MEDNIK syndrome: a novel defect of copper metabolism treatable by zinc acetate therapy

Abstract

MEDNIK syndrome-acronym for mental retardation, enteropathy, deafness, neuropathy, ichthyosis, keratodermia-is caused by AP1S1 gene mutations, encoding σ1A, the small subunit of the adaptor protein 1 complex, which plays a crucial role in clathrin coat assembly and mediates trafficking between trans-Golgi network, endosomes and the plasma membrane. MEDNIK syndrome was first reported in a few French-Canadian families sharing common ancestors, presenting a complex neurocutaneous phenotype, but its pathogenesis is not completely understood. A Sephardic-Jewish patient, carrying a new AP1S1 homozygous mutation, showed severe perturbations of copper metabolism with hypocupremia, hypoceruloplasminemia and liver copper accumulation, along with intrahepatic cholestasis. Zinc acetate treatment strikingly improved clinical conditions, as well as liver copper and bile-acid overload. We evaluated copper-related metabolites and liver function retrospectively in the original French-Canadian patient series. Intracellular copper metabolism and subcellular localization and function of copper pump ATP7A were investigated in patient fibroblasts. Copper metabolism perturbation and hepatopathy were confirmed in all patients. Studies in mutant fibroblasts showed abnormal copper incorporation and retention, reduced expression of copper-dependent enzymes cytochrome-c-oxidase and Cu/Zn superoxide dismutase, and aberrant intracellular trafficking of Menkes protein ATP7A, which normalized after rescue experiments expressing wild-type AP1S1 gene. We solved the pathogenetic mechanism of MEDNIK syndrome, demonstrating that AP1S1 regulates intracellular copper machinery mediated by copper-pump proteins. This multisystem disease is characterized by a unique picture, combining clinical and biochemical signs of both Menkes and Wilson's diseases, in which liver copper overload is treatable by zinc acetate therapy, and can now be listed as a copper metabolism defect in humans. Our results may also contribute to understand the mechanism(s) of intracellular trafficking of copper pumps.

Adaptor Protein-1 Complex Affects the Endocytic Trafficking and Function of Peptidylglycine α-Amidating Monooxygenase, a Luminal Cuproenzyme

“…Despite normal intracellular copper levels, copper delivery to ceruloplasmin is impaired in MEDNIK patients (14). Analysis of fibroblasts from MEDNIK patients showed that ATP7A accumulates at the cell periphery instead of concentrating in the Golgi region (14). A similar shift in ATP7A localization was observed in HeLa cells with reduced levels of 1A, a medium subunit, consistent with the need for all four subunits (6).…”

“…In sera from MEDNIK patients, ceruloplasmin, a copper-dependent secreted ferroxidase, and total copper levels are low, whereas free copper levels are elevated (1,14). The major carrier of copper in serum, ceruloplasmin, incorporates copper during its biosynthesis; lacking copper, apoceruloplasmin is rapidly degraded in plasma (15).…”

“…The major carrier of copper in serum, ceruloplasmin, incorporates copper during its biosynthesis; lacking copper, apoceruloplasmin is rapidly degraded in plasma (15). Despite normal intracellular copper levels, copper delivery to ceruloplasmin is impaired in MEDNIK patients (14). Analysis of fibroblasts from MEDNIK patients showed that ATP7A accumulates at the cell periphery instead of concentrating in the Golgi region (14).…”

“…Using shRNA to decrease expression of the 1A subunit to half, AP-1 was shown to play an essential role in secretory granule biogenesis and PAM trafficking in AtT-20 mouse corticotrope tumor cells (6,7,14). Although ATP7A trafficking in neuroendocrine cells has not been extensively explored, studies in other cell types predict an essential role for AP-1 (6,14,16,17).…”

“…Although ATP7A trafficking in neuroendocrine cells has not been extensively explored, studies in other cell types predict an essential role for AP-1 (6,14,16,17). AtT-20 cells produce proopiomelanocortin (POMC), a prohormone that is cleaved sequentially into a well characterized set of biosynthetic intermediates and product peptides that are either released constitutively or stored in secretory granules for release in response to secretagogue (Fig.…”