BioWorld International Correspondent
LONDON - A greater understanding of Rett syndrome, a genetic disease that affects one in 10,000 baby girls, is likely to flow from the availability of a new knockout mouse that mimics its symptoms. Stocks of the mouse are currently being lodged at the Jackson Laboratory in Bar Harbour, Maine, to allow researchers ready access to the new model.
The mouse lacks a functional gene for the protein called MECP2. Although previous attempts to make mice in which the gene for MECP2 had been knocked out had failed, leading researchers to conclude that the gene's product was essential for embryogenesis. This latest attempt, which uses the conditional knockout approach, resulted in live-born animals.
Adrian Bird, Buchanan professor of Genetics at the University of Edinburgh in Edinburgh, UK, told BioWorld International: "The most interesting feature about these mice is that they develop symptoms similar to those of humans with Rett syndrome at about the same time. In both mice and humans, symptoms begin to show at about 6 months of age, so we believe that we have a really useful model for the disease."
Rett syndrome is primarily a neurological disorder. The gene responsible for it was known to be present on the X chromosome. Most cases are sporadic rather than familial.
Males with Rett syndrome die soon after birth. In affected females, because of the phenomenon known as X inactivation, in which one copy of the two X chromosomes in each cell is randomly inactivated, only approximately half of the body's cells are expressing the mutant gene. The rest make normal protein, thus allowing the individual to survive.
The disease is a devastating one, however. Girls born with it develop normally until at some point between 6 and 18 months they stop making developmental progress and instead regress. They go on to develop severe mental retardation and are never able to speak or walk, although they may survive for several decades.
In September 1999, researchers reported that the gene responsible for Rett syndrome was that encoding MECP2. Bird and his colleagues, who had purified and characterized MECP2 (which stands for methyl CPG binding protein 2) and saw it had a role in silencing gene expression by binding to DNA, found this report of great interest. They set out to make a knockout mouse that would tell them more about the role of MECP2 in living animals.
Their first attempts failed, as they were unable to make chimeric embryos using embryonic stem cells that lacked the gene for MECP2.
So they decided to try a conditional knockout instead. This time, they succeeded in obtaining live animals. Their results are described in the March issue of Nature Genetics in a paper titled, "A mouse Mecp2-null mutation causes neurological symptoms that mimic Rett syndrome." Rudolf Jaenisch and colleagues from the Massachusetts Institute of Technology describe similar findings in an accompanying paper in the same issue of Nature Genetics, titled "Deficiency of methyl-CPG binding protein-2 in CNS neurons results in a Rett-like phenotype in mice."
The group carried out two conditional knockouts. In the first, the gene was turned off during very early development. In the second, the gene was selectively turned off at birth only in neuronal and glial cells. In both groups of animals the resulting phenotype was the same.
Bird told BioWorld International: "We found that the male mice, which had no MECP2 at all, were fine at birth. But six weeks after birth, they suddenly became immobile, and soon afterward lost weight and died at about 10 weeks. The picture with the heterozygous females, which have the same genetic constitution as real patients with Rett syndrome, was slightly different. They looked normal initially and had up to three litters. But from 6 months of age onward, they started to get the same symptoms as the males had had. Instead of declining to death, their condition stabilized but they have severe mental problems and other symptoms similar to the human condition."
Bird predicted that one day it may be possible to provide treatment for Rett syndrome. "In theory," he said, "it should be possible to dig out the silent copy of the gene on the inactive X chromosome and reactivate it during the symptom-free period." He said, however, that it might not be easy to devise a way of screening for girls who are going to develop Rett syndrome, partly because many cases identified so far have been due to different mutations.
Next, he and his colleagues plan to study gene expression in the mutant mice. Bird concluded: "If you take away something that represses gene expression and you get an abnormal phenotype, it is probably because you are failing to repress something that this protein normally represses. We're now trying to find out what that is by looking in the brain for genes that are on, and that should not be."