Genes or environment?
It is not about the disease itself that I want to write – instead about how genetic and environmental factors contribute to the development of the disease. I think that it is exemplary for how many of these diseases develop, and so it’s useful to keep this in mind whenever a discussion springs up along the lines of “are genetic factors more important or environmental?”. The answer to this is akin to what, IIRC, the now Dutch minister of education Ronald Plasterk said when asked what the relative importance was between nature and nurture: 50%/50%, or 90%/10%, or even 10%/90%? He said (I paraphrase from memory): “that is a stupid question – they both are 100% important”.
“MS pathogenesis seems to involve both genetic susceptibility and environmental risk factors. Three sequential factors are implicated in the environmental risk. The first acts near birth, the second acts during childhood, and the third acts long thereafter. Two candidate factors (vitamin D deficiency and Epstein-Barr viral infection) seem well suited to the first two environmental events.”
So that’s the basis, as I said above: both genetic and environmental factors are involved.
“(the model developed in this article) suggests that genetic susceptibility is overwhelmingly the most important determinant of MS pathogenesis. Indeed, over 99% of individuals seem genetically incapable of developing MS, regardless of what environmental exposures they experience.” (emphasis mine)
And the first outcome is that genetics are indeed 100% important in that, if you have the correct genes – as over 99% of the population seems to do – you do not get MS, full stop.
However, there is of course at the same time this (as stated above):
“As noted, environmental risk seems to result from three sequential components of environmental exposure.”
That is, even if you are part of that <1% of the population theoretically (i.e. genetically) susceptible to MS – you still will not get it unless the three environmental components are there. In other words: the environment is also 100% important in developing MS symptoms!
Keep that in mind…
I think that that is important to realise for many related subjects, too: genetics and environment, or nature and nurture, are not two separate entities that both have a limited effect, such as is suggested when they are assigned a “50/50 shared” importance. They also do not work independent of each other, something that can easily be misunderstood from the same “50/50” type of division: cut away the environment “and you reduce the risk by half”; have the right genes and you still have half the chance of “going wrong” (or whatever is the event) – no, wrong. Cut one out and everything collapses: both are 100% important.
…but don’t overdo it
There are of course numerous inherited diseases in which the environmental factor is negligible or even nonexistent. Several cancer-related syndromes such as Lynch syndrome/HNPCC, “familial” breast cancer, and Li-Fraumeni syndrome are examples. True, not everyone who is genetically predisposed with such a syndrome gets the disease – but that is purely a chance process, largely if not completely independent of environment (it would lead me a bit far to explain why that is).
Inversely, the inherited disorder on which I did my Ph.D., xeroderma pigmentosum, increases the chance of getting skin cancer (by “virtue” of exposure to UV-light). In this case, one cannot argue that genetics are 100% important – because, given enough exposure to UV, everyone can develop skin cancer.
But that still leaves enough diseases for which genetics and environment are indeed both 100% important. Keep a lookout for them!