( They sleep together eat together and ...)How can identical twins be genetically different?

Scientists at the University of Michigan Medical School are just beginning to understand how two people who are so similar biologically can be so different when it comes to the development of diseases like rheumatoid arthritis.

U-M researchers have discovered three genes that are over-expressed in rheumatoid arthritis, or RA, that were not known to be associated with the disease before. They also found that non-genetic factors influenced the expression of these genes and that the expression patterns varied between identical twins where only one twin had RA. Results of the U-M study were published in the July issue of Arthritis and Rheumatism.

RA is a chronic inflammatory disease that damages joints. RA causes pain, loss of movement, and bone deformities. It affects 2.1 million Americans. There are many genetic factors that put people at a high-risk for developing RA, yet only 15 percent of identical twins will both develop it.

Scientists compared gene expression patterns of 11 pairs of monozygotic twins, who shared the same egg and were genetically identical, but only one of them had RA. They found three new genes that were significantly over-expressed in the twin with RA compared to the one without the disease. This is the first report for RA that examines gene expression patterns in monozygotic twins.

"This is the crux of the issue we are trying to address in RA -- how two patients can have the same genes but different disease outcomes. Identical twins represent the best experimental system to address this question," says Joseph Holoshitz, M.D., an associate professor of internal medicine at U-M Medical School and co-author of the study.

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Source:University of Michigan Health System