Sometimes it’s difficult to measure exactly what we want to measure so we measure something else as an alternative. On occasion, that leads to headlines in the national press that make scientists sound crazy, if its not reported accurately. For example, scientist recently developed animals which glow in the dark, by injecting DNA from jellyfish into them. It sounds crazy, doesn’t it? Why would scientists do such a thing? Are we trying to create some kind of Frankenstein-like monster?
Well, it’s not crazy, there is a good reason for it, and it highlight another fundamental scientific principle. To understand this, you need to know that DNA contains instructions for making proteins, and if that DNA is introduced into a cell, the cell will now start to make that protein.
Some drugs are large proteins, and these are difficult for us to make in a laboratory.
One solution to this is to make them in animals. As an example, ATryn is a drug which is used to prevent blood from clotting, and it is obtained from goats’ milk. The goats don’t normally make ATryn, but they have been genetically altered to do so. This is done by introducing into the goats the DNA that contains the instructions for making ATryn. The goat’s cells then make ATryn and it can then be purified from the goat’s milk.
To make drugs like this, we need methods to get the DNA into the animal’s cells in the first place. Similar methods may also be useful for fixing faulty genes in human patients. The problem,though, is that methods we have to do this at the moment are not very efficient and so scientists would like to find better methods.
That is where the jellyfish come in. Jellyfish just happen to make a fluorescent protein. The fluorescent protein gives off light when ultraviolet light is shined on to it (it “fluoresces”). So an easy way to test different methods for getting DNA into animals is to use the DNA from jellyfish that encodes the fluorescent protein. We can test different methods for introducing the DNA into the animals and see if were successful just by seeing if the animals fluoresce (“glow in the dark”). Once we have done that, we can then use the method we have developed to transfer DNA encoding the protein drugs. So the “glow in the dark” protein is just being used as an easy way to measure the success of the experiments. If you like it’s a “marker” for how well the experiment worked. Scientists very often use “markers” like this to make their work more efficient.
Sometimes we make use of “markers” because they are the only thing that is feasible to use. Suppose we want to develop a new drug to reduce cholesterol, because we think high levels of cholesterol can ultimately lead to death from heart attacks. What we would really like to do is to test the drug in patients and see if it reduces deaths from heart attacks. The problem with that approach is that it could take decades for high cholesterol to cause heart disease, so we might have to test the drug for more than 10 years to find out if it reduces the number of patients who suffer heart attacks. Can’t we do things more quickly? Well, we could test the drug and see if instead it reduces cholesterol in patients, in the expectation that lower cholesterol will result in fewer deaths from heart attacks. When we measure cholesterol in this way, it’s a “surrogate” for what we really want to measure (a drop in patient deaths).