By Kelly A. Reynolds, MSPH, PhD
The process of setting drinking water regulatory standards promises to become more high tech. It has long been recognized that sensitive subpopulations (i.e., infants and children, elderly and immunocompro-mised) are more vulnerable to adverse outcomes from exposures to drinking water pollutants and sometimes require additional means of protection. Identifying individual vulnerabilities at a genetic level, however, is now plausible. The following is a discussion of the impact the genetic revolution promises to have on the drinking water treatment industry.
Unprecedented discoveries
The Department of Energy’s Human Genome Project was an internationally funded effort aimed at identifying the excess of 25,000 genes that make up human DNA.4 Other goals of the project were to characterize the genetic sequence of some three billion chemical base pairs (the subunits of DNA) and to create an accessible database where this information could be utilized for the improvement of related technologies. The project began in 1990 and was completed in 2003, two years ahead of the estimated schedule and only 50 years after Watson and Crick’s momentous discovery of DNA. In 1999 the first human chromosome was sequenced and numerous others have since been characterized at their most basic structural level, including most recently, chromosome 13—a region of 95.5 million bases that carries genes associated with some types of breast cancer, bipolar disorder and schizophrenia.
Realizing the potential applications of the wealth of information the Human Genome Project data provides is staggering. An article for Science (2003)1 appropriately describes the phenomenom of a genome revolution. While the relative benefits to medical diagnostics, disease prevention and treatment identification are obvious, many other industries have been, or will be, greatly impacted by the current molecular era, including the drinking water industry.
Genetics 101
Cells are the basic building blocks of all living things. Within cells are the genetic codes necessary to instruct subsequent cellular processes. These codes are contained within the chemical structure of deoxyribonucleic acid, or DNA, of the cells. All DNA is made up of the same primary structural agents known as ‘bases’. These bases are called adenine, thymine, guanine and cytosine (A, T, G, C). Each DNA is made of two individual strands of bases that pair together to form the classic double helix shape. The order and number of these base pairs codes for the development of specific organisms and individual traits. Microorganisms contain hundreds of thousands of DNA base pairs while humans have billions.
An organism’s complete DNA structure is known as the genome. The human genome is constructed into 24 distinct chromosomes, each with thousands of genes. The primary function of genes is to house the instructions for making proteins. Proteins then serve to perform the majority of our body’s functions.
Science today allows for detection of the specific genomic sequence of any organism. Sequencing can also detect abnormalities or alterations in a genome. Alterations in the base sequence that composes a gene, can result in a change in the instructions for protein function. These alterations are called mutations. Some mutations have little or no observable effect on the protein and subsequent function, while others are associated with mild to serious physical abnormalities and/or disease.
Minimizing our risk of certain medical conditions involves recognition of both our genetic susceptibility and our environmental exposures. For example, we cannot alter our genetic (hereditary) risk of getting breast cancer, but we can reduce the overall risk by avoiding other compounding factors, such as smoking. This relationship between genes and the environment is important for risk characterization and exposure assessment.
A genetic link to water quality
In the early 90s, molecular methods, detecting the RNA or DNA of human disease-causing microbes, were increasingly applied to the monitoring of drinking water. Presently, the method is widely used for detecting viral, bacterial and protozoan pathogens and has helped to identify emerging waterborne pathogens that were previously unrecognized or underestimated.
In addition to the monitoring of drinking water contaminants, molecular research is now being used to evaluate the variable impact of human exposure to these contaminants. As evidenced by two recently published scientific studies, the sequence of individual genes can help to predict one’s susceptibility to an adverse outcome, following exposure to known waterborne contaminants.
Two recent (2005) studies support that genetic interindividuality should be considered when evaluating the health impact of environmental contaminants. One study identified a genetic association with arsenic metabolism among children.2 Arsenic is found in nature at low levels and makes its way to source waters via the weathering of rocks and erosion. Certain areas of the U.S. contain mineral deposits with high levels of arsenic that can leach into drinking water sources as groundwater flows through these deposits. Health effects from arsenic exposure include skin damage, circulatory system problems and an increased cancer risk, especially of the skin, bladder and lungs. The effect of arsenic exposure on any given individual is directly related to the ingested dose, duration of exposure, form of the arsenic compound and the immunological status of the person exposed, where age and general health play key roles. New research points to a genetic predisposition to children’s ability to handle arsenic.
Another study found a genetic link between perchlorate exposures and adverse outcomes.3 Perchlorate contamination has been identified in at least 25 states and is a concern in groundwater. Perchlorate is used in the manufacture of solid rocket fuels, missiles, fireworks and other explosives, lubricating oils, tanning and leather finishing, electroplating, rubber manufacture and the production of paints and enamels. The compound is highly soluble in water and degrades slowly in the environment. Exposure to high doses of perchlorate can effect thyroid hormone production, which is necessary for normal growth and development in infants and fetuses.
Both studies support that technological advances in genomic sequencing can be used to identify biomarkers that will help define additional subpopulations that are likely to experience increased adverse effects from certain waterborne contaminant exposures. Such an outcome promises to have a significant impact on the operations of public health and regulatory officials.
Future implications
Standards set in the water quality industry are developed with the general population in mind; however, often with a margin of safety built in. Additionally, consideration is given for increased risk in sensitive populations (young, elderly, immunocompromised). The relationship between genetic variability and susceptibility to waterborne contaminants among the general population has not been specifically addressed. Future studies are expected to identify additional subpopulations for consideration of water quality regulatory standards.
Recognizing the potential for known, as well as unforeseen, applications of the human genetic database, DOE and NIH genome programs reserved three percent to five percent of their respective total annual budgets for the study of the project’s ethical, legal and social issues. Added to the list of questions may be the relative implications in determining future environmental contaminant exposures and the responsibility of regulatory agencies in balancing water quality issues with technology from the genetic revolution.
References
- Frazier et al., 2003. Realizing the Potential of the Genome Revolution: The Genomes to Life Program, Science 300, 290.
- Meza, et al., 2005. Developmentally restricted genetic determinants of human arsenic metabolism: association between urinary methylated arsenic and CYT19 polymorphisms in children. Environmental Health Perspectives. 113: 775-781.
- Scinicariello et al., 2005. Genetic factors that might lead to differenct responses in individuals exposed to perchlorate. Environmental Health Perspectives. 113: 1479-1484.
- U.S. Department of Energy Office of Science. Human Genome Project Information, Doegenomes.org
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P technical review committee since 1997. She can be reached via email, [email protected]