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Abstract
Regulating metals in humans leaves little room for error. A recent Metals and Genetics conference in Toronto highlighted what goes wrong when the scales are tipped by genetic mutations.
Inside the human body, metals function in a delicate balance: They are essential to life, but too little or too much can be devastating to the system. Time after time, during the second quadrennial Metals and Genetics conference held May 26-28, 1998, in Toronto, speakers recounted human stories illustrating this devastation.
Perhaps most poignant was the ceremony honoring
Karen Wetterhahn, a
scientist who died last year after a long fight against severe mercury
poisoning. Some dimethylmercury
with which she had worked in her lab had
seeped through her glove, giving her a lethal dose in seconds. Commemorating
her dedication to her field gave the meeting a bittersweet start - and a
context larger than the science itself.
The host of the symposium, Bibudahendra Sarkar of the Hospital for Sick Children - the site of the proceedings - has devoted his career to the interactions of metals such as zinc, nickel, and copper with proteins and nucleic acids. Guided by the late Andrew Sass-Korstak, Sarkar formulated for the juvenile copper-transport disorder Menkes disease a treatment that has brightened the future for many children [1].
Another copper-related disorder is Wilson disease (WD), which is caused by mutations in a gene called ATP7B. The gene was isolated in the laboratory of Diane Cox of the University of Alberta at Edmonton, who was present to discuss the mysteries that continue to surround it.
ATP7B is a member of the P-type ATPases, a family of metal-transport
enzymes, and is involved in the transport of copper away from the liver.
Curiously, although mutations in ATP7B can lead to toxic copper buildup in
the liver, none of its over 70 known mutations occur within any of the six
copper-binding domains (CBDs). Cox is categorizing the mutations into
haplotypes, hoping to explain their effects.
Mutations of the individual CBDs might exist yet not be manifested as disease, she suggested, because of some redundancy between the many different CBDs. Cox also described model systems now being used to analyze WD, including CCC2, a homologue of ATP7B present in yeast. Currently, Cox and her group are transfecting human ATP7B into CCC2 knockout yeast, to investigate which changes in ATP7B are disease-causing mutations and which are innocuous polymorphisms.
The Menkes disease for which Sarkar devised a treatment is caused by
mutations in another member of the same ATPase family, a protein called
ATP7A. It has a similar sequence structure to ATP7B, including the six CBDs.
Bibudahendra Sarkar himself gave a molecular portrait of the Menkes-disease
protein, concentrating on its metal-binding characteristics. Using zinc
blots, Sarkar described competition for binding between various metals
including gold and mercury. (Copper, naturally, binds the best.) Neutron
activation analysis indicates that the Cu:protein ratio is about 7:1,
corresponding with the six CBDs in each protein molecule.
Zeynep Tumer of the University of Copenhagen presented the findings of her group and of collaborator Nina Horn on the mutations in ATP7A which are responsible for Menkes disease [2]. They block absorption of copper in the gastrointestinal tract, leading to storage of copper in a complex with metallothionine (Cu-MT). This complex steals the copper away from copper-containing enzymes such as lysyl oxidase, cytochrome C oxidase, and superoxide dismutase, which are important in development. However, the fact that copper is sequestered rather than circulating freely may explain why Menkes disease is less toxic than Wilson disease.
Surprisingly, as with ATP7B, the majority of point mutations among the 191
known mutations in ATP7A occur outside the CBDs. The analysis of these
mutations in ATP7A has not yet turned up an explanation, but it has led to
genetic tests for Menkes disease. Tumer told of a family with a history of
Menkes that terminated a pregnancy after a traditional test found elevated
copper levels in the fetus. Subsequent DNA testing revealed no sign of
Menkes in a later fetus - nor in
the earlier one. Apparently the original, non-genetic test had some copper
contamination - a tragic testament to the value of genetic research.
Stuart Tanner of the University of Sheffield (United Kingdom) described a copper-related childhood liver disease whose genetics is less well understood. There are cases of copper overload - including Indian childhood cirrhosis (ICC) [3], first described in pockets of Indian children who had been drinking excessive quantities of copper in contaminated water, but also reported in Germany and Australia, caused by low-pH well water flowing through copper pipes. In contrast, Tyrolean childhood cirrhosis (TCC) is an example of gene-environment interaction, not mere overdose. Tanner told of babies who were fed a formula that had turned green because copper leached from the cooking pots. Yet not every one of them showed signs of TCC. In other cases, there seems to be no environmental overload at all, implying a genetic cause. The specific genes have yet to be isolated, and analyses for mutations in known genes such as ATP7B have so far come up negative.
Dipankar Chakraborti of
Jadavpur University
(Calcutta) vividly described a
study of environmental overload involving arsenic rather than copper.
Working with Sarkar, Charkraborti was able to trace arsenic poisoning that
has affected over 50 million people in West Bengal and parts of Bangladesh.
To ensure clean drinking and irrigation water, tube wells have been
installed across remote parts of India and Bangladesh, thanks to
well-meaning Western aid. Many people in these areas began to experience
skin lesions similar to leprosy. The tube wells have drained the aquifer to
the point that arsenic salts in the underlying strata are being oxidized,
producing free arsenic that dissolves into the water systems. Nothing short
of a complete reexamination of surface and rainwater usage, as well as
proper watershed management, will reverse this poisonous development.
Metals have a sobering history in human biology, but there are grounds for optimism. In the four years since the previous conference on the same topic, several genes related to metal disorders have been isolated, and many metal-related biochemical pathways are starting to be elucidated. During a ceremony at the symposium's banquet, which featured the tones of an Alpen horn and a kissing of the cod (actually a salmon; cod have been overfished), attendees spoke of looking forward to reports of a deeper understanding of metal and disease, four years hence, at their next gathering.
Randall C. Willis is a scientist in the Department of Biochemistry Research at Toronto's Hospital for Sick Children, where he spends his days expressing and purifying proteins for NMR structural analysis.
Evolution of Our Understanding of Methylmercury as a Health Threat - a discussion by Chiho Watanabe and Hiroshi Satoh of the research carried out on this dangerous chemical.
Menkes disease and Wilson disease - the OMIM entries for these disorders provide clinical background information and summaries of recent research findings. For links to molecular information about the genes behind these diseases, see NCBI's ATP7B and ATP7A entries.
Cu Transport in Saccharomyces cerevisiae - a brief introduction to the transport of Cu in this model organism, including a discussion of CCC2 and a helpful illustration.
Arsenic in Ground Water in Six Districts of West Bengal, India - part of the extensive West Bengal and Bangladesh Arsenic Crisis Information Centre, this is an online version of Dipankar Chakraborti's paper in environmental geochemistry and health.
Web sites mentioned in this column: