rs8018720 — SEC23A

SEC23A and the Secretory Route to Vitamin D Status

Most vitamin D research focuses on the four classic loci — the skin synthesis enzyme DHCR7, the liver hydroxylase CYP2R1, the transport protein GC, and the catabolic enzyme CYP24A1. In 2018, a large genome-wide association study added two new players to this network, one of which sits in an unexpected place: a gene encoding a structural component of the cell's internal postal system rather than a dedicated vitamin D enzyme.

SEC23A¹¹ SEC23A
SEC23 homolog A, a component of the COPII coat complex that mediates the first step of the secretory pathway — packaging newly synthesized proteins into vesicles that bud off the endoplasmic reticulum and travel to the Golgi apparatus is a core component of the COPII coat complex²² COPII coat complex
Coat Protein complex II; a set of five proteins (SAR1, SEC23, SEC24, SEC13, SEC31) that polymerize around ER membranes to form transport vesicles carrying newly made secretory and membrane proteins toward the Golgi and ultimately to their final destinations — including the cell surface and the bloodstream, the protein machinery responsible for shipping newly synthesized proteins from the endoplasmic reticulum to the Golgi apparatus. The rs8018720 variant changes a single amino acid in SEC23A (leucine to valine at position 211), and carriers of the minor G allele show measurably higher circulating 25-hydroxyvitamin D concentrations.

The Mechanism

The precise molecular mechanism remains incompletely understood, which is why this variant carries an emerging evidence grade. However, the biological plausibility is straightforward: several proteins central to vitamin D metabolism are secreted proteins that transit the ER-Golgi pathway. The most prominent candidate is GC-globulin³³ GC-globulin
Also called vitamin D binding protein (VDBP or DBP); a liver-derived plasma protein that carries approximately 85–90% of circulating 25(OH)D. GC must pass through the COPII pathway to be secreted into the bloodstream (vitamin D binding protein, encoded by the GC gene), a liver-synthesized protein that carries approximately 85–90% of circulating 25(OH)D in the bloodstream. If the Leu211Val substitution in SEC23A alters the efficiency or cargo selectivity of COPII vesicle formation, it could change how much GC — and therefore how much vitamin D — is delivered to circulation.

Similarly, the 25-hydroxylase CYP2R1⁴⁴ 25-hydroxylase CYP2R1
The predominant liver enzyme responsible for the first activation step of vitamin D: converting cholecalciferol (D3) to 25-hydroxyvitamin D (calcidiol), the major circulating form measured in blood tests is an ER-resident enzyme whose substrate and product must transit the secretory pathway; any change in ER membrane dynamics or vesicle budding could influence this process. The Leu211 position is within a functional domain of SEC23A involved in GTPase activation of SAR1 (the molecular switch that triggers COPII assembly), making it structurally plausible that the Val substitution modestly alters COPII efficiency.

The Evidence

The discovery GWAS by Jiang and colleagues⁵⁵ discovery GWAS by Jiang and colleagues
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 expanded an earlier discovery sample from 16,125 to 79,366 European-ancestry individuals and identified rs8018720 at genome-wide significance (P = 4.7×10⁻⁹). This brought the total number of confirmed vitamin D loci from four to six. The G allele, carried by approximately 17% of Europeans, was associated with higher circulating 25(OH)D concentrations. The study also found that vitamin D genetic signals cluster preferentially in immune and hematopoietic tissues, consistent with vitamin D's broad immunomodulatory role.

Beyond the discovery study, rs8018720 has been incorporated into vitamin D genetic risk scores (GRS) used in Mendelian randomization analyses. A study of Barrett's esophagus and esophageal adenocarcinoma⁶⁶ study of Barrett's esophagus and esophageal adenocarcinoma
Dong J et al. No Association Between Vitamin D Status and Risk of Barrett's Esophagus or Esophageal Adenocarcinoma: A Mendelian Randomization Study. Clin Gastroenterol Hepatol, 2019 included this variant among six SNPs in a GRS involving 6,167 cases and 17,159 controls, finding no causal effect. A meta-analysis of genetic evidence for vitamin D and type 1 diabetes⁷⁷ meta-analysis of genetic evidence for vitamin D and type 1 diabetes
Najjar L et al. Nutrients, 2021 similarly found no pooled association (OR 0.97–1.02) with seven vitamin D SNPs including rs8018720 — an informative null result showing the vitamin D effect is not large enough to drive T1D risk through this variant alone.

A 2022 autoimmune study⁸⁸ 2022 autoimmune study
Vanderlinden LA et al. Frontiers in Immunology, 2022 found that the C allele at rs8018720 was associated with rheumatoid arthritis autoantibodies among first-degree relatives of RA/SLE probands (OR = 0.65, 95% CI 0.43–0.99), suggesting a modest immunological signal consistent with the known role of vitamin D in autoimmune regulation.

It is important to calibrate expectations: the effect size at this locus is small. The per-allele change in 25(OH)D is likely in the range of 1–3 nmol/L, smaller than the major loci (GC, CYP2R1, DHCR7, CYP24A1). The variant's clinical relevance is primarily as a contributor to cumulative genetic risk rather than as a stand-alone determinant of vitamin D status.

Practical Actions

The C allele at rs8018720 is the common form in most populations (~82% globally). Most people carry CC and have vitamin D levels reflecting typical SEC23A function. Carriers of the G allele (CG or GG) have a mild genetic tendency toward higher circulating 25(OH)D, which may provide a slight buffer against vitamin D insufficiency.

Because the effect size is modest and the mechanism is unproven, this variant should be interpreted as one data point in a broader vitamin D status picture. Blood testing remains the most important tool for individual vitamin D management. Supplementation with cholecalciferol (D3) bypasses the upstream synthesis and transport processes entirely, making it an effective intervention regardless of this genotype.

Interactions

SEC23A's proposed mechanism — influencing the secretory efficiency of GC-globulin (vitamin D binding protein) — means it may interact most meaningfully with variants in the GC gene itself (rs7041, rs4588). If SEC23A is secreting GC less efficiently AND that GC has altered vitamin D binding properties, the combined effect could be more pronounced than either variant alone. CYP2R1 (rs10741657, rs6994076) and CYP24A1 (rs6013897) remain the highest-impact partners for vitamin D pathway interactions. The GWAS that discovered rs8018720 also identified AMDHD1 (rs10745742) as a second novel locus in the same study.