The Znt4 mutation inlethal milk mice affects intestinal zinc homeostasis through the expression of other Zn transporters.

The lethal milk mouse syndrome is caused by a point mutation in the zinc transporter gene ZnT4 resulting in defective zinc secretion in the milk of homozygous mutant dams. Pups of any genotype fed solely on lm milk die within the first two weeks of neonatal life, displaying zinc deficiency symptoms. Homozygous mutant pups survive when foster nursed by wild type dams and show signs of mild zinc deficiency in adulthood. To further investigate the role of ZnT4 in zinc secretion in the intestinal epithelium, we have studied the expression by real time quantitative PCR of mutant ZnT4 and of other zinc transporters of the Zip and ZnT families, in the jejunum of homozygous lm mice and of the isogenic wild type strain C57BL/ 6J. We report in this paper that expression of the mutant ZnT4 mRNA, carrying a premature translational termination codon (ZnT4/lm), is almost absent in tissues from lm mice, probably as a result of degradation by the Nonsense Mediated mRNA Decay (NMD) Pathway. In the jejunum of mutant mice, we also observed decreased expression of the uptake zinc transporter Zip4, paralleled by increased levels of both metallothionein genes MTI and MTII. Zinc supplementation of lm mice in the drinking water did not result in further decrease of Zip4 expression, but led to full induction of MT mRNAs. These results lead us to conclude that, although in the enterocytes of lm mice the absence of the zinc secretion activity mediated by ZnT4 results in increased intracellular zinc concentration, other zinc efflux activities are able to maintain the level of zinc ions below the threshold necessary for full induction of metallothioneins.