Individual genetic variation
The origins of people's different responses to drug treatment lie in the genetic variances between them.
The Human Genome Project solved the sequence of the about three billion chemical 'letters' (nucleotides) from the human genome. What it produced, though, was an average sequence - and all of us differs slightly from this average.
Since the Human Genome Project was completed, research has aimed at genetic variation between human beings and working out which variants are important to our health; which affect our odds of developing a particular disease or reacting badly to a drug.
Two types of variation are routine in the genome:
changes inside single letters (so-called 'single nucleotide polymorphisms', or perhaps SNPs, known as 'snips')
adjustments affecting blocks of DNA; someone could be missing the block entirely (a deletion), while some might have one, two, three and up copies. This is known because copy number variation.
SNPs are like changing one particular letter in the metaphorical 'book involving life', while copy number variation is compared to whole paragraphs or pages being lost or duplicated.
SNPs have been known about for years, but copy number variation can be less well understood. It is apparently quite common - affecting around 12% from the genome - and has already been found to cause a selection of conditions.
Genetically speaking, humans are around 99. 6% identical to another. In the 0. 4% from the genome that differs are hereditary features that affect our susceptibility to disease or response to drugs. By analysing SNPs (or backup number variation), researchers can seek out these features, which are called genetic markers. This has been possible by advances in technology, which allow hundreds of a huge number of SNPs to be analysed simultaneously - this is known because high-throughput genotyping.
Guilt by affiliation
In these experiments, researchers are looking for an association between a hereditary marker and disease susceptibility (or undesirable reaction). This means that a certain form of a particular marker is found more often in people with one form of reaction to a drug than it really is in people with another sort.
Because all humans are therefore similar genetically, people with a disease may share many genetic markers that are nothing related to their disease. How is it possible to learn which are important to the disease and which are just shared by chance?
The trick is to review the genetic make-up of individuals who have a specific disease and people who do not. If a particular genetic marker exists in, 80 percent of individuals, for example, but only 20 percent of a healthy population, it can become a sign that the marker is increasing raise the risk of disease.
To reduce the possibility of chance correlations, large groups of patients must be studied. A common approach is in order to compare patients with unaffected those who are as similar as possible to the patient (the case-control approach). It is also valuable to analyse as many genetic markers as you can.
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