rs1532423 — CA1

CA1 rs1532423 — A Zinc-Sequestration Variant in the Carbonic Anhydrase Cluster

Most serum zinc tests measure what's circulating in your plasma, but your body's largest zinc reservoir sits inside your red blood cells. The CA1 gene encodes carbonic anhydrase 1¹¹ carbonic anhydrase 1
A zinc-containing metalloenzyme found at extraordinarily high concentrations inside red blood cells — approximately 0.5 mM, or nearly 50 mg of CA1 protein per 100 mL of packed red cells, making it one of the most abundant proteins in the human erythrocyte, an enzyme whose catalytic core is built around a single tightly-bound zinc ion. This puts CA1 at the interface of zinc biology and red blood cell metabolism in a way that few other genes can.

The rs1532423 variant, located within an intron of CA1 on chromosome 8q21.2, was identified in the first genome-wide association study of blood trace elements as one of only three loci reaching genome-wide significance for circulating zinc — alongside a variant in PPCDC on chromosome 15 and one on the X chromosome. The P-value of 6.40 × 10⁻¹² from 5,477 Australian and UK participants leaves no doubt about the association; the molecular pathway it reflects is the more interesting question.

The Mechanism

CA1 and its closely related paralog CA2 sit side-by-side on chromosome 8q21.2, with CA2 (carbonic anhydrase 2) located approximately 108 kilobases downstream of rs1532423. Both enzymes catalyze the reversible hydration of carbon dioxide into bicarbonate and a proton — a reaction critical for gas exchange, pH regulation, and bicarbonate reabsorption in the kidney proximal tubule.

The key zinc biology is straightforward: each molecule of CA1 coordinates one zinc ion at its active site, using it as a Lewis acid to polarize the water molecule that attacks CO₂. Red blood cells contain roughly 330 mg/dL of hemoglobin and approximately 15–25 mg of carbonic anhydrase per 100 mL²² roughly 330 mg/dL of hemoglobin and approximately 15–25 mg of carbonic anhydrase per 100 mL
By mass, carbonic anhydrase constitutes 1–2% of total erythrocyte protein, making it the dominant zinc-chelating protein in whole blood. Because CA1 is expressed at this enormous concentration exclusively inside erythrocytes, any variant that alters CA1 expression or enzyme turnover directly changes how much zinc is sequestered in red blood cells versus circulating freely in plasma.

The rs1532423 variant is intronic and does not change any amino acid in CA1 or CA2. Intronic variants in this region can influence zinc-handling through at least two mechanisms: (1) altered transcription factor binding in regulatory elements embedded in the intron, changing CA1 expression level and thus erythrocyte zinc capacity; (2) modified splicing efficiency for adjacent exons, changing the proportion of active versus inactive CA1 isoforms. The precise mechanism has not been established in functional studies.

The Evidence

The foundational evidence comes from Evans DM et al. 2013³³ Evans DM et al. 2013
Evans DM, Zhu G, Dy V, et al. Genome-wide association study identifies loci affecting blood copper, selenium and zinc. Hum Mol Genet, 2013, a study that combined two cohorts from the Queensland Institute of Medical Research (QIMR, 4,012 Australian twins and siblings) and the ALSPAC study (1,465 UK pregnant women) in a GWAS of trace element concentrations in whole blood. For zinc, three independent loci reached genome-wide significance. The chromosome 8 signal, marked by rs1532423, achieved P = 6.40 × 10⁻¹² — the most significant of the three zinc loci. The authors specifically noted that "the chromosome 8 locus for Zn contains multiple genes for the Zn-containing enzyme carbonic anhydrase," identifying the erythrocyte carbonic anhydrase system as the probable biological explanation.

rs1532423 has since been used as a standard genetic instrument for zinc status in Mendelian randomization studies⁴⁴ Mendelian randomization studies
A method that uses genetic variants as natural experiments to test whether a biomarker causally influences a health outcome, analogous to a randomized trial but using nature's randomization of alleles at conception. A 2018 two-sample MR analysis⁵⁵ 2018 two-sample MR analysis
Thun GA et al. Effects of copper and zinc on ischemic heart disease and myocardial infarction: a Mendelian randomization study. Am J Clin Nutr, 2018 found that genetically instrumented higher zinc was associated with slightly elevated ischemic heart disease risk (OR 1.06; 95% CI 1.02–1.11). This finding illustrates that zinc behaves on a U-shaped curve: both deficiency and excess associate with adverse outcomes, and the goal is optimization rather than maximization.

In COVID-19 pandemic research, Li et al. 2022⁶⁶ Li et al. 2022
Li Z et al. Genetically Predicted Circulating Concentrations of Micronutrients and COVID-19 Susceptibility and Severity. Front Nutr, 2022 included rs1532423 alongside rs2120019 as genetic instruments for zinc, finding limited evidence that genetically predicted zinc levels causally alter COVID-19 susceptibility or hospitalization (OR 1.06, 95% CI 0.81–1.39), though the study was underpowered for severe outcomes.

Practical Actions

The clinical significance of rs1532423 is that it shifts where in the blood zinc resides — and therefore what standard plasma zinc tests may or may not reveal. Because CA1 binds zinc inside erythrocytes, variants that reduce CA1 expression could lower whole-blood zinc without affecting plasma zinc (and vice versa). This is relevant for interpreting zinc blood tests: whole-blood zinc captures the erythrocyte pool; plasma zinc reflects extracellular status.

For dietary management, the fundamental actions are the same regardless of which allele drives the effect: ensure consistent dietary zinc adequacy, choose bioavailable forms, and monitor status periodically if diet is restrictive or if symptoms suggest deficiency (frequent infections, impaired wound healing, altered taste or smell). Supplementation should be calibrated to confirmed deficiency, not to genotype alone — Mendelian randomization data argues against pushing zinc above the population-normal range.

Interactions

rs1532423 is one of three established genome-wide significant zinc loci. The other two are rs2120019 (PPCDC, chromosome 15) and rs11638477 (chromosome X). All three are independent signals — they operate through different biological mechanisms and their effects on zinc status would compound. Individuals carrying zinc-lowering alleles at two or more of these loci have a larger genetically anchored downward shift in zinc than those carrying a single zinc variant.

CA2, located 108 kb downstream of rs1532423 on the same chromosome, encodes carbonic anhydrase 2 — an enzyme expressed in the kidney proximal tubule where it regulates bicarbonate reabsorption and mineral handling. Pathogenic CA2 mutations cause osteopetrosis with renal tubular acidosis (OMIM 259730), a rare recessive condition. rs1532423 has no known effect on this pathway, but the proximity means variants in strong LD with rs1532423 might also tag CA2 regulatory regions; this has not been specifically studied.