Epigenetics
Enhancers: The Genomic Switches at the Heart of Kabuki
A sweeping review reveals how non-coding DNA "enhancers" control brain development, learning, and disease — and why KMT2D and KDM6A sit at the very center of this machinery
What are enhancers, actually?
Glad you asked!
Enhancers are short sections of DNA that act like remote controls for genes.
They can turn a gene on or off from far away, ensuring it's active at the right time and place.
OK. But so why are they so active in our brains?
In the brain, enhancers create incredible cellular diversity by precisely controlling which genes are active in different types of neurons. They are also crucial for learning and memory, as they help neurons respond dynamically to new experiences.
What's the Kabuki angle here?
The Kabuki genes KMT2D and KDM6A help prepare enhancers for action. Without them, enhancers can't properly activate the genes needed for brain development and learning, contributing to Kabuki symptoms.
Got it. Have to ask though: could enhancers be a target for therapy?
Conceivably. New tools can target specific enhancers to correct gene activity. Since enhancers control entire groups of genes, fixing one could help correct many related problems at once.
Kabuki syndrome is caused by mutations in genes that help control how other genes are turned on and off. But where, exactly, does that control happen? A growing body of research points to a vast, overlooked layer of the genome called enhancers — and a 2019 review paper maps out why they matter so much for brain development, learning, and disease.
Enhancers are short stretches of DNA that don't code for any protein themselves. Instead, they act as remote-control switches, dialing up the activity of nearby genes at just the right time, in just the right cells.
The brain relies on these switches more than almost any other organ. Enhancers help determine which genes fire during brain development, which stay accessible in adult neurons, and which flip on in response to experience. When the proteins that maintain enhancers are disrupted — as they are in Kabuki syndrome — the consequences ripple across the genome.
Why enhancers matter for Kabuki
Here's the connection. The two genes linked to Kabuki syndrome — KMT2D and KDM6A — both produce proteins that chemically mark enhancers to keep them in a ready or active state. Specifically, Kmt2d (the protein made by KMT2D) is one of a small group of enzymes that place a key chemical tag on enhancers, a tag that primes them for activation. When that tagging goes wrong, enhancers across the genome can lose their ability to switch genes on properly.
This isn't unique to Kabuki. The review highlights a striking pattern: mutations in any of the major proteins that build, maintain, or connect enhancers tend to cause cognitive disorders. Mutations in the proteins that acetylate (activate) enhancers cause Rubinstein-Taybi syndrome. Mutations in the ring-shaped complex that physically connects enhancers to their target genes cause a group of conditions called Cohesinopathies. Mutations in a protein that helps enhancers loop to the right gene cause intellectual disability. Kabuki syndrome fits squarely into this family of "enhancer disorders.
What are enhancers, exactly?
Think of the genome as a vast library. Genes are the books — they contain the instructions for making proteins. But enhancers are the librarians: they decide which books get pulled off the shelf, in which room, and when.
Enhancers sit in the non-coding part of DNA — the 98% of the genome that doesn't directly build proteins. Current estimates suggest vertebrate genomes contain tens of thousands, possibly millions, of active enhancers. That's a lot of librarians. And the majority of disease-linked genetic variants found in large population studies fall into these non-coding regions, with a growing number tied specifically to enhancer function.
An enhancer can sit thousands — even millions — of DNA bases away from the gene it controls. It reaches its target gene by physically looping through three-dimensional space, bringing the switch and the gene into direct contact. Several molecular machines stabilize these loops, including a ring-shaped complex that literally wraps around two segments of DNA to hold them together.
How enhancers get turned on
Enhancers don't just sit there in an on-or-off state. They go through stages. A "poised" enhancer carries one set of chemical marks, signaling it is ready but not yet active. An "active" enhancer carries a different mark — a chemical tag called acetylation — along with binding by specific activating proteins. The transition from poised to active requires the arrival of the right combination of transcription factors — proteins that read DNA sequences and recruit the machinery needed to start a gene.
This combinatorial logic is what gives enhancers their remarkable specificity. Different combinations of transcription factors landing on the same enhancer in different cell types produce different gene activity patterns. That's how a single genome can build hundreds of distinct brain cell types, each with its own gene expression profile.
Once active, enhancers also produce their own small RNA transcripts, called enhancer RNAs. These were once dismissed as biological noise, but a decade of research has shown they play real functional roles — interacting with the transcription machinery and helping sustain the active state of the enhancer.
Enhancers in brain development
A landmark study tracked enhancer activity across seven stages of mouse development and found that 85% of brain enhancers were active only during specific developmental windows. Early-active enhancers were enriched for factors controlling neuronal differentiation, while later-active enhancers were enriched for factors involved in synaptic transmission, learning, and memory. This means the brain's enhancer landscape is not static — it shifts dramatically as development unfolds.
Enhancers are also remarkably specific to brain regions. When researchers microdissected subregions of the mouse cortex and profiled their active enhancers, about a third were specific to a single subregion. Within those, some enhancers showed specificity not just for a brain region but for a particular cell type within that region. A human study mapping chromatin accessibility across 14 brain regions confirmed this: many enhancers were found exclusively in one cell type — dopamine neurons, pyramidal neurons, or non-neuronal cells.
This exquisite specificity helps explain why Kabuki syndrome, despite being caused by a mutation present in every cell, produces its most prominent effects in particular tissues and developmental stages.
Enhancers respond to experience
In the adult brain, enhancers don't just set up the wiring — they help it adapt. A pioneering study identified roughly 12,000 enhancers in mouse neurons that responded to neuronal stimulation. Activation of these enhancers was accompanied by recruitment of key activating proteins to genes involved in the brain's immediate response to stimuli.
Crucially, different types of stimulation activated different combinations of enhancers. Researchers studying five enhancers around a single important gene found that each enhancer responded differently depending on the stimulus — electrical depolarization, growth factor signaling, or a chemical messenger. In living mice, different sensory experiences activated different enhancer combinations at the same gene. This suggests enhancers help the brain distinguish between types of experience and fine-tune its transcriptional response accordingly.
This has direct relevance for Kabuki. If the molecular machinery that activates enhancers is impaired, the brain's ability to mount precise transcriptional responses to experience — the molecular foundation of learning and memory — could be compromised.
Super enhancers: master regulators
Some enhancers work not alone but in clusters called super enhancers. These clusters are larger, bind more transcription factors and co-activators, and can regulate entire groups of functionally related genes at once. Super enhancer-regulated genes tend to be those that require the most precise coordination — genes active in specific cell types, at specific developmental stages, or in response to particular signals.
Mutations that impair super enhancers have been linked to cancer, diabetes, and Alzheimer's disease. In Alzheimer's specifically, researchers identified five disease-associated variants sitting within super enhancer regions active in the brain. Because super enhancers control whole programs of gene expression, even subtle disruption can cascade across multiple genes simultaneously.
Recent work has revealed that super enhancers may function through a physical mechanism called phase separation — forming dense molecular droplets in the nucleus that concentrate all the components needed for gene activation into a tiny space. Disrupting these droplets caused loss of key activating proteins and the transcription machinery at super enhancers and their target genes.
Enhancers and disease
The review catalogues a growing list of brain disorders linked to enhancer disruption. In autism spectrum disorder, a prominent risk region sits between two genes and shows enhancer activity in cortical, striatal, and cerebellar neurons. In Fragile X syndrome, a repeat expansion disrupts the boundaries that organize enhancer loops, potentially altering gene expression. In Parkinson's disease, a disease-associated variant falls within an enhancer inside one of the key risk genes, disrupting transcription factor binding. In depression, variants within a specific enhancer reduce its ability to activate its target gene.
Across all these conditions, the theme is consistent: genetic changes in enhancer regions, or in the proteins that regulate enhancers, can disrupt the precise gene expression programs the brain depends on.
Therapeutic possibilities
The review closes with a forward look that should interest Kabuki families. Because enhancers are modular and specific, they offer multiple potential points of therapeutic intervention.
New gene-editing tools can now be directed to specific enhancer sites to activate them and their downstream gene programs. Because many enhancers — especially super enhancers — control groups of related genes, targeting a single enhancer cluster could potentially correct expression of an entire disrupted pathway at once.
Three practical approaches are highlighted: antisense molecules that modulate enhancer RNAs, small-molecule drugs targeting enhancer-linked proteins (like the acetylating enzymes disrupted in Rubinstein-Taybi and potentially relevant in Kabuki), and using enhancer sequences themselves to drive gene therapy in a cell-type-specific way. The cell-type specificity of enhancers is a particular asset — it could allow therapies to target gene expression only in affected cell types or brain regions, combined with inducible systems for precise timing.
Much remains unknown — including how to assign specific functions to all identified enhancers, the exact sequence of molecular events in enhancer activation, and how far these therapeutic ideas can go. But the field is moving fast, and the convergence of enhancer biology with Kabuki research offers real reasons for cautious optimism.
Deeper concepts
These ideas from the paper require more specialized vocabulary than fits the main article:
• Master transcription factors like Oct4, Sox2 and Nanog predict enhancer activity in stem cells, and their function depends on recruiting coactivators with chromatin remodeling activity.
• Enhancers are characterized by high ratios of H3K4me1/me2 relative to H3K4me3, the opposite pattern found at gene promoters.
• Histones at enhancers are enriched for H2A. Z and H3. 3 variants, which are less stable and easier to displace, allowing more dynamic access for transcription factors.
• Enhancers and promoters are not rigidly distinct categories: some enhancers have promoter-like activity, some promoters function as enhancers, and elements can be "repurposed" between species.
• Phase-separated condensates at super enhancers were demonstrated by showing that Mediator and BRD4 associate with super enhancers in nuclear condensates; disrupting these condensates with 1,6-Hexanediol abolished key protein binding.
• The transcription factor Isl-1 switches binding partners at motor neuron enhancers depending on developmental stage, illustrating how enhancer regulation shifts over time.
• Reelin, an extracellular signaling protein, activates a subset of neuronal enhancers that translate synaptic inputs into transcriptional programs for synaptic plasticity.
• At the Arc gene, neuronal activity induces enhancer-promoter interactions, and enhancer RNA production is required for release of a transcriptional brake (NELF) to initiate gene transcription.
generated by Claude