FUT2 Gly258Ser

FUT2 Gly258Ser — A Second Handle on Your Vitamin B12 Levels

Your circulating vitamin B12 is not simply a readout of how much B12 you eat —
it is actively shaped by the proteins that carry B12 in your blood and determine
how quickly it is cleared from circulation. The FUT2 gene encodes
alpha-(1,2)-fucosyltransferase 2 | An enzyme that adds fucose sugar residues
to glycan chains on cell surfaces and on secreted proteins, creating the H blood
group antigen on mucosal surfaces and modifying the glycosylation of carrier
proteins like haptocorrin, an enzyme that influences B12 metabolism through
a specific post-translational modification of haptocorrin — the protein that
binds most of the B12 in your blood.
The Gly258Ser variant (rs602662) changes a glycine to a serine at amino acid
position 258 of the FUT2 protein. People with the G allele (Gly258) carry a
form of FUT2 that more actively fucosylates haptocorrin, leading to lower
measured serum B12. People with two A alleles (Ser258) have altered enzyme
activity, less fucosylation of haptocorrin, and paradoxically higher circulating
B12 measurements. This variant is in strong
linkage disequilibrium | When two SNPs are inherited together so frequently that
knowing one predicts the other — in this case r² = 0.76-0.92 in Europeans
with rs601338 (the W143X nonsense mutation that determines classical secretor
status), but represents an independent missense change with its own protein-level
consequences on enzymatic activity.

The Mechanism

FUT2 fucosylates haptocorrin | Also called transcobalamin I or R-binder — a
heavily glycosylated protein synthesized mainly by salivary glands and gastric
mucosa that binds B12 in the gut to protect it from acid degradation, then
releases it in the small intestine. In the bloodstream, haptocorrin carries
approximately 70-80% of total serum B12, adding fucose residues to its
complex glycan chains. This fucosylation affects how haptocorrin is recognized
and cleared by the liver.
The asialoglycoprotein receptor | A lectin receptor on hepatocytes that
preferentially binds and internalizes glycoproteins bearing exposed galactose
or N-acetylgalactosamine residues — residues that become exposed when the
terminal sialic acid is removed. Fucosylation competes with sialylation on
these glycan positions, altering which glycoforms predominate (ASGR) on
liver cells is responsible for clearing haptocorrin from the bloodstream. The
degree of fucosylation alters the rate of this hepatic clearance. In carriers
of the G allele (Gly258, higher FUT2 activity), haptocorrin is more thoroughly
fucosylated, shifts toward the TCIII glycoform, and is cleared more efficiently
from the blood — resulting in lower measured serum B12. In AA individuals
(Ser258, lower FUT2 activity on this residue), haptocorrin retains more
sialylation, resists ASGR-mediated clearance, and accumulates to higher levels
in the bloodstream.
Critically, Velkova et al. 2017 | Velkova A et al. The FUT2 secretor variant
p.Trp154Ter influences serum vitamin B12 concentration via holo-haptocorrin,
but not holo-transcobalamin, and is associated with haptocorrin glycosylation.
Hum Mol Genet, 2017 demonstrated
that these FUT2 variants only affect holo-haptocorrin, not
holo-transcobalamin | The approximately 20-30% of blood B12 bound to
transcobalamin II — the only form actively taken up by cells via the TCN2
receptor. Only holoTC reflects the B12 available for cellular use; haptocorrin-
bound B12 is not accessible to most cells — the biologically active fraction.
This is the critical practical implication: standard serum B12 tests measure
total B12 (mostly haptocorrin-bound), and GG individuals with lower total B12
may actually have normal cellular B12 availability, while standard tests can
give an artificially low result. Conversely, AA individuals with higher total
B12 measurements may have perfectly normal cellular B12 availability.

The Evidence

Two independent GWAS studies identified rs602662 as a top hit for plasma vitamin
B12. Tanaka et al. 2009 | Tanaka T et al. Genome-wide association study of
vitamin B6, vitamin B12, folate, and homocysteine blood concentrations. Am J
Hum Genet, 2009 analyzed 3,622
participants in three Italian cohorts and found rs602662 to be the single
strongest genetic association with vitamin B12 (p = 2.83 x 10^-20). The combined
analysis by Hazra et al. 2008 | Hazra A et al. Common variants of FUT2 are
associated with plasma vitamin B12 levels. Nat Genet,
2008 in 2,717 women identified the FUT2
locus (lead SNP rs492602, in strong LD with rs602662) at p = 5.36 × 10⁻¹⁷, with
GG individuals having substantially lower B12 than the AA group.
A 2024 study in kidney transplant patients Kotowski et al. |
Kotowski M et al. The Importance of the FUT2 rs602662 Polymorphism
in the Risk of Cardiovascular Complications in Patients after Kidney
Transplantation. Int J Mol Sci, 2024
found that the G allele was present in 65% of hypertensive patients versus 56%
of normotensive patients — consistent with the lower B12 leading to higher
homocysteine and greater cardiovascular risk.
A metabolic study de Luis et al. 2022 | de Luis DA et al. Effect of the
variant rs602662 of FUT2 gene on anthropometric and metabolic parameters in a
Caucasian obese population. Eur Rev Med Pharmacol Sci,
2022 found that AA homozygotes had
significantly lower BMI, better lipid profiles, lower fasting glucose, reduced
insulin resistance, and a 72% lower metabolic syndrome risk (OR 0.28) compared
to GG+GA carriers — an observation that may connect FUT2 biology to wider
metabolic regulation.

Practical Implications

The key practical point for GG individuals (and to a lesser extent GA carriers)
is awareness: if your standard serum B12 test comes back in the lower-normal
range, your result may be influenced by your genotype rather than your dietary
intake alone. Requesting a holotranscobalamin | Also called "active B12" or
holoTC — the B12 fraction bound to transcobalamin II that is available for
cellular uptake. Normal range is typically above 35-50 pmol/L test instead
of, or alongside, total serum B12 gives a more accurate picture of your functional
B12 status. Alternatively, methylmalonic acid (MMA) — which rises specifically
when cells lack functional B12 — provides a direct functional readout.
For AA individuals, the opposite consideration applies: your total serum B12 may
read higher than average, but this primarily reflects haptocorrin-bound B12
in circulation, not improved cellular availability. Standard B12 testing can be
misleadingly reassuring if cellular deficiency is a concern.

Interactions

rs602662 is in strong linkage disequilibrium with rs601338 (W143X, the primary
European secretor variant) with r² = 0.76-0.92 in Europeans. Together these
variants capture FUT2 enzymatic activity from two different positions in the
protein — rs601338 causes complete truncation at Trp143, while rs602662 alters
activity at Gly258. The associated B12-lowering effects of both variants are
mediated through the same haptocorrin glycosylation mechanism, and their effects
largely overlap in European populations.
For individuals of East Asian ancestry, the classical W143X non-secretor allele
(rs601338) is nearly absent. A different FUT2 variant, rs1047781 (A385T, Ile129Phe),
is the primary East Asian non-secretor allele. The rs602662 Gly258Ser change is
correspondingly rare in East Asian populations, making this variant much less
informative in that ancestry context.
The downstream effect of lower B12 from GG genotypes intersects with one-carbon
metabolism. Vitamin B12 is required for the methionine synthase reaction that
converts homocysteine back to methionine. Chronically lower B12 can raise
homocysteine, increasing cardiovascular and neurological risk. This interaction
is especially relevant in individuals who also carry MTHFR variants (rs1801133
C677T or rs1801131 A1298C) that impair folate-driven remethylation — the two
pathways jointly determine homocysteine levels.