rs1871534 — SLC39A4 Leu372Val

SLC39A4 Leu372Val — The World's Most Population-Differentiated Common SNP

Every milligram of zinc you absorb from food passes through a single gateway in the intestinal wall: a protein called ZIP4¹¹ ZIP4
Zinc-Iron transporter Protein 4, encoded by SLC39A4 on chromosome 8q24.3; expressed at the apical membrane of duodenal and jejunal enterocytes; the sole high-capacity zinc importer in the mammalian gut. When ZIP4 stops working entirely — through rare pathogenic mutations — the result is acrodermatitis enteropathica, a severe inherited zinc deficiency disease. But rs1871534 is not one of those rare mutations. It is one of the most common SNPs in the human genome, and its story is one of the most striking examples of recent positive selection in human evolution.

The rs1871534 variant swaps a leucine for a valine at position 372 of the ZIP4 protein (Leu372Val). The valine form — the C allele on the plus strand — is carried by essentially every person of West African descent and by virtually nobody of European or East Asian ancestry. The [FST | a measure of population differentiation ranging from 0 (identical frequency) to 1 (completely different); values above 0.90 are extremely rare for common SNPs] between Europeans and Yorubans (West Africans) for this variant is 0.99999977 — the most differentiated common SNP in the genome at the time of its discovery.

The Mechanism

Engelken et al. (2014)²² Engelken et al. (2014) investigated why this SNP shows such extreme population differentiation. They expressed both the Leu372 and Val372 forms of ZIP4 in HeLa cells and measured three outcomes: protein levels at the cell surface, baseline intracellular zinc, and zinc uptake rate. Val372 (the West African form) showed significantly reduced surface expression, lower basal intracellular zinc, and reduced zinc uptake compared to Leu372. The variant does not eliminate ZIP4 function — it reduces its efficiency.

ZIP4 is regulated through zinc-dependent endocytosis: when zinc is abundant, ZIP4 is pulled off the cell surface and degraded; when zinc is scarce, ZIP4 is rapidly trafficked back to the apical membrane to capture more zinc. The Leu372Val substitution sits in a transmembrane domain of the protein and appears to alter the protein's stability at the cell surface — [effectively reducing the maximum capacity of the intestinal zinc absorption system | Wang et al. 2004 showed that reduced surface expression is the primary mechanism by which ZIP4 missense variants impair transport (PMID 14709598)].

The Evidence

The key study is the 2014 analysis by Engelken and colleagues³³ Engelken and colleagues
Engelken J et al. Extreme population differences in the human zinc transporter ZIP4 (SLC39A4) are explained by positive selection in Sub-Saharan Africa. PLoS Genet, 2014. Using coalescent simulations that accounted for local recombination hotspots, they demonstrated that the extreme allele frequency differences cannot be explained by genetic drift alone — directional selection favoring the Val372 allele in sub-Saharan Africa with a selection coefficient of approximately 0.5% is the most parsimonious explanation. This is a modest but sustained selective advantage, consistent with the allele rising to near-fixation over thousands of generations.

Why would reduced zinc absorption be advantageous? Zinc is essential for many bacterial and parasitic pathogens. The human immune system uses zinc-starvation as a front-line antimicrobial weapon — macrophages deliberately flood zinc into vesicles containing intracellular bacteria to kill them. The authors hypothesize that reduced intestinal zinc uptake by Val372-ZIP4 may also reduce systemic zinc availability to pathogens, conferring a survival advantage in high-pathogen-burden environments like sub-Saharan Africa. This hypothesis remains speculative — no direct in vivo evidence in humans has yet tested it — but it is consistent with the geographic distribution of selection and with what is known about nutritional immunity.

The functional consequence for the individual: Val372/Val372 (CC) homozygotes absorb zinc less efficiently than Leu372/Leu372 (GG) carriers. At typical dietary zinc intakes this may not produce frank deficiency, but it creates a narrower margin — particularly on high-phytate diets that already impair zinc bioavailability.

Practical Actions

For CC carriers (almost exclusively of West African or recent African ancestry), the gap between dietary zinc intake and actual absorption is wider than for GG carriers. Phytate-rich staple diets — common in sub-Saharan Africa — compound this by further reducing bioavailability. The most direct interventions are dietary: prioritise animal-source zinc (which bypasses phytate inhibition) and reduce phytate intake through food preparation techniques. Monitoring serum zinc provides an objective check on zinc adequacy.

For CG heterozygotes, a modest intermediate effect on ZIP4 surface expression is expected; the practical relevance is smaller but the same dietary principles apply.

Interactions

With SLC30A1 (ZnT1, rs3738198): ZIP4 handles zinc import at the apical membrane; ZnT1 handles export at the basolateral membrane. Individuals carrying reduced-function alleles at both transporters face a double constraint on net zinc delivery to the portal circulation.

With dietary phytate: The gene-diet interaction is the dominant modifiable factor. Phytate in legumes, wholegrains, and maize-based staples forms insoluble zinc-phytate complexes in the gut, reducing absorption to 10–15% vs 25–40% for animal-source zinc. For CC carriers, this interaction is clinically meaningful — phytate in the context of reduced ZIP4 capacity compounds into significant functional zinc inadequacy.

With pathogenic SLC39A4 variants: Compound heterozygosity — one Leu372Val allele plus one pathogenic acrodermatitis enteropathica allele on the other chromosome — has not been systematically studied but is theoretically possible. Because the pathogenic variants (p.Arg95Cys, p.Gln278His, etc.) cause null or near-null ZIP4 function, the Leu372Val allele would provide residual function on that chromosome.