Epigenetics
Why Kabuki (and Other Epigenetic Disorders) Impact the Brain
A deep, comparative study of Kabuki, Wiedemann-Steiner, Rubinstein-Taybi, and other "MDEMs" reveals shared and divergent cognitive profiles — and argues these rare conditions are a Rosetta Stone for understanding cognition itself.
A new review paper makes a compelling case that rare genetic conditions—including Kabuki Syndrome—are becoming a powerful lens for understanding how the brain develops the capacity to think, learn, and behave. The disorders in question all share a common thread: they're caused by mutations in genes that control the epigenetic machinery, the cellular equipment that decides which genes get switched on or off. And nearly all of them come with cognitive and behavioral challenges as core features.
The paper surveys what's known about the cognitive and behavioral profiles of several of these conditions, comparing them side by side. The goal isn't just to catalog symptoms. It's to use the similarities and differences between these syndromes to reverse-engineer how epigenetic regulation shapes brain function—in sickness and in health.
Let's start with the basics. Epigenetics refers to heritable changes in gene expression that happen without altering the DNA sequence itself. Think of it this way: if your genome is a piano, epigenetics determines which keys get played and which stay silent. The term was coined back in 1942 by Conrad Waddington, who wanted a word for biological events shaped by external factors.
How the machinery works
The cell has an elaborate toolkit for controlling gene expression epigenetically. There are four main mechanisms: DNA methylation (adding chemical "mute" tags to DNA), histone modifications (adding or removing chemical tags on the protein spools that DNA wraps around), chromatin remodeling (physically reshuffling how DNA is packaged), and RNA-based mechanisms. Together, these processes govern critical events like early development, X-inactivation (silencing one X chromosome in females), genomic imprinting (where only one parent's copy of a gene is active), and tissue-specific gene regulation.
The paper organizes the proteins that carry out these jobs into four roles. Writers attach chemical marks—mainly methylation and acetylation—to DNA or histones. Erasers remove those marks. Readers recognize specific marks and recruit other molecular machinery to act on them. And chromatin remodelers—belonging to four structural families called SWI/SNF, ISWI, INO80, and CHD—use cellular energy (ATP) to physically rearrange chromatin structure, either opening it up for gene expression or packing it tighter to silence genes.
Why the brain is especially vulnerable
All of these mechanisms matter enormously for the brain. Many of the genes implicated in neurodevelopmental disabilities converge on shared pathways: the birth of new neurons (neurogenesis), the proliferation and specialization of neural precursor cells, the migration of neurons to their correct positions, the formation and function of synapses, and the balance between excitatory and inhibitory signaling. Epigenetic regulation plays a crucial role in every one of those processes.
DNA methylation—the most studied epigenetic mark—is especially important here. The most common form occurs at specific two-letter sequences in the genome called CpG dinucleotides, where it typically acts to silence nearby genes. Different brain regions and cell types carry distinct methylation signatures, which start out flexible and become locked in over the course of development. There's also a less-studied form called non-CpG methylation that is specifically enriched in brain tissue, accumulates from development into adulthood, and is crucial for both neurogenesis and the maturation of neural precursor cells.
Histone modifications are equally vital. Depending on the type and location of the mark, they can open chromatin to allow gene expression or close it to silence genes. Disruption of histone methylation, demethylation, acetylation, or deacetylation leads to neurodevelopmental abnormalities. For example, deleting a key histone-modifying protein called Dpy30 in mouse brains causes deficits in the production of both neurons and glial cells (the brain's support cells). Another histone modifier, Ezh2, stimulates neurogenesis and the proper differentiation of neural precursor cells into mature neurons and glia. Even the balance of histone acetylation must be carefully choreographed: an orchestrated dip and then rise in acetylation at a specific histone position (H3K9) is required for early neural differentiation, and if that rhythm is disrupted, differentiation stalls or mistimes.
Chromatin remodeling adds yet another layer. The physical restructuring of chromatin is critical for the transition from neural precursor cells to their final, specialized forms. During differentiation, certain chromatin regions become increasingly compressed, permanently shutting off genes tied to the precursor cell state. Specific chromatin remodeler proteins are essential for proper brain development and neuronal differentiation. In mice, for instance, loss of one copy of the remodeler Chd2 specifically reduces production of GABAergic neurons—the brain's primary inhibitory neurons—significantly altering the cellular composition of the cortex. Knockout of Ctcf, a key protein for 3D chromatin organization, leads to increased cell death and disorganization of the forebrain. In humans, loss-of-function mutations in either CTCF or CHD2 cause neurodevelopmental disorders featuring intellectual disability, seizures, and behavioral issues.
A special class of genetic disorders
The conditions covered in this review belong to a group called Mendelian disorders of the epigenetic machinery (MDEMs). These are caused by mutations—most often the loss of just one working copy—of genes encoding epigenetic proteins. Because the mutated protein's normal job is to regulate other genes epigenetically, the downstream effects ripple across the expression of many genes.
What makes MDEMs unusual among genetic diseases is their dosage sensitivity. About 80% of all Mendelian diseases caused by enzyme deficiency follow a recessive inheritance pattern, meaning you need two bad copies to get sick. But 86% of MDEMs are dominantly inherited—losing just one copy is enough to cause disease. Epigenetic machinery genes score significantly higher on measures of loss-of-function intolerance compared to all other genes, with chromatin remodelers being the most intolerant group of all. Many of these proteins also carry dual epigenetic functions—for example, both an enzymatic domain and a reader domain—which may help explain why even partial loss is so damaging.
The vast majority of MDEMs are associated with neurological dysfunction, specifically neurodevelopmental disorders. Together, they account for a large proportion of genetic intellectual disability. Several of the disease-causing histone modifiers share overlapping molecular mechanisms, and many of these proteins work together in complexes or are codependent. This creates an opportunity: if deep phenotyping reveals that MDEMs caused by proteins with overlapping or codependent functions share similar cognitive or behavioral profiles, it can teach us how those proteins and their epigenetic mechanisms function in both developing and adult brains.
DNA methylation disorders: Tatton-Brown and Beck-Fahrner syndromes
Tatton-Brown syndrome (TBRS) is an overgrowth disorder caused by loss-of-function variants in DNMT3A, a gene encoding a DNA methyltransferase—a "writer" that adds methyl marks to DNA. The majority of individuals with TBRS have intellectual impairment in the moderate range. They may show stronger verbal reasoning than nonverbal or spatial processing skills.
TBRS carries a notably high incidence of autism spectrum disorder (ASD), estimated at 36–44%. Neuropsychiatric concerns including psychotic disorders or schizophrenia have been reported in 5–42% of published samples. Aggressive behaviors, stereotypic behaviors, obsessive-compulsive behaviors, anxiety, features of ADHD, and neurodevelopmental regression have also been observed. Mouse models carrying a heterozygous Dnmt3a mutation mirror several of these features, showing increased anxiety, abnormal and reduced social behaviors, and reduced social drive, though cognition was less affected.
Beck-Fahrner syndrome (BEFAHRS) sits on the opposite side of the DNA methylation coin: it's the first identified Mendelian disorder of DNA demethylation, caused by deficiency of TET3, an "eraser" enzyme that removes methyl marks. All 11 reported cases had global developmental delay or intellectual disability, most had low muscle tone or joint hypermobility (9 of 11), and a little over half presented with autistic features (6 of 11).
Mouse studies of Tet3 deficiency point to increased anxiety and fear generalization, as well as impaired spatial orientation and short-term memory—implicating the hippocampus and prefrontal cortex. Expression of genes involved in memory formation is regulated by Tet3 levels. These findings suggest hippocampal functions including spatial processing and semantic memory formation may be particularly affected in BEFAHRS, and that anxiety and autistic traits may be common features.
A chromatin remodeling disorder: CHARGE syndrome
CHARGE syndrome is caused, in the majority of cases, by heterozygous mutations in CHD7, a chromatin remodeler. It is characterized by growth retardation and multiple organ anomalies affecting the heart, nasal passages (choanae), genital/urinary systems, ears, and eyes (ocular coloboma).
Cognitive functioning varies widely, from impaired to broadly average, though nearly half of those with CHARGE syndrome have intellectual impairment. About a third have hearing loss, which may contribute to speech and language delay. Cognitive testing reveals primary weaknesses in visuospatial construction, sequential processing, and selective attention, alongside relative strengths in semantic skills, logical reasoning, and planning. Brain malformations, microcephaly, and low vision are prognostic predictors of intellectual functioning.
The behavioral profile of CHARGE syndrome is complex. Obsessive-compulsive disorder (43–49%) and anxiety disorder (37–53%) are common, with potentially high overlap between the two. Depression (8–24%) and ADHD (26–34%) are also frequently found. Self-injurious behaviors (40–54%) and aggressive behaviors (38–53%) are prevalent. Those with dual sensory impairment—meaning both deaf and blind—generally present with more challenging behaviors. Autistic behaviors are reported in 26–43% of individuals, though some research suggests these features may stem from sensory deficits rather than the social impairment seen in idiopathic ASD.
Histone acetylation disorders: Rubinstein-Taybi and KAT6A syndromes
Rubinstein-Taybi syndrome (RTS) and KAT6A syndrome both involve variants in genes that regulate gene expression through histone acetylation—the addition of acetyl groups to histones, typically associated with switching genes on. RTS is caused by heterozygous mutations in CREBBP or EP300. KAT6A syndrome results from pathogenic variants in KAT6A, which normally encodes a lysine acetyltransferase. Despite their shared disruption of histone acetylation, the two syndromes have distinct cognitive and behavioral profiles.
In RTS, those with EP300 mutations (less than 10% of cases) tend to have milder intellectual disability than those with CREBBP variants. Individuals with RTS show relative strengths in behavior regulation, social communication, and affinity for interacting with others, even in the context of severe intellectual impairment. Verbal skills tend to be stronger than nonverbal and spatial skills. Behaviorally, attention problems, hyperactivity, and motor stereotypies (repetitive movements) are prevalent. Between 37–43% meet the clinical cutoff for ASD on screening tools. Elevated rates of anxiety, obsessive-compulsive disorder, and depression have been reported, particularly with increasing age.
KAT6A syndrome, by contrast, uniformly features intellectual disability and developmental delay. The most well-documented hallmark is severe receptive and expressive language and communication deficits—nearly 70% of affected individuals are minimally verbal. Comprehension skills are more preserved than expressive language. About 25–33% carry a diagnosis of ASD. Notably, emotional disturbances appear to be less of a concern.
Histone methylation disorders: Kabuki Syndrome and Wiedemann-Steiner syndrome
Kabuki Syndrome and Wiedemann-Steiner syndrome (WSS) are both caused by pathogenic variants in genes from the same family of histone methyltransferases—"writer" proteins called KMTs. Most cases of Kabuki Syndrome result from heterozygous variants in KMT2D (about 80%) or KDM6A (5–10%). WSS is due to haploinsufficiency (having only one working copy) of KMT2A. Both disorders disrupt histone methylation and chromatin remodeling, and both KMT2D and KMT2A also carry reader domains.
Intellectual functioning in Kabuki Syndrome ranges from severe impairment to average, with mild to moderate intellectual disability making up the majority. Estimates for WSS are similar: moderate intellectual disability to average intelligence, with most individuals in the mild to moderate range.
Here's where it gets interesting. Emerging evidence suggests that Kabuki Syndrome and WSS share relative weaknesses in nonverbal reasoning, visuoconstruction, and visuospatial skills. This pattern implicates abnormal development of the hippocampal formation—a brain region essential for spatial processing and memory—as the potential shared underpinning of both syndromes. But the two conditions diverge in other cognitive domains: executive functioning and working memory are generally less affected in Kabuki Syndrome, whereas recent case series suggest these are relative challenges in WSS.
Behaviorally, the two syndromes also diverge in notable ways. Kabuki Syndrome is generally associated with few behavioral problems, with anxiety, obsessive behaviors, and attention difficulties most often observed. Social skills are considered a relative strength—individuals are often described as sweet and affectionate, with relatively preserved social adaptive skills, though pragmatic language can be challenging. WSS presents with elevated rates of anxiety and behaviors concerning for ADHD. Unlike the low levels of aggression seen in Kabuki Syndrome, relatively high rates of aggression and conduct problems have been reported in WSS, which may be linked to the poorer executive functioning skills seen in that population. That said, new evidence highlights high sociability and strong prosocial skills in WSS as well, regardless of intellectual disability or ASD diagnosis.
Where this leads
The paper argues that these rare genetic conditions aren't just medical curiosities—they're a systematic way to decode the epigenetic underpinnings of cognition and behavior. DNA methylation, in particular, may hold a valuable key. Most MDEMs (and some other genetic neurodevelopmental syndromes like fragile X) have very specific DNA methylation patterns—called episignatures—at certain genome regions, distinctive enough to be used for diagnosis. Even more intriguingly, subtle variations within these episignatures may correlate with aspects of the cognitive and behavioral profile of a given individual. This means methylation signatures could eventually be used not just for diagnosis, but for prognosis and therapeutic monitoring.
To get there, the field needs larger sample sizes of individuals with MDEMs to undergo systematic deep phenotyping combined with genome-wide DNA methylation analysis. Such work could identify common signature loci and attributes for certain neurodevelopmental traits. And the implications extend far beyond rare disease: the findings and methods developed for MDEMs may be applied to much broader categories of neurodevelopmental and neuropsychiatric disorders.
The reason for that broader relevance is straightforward. Numerous conditions—from autism to epilepsy to schizophrenia—have been linked to epigenetic machinery dysfunction. Therapies developed for MDEMs to restore epigenetic balance may prove beneficial for these far more common conditions as well. In other words, research on rare disorders like Kabuki Syndrome is likely to lead to novel and effective therapies for neurodevelopmental and neuropsychiatric disorders more broadly.
Other findings:
• DNA methylation is the mechanism by which genes are imprinted—the process by which only one copy of certain genes is active, depending on which parent it came from. Imprinting disorders occur when this expression is disrupted by sequence changes or epigenetic misregulation. Prader-Willi syndrome and Angelman syndrome highlight the vital role of imprinting on gene expression and downstream cognitive and behavioral functioning.
• Variants causing alterations in the normal two-copy (biallelic) state of genes encoding epigenetic regulators often lead to disease.
• Imprinting refers to the process by which only one allele (copy) of certain autosomal genes is expressed, depending on whether it was inherited from the mother or the father.
• Methyl CpG binding proteins are reader proteins that bind to methylated DNA and recruit chromatin remodeling complexes to repress gene expression.
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