Defective localization of the Wilson disease protein (ATP7B) in the mammary gland of the toxic milk mouse and the effects of copper supplementation

Toxic milk (tx) is a copper disorder of mice that causes a hepatic accumula tion of copper similar to that seen in patients with Wilson disease. Both d isorders are caused by a defect in the ATP7B copper-transporting ATPase. A feature of the tx phenotype is the production of copper-deficient milk by l actating dams homozygous for the tx mutation; the milk is lethal to the pup s. It has not been determined whether the production of copper-deficient mi lk is a direct consequence of impaired expression of ATP7B protein in the m ammary gland. With the use of immunohistochemistry, our study demonstrated that the ATP7B protein was mislocalized in the lactating tx mouse mammary g land, which would explain the inability of the tx mouse to secrete normal a mounts of copper in milk. Confocal microscopy analysis showed that, in the lactating tx mammary gland, ATP7B was predominantly perinuclear in comparis on with the diffuse, cytoplasmic localization of ATP7B in the lactating nor mal mammary gland. Lactating tx mice showed impaired delivery of copper fro m the mammary gland to the milk and this was not ameliorated by dietary cop per supplementation. In contrast, the normal mouse mammary gland responded to increased dietary copper by increasing the amount of copper in milk. A c hange in the distribution of the ATP7B protein from perinuclear in the non- lactating gland to a diffuse, cytoplasmic localization in the lactating gla nd of the normal (DL) mouse suggests that the relocalization of APT7B is a physiological process that accompanies lactation. We conclude that the impa ired copper transport from the mammary gland into milk in lactating tx mice is related to the mislocalization of ATP7B.