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What’s Making MeCP2 Toxic in Duplication Syndrome?

I’m a post-doc in the lab of Adrian Bird. The lab has historically studied MeCP2 in the context of Rett Syndrome – but now we are using everything we’ve discovered about MeCP2 to also better understand its role in MECP2 Duplication Syndrome (MDS).

Professor Bird discovered MeCP2 in the early 1990s. Since then we have learned a lot about what it does, how it works, and which parts of it are important. We know that in neurons, MeCP2 is very abundant. We also know that it is very important to have the right amount of MeCP2. Too little of it causes Rett Syndrome, and too much of it causes MeCP2 Duplication Syndrome (MDS)

Our lab wanted to know if the problems caused by too much MeCP2 are simply a consequence of too much protein in neurons or if there were specific regions within MeCP2 which are important for MDS. We have recently published this work.

In order to learn more about MeCP2’s role in MDS we developed a series of mouse models. The models were based on one originally developed in the Jaenisch lab. In these mice, an extra copy of MECP2 is added into a gene called Tau – so you can have up to three MECP2 copies in the mouse; one from its normal location, and one or two from the Tau locus.

The experiments showed that mice which had two Tau-MeCP2 genes and the normal MeCP2 protein were very sick – clearly demonstrating that too much MeCP2 had a detrimental effect.

We then inserted mutations into the Tau-MeCP2 gene – mutations which frequently occur in Rett Syndrome patients and which deactivate one of two critical regions of MeCP2: the NID (NCoR Interaction Domain) or the MBD (methyl-CpG-binding domain).

We started by inserting a mutation that inactivates the NID, Tau-MeCP2-R306C. Mice which don’t have any normal MeCP2, but only have Tau-MeCP2-R306C developed Rett like symptoms, similar to knock-in mouse models of MeCP2-R306C.

We then analysed mice which had normal MeCP2, plus 2 copies of Tau-MeCP2-R306C. Interestingly the mice were pretty normal. This clearly showed that the problems in MDS are not just because of too much MeCP2 protein – the extra MeCP2 needs to be functional.

Next, we repeated the experiment with Rett mutations that destroy the MBD – R133C and T158M. Mice with normal MeCP2 and 2 copies of either Tau-MeCP2-R133C or Tau-MeCP2-T158M survived, but showed a hindlimb clasping phenotype. This suggests that having surplus MeCP2 with a mutated MBD but a functional NID causes problems.

This series of experiments demonstrate that the NID is the key part of MeCP2 which mediates the toxic effect of MeCP2 in MDS.

The NID allows MeCP2 to bind to a big protein complex called NCoR which contains a lot of different proteins, including one called HDAC3. We hypothesized that perhaps MDS is the result of too much MeCP2 recruiting too much HDAC3 – so if we could reduce the activity of HDAC3, this should counteract having too much MeCP2.

We therefore crossed our Tau-MeCP2 mice with mice from the Lazar lab, which have reduced HDAC3 activity. Against our prediction, this did not rescue the lethal phenotype observed for Tau-MeCP2. This showed us, that HDAC3 is not the crucial NCoR component.

Future work will focus on other NCoR components, and their potential roles in MDS. Identifying the part of NCoR which mediates the toxicity could lead to the development of drugs specifically reducing the function of this component – and by that hopefully help in treating MeCP2 Duplication patients.