rs842999 — GC

GC rs842999 — A Second Intronic Tag for Vitamin D Transport Capacity

Circulating vitamin D travels through the bloodstream almost entirely in a carrier-bound form — roughly 85-90% of the 25-hydroxyvitamin D 25(OH)D¹¹ 25(OH)D
The major circulating form of vitamin D measured by standard blood tests. It reflects total body vitamin D stores and is the number your doctor reports your doctor measures is hitched to vitamin D binding protein (VDBP)²² vitamin D binding protein (VDBP)
Also called group-specific component, or GC. A 58-kDa glycoprotein produced in the liver that is the primary transporter of vitamin D metabolites in blood. VDBP concentration and binding affinity are the two largest determinants of total circulating 25(OH)D, encoded by the GC gene on chromosome 4. The GC locus is the single strongest genetic determinant of circulating 25(OH)D in genome-wide studies — dwarfing contributions from vitamin D synthesis, activation, and receptor genes.

rs842999 is one of several independent intronic variants at the GC locus that tag haplotypes influencing VDBP expression level or binding efficiency. Unlike the missense variants rs7041 (Asp432Glu) and rs4588 (Thr436Lys), which define the classical Gc1f, Gc1s, and Gc2 protein isoforms, rs842999 does not change the amino acid sequence. Instead, it marks a haplotype block associated with altered gene expression or alternative splicing — the exact regulatory mechanism has not been characterized at a molecular level. In Danish family cohorts, rs842999 and rs4588 were the two GC variants most strongly driving the gene-level association with serum 25(OH)D, suggesting the two carry partially independent information about VDBP function.

The Mechanism

rs842999 sits in an intron of the GC gene on the minus strand at GRCh38 chr4:71745973. The G allele is the reference and the population-major allele in most ancestries; the C allele is the minor allele and functions as the risk allele associated with lower 25(OH)D. Because GC encodes VDBP — the protein that determines how much total vitamin D circulates in blood — variants that reduce VDBP expression or binding affinity translate directly into lower measured 25(OH)D on standard blood tests, even when vitamin D intake or sun exposure is the same as someone without the variant.

The C allele is common in European populations (~45% allele frequency) but is effectively absent in African populations (<0.5%) and very rare in East Asian populations. This population distribution mirrors the broader pattern at the GC locus, where C-allele haplotypes at intronic tag variants (including rs842999 and rs2282679) are markedly enriched in European ancestry groups.

The Evidence

The strongest evidence comes from a series of Danish family studies examining genetic predictors of vitamin D status. In the PLoS One 2014 study by Nissen et al.³³ Nissen et al.
Nissen J et al. Common variants in CYP2R1 and GC genes predict vitamin D concentrations in healthy Danish children and adults. PLoS One, 2014 (758 participants from 201 families), rs842999 and rs4588 were specifically identified as the GC variants most responsible for the gene-level 25(OH)D association, with a dose-dependent relationship: carriers of two G-alleles had higher 25(OH)D than one-G carriers, who had higher levels than zero-G carriers.

A companion study in the same cohort, Nissen et al. Genes & Nutrition 2014⁴⁴ Nissen et al. Genes & Nutrition 2014
Nissen J et al. Real-life use of vitamin D3-fortified bread and milk during a winter season: the effects of CYP2R1 and GC genes on 25-hydroxyvitamin D concentrations in Danish families, the VitmaD study. Genes Nutr, 2014, built a 4-SNP genetic risk score from rs10741657 (CYP2R1), rs10766197 (CYP2R1), rs4588 (GC), and rs842999 (GC), counting the C/A alleles at rs842999 as risk alleles. Across 0-8 risk alleles, there was a statistically significant negative linear trend with 25(OH)D concentrations (p < 0.0001), confirming that rs842999 contributes independently to genetic vitamin D risk.

The 2015 Am J Clin Nutr study by Nissen et al.⁵⁵ Nissen et al.
Nissen J et al. Common variants in CYP2R1 and GC genes are both determinants of serum 25-hydroxyvitamin D concentrations after UVB irradiation and after consumption of vitamin D3-fortified bread and milk during winter in Denmark. Am J Clin Nutr, 2015 extended these findings, showing that rs842999 and rs4588 in GC jointly predict 25(OH)D levels both after UVB irradiation (simulated sun exposure) and after 6 months of consuming vitamin D3-fortified foods, suggesting the genetic effect persists regardless of the vitamin D source (endogenous vs. dietary).

In a large Arizona cohort of 1,439 participants from two colorectal neoplasia trials, Hibler et al.⁶⁶ Hibler et al.
Hibler EA et al. Polymorphic variation in the GC and CASR genes and associations with vitamin D metabolite concentration and metachronous colorectal neoplasia. Cancer Epidemiol Biomarkers Prev, 2012 reported that rs842999 was one of seven GC polymorphisms significantly associated with circulating 25(OH)D concentrations after adjustment (P < 0.01).

Practical Actions

Carriers of the C allele — particularly CC homozygotes — are likely to have lower baseline 25(OH)D for a given level of sun exposure and dietary intake. The practical response is the same as for other GC locus variants: check your 25(OH)D level, and if it is below the optimal range (75-100 nmol/L / 30-40 ng/mL), supplement with vitamin D3 at doses calibrated to your measured level. The key distinction from the population average is that CC carriers typically need higher maintenance doses to sustain the same 25(OH)D concentration as GG carriers. Studies using genetic risk scores that include rs842999 found that high-risk individuals show reduced responsiveness to vitamin D fortification, meaning the same supplement dose raises 25(OH)D less in CC carriers than in GG carriers.

Interactions

rs842999 is in partial linkage disequilibrium with rs2282679 (the strongest GWAS signal at the GC locus, r² varies by population) and with rs4588 (the Thr436Lys missense variant defining the Gc2 isoform). The three variants are not fully redundant — Nissen et al. showed rs842999 and rs4588 were independently selected as the top GC predictors, suggesting they tag distinct aspects of GC haplotype variation. A full vitamin D genetic risk assessment ideally includes rs842999 alongside rs4588 or rs2282679 and rs7041.

GC variants interact with CYP2R1 (rs10741657, the major vitamin D 25-hydroxylase SNP) in a way that compounds risk: individuals with risk alleles at both the synthesis step (CYP2R1) and the transport step (GC) have the lowest 25(OH)D and the least responsiveness to supplementation in the Danish studies. VDR variants (particularly rs2228570 / Fok1) modulate downstream vitamin D receptor signaling, so GC + VDR compound genotypes are worth assessing for people with persistent vitamin D insufficiency despite adequate supplementation.