The Human Genome Project was an epic race to sequence the entire human genome, a feat that took almost thirteen years to complete, in 2003. Now genome sequencing is faster, cheaper, and more accessible. Doctors can request that a patient’s genome be sequenced to help determine whether symptoms are due to a genetic disease. Or, for those who prefer the at-home version, for several hundred dollars and a vial of saliva, an online company will sequence portions of your genome and determine your propensity for more than 200 traits and health risks. It seems as though the Human Genome Project is finally paying off and the idea of genetically-tailored medicine is right around the corner.
Some hope such developments may lead to the kind of drive-through genome sequencing seen in the movie GATTACA, in which a person’s genetic code can reveal everything from male pattern baldness, to heart disease, to a person’s projected death date. However, the possibility of technology like this also prompts fears that your genetic information could fall into the wrong hands, or that knowing too much about your own future could negatively impact quality of life. Could insurance companies charge more based on the presence of certain genetic variants? Could employers choose whether to hire (or fire) someone based on their genetics? And, perhaps most unsettling, could genome sequencing eventually predict when and how a person will die?[1]
One could multiply such rhetorical questions, but it is worth considering whether such concerns are merited or whether genetic testing is little more than a technologically advanced version of palm reading. Let’s separate the science from the hype.
First, there are indeed genetic markers for certain diseases. Take Huntington’s disease, for example. A person with Huntington’s has an inordinate number of a particular trinucleotide repeated sequence on chromosome 4. These sequences have been studied extensively, and now genetic tests can determine if an individual will develop Huntington’s later in life. However, Huntington’s is the exception when it comes to the relationship between genetic markers and diseases. In most cases, if someone has a particular genetic marker for a disease, it does not follow that she will inevitably develop the disease. Genes must be expressed, and expression depends on a number of factors, including epigenetic factors that have nothing to do with one’s sequence of A’s, T’s, G’s, or C’s.[2] Identical twin studies have shown that even when two individuals begin life with the same genetic sequence, epigenetic factors and mutations that occur throughout life mean gene expression may be different for each individual.[3]
Second, genetic tests are based on a comparison to a norm, which is often based on relatively few genetic sequences. In reality, most people have some mutations or rare variants within their genetic code, and some rare variants are more common in certain ethnic groups or populations than others, making individual genomes less similar than was once thought. An article in The Scientist points out the trouble with current genetic sequencing:
Indeed, many of the studies that were done over the past decade to identify and measure the effects of genetic change were carried out using tools that were created with the assumption that genetic differences are rare. The most common tools are all based on a single human reference genome sequence that was put together nearly 10 years ago.[4]
Comparing genetic sequences to such a limited number of references causes minor variations to yield inconclusive or misleading results. For genetic tests to provide helpful information, they should be compared to a large number of reference sequences, with relevant variations taken into account.
Given these considerations, genetic testing is usually not very effective in predicting the future, but it can provide some helpful information for the present. Genetic sequencing can indicate if someone has a higher probability of getting certain diseases, particularly those diseases that have been well-studied. But probability does not equal inevitability. Risk probabilities ascertained through genetic testing may motivate some people to make healthy lifestyle decisions, but risk for some diseases may not be affected by lifestyle choice. For instance, is it beneficial to learn one might have a propensity for that kind of disease, or will this lead primarily to fruitless (and often unnecessary) worry? Alternatively, genetic sequencing can be used to look for a disease after a person has displayed symptoms. In the case of Adam Foyes, one of many cases highlighted in Time magazine’s cover story on genetic sequencing, doctors had the sick child’s genome sequenced to check for known variants, resulting in a diagnosis.[5] So, while genome sequencing is only moderately helpful for directing future healthcare, it may be more helpful in diagnosing rare genetic diseases, thus eliminating the need and expense of other exploratory procedures.
On the other hand, there are significant deficiencies in our current level of knowledge and ability with genetic sequencing. A survey conducted by United Healthcare says that only 1 in 5 of its doctors believes genetic testing will lead to lower medical costs.[6] Most believe it will lead to unnecessary tests. Additionally, doctors are unsure how much information to give patients. Should doctors tell patients if they find a marker for breast cancer or dementia risk, when the original motivation for testing was something completely different? And many question whether it is fair to a child for her parents to know this kind of information about her.
One of the motivations behind the Human Genome Project was to develop cures for diseases. The idea was that pinpointing the genetic markers for a disease would lead to development of tailor-made medicines. Even more hopeful is the prospect of replacing faulty genes with good ones, removing the problem completely. As it turns out, however, not all diseases are genetically-based, and those that are may be caused by multiple sites within the genome or related to epigenetic factors rather than the actual gene sequence. In addition, replacing a person’s faulty genetic sequence with a healthy one is not a simple task, and the consequences of doing so may be more harmful than the disease itself.
Overall, genetic testing is a powerful tool. But like any other tool, it is most helpful when used at the appropriate time to do the right job. As we pursue the benefits of genetic testing, we should keep in mind that many limitations remain as the results are still difficult to interpret, and it often raises more questions than it answers.
[1] The first two of these concerns ultimately led to the passage of the Genetic Information Nondiscrimination Act of 2008 (GINA).
[2] E. R. Gibney and C. M. Nolan, “Epigenetics and Gene Expression,” 105, no. 4-13 (2010): htp://www.nature.com/hdy/journal/v105/n1/full/hdy201054a.html (accessed January 15, 2013).
[3] Sujit Maiti, Kiran Halagur Bhoge Gowda Kumar, Christiana A. Castellani, Richard O’Reilly, and Shiva M. Singh, “Ontogenetic De Novo Copy Number Variations (CNVs) as a Source of Genetic Individuality: Studies on Two Families with MSD Twins for Schizophrenia,” PLoS ONE 6(3): e17125 (2011): http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0017125. Science Daily Release: http://www.sciencedaily.com/releases/2011/03/110328151740.htm.
[4] Todd Smith and Sandra Porter, “Genetic Inequality,” The Scientist (December 1, 2012): www.the-scientist.com/?articles.view/articleNo/33364/title/Genomic-Inequality/ (accessed January 15, 2013).
[5] Bonnie Rochman, “Researchers Solve the Mystery of Child’s Illness,” TIME (November 8, 2012): http://healthland.time.com/2012/11/08/researchers-solve-the-mystery-of-childs-illness/ (accessed January 15, 2013).
[6] Bonnie Rochman, “Why Cheaper Genetic Testing Could Cost Us a Fortune,” TIME (October 26, 2012): http://healthland.time.com/2012/10/26/why-cheaper-genetic-testing-could-cost-us-a-fortune/ (accessed January 15, 2013).
Heather Zeiger, "An Update on Gentetic Testing: Seperating the Hope from the Hype,” Dignitas 19, no. 4 (2012): 8–9.
