MeCP2 is a microsatellite binding protein that protects CA repeats from nucleosome invasion

The Rett syndrome protein MeCP2 was described as a methyl-CpG-binding protein, but its exact function remains unknown. Here we show that mouse MeCP2 is a microsatellite binding protein that specifically recognizes hydroxymethylated CA repeats. Depletion of MeCP2 alters chromatin organization of CA repeats and lamina-associated domains and results in nucleosome accumulation on CA repeats and genome-wide transcriptional dysregulation. The structure of MeCP2 in complex with a hydroxymethylated CA repeat reveals a characteristic DNA shape, with considerably modified geometry at the 5-hydroxymethylcytosine, which is recognized specifically by Arg133, a key residue whose mutation causes Rett syndrome. Our work identifies MeCP2 as a microsatellite DNA binding protein that targets the 5hmC-modified CA-rich strand and maintains genome regions nucleosome-free, suggesting a role for MeCP2 dysfunction in Rett syndrome.

The methyl CpG-binding protein MeCP2 is over-expressed in brain metastatic breast cancer.

Control of gene expression includes regulation by trans-acting transcription factors and cis-acting epigenetic regulation by chemical modification of histones and the DNA (1, 2). Brain metastases are a clinical problem in patients with breast cancer (3-5). We mined published microarray data (6-8) to discover genes associated with brain metastasis in patients with brain metastatic breast cancer. We found that the gene encoding the methyl CpG-binding protein MeCP2, a molecule with critical epigenetic functions in the brain (9, 10), was among the genes most differentially expressed in the brain metastases of patients with brain metastatic breast cancer. MeCP2 may be of relevance to the biology underlying metastasis to the brain, and it may be of relevance as a potential therapeutic target in patients with intractable disease.

Neuronal non-CG methylation is an essential target for MeCP2 function

SUMMARYDNA methylation is implicated in neuronal biology via the protein MeCP2, mutation of which causes Rett syndrome. MeCP2 recruits the NCOR1/2 corepressor complexes to methylated cytosine in the CG dinucleotide, but also to non-CG methylation, which is abundant specifically in neuronal genomes. To test the biological significance of its dual binding specificity, we replaced the MeCP2 DNA binding domain with an orthologous domain whose specificity is restricted to mCG motifs. Knock-in mice expressing the domain-swap protein displayed severe Rett syndrome-like phenotypes, demonstrating that interaction with sites of non-CG methylation, specifically the mCAC trinucleotide, is critical for normal brain function. The results support the notion that the delayed onset of Rett syndrome is due to the late accumulation of both mCAC and its reader MeCP2. Intriguingly, genes dysregulated in both Mecp2-null and domain-swap mice are implicated in other neurological disorders, potentially highlighting targets of particular relevance to the Rett syndrome phenotype.

KMT5C displays robust retention and liquid-like behavior in phase separated heterochromatin

The pericentromere exists as a distinct chromatin compartment that is thought to form by a process of phase separation. This reflects the ability of the heterochromatin protein CBX5 (aka HP1α) to form liquid condensates that encapsulate pericentromeres.1,2 In general, phase separation compartmentalizes specific activities within the cell, but unlike membrane-bound organelles, their contents rapidly exchange with their surroundings.3 Here, we describe a novel state for the lysine methyltransferase KMT5C where it diffuses within condensates of pericentromeric heterochromatin but undergoes strikingly limited nucleoplasmic exchange, revealing a barrier to exit similar to that of biological membranes. This liquid-like behavior maps to a discrete protein segment with a small number of conserved sequence features and containing separable determinants for localization and retention that cooperate to confer strict spatial control. Accordingly, loss of KMT5C retention led to aberrant spreading of its catalytic product (H4K20me3) throughout the nucleus. We further found that KMT5C retention was reversible in response to chromatin state, which differed markedly for CBX5 and the methyl-CpG binding protein MeCP2, revealing considerable plasticity in the control of these phase separated assemblies. Our results establish that KMT5C represents a precedent in the biological phase separation4 continuum that confers robust spatial constraint of a protein and its catalytic activity without progression to a gel or solid.

Maternal experience-dependent cortical plasticity in mice is circuit- and stimulus-specific and requires MECP2

ABSTRACTThe neurodevelopmental disorder Rett syndrome is caused by mutations in the gene Mecp2. Misexpression of the protein MECP2 is thought to contribute to neuropathology by causing dysregulation of plasticity. Female heterozygous Mecp2 mutants (Mecp2het) failed to acquire a learned maternal retrieval behavior when exposed to pups, an effect linked to disruption of parvalbumin-expressing inhibitory interneurons (PV+) in the auditory cortex. However, the consequences of dysregulated PV+ networks during early maternal experience for auditory cortical sensory activity are unknown. Here we show that maternal experience in wild-type adult female mice (Mecp2wt) triggers suppression of PV+ auditory responses. We also observe concomitant disinhibition of auditory responses in deep-layer pyramidal neurons that is selective for behaviorally-relevant pup vocalizations. These neurons also exhibit sharpened tuning for pup vocalizations following maternal experience. All of these neuronal changes are abolished in Mecp2het, yet a genetic manipulation of GABAergic networks that restores accurate retrieval behavior in Mecp2het also restores maternal experience-dependent plasticity of PV+. Our data are consistent with a growing body of evidence that cortical networks are particularly vulnerable to mutations of Mecp2 in PV+ neurons.