Hey Donna!
There are a variety of epidemiological studies out there that examine statistically the relationship between MDS and various exposures, professions, and aspects of lifestyle.
Many studies point to a link with benzene and other chemicals.
This 2009 US study finds that being overweight and smoking are risk factors.
This 2001 French study suggests one might want to consider some line of work other than being a technical sales rep. It's not clear if the salesmen were at greater risk because they spent a lot of time in factories around chemicals or spent so much time wining and dining their sales prospects that the ate, drank,and smoked too much.
As Neil suggests, these kind of statistical studies are really all we are likely to get, since we can't go experimenting on folks to see if we can give them MDS.
I think there are a couple of other problems, as well. MDS is a convenient label, but any definition of "myelodysplastic syndromes" begins with the phrase "a set of related disorders." In other words, there's more than one type of MDS, those types might not even be that closely related, and so they might not be caused by the same thing. That complicates the statistician's task of finding a cause. You might have a version of MDS caused by an autoimmune issue; mine might be caused by an inherited genetic mutation; and Joe's might be the result of a lifetime of smoking (or selling industrial machinery to Frenchmen).
The other problem is that a lot of the types of MDS, like other cancers, seem to be about genetic mutations. And lots and lots of things, from too much sun to too many beers, can make human cells mutate. We are veritable mutation machines. That's how we got these opposable thumbs, why we walk upright, and how we got these big brains that can ponder questions about why we get MDS.
The way we tend to think about disease is that it's caused by an outside invader -- the way that an unfriendly microbe causes smallpox. But most cancers aren't invaders. They are our very own cells, gone bad, which is what makes the disease so hard to treat.
The ability of our cells to mutate -- and to capitalize on useful mutations -- is what made us who we are. It's a great gift. But it has a dark side, and that dark side is what we call "cancer."
If you look at it that way, finding a "cause" for MDS or other cancers is a very difficult enterprise, because the mysterious cause of cancer is all wrapped up in the mystery of life itself.
Take care!
Greg