Compound Heterozygous Mutations in SLC30A2/ZnT2 Results in Low Milk Zinc Concentrations: A Novel Mechanism for Zinc Deficiency in a Breast-Fed Infant

Abstract

Zinc concentrations in breast milk are considerably higher than those of the maternal serum, to meet the infant's requirements for normal growth and development. Thus, effective mechanisms ensuring secretion of large amounts of zinc into the milk operate in mammary epithelial cells during lactation. ZnT2 was recently found to play an essential role in the secretion of zinc into milk. Heterozygous mutations of human ZnT2 (hZnT2), including H54R and G87R, in mothers result in low (>75% reduction) secretion of zinc into the breast milk, and infants fed on the milk develop transient neonatal zinc deficiency. We identified two novel missense mutations in the SLC30A2/ZnT2 gene in a Japanese mother with low milk zinc concentrations (>90% reduction) whose infant developed severe zinc deficiency; a T to C transition (c.454T>C) at exon 4, which substitutes a tryptophan residue with an arginine residue (W152R), and a C to T transition (c.887C>T) at exon 7, which substitutes a serine residue with a leucine residue (S296L). Biochemical characterization using zinc-sensitive DT40 cells indicated that the W152R mutation abolished the abilities to transport zinc and to form a dimer complex, indicating a loss-of-function mutation. The S296L mutation retained both abilities but was extremely destabilized. The two mutations were found on different alleles, indicating that the genotype of the mother with low milk zinc was compound heterozygous. These results show novel compound heterozygous mutations in the SLC30A2/ZnT2 gene causing zinc deficiency in a breast-fed infant.

Essential Role for Zinc Transporter 2 (ZnT2)-mediated Zinc Transport in Mammary Gland Development and Function during Lactation

“…Lactating ZnT2-null Mice Accumulate Zinc and Have Profound Defects in Morphology and Impaired MEC Differentiation-Consistent with results in nulliparous ZnT2ko mice and previous reports in breastfeeding women with heterozygous mutations in ZnT2 (11,17), overall zinc status as measured by plasma (wt, 13.08 mol/liter Ϯ 1.4; ZnT2ko, 13.95 mol/liter Ϯ 1.3) and liver (wt, 32.38 g Zn/g of tissue Ϯ 2.6; ZnT2ko, 31.78 g Zn/g of tissue Ϯ 4.5) zinc concentration was not altered in the lactating ZnT2ko mice. Additionally, similar to previous reports in rats and mice (31,32), ZnT2 was localized intracellularly and associated with the apical membrane of MECs in the mammary gland during lactation (Fig.…”

“…ZnT2 is expressed in the mammary gland (47), and previous studies have focused on its role in importing zinc into vesicles (10, 13) for secretion into milk during lactation (11,16,17). ZnT2 is also found in mitochondria and vesicles in non-lactating mammary glands; however, the physiological role of ZnT2-mediated zinc transport beyond that of zinc secretion into milk is almost entirely unknown.…”

“…However, we chose to focus on the loss of ZnT2 function on the lactating mammary gland as previous studies clearly show that ZnT2 imports zinc into secretory vesicles, thus playing a vital role in zinc secretion into milk during lactation (10,13,16,17). Although milk zinc concentration was significantly reduced in ZnT2-null mice, much more profound was the accumulation of zinc and the morphological defects observed in the lactating mammary glands that indicate impaired mammary differentiation.…”

“…9A). Because women with mutations in ZnT2 have a much greater (50 -90%) reduction in milk zinc concentration (10,11,13,16,17), we expected to see a greater loss of zinc transfer into milk in ZnT2ko mice. Given this unexpected finding, we tested the hypothesis that ZnT4 expression was augmented to compensate for the lack of ZnT2, as we and others (24,46) have shown that ZnT4 is important for zinc secretion into milk.…”

Altered expression of two zinc transporters, SLC30A5 and SLC30A6, underlies a mammary gland disorder of reduced zinc secretion into milk

“…The production of zinc-deficient milk was also linked to a glycine to arginine substitution (G87R) (Lasry et al 2012). Two novel missense mutations in the SLC30A2 (ZnT2) genes were reported in a Japanese mother with milk zinc levels reduced below 90 % of normal (Itsumura et al 2013). …”