Among people who have multiple sclerosis (MS), women are more likely than men to have a gene associated with the condition, a new study shows.

Further, scientists believe that factors in the environment interact with patients' DNA in the region of this gene, according to a statement from the American Academy of Neurology.

The number of people diagnosed with MS has been rising, and the rate has been rising faster for women than men, the researchers said.

The cause of MS is not known, but evidence suggests that it is triggered by environmental factors in people who are genetically susceptible to the disease. The main gene associated with MS is called the human leukocyte antigen (HLA) class II gene, but most of the risk for developing the disease comes from the interaction of both parental genes, the researchers said.

The researchers examined the HLA genes of 1,055 families that had more than one person with MS in the family. In total, the researchers looked at the HLA genes in 7,093 people, including 2,127 people with MS. The researchers examined whether those with MS had inherited the susceptibility gene from their mother or their father, and what the relationships were between people in the same family with MS.

The researchers found that women with MS were 1.4 times more likely to have the HLA gene variant associated with MS than men with MS. A total of 919 women and 302 men had the HLA gene variant, compared to 626 women and 280 men who did not have the gene variant. This fits with other work by these researchers that showed that the environment interacts with this gene region, and changes a person's risk for developing the disease.

The researchers said the environment seems to be affecting a person's risk for developing the disease through an epigenetic mechanism a change not in the gene itself, but in other factors within the cell that govern the gene's expression.

"Our findings also show women with the HLA gene variant are more likely to transmit the gene variant to other women in their families than to men," said study researcher Dr. George C. Ebers, of the University of Oxford in the United Kingdom.

The researchers also determined that second-degree relatives, such as aunts and their nieces or nephews, were more likely to inherit the gene variant than first-degree relatives such as siblings or parents and children.

"It appears that the less the genetic sharing between individuals, the higher the interaction is between female sex and inheritance of the HLA gene variant," said Dr. Orhun Kantarci, of the Mayo Clinic in Rochester, Minn., who wrote an editorial on the study. "These findings pave the way for future studies of these genes, hopefully to advance our understanding of inheritance of complex diseases such as MS."

The study published online today (Jan. 5) in the journal Neurology, and was funded by the Multiple Sclerosis Society of Canada and the Multiple Sclerosis Society of the United Kingdom.