A recent study published in the April 28th issue of Nature is befuddling to those looking for answers about multiple sclerosis (MS) on the genetic level.
Scientists recruited three pairs of MS discordant identical twins (one of each pair had MS and one did not) to provide samples of DNA. The researchers then sequenced the entire genome for each of the participants - 2.8 billion "base pairs" for each one, to be exact - and compared the twins' DNA to one another.
Guess what they found? No difference in the DNA. None at all.
The scientists even conducted a very detailed investigation of immune cells, as MS is though by most experts to be an autoimmune disease, basically sequencing those and looking for differences. Still, no differences.
Although it would be tempting to say that this study (which cost 1.5 million dollars and took 18 months to complete the sequencing) provided no information, the fact is that it did tell us something - that there is apparently no direct genetic cause of MS. Otherwise, there would have been a difference in the DNA between the twins.
This doesn't mean that genetics doesn't play a role in MS, however. It could be a situation where there is a genetic component that is "necessary, but not sufficient" and requires an environmental factor to start the process that leads to MS, like an infection that one twin was exposed to and the other wasn't.
It is estimated that the likelihood of an identical twin developing MS if the other twin has MS is 30%. This falls to 4% likelihood in a non-identical twin and less than 3% in a non-twin sibling.
This is the first time that the entire genome of twins with autoimmune disease has been sequenced.
Source: Sergio E. Baranzini, Joann Mudge, Jennifer C. van Velkinburgh, et al. "Genome, epigenome and RNA sequences of monozygotic twins discordant for multiple sclerosis." Nature 464, 1351-1356 (29 April 2010).