Cellular copper levels determine the phenotype of the Arg 875 variant of ATP7B/Wilson disease protein

Abstract

In human disorders, the genotype-phenotype relationships are often complex and influenced by genetic and/or environmental factors. Wilson disease (WD) is a monogenic disorder caused by mutations in the copper-transporting P-type ATPase ATP7B. WD shows significant phenotypic diversity even in patients carrying identical mutations; the basis for such diverse manifestations is unknown. We demonstrate that the 2623A/G polymorphism (producing the Gly⁸⁷⁵→Arg substitution in the A-domain of ATP7B) drastically alters the intracellular properties of ATP7B, whereas copper reverses the effects. Under basal conditions, the common Gly⁸⁷⁵ variant of ATP7B is targeted to the trans-Golgi network (TGN) and transports copper into the TGN lumen. In contrast, the Arg⁸⁷⁵ variant is located in the endoplasmic reticulum (ER) and does not deliver copper to the TGN. Elevated copper corrects the ATP7B-Arg⁸⁷⁵ phenotype. Addition of only 0.5–5 μM copper triggers the exit of ATP7B-Arg⁸⁷⁵ from the ER and restores copper delivery to the TGN. Analysis of the recombinant A-domains by NMR suggests that the ER retention of ATP7B-Arg⁸⁷⁵ is attributable to increased unfolding of the Arg⁸⁷⁵-containing A-domain. Copper is not required for the folding of ATP7B-Arg⁸⁷⁵ during biosynthesis, but it stabilizes protein and stimulates its activity. A chemotherapeutical drug, cisplatin, that mimics a copper-bound state of ATP7B also corrects the “disease-like” phenotype of ATP7B-Arg⁸⁷⁵ and promotes its TGN targeting and transport function. We conclude that in populations harboring the Arg⁸⁷⁵ polymorphism, the levels of bioavailable copper may play a vital role in the manifestations of WD.