That its presence had been overlooked for so long also suggested that molecular biologists needed to rethink some of their experimental methods. The Heintz lab had long emphasized the use of cells taken from animals, but if Kriaucionis had instead relied on immortalized neuronal cell lines, which are more convenient to use but differ subtly from normal neurons, he would have found no hmC. He also determined that the standard sequencing technique for mapping mC in cellular genomes could not distinguish hmC from ordinary mC.

As it turned out, other researchers had been on a similar epigenetic quest. A week after Heintz and Kriaucionis submitted their paper to Science, a group led by Harvard Medical School scientist Anjana Rao submitted their own closely related finding. Hydroxymethylcytosine, they reported, is present in mouse embryonic stem cells and is converted from mC by an enzyme found in humans as well as mice. “Her discovery of an enzyme that actually does this hydroxylation is critically important, for it tells us that there's a dedicated biochemical mechanism for producing this modification,” Heintz says.

The two papers appeared together in Science on May 15, 2009, and both labs are now trying to understand hmC's functions in different cell types. Some evidence already suggests that hmC can reverse mC's usual gene-silencing function, although Heintz thinks it could have other, more interesting epigenetic roles. To help clear up the mystery, he and Kriaucionis are first trying to map hmC's distribution in the DNA of various cell types—a goal complicated by the lack of a high-resolution method that can separate hmC from mC. “It is critical to develop a new chemical strategy to allow us to precisely map the locations of both of these marks in the genome,” says Heintz. The two researchers also have begun collaborating with Rao's group to manipulate hmC levels in different types of mouse neurons to see how it affects their gene expression.

The apparent significance of hmC has drawn keen interest from other researchers, too. Based on queries she's had from other scientists, Rao says “at least fifty other labs are interested in this now.”

	PAGE 2 OF 2	Back