Loss of function of the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2) causes the progressive neurological disorder Rett syndrome (RTT). Conversely, duplication or triplication of Xq28 causes an equally wide-ranging progressive neurological disorder, MECP2 duplication syndrome, whose features overlap somewhat with RTT. To understand which MeCP2 functions cause toxicity in the duplication syndrome, we generated mouse models expressing endogenous Mecp2 along with a RTT-causing mutation in either the methyl-CpG binding domain (MBD) or the transcriptional repression domain (TRD). We determined that both the MBD and TRD must function for doubling MeCP2 to be toxic. Mutating the MBD reproduces the null phenotype and expressing the TRD mutant produces milder RTT phenotypes, yet both mutations are harmless when expressed with endogenous Mecp2. Surprisingly, mutating the TRD is more detrimental than deleting the entire C-terminus, indicating a dominant-negative effect on MeCP2 function, likely due to the disruption of a basic cluster.
Rett syndrome is a disorder that affects the development of the brain after birth. Infants with this condition develop as normal until they are 6–18 months old, when the development of their language and motor skills stops, or even regresses. Most cases of Rett syndrome are caused by mutations in a gene called MECP2.
If an individual mistakenly inherits an extra copy of the MECP2 gene, it can cause another developmental disorder called MECP2 duplication syndrome. This condition, which also affects the brain, gets worse over time and shares many features with Rett syndrome. The extra copy of the MECP2 gene leads to the production of too much MeCP2 protein. However, how doubling the level of this protein causes the syndrome and, in particular, which parts of the protein are involved are unknown.
Previously, researchers engineered mice that expressed a copy of the human MECP2 gene alongside their own version of the gene. These mice developed a condition similar to MECP2 duplication syndrome and many of these mice suffered from seizures and died within their first year.
Heckman et al. have now engineered mice that also have an extra human MECP2 gene but with one of two mutations that cause Rett syndrome in humans. Some mice had a mutation in a part of the MeCP2 protein that binds to DNA that is marked with small chemical tags called methyl groups. Other mice had a mutation in a domain of the protein that works to switch off genes. Heckman et al. found that mice with extra MeCP2 protein with either of these two mutations were as healthy as normal mice and showed none of the signs of MECP2 duplication syndrome. This indicates that both of these domains must be intact for doubling the levels of the MeCP2 protein to be harmful. Furthermore, Heckman et al. discovered that the mutation in the part of MeCP2 that works to switch genes off also reduces the protein's ability to bind to DNA.
The next challenge is to understand the mechanism by which doubling the levels of this protein causes harm to the brain. Further work is also needed to uncover why having too much MeCP2 protein or none at all cause syndromes that share many features.