A new study concludes that young adults genetically predisposed to developing multiple sclerosis, a crippling neurodegenerative disease, could be at greater risk of developing that condition because of environmental factors.
Multiple sclerosis, or M.S.,affects an estimated 1.2 million people worldwide. The progressive disease leaves many of its victims unable to walk unassisted. It mostly strikes women of northern European descent.
M.S. is an auto-immune disorder, caused when the patient's own immune system attacks the myelin sheath that covers human nerve cells. Myelin plays a vital role in speeding nerve impulses to the rest of the body and its destruction can eventually cause disability.
Researchers have long suspected that M.S. is caused in people who are predisposed to the disease by a combination of inherited genetic mutations and an environmental trigger, such as a viral infection or living in temperate climates since the disease is rare in those of African and Middle Eastern descent who live close to the Equator.
Now, a team of scientists has discovered that some people with genetic mutations for M.S. in their DNA never develop the disease. They conducted an exhaustive genetic analysis of three sets of identical twins - including two female pairs - and found no biological explanation for why, in 30 percent of the cases, one twin developed MS and the other one did not.
"This points to some novel and fairly powerful effect on top of the genetic risk factors which seems to spell out whether you're going to develop multiple sclerosis," said Stephen Kingsmore, a genetic researcher at the National Center for Genome Resources in Santa Fe, New Mexico, who led the study.
While the genetic analysis revealed nothing at the genetic level that would cause one twin to develop MS while sparing the other, Kingsmore says the study at least confirms that MS is triggered by a combination of environmental and genetic risk factors.
"We don't really know why one of them didn't develop multiple sclerosis, given that they both had the same (biological) risk factors and indeed they both had much of the same environment. Because if you think about it, they would have been born on the same day to the same parents and lived in the same house and had all of the same childhood environmental things in common," he said.
But scientists remain clueless as to what, precisely, that "powerful effect" might be. Kingsmore says the genetic map they created of the M.S. twins is the first complete DNA analysis of identical twin pairs, women and an autoimmune disease.
Kingsmore says the work could help scientists interested in other autoimmune diseases, such as lupus or juvenile diabetes, delve deeper into their causes.
An article describing the study and the full genetic sequence of multiple sclerosis is published this week in the journal Nature.