Methyl-CpG-binding domain as a protein interaction partner in promoter regulation and neurodevelopment through evolutionary expanded entanglement.

There is increasing evidence that the methyl-binding domain (MBD) is a protein-protein interaction motif that can function independently of methylated DNA binding. The MBD proteins found throughout plants and invertebrates duplicated into multiple vertebrate DNA and non-DNA-binding members (MBD1, MBD2, MBD3, MBD4, MBD5, MBD6, MECP2, BAZ2A, BAZ2B, SETDB1, and SETDB2). Although many invertebrate species possess MBD proteins that can bind and recognize DNA methylation, the DNA-binding function has been independently lost multiple times, with only minor alterations to the protein interaction residues. The nucleosome remodeling and deacetylase (NuRD) complex, which interacts with MBD2/3 and is colocalized with MBD1/4 ChIP-Seq, is maintained in species where MBD2/3 cannot bind to DNA. NuRD ChIP-seq data from HepG2 cell line, human induced pluripotent stem cells (iPSCs), and human iPSC-derived liver cells suggest that the NuRD complex is highly localized to nonmethylated CpG-rich housekeeping gene promoter elements, which are essential in organogenesis and maintained within the MBD2/3 non-DNA-binding system. Integration of MBD interaction proteins and NuRD gene expression from >115 million cells of single-cell RNA-seq, along with thousands of bulk tissue profiles, highlights a critical role of MBD3, MECP2, and GATAD2B in brain development and intellectual disability syndromes that is maintained throughout invertebrate neural development and likely involves evolutionary expanded entanglement as the vertebrate MBD proteins expanded. This work suggests that MBD has a largely unexplored role as a critical protein interaction motif that is evolutionarily conserved for regulating enhancers and promoters. The evolution of the methyl-binding domain (MBD) suggests a shared function in gene regulation, from plants to humans, with the conservation of non-DNA-interacting amino acids critical for protein-protein interactions. The MBD-regulated NuRD complex localizes to promoters of housekeeping genes with hypomethylated CpG islands. Expression profiles suggest a shared role for NuRD complex components in neurodevelopment, where the MBD3 and GATAD2B subcomplex of NuRD may be underexplored due to its non-DNA-binding biology.