Genetic differences contribute to the variation among individuals in coping with environmental stresses, such as nutritional deficiency. Copper is an essential nutrient, and disorders of copper metabolism can lead to severe neuronal, muscular and pigmentation clinical symptoms. Genetic mutations in the copper transporter ATP7A lead to a copper-deficiency syndrome called Menkes disease, and copper deficiency can also develop after intestinal bypass surgery or an excess intake of iron. However, many causes of copper deficiency are unknown, and genetic factors might contribute to the sensitivity of some individuals to reduced nutritional availability of copper.

Many crucial enzymes require copper as a cofactor, and melanocytes have a specific requirement for copper in the maturation of pigmented melanosomes. Here, the authors use a zebrafish- and yeast-based approach to identify genetic pathways that modulate melanocyte pigmentation in conditions of copper nutritional deficiency. First, they carry out a chemical screen for small molecules that affect copper homeostasis in zebrafish on the basis of a hypopigmentation phenotype. Next, they use budding yeast to systematically map the genetic pathways that underlie sensitivity to the small molecules identified in the zebrafish-based chemical screen. They then demonstrate that two genes encoding intracellular transport proteins identified in the yeast genetic screen are physiologically relevant, as their zebrafish orthologs, aps1s and ap3s2, are involved in sensitizing zebrafish melanocytes to hypopigmentation in conditions of mild copper deficiency. Notably, the authors also use the zebrafish-yeast approach to identify an off-target effect for a small molecule commonly used in cell-signaling studies, the MEK inhibitor U0126, which was previously not known to affect pathways of copper metabolism.

The copper-gene interactions identified in this study illustrate the larger issue of how disease susceptibility can be underpinned by complex environment-gene interactions. This zebrafish-yeast approach will be applicable to the dissection of many complex disease-gene networks, particularly because such networks are enriched for highly conserved genes. This approach is also well suited for investigating gene-environment interactions, which are a challenge to assess given that modest genetic effects can be difficult to identify through classical quantitative methods. Importantly, the zebrafish-yeast approach provides a systematic means by which to elucidate the in vivo actions of small molecules in a vertebrate system, which is a challenge in chemical biology applications. Finally, the identification of copper-metabolism pathways in zebrafish is a starting point for exploring the role of analogous pathways in human diseases of copper deficiency.