It is hard to imagine how this could happen.
Let’s assume that all the technical obstacles that now plague gene therapy[1] are resolved. In other words, that we learn how to deliver genes efficiently to the appropriate tissues without any side effects. Gene therapy would still be appropriate for only a tiny fraction of human diseases.
This is because few diseases have a simple cause-effect relationship with a definable set of genes and their variants. Take heart disease, for example. It is the number one cause of death worldwide. Family history is a strong risk factor for heart disease. But the risk added by any one gene that is linked to heart disease tends to be miniscule, despite the hype that attends their discovery. This headline, from a credible news source is a good example[2] :
Sounds like a big deal, right?. But read down the article and you find that people who carry two copies of this gene have a risk of heart attack that increases from 10% to 14%. Not so impressive. And if recent history is any guide, even this small effect may disappear upon further study. The repeatability of genome-wide association studies is not great[3] , false-positives are common[4] , and retractions are numerous.
The problem is that too many factors confound the association of disease with genes – other genes (in their multitudinous combinations), epigenetic effects, environment and history.
We like to think that there is a straight-line association between genes and disease. Scientists are conditioned to think this way because that’s what we learned in our genetics courses: the one gene-one enzyme model of metabolic pathways in fungi and bacteria. Although these mechanisms are elegant, they are not representative, not even in bacteria. When Craig Venter’s lab set out to create a minimal bacterial genome, they found that about a third of the genes required were of unknown function[5] . That’s in a genome of about 480 genes, for an organism that is provided with all necessary nutrients and has no competition. The amount of genetic “dark matter” in any real-life organism is surely much greater.
Gene therapy has an important role to play in medicine – there really are a number of diseases with simple gene-disease associations. But these are a distinct minority. The classic pharmaceutical paradigm is not likely to be eclipsed soon or ever.
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Footnotes
[1] Gene therapy clinical trials worldwide to 2012 – an update
[2] Strong DNA link found to heart attacks
[3] http://www.ncbi.nlm.nih.gov/pmc/…
[4] Genetic associations: false or true?
[5] Design and synthesis of a minimal bacterial genome