rs121434291 — SLC39A4 SLC39A4 zinc transporter variant

SLC39A4 G305D — A Pathogenic ZIP4 Zinc Transporter Variant

Every cell in the body depends on zinc for more than 300 enzymes and 2,000+ transcription factors, yet the human body has no dedicated zinc storage organ — it must be continuously absorbed from food. In the intestine, that absorption flows primarily through a single gateway: ZIP4¹¹ ZIP4
The Zrt/Irt-like protein 4, encoded by SLC39A4 on chromosome 8q24.3, is the primary zinc importer on the apical surface of duodenal and jejunal enterocytes. When both copies of the SLC39A4 gene carry loss-of-function variants, dietary zinc cannot cross the gut wall. The consequence — hereditary acrodermatitis enteropathica (AE) — is a severe systemic zinc deficiency that is uniformly fatal without treatment but fully manageable with lifelong oral zinc supplementation.

The rs121434291 variant (C>T on the GRCh38 plus strand) replaces glycine at position 305 of the ZIP4 protein with aspartate (p.Gly305Asp; also annotated as p.Gly330Asp in the longer transcript isoform). Glycine 305 lies in a region of ZIP4 critical for normal protein folding and zinc transport function. The introduction of aspartate — a negatively charged, polar residue — in place of the structurally neutral glycine is predicted to disrupt the protein's tertiary structure and abolish zinc transport activity. An alternative alternate allele at the same position (C>A, p.Gly305Val) exists at similarly rare global frequency (~0.000004 in gnomAD exomes) and is expected to be pathogenic by the same mechanism, though it has not been independently classified in ClinVar.

The Mechanism

ZIP4 is expressed on the apical (luminal-facing) membrane of enterocytes, with expression upregulated when zinc levels fall. Its function is to move zinc ions from the intestinal lumen into absorptive cells, from where zinc enters circulation via basolateral zinc exporters. The Gly305Asp missense substitutes a bulky, charged residue into a structurally sensitive region of the ZIP4 protein. Because AE is autosomal recessive²² autosomal recessive
Both gene copies must be non-functional for disease; one functional copy is sufficient for normal zinc absorption, a single defective copy has no measurable impact on zinc status. Homozygotes and compound heterozygotes — who lose all functional ZIP4 activity — suffer progressive systemic zinc deficiency within weeks of birth, since the body cannot synthesise or store meaningful zinc reserves.

The Evidence

Wang et al. (2002)³³ Wang et al. (2002)
Wang K et al. A novel member of a zinc transporter family is defective in acrodermatitis enteropathica. Am J Hum Genet, 2002 identified SLC39A4 (encoding hZIP4) as the gene defective in AE by positional cloning and functional characterisation, establishing that pathogenic variants throughout the gene eliminate intestinal zinc uptake. The rs121434291 T allele (p.Gly305Asp) is classified Pathogenic in ClinVar (RCV000003720) on the basis of this gene identification and subsequent case-series evidence.

Küry et al. (2003)⁴⁴ Küry et al. (2003)
Küry S et al. Mutation spectrum of human SLC39A4 in a panel of patients with acrodermatitis enteropathica. Hum Mutat, 2003 documented seven additional SLC39A4 mutations in 12 AE families from France, Tunisia, Austria, and Lithuania — including missense, nonsense, and splice-site variants — confirming that pathogenic variants are distributed throughout the ZIP4 protein and that all result in clinically indistinguishable AE phenotype.

A later mutation update by Schmitt et al. (2009)⁵⁵ Schmitt et al. (2009)
Schmitt S et al. An update on mutations of the SLC39A4 gene in acrodermatitis enteropathica. Hum Mutat, 2009 catalogued 31 pathogenic SLC39A4 variants, confirming missense mutations as the most common class. Clinically, untreated AE presents in formula-fed infants within the first 4–10 weeks of life with a triad of acral and perioral dermatitis, diarrhoea, and alopecia. Breast-fed infants are typically protected by the high zinc bioavailability of breast milk and present at weaning. Without supplementation the disease is fatal; with it, prognosis is excellent.

Practical Implications

Oral zinc supplementation fully corrects the phenotype in homozygous AE patients. Treatment is initiated at 5–10 mg/kg/day of elemental zinc during the acute phase, then reduced to a maintenance dose of 1–2 mg/kg/day for life. Doses must be adjusted upward during growth phases, illness, and pregnancy. Regular monitoring of serum zinc is essential to avoid both deficiency relapse and zinc toxicity from over-supplementation.

Carriers (heterozygotes) are clinically unaffected under normal dietary conditions, but this variant is important for family planning: two carrier parents have a 25% probability of having an affected child with each pregnancy.

Interactions

AE illustrates how completely the body's zinc economy depends on ZIP4. Variants in other SLC39A (ZIP family) and SLC30A (ZnT family) genes modulate zinc homeostasis but do not cause AE. Dietary phytates in cereals and legumes form insoluble zinc complexes that compete with ZIP4-mediated uptake; this is especially relevant for heterozygous carriers whose single functional ZIP4 copy must operate efficiently. Co-administration of oral zinc with quinolone antibiotics (ciprofloxacin) or tetracyclines (doxycycline) should be separated by at least 2 hours to avoid chelation interactions that reduce absorption of both compounds.

Compound heterozygosity — carrying one copy of the Gly305Asp allele (rs121434291) on one chromosome and a different SLC39A4 pathogenic variant on the other — causes full AE and would appear as a CT genotype at this locus. A closely related variant, rs121434288 (p.Gly501Arg), affects a different conserved residue in the ZIP4 transmembrane domain and is catalogued separately in GeneOps.