The air and water grow heavier with the debris of our spectacular civilization.
Lyndon B. Johnson
message to Congress, 30 January 1967
Meeting of the Minds on Mercury
In an attempt to reconcile a body of controversial and conflicting scientific evidence, scientists and government representatives gathered 18-20 November 1998 in Raleigh, North Carolina, to discuss several studies on the health effects of methylmercury (MeHg), an organic form of mercury that bioaccumulates in fish and other seafood. The workshop was organized at the request of the White House Office of Science and Technology Policy, and was chaired by the NIEHS.
For the first time, researchers compared at once the studies conducted to date on populations in Iraq, the Faroe Islands, the Seychelles, and the Amazon Basin. Because of their seemingly contradictory findings, these studies have stirred up controversy over how high the acceptable threshold for exposure to MeHg should be set.
The original Iraqi study, by Thomas Clarkson of the University of Rochester in New York, looked at 84 mother-child pairs who had been poisoned by consuming contaminated grain in the winter of 1971-1972. This study found that MeHg in the mothers' hair at levels over 10 parts per million appeared to be related to neurodevelopmental abnormalities in offspring. Clarkson and colleague Philip Davidson undertook the Seychelles study in 1989 to test the Iraqi findings on a larger population, but the prospective, longitudinal main study of 779 children to age five showed no association with neurodevelopment. The scientists concluded that the low levels of mercury found in the ocean fish eaten by the Seychellois pose no health risks.
In the Faroe Islands, Philippe Grandjean and colleagues from the University of Odense in Denmark administered a battery of neurodevelopmental tests to 917 seven-year-olds whose mothers showed hair mercury concentrations of 10-20 µg/g. The team found a correlation between mild neurodevelopmental deficits and maternal MeHg exposure, indicating that MeHg exposure in utero may cause negative health effects.
Studies conducted by Donna Mergler of the Université du Québec in Montréal, Canada, among adult tribespeople of the Amazon Basin found that adults with levels of hair mercury below the currently recognized threshold of 50 µg/g demonstrated reduced nervous system function. A second study confirmed that hair mercury levels were significantly higher for those subjects with reduced motor control and restricted visual fields.
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Smoked fish. Federal agencies may be close to a coordinated standard for methylmercury, an organic pollutant that accumulates in fish. |
Five expert panels were convened at the workshop to discuss the studies in terms of a framework of questions. The exposure panel looked at relative exposures to organic and inorganic mercury, the sources of the exposures, and whether there were significant confounders. The neurobehavioral end points panel considered the identified health end points and how they were tested, and whether the tests used in the different studies are comparable. The confounders and variables panel scrutinized possible confounders and variability within the study populations. The design and statistics panel compared the statistical design and research protocol of each study and assessed their strengths and weaknesses. A fifth experimental panel looked at the contributions of animal and experimental studies toward interpreting the human studies.
The panels stated unanimously that the studies were of high scientific value, and that their results are credible. No obvious reason for the inconsistencies among the studies' findings was identified. The contradictory findings may be due to dietary confounders or to ethnic differences in response to MeHg. In addition, the Faroes and Seychelles studies looked at different health end points, conceivably leading to disparate results.
Based on the studies as a whole, MeHg must be considered a neurodevelopmental neurotoxin, although the effects of exposure to low doses is still unclear. The panels called for reevaluation of the two studies using similar assessment techniques and statistical methods to measure parallel health end points. It was also suggested that researchers might test other systems besides the neurodevelopmental system--such as the renal, hematological, and immune systems--for assistance in developing thresholds.
Only days before the workshop, the EPA announced that coal-fired power plants must now measure the mercury content of the coal they burn, and a randomly selected sample of 75 plants must conduct smokestack testing to establish the amount and type of mercury emissions, with the results to be made public beginning in early 2000. This unprecedented move is part of a larger strategic plan to reduce emissions of several persistent, bioaccumulative, and toxic pollutants. It is expected that the panel comments and workshop report will significantly influence future public health policy decisions by the EPA and other agencies.
Some groups feel these steps aren't coming a minute too soon. According to a report entitled Poisoning Our Future: The Dangerous Legacy of Persistent Toxic Chemicals, released in November 1998 by U.S. PIRG (the national lobbying office for state-level public interest research groups) and the National Environmental Trust (a nonprofit educational organization), nearly 20 million pounds of mercury and other toxic pollutants are released into the air, land, and water each year. Because reporting thresholds are set too high, the report says, this is only a fraction of the real sum of pollution going into the environment.
George Lucier, director of the NIEHS Environmental Toxicology Program and cochair of the workshop, was pleased with the outcome of the workshop. "This is exactly the kind of thorough analysis that's needed to provide the science base necessary for making public health decisions," he says.
New Understanding of Algae
Within one week, British botanist Brian A. Whitton received two phone calls about cyanobacteria in local reservoirs--one caller wanted to know how to eradicate a potentially toxic bloom, the other wanted advice on marketing a related species as a health food. The anecdote, shared by Whitton at the 4th International Conference on Toxic Cyanobacteria, held 27 September-1 October 1998 at the Duke University Marine Laboratory in Beaufort, North Carolina, points to the complexities in understanding such organisms; some contain potent toxins and others promise health and therapeutic benefits.
An unprecedented number of laboratories worldwide are studying cyanobacteria, also known as blue-green algae, and participation at the September conference ballooned from early predictions, said conference cochair Wayne Carmichael, a cyanobacteria researcher and professor of aquatic biology and toxicology at Wright State University in Dayton, Ohio.
Microcystins were a common theme among many presentations. Toxic substances manufactured in the cells of some cyanobacteria, microcystins have been blamed for the liver failure and subsequent death of 75 dialysis patients at a clinic in Caruaru, Brazil, in 1996, said Carmichael. Other patients suffered nausea, vomiting, headache, and visual disruption, including blindness. The poisonings occurred in spite of water treatment methods in place that should have removed the contaminants from the clinic's water supply. Although microcystins have also been suspected in dialysis clinic deaths in Portugal, the Brazil fatalities were the only ones to date documented by analyses of blood serum and tissue samples.
Could this scenario occur in the United States or other developed nations? Cyanobacteria appear around the world in freshwater lakes and brackish waterways; some even tolerate seawater, and a few are terrestrial. Aquatic blooms flourish in the presence of increased nutrients, especially phosphorus. Carmichael reminded conference participants that water treatment procedures are only as good as the upkeep that accompanies them.
"We tend to pride ourselves on the fact that we have better water treatment [in the developed world]," he said. "But if you look at the water treatment process in place here, it's not much different than any other place in the world. I'm told that in most dialysis clinics in the United States, it's not a fail-safe procedure, that it needs to be maintained properly."
In a study of selected water utilities throughout the United States and Canada during 1996 and 1997, Carmichael and his colleagues found microcystins in 65% of water samples that had not undergone treatment. A third of the positive samples registered levels of microcystins in excess of the recently adopted World Health Organization (WHO) drinking water standard of 1.0 µg/l of total microcystin. In a few cases, treated water samples contained microcystin levels higher than the WHO guideline.
Cyanobacteria species number in the thousands, and it is not known what percentage produce toxins. Besides microcystins, which attack the liver, other cyanotoxins have been identified that affect the nervous system and cause skin rashes. For more than a century, blue-green algae have been implicated in the deaths of livestock and other animals that drink contaminated water. People have historically avoided drinking from and bathing in lakes and other waters laden with surface scum, primarily because the algae produce a bad taste and odor.
Blue-green bombshell. Cyanobacteria such as Microcystis aeruginosa (l) and Nodularia (r) contaminate much of the world's water supply with toxins that cause serious health effects.
(l) Colin Carbis, (r) David Maddison
"Decreasing water quality throughout the world means that these blooms are present more often and for a longer duration, and people are using marginal water supplies more than they did in the past. They are forced to bathe in a bloom or to use water from a bloom," says Carmichael. "We are now starting to see acute poisonings, contact irritations, accidental ingestion, and low-dose exposure [that may result in] liver cancer."
Ian Falconer, a professor at the University of Adelaide Medical School in Australia, said reports of contaminated drinking water are pushing blue-green algae into the spotlight. "Lead, arsenic, and things like that are known environmental contaminants," Falconer said, "where cyanobacteria have only really become significant through eutrophication. There is an issue everywhere with respect to the water industry in meeting the WHO guidelines. The other issue is whether the most sophisticated treatments, which are more expensive, are actually [being used]."
Scientists from 35 countries made up the more than 150 participants at the conference, and presented on a range of topics including laboratory toxicity and developmental tests in animals, molecular analysis, toxin detection, regulation, and ecological studies. The beneficial aspects of cyanobacteria were also represented. While such algae as Microcystis, Anabaena, and Nodularia produce chemicals lethal to humans and animals, some blue-green algae produce compounds that have anticarcinogenic, antibacterial, or antifungal properties.
Extracts of a blue-green algae have produced the antitumor drug cryptophycin, which is currently in clinical trials, said Richard Moore, a professor of chemistry at the University of Hawaii in Honolulu. Extracts from a marine blue-green algae collected in Guam have also exhibited antitumor properties, and appear to be chemically similar to dolastatin 10, a compound derived from marine mollusks that is also undergoing clinical trials as a cancer drug, Moore said.
Consumption of blue-green algae has also been touted as a health benefit by the dietary supplement industry. More than 1 million people in the United States and Canada consume blue-green algae in capsule or tablet form. Much of the product is harvested from blooms of Aphanizomenon in Oregon's Klamath Lake. Concurrent toxic blooms of Microcystis aeruginosa in 1996 prompted consumer concern about the safety of the product. Health officials subsequently determined a standard of 1 µg/g (equal to 1 ppm) to be a safe level of microcystins in supplements, said Duncan Gilroy, a public health toxicologist with the Oregon Health Division in Portland. But 50 of 67 samples obtained from harvesters, wholesalers, and retail outlets exceeded 1 ppm, he said. "We are concerned about people getting exposure to even low levels of microcystin for the long term," said Gilroy.
Alzheimer's Disease: A Basis in Bacteria?
According to a study published in the June 1998 issue of Medical Microbiology and Immunology, the bacterium Chlamydia pneumoniae may be linked with, or possibly even responsible for, Alzheimer's disease, a form of dementia that currently affects approximately 4 million U.S. citizens. Brian J. Balin, an associate professor at the Philadelphia College of Osteopathic Medicine in Pennsylvania, and Alan P. Hudson, an associate professor in the department of microbiology and immunology at Wayne State University School of Medicine in Detroit, Michigan, led what may prove to be a huge step forwarding in understanding Alzheimer's. Says Balin, "We don't have a definite cause yet, but we definitely have a potential cause or risk factor for the disease."
C. pneumoniae is the cousin of C. trachomatis, which causes both the sexually transmitted infection commonly known as chlamydia and trachoma, a form of blindness. C. pneumoniae is a pervasive bacterium, appearing in 40-70% of adults. Unlike most other bacteria, C. pneumoniae can only live inside a cell and draws its energy from the cell's supply of adenosine triphosphate. The bacterium is spread through sneezing and coughing, and causes diseases such as sinusitis and bronchitis.
In studying the relationship between C. pneumoniae and late-onset Alzheimer's, Balin and Hudson used postmortem samples from various regions of the brains of patients both with and without Alzheimer's. From each sample, the scientists prepared samples of nucleic acid, which were screened by polymerase chain reaction assay for the presence of two C. pneumoniae marker genes. Analyses showed that samples from 17 of the 19 brains with Alzheimer's tested positive for the bacteria, while only 1 of 19 brain samples without Alzheimer's tested positive.
The C. pneumoniae cells were found to be concentrated in the microglial and astroglial cells. (The microglia function as an immune system within the brain, generating severe inflammation when infected, while the astroglia promote neuronal cell maintenance and growth.) The organism was also found to be concentrated in brain regions marked by neuropathologies characteristic of Alzheimer's, such as neurofibrillary tangles, composed of the cytoskeletal proteins of damaged neurons, and neuritic senile plaques, caused by the accumulation of the peptide ß-amyloid--believed by some scientists to be secreted by the astroglia.
Balin and Hudson think the deposition and processing of ß-amyloid may begin after an initial infection with C. pneumoniae. "Where you have certain types of pathology, that's where you have accumulation of the organism," says Balin. This raises the possibility that the damaging plaques and tangles may form in response to infection with C. pneumoniae. Balin also believes the organism may live in people's cells for decades, causing a slow, chronic immune response. By hiding out in the body, he posits, the bacterium escapes surveillance as a foreign body.
There may be additional factors acting in relationship with C. pneumoniae. "If everyone gets C. pneumoniae, why doesn't everyone also get Alzheimer's disease?" asks Hudson. "There are unidentified genetic factors working in relationship with C. pneumoniae." One possible partner is apolipoprotein E, a serum protein used in lipid transport. Each person's DNA contains some combination of two of five different alleles of apolipoprotein E; while the APOE4 allele occurs in 13-15% of the general population, it turns up in 60% of Alzheimer's patients. Hudson and Balin think APOE4 may act in conjunction with C. pneumoniae infection to cause Alzheimer's. Says Balin, "We have a realistic mechanism for allowing the organism to enter cells in certain people."
The scientists also performed electron- and immunoelectron-microscopic studies on tissue samples from both Alzheimer's and non-Alzheimer's brains. These studies also identified C. pneumoniae organisms in the Alzheimer's samples but not in the non-Alzheimer's samples. Culture studies of a subset of Alzheimer's brain tissues for C. pneumoniae were strongly positive, while identical analyses of non-Alzheimer's brain tissues were negative.
One of the next steps for the scientists is to investigate how C. pneumoniae infiltrates the nervous system. Balin plans to examine postautopsy brain samples to determine the earliest stages of late-onset Alzheimer's and identify whether the organism is present in those stages. Other future projects include a cell culture study to demonstrate which specific cellular mediators are turned on by the C. pneumoniae organism, and developing methods for diagnosing C. pneumoniae infection. The findings of this research could result in clinical trials of antibiotics and antiinflammatory agents to treat Alzheimer's in the near future. Says Balin, "If we can block the stimulus for inflammation and inflammation itself, we may be able to prevent damage." At the least, says Hudson, damage could be postponed or ameliorated.
Gene-Toxicant Link to Parkinson's Disease
The combination of pesticide exposure and a particular genetic trait may increase the risk that people with Parkinson's disease will develop dementia. That's the conclusion of a preliminary study conducted by researchers at the University of Kansas Medical Center in Kansas City, Barrow Neurological Institute in Phoenix, Arizona, and the Veterans Affairs Puget Sound Health Care System in Seattle, Washington. The study, published in the March-April 1998 issue of Neuroepidemiology, looked for possible causes of the dementia that occurs in about one-fifth of all Parkinson's patients.
This is your brain on pesticides? New research suggests exposure to pesticides may be linked with Parkinson's disease.
Parkinson's disease is a brain disorder that causes trembling, slow movements, a rigid posture, and an unbalanced, shuffling walk. The subjects of this study were 94 patients with Parkinson's disease, 43 of whom had dementia and 51 of whom did not. The researchers looked at various environmental, sociodemographic, and clinical variables, as well as three suspected gene markers. As in previous research, they found that lower educational level, older age at disease onset, and greater motor impairment were individual risk factors for dementia. For the first time, they also showed that patients who had been exposed to pesticides in the past and who had a gene known as the CYP 2D6 29B+ allele were three times more likely to develop dementia than patients who lacked this combination of factors.
"This study was the first attempt in Parkinson's disease to see if we could identify interactions between unique genetic makeups and exposure to environmental toxins," says coauthor Matthias Kurth, director of clinical science for Axys Pharmaceuticals in La Jolla, California. For purposes of the study, the researchers defined pesticide exposure as pesticide use for more than 20 days in any given year, a level they say is based on typical use of the chemicals in commercial grain farming. The allele in question activates a series of enzymes in the liver that metabolize and detoxify chemicals that enter the body. However, up to 10% of Caucasians have a mutant version of the allele, which results in no activity of this enzyme system. The hypothesis is that such people might be particularly susceptible to environmental toxicants because of their inability to detoxify chemicals.
While provocative, this study does have some important limitations. Its case-control design is less than ideal for studying risk factors in Parkinson's disease, because symptoms of the disease may change over time. Thus, some of the subjects in the no-dementia control group may later develop dementia. Another drawback is the study's reliance on self-reports from dementia patients. Yet another concern is "the absence of predictive value for either pesticide exposure or the allele taken alone," says Jean Harry, a group leader in the NIEHS Environmental Toxicology Program. Only the interaction between these variables was statistically significant. In addition, Harry notes that the study used "a crude estimate of pesticide exposure." Still, she says, the results offer interesting data for further evaluation.
Clearly, the gene-toxicant interaction still needs to be confirmed by further research, ideally a larger, prospective study of Parkinson's-related dementia. Several previous studies found an association between Parkinson's disease and pesticides, other toxicants, and rural living, but so far no specific pesticide or class of environmental toxicants has been definitively linked to the disease. "Overall, past studies support the notion that pesticide exposure is a risk factor for Parkinson's disease, but only in a subset of individuals," says Kurth. In fact, he says, "Most Parkinson's patients do not have a history of pesticide exposure, and the recognition of Parkinson's disease predates the development of these compounds." The gene-toxicant interaction is another tantalizing clue in the quest for the causes of Parkinson's disease in its various forms, but only time will tell whether it is a true clue or a red herring.
As the Human Genome Project steadily rolls forward, much of the information on human chromosomes is being catalogued in digital databases such as GenBank, which now contains well over 141 million base pairs' worth of information on human DNA.
The problem facing scientists now is not so much how to gather genetic information, but rather how to take advantage of the data that have already been harvested. This is where researchers such as Harold "Skip" Garner are trying to make a difference. With the help of a Hewlett-Packard supercomputer and the World Wide Web, Garner and his colleagues at the University of Texas Southwestern Medical Center in Dallas are gathering useful information from GenBank and making it available to researchers across the globe.
Especially interesting to scientists that study genetically influenced diseases are the pieces of human DNA that tend to vary among individuals. Differences in these regions--called polymorphisms--can be telltale signs that a person carries a genetic disease or is genetically susceptible to getting a disease. Because GenBank contains many duplicate bits of DNA data that come from the same part of the same chromosome but from different individuals, analysis of the database can show where polymorphisms tend to occur.
Gleaning the locations of polymorphisms from the data in GenBank is the purpose of the Polymorphic Marker Predictions of Ubiquitous Simple Sequences, or POMPOUS, a suite of computer programs designed by Garner and his colleagues. Thus far, POMPOUS has found 13,261 polymorphic regions in the human genome. Information about these regions, including their nucleotide sequences, is available on the Garner laboratory home page at http://pompous.swmed.edu.
By clicking the POMPOUS link on the Garner home page and then following the link named Analysis of Your Sequence by POMPOUS Server, researchers can also make use of the supercomputer's eight processors and 0.5 gigabytes of RAM to compare their own data to GenBank. Not only will the computer find known polymorphic regions in the supplied DNA sequence (up to 32,000 bases long), it will also return a pair of primers for each. Primers are unique DNA sequences used to isolate the polymorphic region for further study. In a test of the software, POMPOUS identified 33 polymorphic regions in a part of a human chromosome implicated in breast and lung cancer. Twenty-two of these were shown to actually vary in a group of 36 cancer patients.
Other software tools for genetic researchers are also available from the Garner site. These can be accessed by following the Computational Biology link on the home page. One of these tools, GeneAlert, assists researchers who are analyzing a known sequence of DNA using the National Center for Biotechnology Information's BLAST programs. GeneAlert will automatically pare down the often exhaustive lists of matching DNA sequences returned from such a search to only those that are relevant, for instance, only those containing a certain keyword such as "human" or "carcinoma." Another software tool available at this site is SIGNAL, a program that can be downloaded and used to make comparisons between DNA and protein sequences. Other tools, including a complex and thorough gene hunting system, are planned for inclusion on the site in the future.
Another highlight of the Garner site is the information on biochips available by following the Chip Based & Nanovolume Biology link. The chips are a promising new technology for sorting DNA samples so that their nucleotide sequences can be identified.
A biochip contains an array of tiny, segregated spots, each with its own sequence of nucleotides. The nucleotide sequences on the biochip, to which the sample DNA strands bond, can be constructed by controlling the reagents and the amount of light that reaches each spot on the chip during construction. Currently, the Garner laboratory is experimenting with a light processing microchip and a reagent delivery system that will allow the researchers to build chips with up to 2 million unique spots. Such chips may be able to efficiently diagnose genetic diseases and identify medically relevant genes.
Last Updated: December 30, 1998