Rett syndrome: insights into genetic, molecular and circuit mechanisms

Abstract

Rett syndrome (RTT) is a severe neurological disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). Almost two decades of research into RTT have greatly advanced our understanding of the function and regulation of the multifunctional protein MeCP2. Here, we review recent advances in understanding how loss of MeCP2 impacts different stages of brain development, discuss recent findings demonstrating the molecular role of MeCP2 as a transcriptional repressor, assess primary and secondary effects of MeCP2 loss and examine how loss of MeCP2 can result in an imbalance of neuronal excitation and inhibition at the circuit level along with dysregulation of activity-dependent mechanisms. These factors present challenges to the search for mechanism-based therapeutics for RTT and suggest specific approaches that may be more effective than others.

Fig. 1.. Effects of MeCP2 loss at different stages of brain development.

Methyl-CpG-binding protein 2 (MeCP2) regulates multiple stages of brain development and function. Loss of MeCP2 results in impaired neurogenesis and differentiation in early development, abnormal neuronal maturation, reduced circuit connectivity and excitation–inhibition (E/I) imbalance — all of which potentially contribute to Rett syndrome (RTT) pathophysiology. Neural progenitor cells (left) differentiate into mature neurons through the development of dendrites and synapses and through experience-dependent and activity-dependent refinement. The proper development of microcircuits comprising pyramidal neurons and inhibitory neurons, including cells expressing somatostatin (SOM), parvalbumin (PV) or vasoactive intestinal peptide (VIP), is required for proper E/I balance. In RTT, the differentiation of neural progenitors is impaired at an early stage of development. At postnatal stages, RTT neurons show smaller soma sizes and underdeveloped dendritic arbors. Altered connectivity and reduced excitation and inhibition lead to E/I imbalance in RTT and RTT models; dashed lines represent altered connection strengths. WT, wild type.

Fig. 2.. A model of MeCP2–DNA interaction

During early development, methyl-CpG-binding protein 2 (MeCP2) binds to genomic methylated CG (mCG) dinucleotide sequences to repress gene expression. During early postnatal life, DNA (cytosine-5)-methyltransferase 3A (DNMT3A) binds across transcribed regions of genes that show low expression and dictates DNA methylation at CA sequences (producing methylated CA (mCA) modifications)[61]. Once methylated, mCA is bound by MeCP2, leading to strong repression of gene transcription[61]. Passive ‘functional demethylation’ can occur through oxidation of mCG to hydroxymethylated CG (hmCG) by the ten-eleven translocation (TET) enzyme. As MeCP2 has a low affinity for hmCG, this modification results in the detachment of MeCP2, leading to derepression of transcription[63].

Fig. 3.. Proposed molecular effects of MeCP2 and their alterations in RTT.

Methyl-CpG-binding protein 2 (MeCP2) has been proposed to recruit the nuclear receptor co-repressor (NCoR)–silencing mediator of retinoic acid (SMRT) complex to methylated DNA to repress transcription[68] or permit transcription[75] through the action of histone deacetylase 3 (HDAC3) depending on different genomic contexts. MeCP2 interacts directly with transducin β-like protein 1 (TBL1) in the NCoR–SMRT complex via its NCoR–SMRT interaction domain (NID)[69]. This function is abolished in NID variants of MeCP2, such as R306C MeCP2. MeCP2 has also been suggested to compact chromatin structure through a basic amino acid cluster in its NID as well as via three AT hooks (not shown)[77]. The carboxyl terminus of MeCP2 binds to the microprocessor complex protein DiGeorge syndrome critical region 8 (DGCR8) and inhibits ribonuclease 3 (DROSHA) from binding; thus, MeCP2 may inhibit the processing of pri-miR-134 (Ref.[9]). In Rett syndrome (RTT) patients with MECP2 mutations that change or truncate the C terminus (for example, the MeCP2 380 truncation), it is expected that MeCP2 binding to DGCR8 is abolished and DROSHA is free to bind and activate pri-miR-134 processing. Other methyl-CpG binding domain (MBD)-affecting or truncation- inducing mutations in MECP2 also lead to upregulations of pri-miR-199 indirectly, via aberrant bone morphogenetic protein 4 (BMP4) and SMAD signalling[41]. Ac, acetyl group; C´, carboxyl terminus; FOXO3, forkhead box protein O3; GPS2, G protein pathway suppressor 2; miRNA, microRNA; TRD, transcriptional repression domain, WT, wild type. Figure adapted from Ref.[53], Macmillan Publishers Limited.

Fig. 4.. Effects of deficiency in inhibitory neurons and MeCP2 excitation–inhibition balance

Deletion of methyl-CpG-binding protein 2 (MeCP2) in all forebrain neurons or only in inhibitory neurons (such as parvalbumin (PV)-positive neurons) reduces GABA synthesis, glutamate decarboxylase 1 (GAD1) and GAD2 levels and miniature inhibitory postsynaptic currents, and thus reduces inhibition onto excitatory pyramidal neurons (green cells). The effectiveness of GABA as an inhibitory neurotransmitter is also altered in MeCP2-deficient neurons. Reduced levels of the K⁺/Cl⁻ exporter KCC2 relative to the Na⁺/K⁺/Cl⁻ symporter NKCC1 have been found in cells with mutations in the gene encoding MeCP2, resulting in neurons with higher levels of intracellular chloride, and hence a reduction in the reversal potential for GABA and reduced inhibitory drive to pyramidal neurons. We suggest that homeostatic compensation for the reduction in inhibition leads to hyperconnectivity of PV⁺ neurons and reduced excitatory drive onto pyramidal neurons, resulting in a joint reduction of excitation and inhibition in cortical circuits. Dashed arrows depict reduced function and solid arrows depict normal function. Ex, excitatory; GABAR, GABA receptor.