GC Thr436Lys

Vitamin D Binding Protein — The Carrier That Shapes Your D Levels

The GC gene encodes vitamin D binding protein (VDBP/DBP) | A 58-kDa glycoprotein
produced mainly by the liver, also called group-specific component (Gc). It carries
85-90% of circulating 25(OH)D and 85% of 1,25(OH)₂D in the bloodstream,
the main transport protein for vitamin D metabolites in the blood. Nearly all circulating
25-hydroxyvitamin D — the form your doctor measures — travels bound to VDBP. A single
nucleotide change at rs4588 swaps a threonine for a lysine at position 436 of the protein,
defining the boundary between the Gc1 and Gc2 isoforms. This amino acid substitution
removes a key O-glycosylation | A post-translational modification where a sugar
(N-acetylgalactosamine) attaches to the threonine at position 436. The Gc2 isoform
(lysine) cannot be glycosylated at this site, altering protein stability and
binding properties site, lowering both the protein's binding affinity for
vitamin D metabolites and its overall serum concentration.

The Mechanism

VDBP exists in three major isoforms defined by two SNPs — rs4588 and
rs7041 | The other key GC variant (Asp432Glu), which together with rs4588 defines
the Gc1f, Gc1s, and Gc2 haplotypes. The rs4588 T allele (Lys436) creates the
Gc2 isoform, while the G allele (Thr436) is shared by both Gc1f and Gc1s isoforms.
The Gc2 protein has lower affinity for 25(OH)D and
1,25(OH)₂D | The active hormonal form of vitamin D (calcitriol), produced in the
kidneys from 25(OH)D compared to the Gc1 variants. Together, rs4588 and rs7041
explain over 50% of the variance in circulating VDBP concentration — a remarkably
large genetic effect for any serum protein.

Because VDBP carries most circulating vitamin D, people with the Gc2 isoform (TT
homozygotes) have measurably lower total 25(OH)D on standard blood tests. However,
the lower binding affinity simultaneously means that a greater proportion of their
vitamin D is in the free or bioavailable | The fraction of 25(OH)D not bound to
VDBP — consisting of the truly free fraction plus the loosely albumin-bound fraction.
This is the portion that can enter cells and exert biological effects form. This
creates an important paradox: a blood test showing "low" total 25(OH)D may not reflect
true vitamin D insufficiency in someone with the Gc2 genotype.

The Evidence

The GC locus was identified as the strongest genetic determinant of circulating 25(OH)D
in the first large GWAS studies | Wang TJ et al. Common genetic determinants of
vitamin D insufficiency: a genome-wide association study. Lancet, 2010
of vitamin D levels. The expanded SUNLIGHT consortium analysis | Jiang X et al.
Genome-wide association study in 79,366 European-ancestry individuals informs the
genetic architecture of 25-hydroxyvitamin D levels. Nat Commun, 2018
of 79,366 Europeans confirmed rs4588 as likely causal at this locus (per-allele
beta = -0.11 standard deviations for 25(OH)D, P = 1.5 x 10⁻¹³). The most recent
mega-GWAS | Revez JA et al. Genome-wide association study identifies 143 loci
associated with 25 hydroxyvitamin D concentration. Nat Commun, 2020
of 417,580 individuals identified 143 loci affecting vitamin D levels, yet GC remained
the single strongest signal in the genome.

A landmark New England Journal of Medicine study | Powe CE et al. Vitamin D-binding
protein and vitamin D status of black Americans and white Americans. N Engl J Med,
2013 demonstrated the clinical relevance
of VDBP genotype. Black Americans had substantially lower total 25(OH)D (15.6 vs 25.8
ng/mL) and lower VDBP levels than White Americans, yet their bioavailable 25(OH)D
concentrations were similar (2.9 vs 3.1 ng/mL, P = 0.71) and their bone mineral
density was higher. The difference was largely explained by the higher frequency of
Gc1f alleles (lower VDBP, lower total D, but adequate free D) in populations of
African descent.

Supplementation studies show that response to vitamin D varies by GC genotype |
Al-Daghri NM et al. Efficacy of vitamin D supplementation according to vitamin
D-binding protein polymorphisms. Nutrition, 2019.
Carriers of the rs4588 TT genotype may show a smaller rise in total 25(OH)D after
standard supplementation, though the clinical significance of this — given the
bioavailability paradox — remains debated.

Practical Implications

The key takeaway for carriers of the T allele is that standard 25(OH)D blood tests may
underestimate your functional vitamin D status. A "low" reading does not necessarily
mean you are deficient in the biologically active form. If your total 25(OH)D is
borderline low (20-30 ng/mL) and you have no symptoms of deficiency (fatigue, bone
pain, muscle weakness), your bioavailable vitamin D may be perfectly adequate.

For TT homozygotes who do show true deficiency with symptoms or very low levels
(below 20 ng/mL), vitamin D3 (cholecalciferol) supplementation remains effective —
you may simply need a higher dose or longer duration to reach the same total 25(OH)D
target on blood tests. Taking vitamin D with a fat-containing meal improves absorption
regardless of genotype.

Interactions

rs4588 is in strong linkage disequilibrium with rs7041 (Asp432Glu), the other major
GC variant. Together they define the three classical VDBP isoforms: Gc1f
(rs7041-T + rs4588-G), Gc1s (rs7041-G + rs4588-G), and Gc2 (rs7041-T + rs4588-T).
The Gc2/2 diplotype (homozygous for both variant alleles) has the lowest VDBP levels
and the greatest reduction in total 25(OH)D, while Gc1f/1f has the highest VDBP
concentration.

Variants in other vitamin D pathway genes — CYP2R1 (hepatic 25-hydroxylation),
DHCR7/NADSYN1 (skin synthesis), and CYP24A1 (degradation) — can compound the
effect of GC variants. Someone who carries both a low-transport GC genotype and
impaired synthesis or hydroxylation variants may be at genuinely higher risk of
functional vitamin D insufficiency.