The discovery in zebrafish could lead to treatments for children with Menkes disease, the researchers reported in the August Cell Metabolism, published by Cell Press. The rare genetic disorder, which is characterized by the inability to acquire copper before birth, affects about one in 300,000 people. Most children with Menkes die within the first decade of life.
The transparent and rapid development of zebrafish embryos permit detailed characterization of deficiencies from the moment of fertilization. The study method could ultimately unravel the interplay between genes and nutrition that can lead to many kinds of developmental abnormalities--advances that might one day allow for personalized prenatal care designed to reduce the risk of birth defects, said pediatrician Jonathan Gitlin of Washington University School of Medicine in St. Louis and the Children's Discovery Institute.
The research group found a role for copper in the formation of the notochord, a structure that, during the process of development in humans and other higher vertebrates, gives rise to the spine. The notochord also plays an important role in determining the developmental fate of other tissues.
"Our observations that copper is essential for notochord structure raise the intriguing possibility that suboptimal copper availability due either to dietary factors or genetic variation during the period of notochord formation or subsequent bone formation may contribute to structural birth defects such as congenital scoliosis" (or curvature of the spine), Gitlin said.
Copper--a nutrient found in foods including lobster, peanuts, and chocolate--is a critical component of several enzymes responsible for biochemical processes including cellular respiration, iron oxidation, and the formation of pigments and connective tissue, Gitlin said. The signs and symptoms of copper deficiency therefore result from the impaired function of such copper-containing enzymes.
Considerable clinical and experimental data had suggested that copper is an essential nutrient for normal development, the researchers said. Children with Menkes disease harbor a mutant version of the gene encoding so-called copper-transport ATPase (ATP7A), leading to symptoms including degeneration of parts of the brain and severe failure to thrive. Similarly, in a variety of mammals, copper deficiency during pregnancy results in loss of the embryo or offspring with severe neurologic impairment and developmental defects of the heart, skeleton, and other tissues.
By studying developing zebrafish treated with drugs that interfered with copper metabolism in the current study, the researchers found that copper deficiency led to a loss of pigment and profoundly altered notochord development. Further examination also revealed other impairments, including abnormal cartilage, vascular, and neurologic development, they reported.
Careful examination of embryos treated with different doses of copper-blocking drugs or for different lengths of time uncovered a hierarchy of copper distribution within the developing embryo. For example, a 10 min drug treatment resulted in absence of pigment but a normal notochord. Extending the treatment to 20 min resulted in loss of pigment and a clearly abnormal notochord, and by 60 min, the treatment led to still other defects.
The team then screened mutant zebrafish for defects like those caused by copper deficiency, identifying one with a "striking resemblance--particularly in its lack of melanin pigment and wavy notochord--to the chemically induced copper-deficient" animals. The zebrafish harbored a mutation in the copper transport gene ATP7A that underlies Menkes disease, the researchers reported. They further showed that insertion of a normal human copy of the ATP7A gene reversed the development defects in the mutant fish.
The discovery of the Menkes fish will now permit screens of pharmaceutical compounds that restore copper enzyme function in the setting of a deficiency of ATP7A, the researchers said. Such drugs would be of immediate clinical relevance for treating Menkes disease.
Similar studies in zebrafish that examine the role of other gene-nutrient interactions in early human development could ultimately lead to improvements in prenatal care with the capacity to reduce the risk of birth defects, the researchers said.
"While it will require an enormous amount of science, this is the first time it is even within our grasp to know how an individual woman's genes might affect her nutritional requirements and the risk that her children might develop a congenital disorder," Gitlin said. "Ultimately, that information could allow us to provide for every woman a recipe for what is right for her pregnancy."
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