Scanning methods involve testing the gene or genes having no assumptions about any previous mutations (3). A major limitation in this method is that only the coding regions, splice sites and promoter regions are scanned (3). This excludes mutations within the regulatory regions other than the promoters, introns, as well as other genes whose protein products could potentially interact with the disease-causing gene. Some of the scanning methods include: nucleotide sequencing, tests of molecular phenotype, protein truncation, and just recently DNA chip technology (3). One of the major obstacles in developing successful screening techniques involves the size and complexity of disease related genes.
The production of DNA chips have evolved along two major pathways: one method uses nucleic acids that have been immobilized on the chip surface sequentially to form oligonucleotides and the other method involves complementary DNA from an individual with a known genetic mutation as a source of prefabricated oligonucleotides (2). In either case, the problem lies with how to attach the nucleic acids or cDNA to the chip.
Chips using nucleic acids are produced using photolithography. Photolithography, according to the Science article by Stephen Fodor, consists of the modification of synthetic linkers, containing photochemically removable protecting groups, attached to a glass substrate, usually a silicon-derivative glass chip. Light is directed at the photolithographic "mask" at specific areas of the chip in order to facilitate the removal of the photoactive groups, yielding 5( hydroxy groups. These modified groups are now capable of binding other nucleotides, generating a highly specific probe, which contains the sequence of a known disease causing genetic mutation.
The other method, described in the DNA Chips and Microassays website, uses purified single-stranded cDNA from an individual with a known genetic disease, requiring the use of touch or fine micropipetting, to spot the cDNA onto the surface of the chip. The cDNA immobilizes on the chip through covalent bonds, due to the positively charged surface, produced by amino silane or polylysine (2). For both types of chips, a potential DNA target sequence, from an asymptomatic individual, is fluorescently tagged and allowed to interact with the probes. Hybridization will occur at complementary sequences between the two samples resulting in a fluorescent image, which is then scanned by a laser beam and analyzed by a computer. The intensity of fluorescent light varies with the strength of the hybridization, thus providing a quantitative 'snapshot' of gene expression (7).
This approach, requiring only minute consumption of chemical reagents and minute preparations of biological samples, can scan more than 400,000 probes placed on a single chip measuring 1.28cm X 1.28cm in size (7). As of now, specific chips are available for as little as $100, but could cost over thousands of dollars, once custom-made chips are available (2). In the future, attempts to design chips using the computer, instead of doing it by hand, will greatly speed up the process allowing companies to make custom chips in one day, as opposed to months, which would lower the cost of production. Consequently, DNA chips could probably sell for about $50, providing access to scientists regardless of their funding situation (10).
Genetic testing has been slow to achieve popularity, partly due to the likelihood of obtaining meaningful data. When there are so many genes interacting together within any given disease, how could one test possibly be able to examine every pathway that could lead to a potential disease? Scientists have produced DNA chips with the capability of containing 20-30,000 different probes per square centimeter (2). This means, virtually all genetic sequences pertaining to a particular disease could be formatted on one DNA chip. This technology could greatly reduce health care costs by reducing the number of visits to the doctor or perhaps even a specialist, by cutting back on the number of useless diagnostic tests, by reducing the amount of needless pharmaceutical prescriptions, or because the chip has a shelf life of up to several months, there should not be a need to throw out expired chips. With the DNA chips intricate design, even hard to find genes found in mRNA species are able to be detected (7).
DNA chips sound simple in concept, but generating probes on a solid array surface requires considerable expertise and "technical wizardry" (2). At present, DNA chips are much too expensive and limited in application, because their use requires prior knowledge of gene sequences and any interactions with other genes, in order to be available for use in medical practices. DNA chip making is a complex process and most of the labor is usually done by high tech and fairly expensive robotics.
Critics of genetic testing believe it is unethical due to the lack of comprehension in the test results. A positive outcome can effect a person's life in important ways: allowing for earlier detection, the possibility of prevention, or the ability to make personal medical and lifestyle decisions. On the other hand, a positive test could also result in potential employment discrimination, ineligibility for health insurance coverage or higher premiums, as well as in physical and psychological strain. The decision is especially wrenching for persons confronted with a disease that can be neither prevented nor cured. For these reasons, many groups and individuals oppose genetic testing. In contrast, advocates believe these concerns can be minimized if potential consumers are educated about the limits of tests and their potential consequences through genetic counseling.
Several public policy measures have been taken in the hopes of preventing genetic discrimination by health insurance companies. After four years of discord, eleven states have now passed bills that would bar health insurance companies from using genetic tests as the sole reason to deny or cut coverage or to charge higher premiums (8). People should not be punished for a hereditary predisposition that may or may not make them sick in the future. Congress in 1995, forbade group health policies in the workplace from denying coverage based on genetic tests (8). The Health Insurance Portability and Accountability Act of 1996 mandated that genetic test results alone cannot be treated as a pre-existing condition and group medical plans cannot force any person to pay a higher premium than people of equal status, based solely on genetic test results (5). These laws are definitely a step in the right direction, however, as genetic testing increases in use, more legislation unquestionably will be needed to fully protect at-risk individuals.
At first, the general public will think of genetic testing as a noble, technical advancement in science, one in which the knowledge gained from these tests could help prevent such painful genetic diseases, such as cystic fibrosis, many of the cancers or storage diseases. When they think of genetic testing, they will have someone in mind who is near and dear to their hearts, who is suffering from a genetic disease, which cannot be effectively treated. These people are the ones in favor of genetic testing, in order to find the disease-causing mutation, which in the future would help gain insight into genetic therapy. This would be fine, if the genetic research would stop there. However, if scientists can find out what causes genetic diseases, why not try to prevent them from ever occurring, by genetically engineering a disease-free human. Scientists could potentially produce a human race that could live well past a century. The world's population has already approximately doubled since the 1950's, how many more people can the Earth support before another mass extinction occurs and wipes out the entire ecosystem. I believe that everyone was put on this Earth for a purpose, whether that means they have to suffer from an illness and die at an early age or they lead a healthy life and live to be one hundred years of age. I consider a true test of the existence of evolution, when we have the knowledge and technical aspects to greatly enhance the human race, but have the common sense and morality to restrain ourselves.
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