rs4752 — GC GC rs4752

The Silent Haplotype Marker in Your Vitamin D Transport System

The GC gene encodes vitamin D binding protein (VDBP/DBP)¹¹ vitamin D binding protein (VDBP/DBP)
A 58-kDa glycoprotein produced mainly by the liver. It carries approximately 85-90% of circulating 25(OH)D and 85% of 1,25(OH)₂D in the bloodstream and acts as the primary transport protein for all vitamin D metabolites, the protein responsible for carrying nearly all circulating vitamin D through the bloodstream. The rs4752 variant is a synonymous substitution at codon 299 — it does not change the amino acid sequence of VDBP but acts as a haplotype-defining marker within the broader genetic architecture of the GC locus, which is the single strongest genetic determinant of circulating vitamin D levels in the human genome.

Unlike the two well-known missense variants rs4588²² rs4588
Thr436Lys — defines the Gc2 isoform; the T allele removes an O-glycosylation site, reducing VDBP binding affinity and rs7041³³ rs7041
Asp432Glu — together with rs4588 defines Gc1f, Gc1s, and Gc2 isoforms, rs4752 does not directly alter VDBP protein function. Instead, its G allele marks specific haplotypes within the GC gene that are associated with altered vitamin D transport, immune dysregulation, and disease susceptibility. The platform's existing entries for rs4588, rs7041, and the intronic GWAS tag rs2282679 cover the primary functional and epidemiological signals at this locus; rs4752 adds granularity by tagging haplotype branches that carry independent disease associations.

The Mechanism

Population genetics analyses of the GC locus have identified at least seven distinct haplotypes, defined by four key polymorphisms: rs4588 and rs7041 (the two missense variants), rs4752 (synonymous at codon 299), and rs3733359 (a non-coding promoter-region variant). Among these seven haplotypes, four encode the Gc1F isoform, two encode Gc1S, and one encodes Gc2. The rs4752-G allele is present in a subset of Gc1F haplotypes — specifically those carrying the derived allele at this locus — and appears within certain Gc1S background haplotypes as well.

Although the Cys299= substitution is synonymous at the protein level, there is evidence that synonymous variants can affect gene expression⁴⁴ evidence that synonymous variants can affect gene expression
Synonymous variants (also called "silent" mutations) can alter mRNA stability, translational efficiency, splicing enhancer or silencer sequences, and miRNA binding sites — all without changing the amino acid sequence. Research on the 2025 GDM study proposed that rs4752, alongside rs7041, may alter post-transcriptional splicing of GC mRNA, potentially affecting VDBP expression levels or isoform ratios rather than protein sequence directly. The rs3733359 companion variant alters transcription factor binding in the promoter region. Together, these variants tune GC expression at both transcriptional and post-transcriptional levels.

The G allele at rs4752 shows striking population stratification: it reaches ~24% in populations of African ancestry but is rare in Europeans (~1%) and South Asians (~0.5%), with intermediate frequencies in East Asians (~9%) and Latinos (~6%). This pattern reflects both ancient population history and the complex selection pressures acting on the GC locus — VDBP plays roles in vitamin D transport, actin scavenging, macrophage activation, and immune modulation, all of which may have been subject to geographically variable selection.

The Evidence

The most direct evidence for clinical consequences of the G allele comes from a case-control study in 223 Korean patients with ankylosing spondylitis⁵⁵ case-control study in 223 Korean patients with ankylosing spondylitis
Jung KH et al. Associations of vitamin D binding protein gene polymorphisms with the development of peripheral arthritis and uveitis in ankylosing spondylitis. J Rheumatol. 2011. The rs4752-G allele was associated with a significantly increased risk of uveitis (OR 2.04, 95% CI 1.12–3.72, p = 0.02). Eight GC polymorphisms including rs4752 and rs3733359 were genotyped, and haplotype analysis identified specific four-SNP combinations that were protective or deleterious for peripheral arthritis and uveitis development. The G allele's association with uveitis likely reflects VDBP's role in immune regulation and macrophage activation rather than a direct effect on vitamin D transport, since VDBP can be enzymatically converted to Gc-MAF⁶⁶ Gc-MAF
GC protein-derived macrophage-activating factor — a potent activator of macrophages and part of innate immune defense against pathogens and tumors, and different GC haplotypes differ in their efficiency of Gc-MAF production.

A 2025 study published in Scientific Reports⁷⁷ 2025 study published in Scientific Reports
Wu et al. GC vitamin D-binding protein gene functional genetic variants and gestational diabetes mellitus risk and prediction. Sci Rep, 2025 examined GC gene variants and gestational diabetes mellitus (GDM) risk in a Chinese cohort. In multifactor dimensionality reduction analysis, the two-locus model combining rs4752 and rs7041 was identified as the best interaction model for GDM prediction. Carriers of the AG genotype at rs4752 had an adjusted OR of 1.58 (95% CI 1.19–2.10, P = 0.001) for GDM compared to AA genotype, and the combined AG/GG genotype showed an adjusted OR of 1.34 (95% CI 1.04–1.71, P = 0.021). This association likely operates through effects on VDBP-mediated vitamin D bioavailability and/or immune modulation, since adequate vitamin D signaling is important for pancreatic beta-cell function and insulin sensitivity.

Population-level genetic studies of the GC locus, including the landmark SUNLIGHT consortium GWAS of 33,996 Europeans⁸⁸ SUNLIGHT consortium GWAS of 33,996 Europeans
Wang TJ et al. Common genetic determinants of vitamin D insufficiency: a genome-wide association study. Lancet, 2010, established the GC locus as the strongest genetic signal for circulating 25(OH)D (P = 1.9×10^-109). While rs4752 is not the primary functional variant at this locus, it participates in the haplotype background that modulates overall GC gene output.

Practical Actions

For most people carrying one G allele (AG heterozygotes), the practical implications overlap substantially with those of other GC variants already in the platform: the G allele tags a haplotype associated with modestly altered VDBP expression, which may contribute to variation in total 25(OH)D levels and immune function. Standard vitamin D status monitoring and attention to supplementation response are the primary actionable takeaways.

The uveitis association in the context of ankylosing spondylitis is particularly relevant for G carriers who already carry HLA-B27 or have axial spondyloarthropathy. Vitamin D sufficiency supports immune regulation and may modulate uveitis risk through multiple pathways including Gc-MAF production and VDR-mediated T-cell regulation.

Interactions

rs4752 sits within a haplotype defined by four GC variants: rs4588 (Thr436Lys), rs7041 (Asp432Glu), rs4752 (Cys299=), and rs3733359 (promoter). The rs4588 and rs7041 entries in this platform cover the primary functional allele effects on VDBP isoform and binding affinity. The rs2282679 intronic entry covers the lead GWAS signal for circulating 25(OH)D. rs4752 adds the haplotype context that distinguishes specific Gc1F sub-haplotypes and links to immune outcomes beyond simple vitamin D transport.

The association between rs4752-G and gestational diabetes risk operates in combination with rs7041 genotype — the two-locus interaction was stronger than either alone, consistent with a compound haplotype effect on VDBP expression and/or vitamin D bioavailability during pregnancy.