rs174547, rs174548, rs174537, rs174553, and rs174561.
Users carrying the T allele at rs174546 are likely to carry risk alleles at these
linked sites on the same chromosomal segment. The variants tag the same underlying
expression phenotype; their individual entries on the platform add resolution to
different functional evidence layers — rs174546 provides the 3'UTR miRNA mechanism
while rs174541 and rs174547 provide intronic regulatory and clinical lipid evidence.

The ELOVL2 variant rs17606561 encodes elongase 2, which converts EPA to DHA
downstream of the FADS1 desaturation step. A user carrying both FADS1 T alleles
(reduced ALA→EPA conversion) and an ELOVL2 impairment (reduced EPA→DHA conversion)
faces sequential blocks in the omega-3 synthesis chain. For this combined genotype,
DHA-targeted supplementation (≥500 mg DHA specifically, not just total EPA+DHA)
addresses the downstream block that EPA supplementation alone would not reach.