Kleefstra syndrome is a rare genetic condition caused by the loss or disruption of one working copy of a gene called EHMT1, located near the tip of chromosome 9. The result is a recognizable pattern of intellectual disability, low muscle tone in infancy, distinctive facial features, and a range of organ and behavioral complications that can evolve over a person’s lifetime. Because EHMT1 encodes a protein that helps control how other genes are switched on and off during brain development, even a partial reduction in its activity has far-reaching consequences. The condition is considered rare, but improved genetic testing has made diagnosis more common than it once was, and an international clinical guideline published in 2025 now provides structured recommendations for care.
What Causes Kleefstra Syndrome
EHMT1 sits in a region of chromosome 9 known as 9q34.3. The syndrome arises when one copy of EHMT1 is either deleted entirely or carries a mutation that prevents the gene from producing a functional protein. In genetics shorthand, this is called haploinsufficiency: one working copy is simply not enough to do the full job. Research comparing patients who have large chromosomal deletions with those who have smaller point mutations in EHMT1 has confirmed that the DNA methylation signatures are essentially the same in both groups, reinforcing the idea that it is the loss of EHMT1 function specifically, rather than neighboring genes, that drives the core syndrome.1Journal of Translational Genetics and Genomics. EHMT1 pathogenic variants and 9q34.3 microdeletions share altered DNA methylation patterns in patients with Kleefstra syndrome
That said, the size of the deletion matters. A large study of 209 individuals found that people with multigene deletions spanning more than just EHMT1 tend to have more severe intellectual disability, shorter stature, and a higher rate of constipation compared to those with mutations limited to EHMT1 alone. About a third of individuals with multigene deletions had severe intellectual disability, compared with roughly one in ten of those with EHMT1-only mutations. Structural heart defects were also more common in the large-deletion group, and all four patients identified with a specific type of heart malformation called an atrioventricular septal defect had deletions larger than one megabase.2American Journal of Human Genetics. Comprehensive clinical and molecular spectrum of Kleefstra syndrome and genotype-phenotype correlations in 209 individuals
How EHMT1 Shapes the Developing Brain
EHMT1 produces a protein that works as part of a two-protein team alongside a close relative called EHMT2 (sometimes referred to as G9a). Together, they chemically modify histone proteins, the molecular spools around which DNA is wound, by attaching methyl groups to a specific spot. This modification acts like a “quiet down” signal, dialing back the activity of nearby genes. The process is critical for fine-tuning which genes are active in neurons and when.3Neuron. EHMT1-Dependent Epigenetic Control of Homeostatic Synaptic Scaling in the Developing Visual Cortex
One of the key roles this system plays is in synaptic scaling, a process by which neurons adjust their sensitivity based on how much input they are receiving. When neural activity drops, EHMT1 and EHMT2 help ramp up synaptic strength so circuits stay functional. Research in visual cortex tissue has shown that the two proteins repress the expression of a growth factor called BDNF during this process, and that losing EHMT1 disrupts the entire scaling program.4Neuron. EHMT1-Dependent Epigenetic Control of Homeostatic Synaptic Scaling in the Developing Visual Cortex
An unexpected wrinkle emerged from studies of mouse models. When EHMT1 levels drop, the brain does not simply end up with less histone methylation across the board. Instead, other enzymes appear to overcompensate, piling on a more extreme form of the same chemical mark. This compensatory cascade may itself contribute to the cognitive problems seen in the syndrome, because the resulting over-methylation silences genes that should be active, including a family of cell-adhesion molecules called protocadherins that help neurons wire up properly.5Nucleic Acids Research. Increased H3K9 methylation and impaired expression of Protocadherins are associated with the cognitive dysfunctions of the Kleefstra syndrome
Recognizing the Clinical Features
The hallmarks of Kleefstra syndrome usually become apparent in the first year or two of life. Low muscle tone, or hypotonia, is often the earliest sign, followed by noticeable delays in motor milestones like sitting and walking. Speech development tends to be significantly delayed; many individuals develop only limited spoken language, though receptive understanding is often stronger than expressive ability.
Facial features become more recognizable over time. They typically include a flat midface, a short nose with an upturned tip, a protruding tongue, widely spaced eyes, and arched eyebrows. These features are subtle in some people and more pronounced in others, and clinicians experienced with the condition can often suspect the diagnosis on appearance alone before genetic testing confirms it. The intellectual disability ranges from mild to severe, with the degree influenced in part by the genetic subtype described above.6PubMed Central. Clinical characteristics and genetic analysis of four pediatric patients with Kleefstra syndrome
Brain imaging results vary widely. About a quarter of individuals have structurally normal scans, while others show abnormalities including enlarged ventricles, white matter signal changes, or an unusually shaped corpus callosum and brainstem.7PubMed Central. Longitudinal imaging in Kleefstra syndrome-Brief report and literature review The imaging findings do not always correlate neatly with the severity of a person’s intellectual disability, which makes MRI useful for ruling out certain complications but unreliable as a predictor of outcome on its own.
Heart, Kidney, and Other Organ Involvement
Kleefstra syndrome is not limited to the brain. Congenital heart defects occur at a high enough rate that routine cardiac screening with echocardiography is recommended for every newly diagnosed individual. The types of heart malformations vary, from relatively mild septal defects to more complex structural problems. Kidney abnormalities, though less common, also occur and should be screened for with ultrasound.8PubMed. Severe neonatal presentation of Kleefstra syndrome in a patient with hypoplastic left heart syndrome and 9q34.3 microdeletion
Other medical features that crop up across the Kleefstra syndrome population include:
- Epilepsy: Seizures affect a substantial minority of individuals, with onset that can range from infancy to later childhood.
- Constipation: Especially common in individuals with large chromosomal deletions, likely reflecting both low muscle tone in the gut and possible autonomic nervous system differences.
- Overweight and obesity: Weight gain becomes a growing concern through childhood and adolescence, possibly related to reduced activity levels, appetite regulation differences, and in some cases medication effects.
- Hearing and vision issues: Recurrent ear infections and refractive errors are reported frequently enough to warrant regular monitoring.
Because the syndrome can affect so many organ systems, the international guideline developed by a consortium of 43 clinical experts and patient representatives from 15 countries includes 66 specific recommendations covering everything from cardiac screening schedules to behavioral interventions.9PubMed Central. International clinical evidence-based guideline for Kleefstra syndrome
Behavioral and Psychiatric Features
Behavioral challenges are among the most impactful aspects of Kleefstra syndrome for both the individual and their family. Research using standardized assessments has found that people with the syndrome tend to show a distinctive profile of adaptive and maladaptive behaviors, one that is recognizably different from profiles seen in other genetic causes of intellectual disability.10PubMed. Adaptive and maladaptive functioning in Kleefstra syndrome compared to other rare genetic disorders with intellectual disabilities Many individuals meet criteria for autism spectrum disorder, and features like rigid adherence to routines and difficulty with transitions are common.
In adolescents and adults, the psychiatric picture can become more complex. A detailed case report described a young man with Kleefstra syndrome who experienced periods of manic-like symptoms, compulsive eating with significant weight gain, addictive and gambling behaviors, unsafe internet use, and severe meltdowns. His symptoms were eventually brought under reasonable control through a combination of behavioral strategies and medication, highlighting that while psychiatric problems in this population can be severe, they are not necessarily untreatable.11PubMed Central. Psychiatric manifestations of Kleefstra syndrome: a case report
The Regression That Can Come With Adolescence
One of the more distressing features of Kleefstra syndrome, and one that families are often unprepared for, is a pattern of behavioral and functional regression that can emerge during the teenage years or early adulthood. An individual who had been making steady, if slow, developmental gains may become increasingly withdrawn, lose previously acquired skills, develop apathy, or show a marked increase in challenging behaviors. Clinicians who have followed patients over time describe this regression as having no clear neurological cause on brain scans, framing it instead as an unexplained neuropsychiatric phenomenon.12PubMed Central. Update on Kleefstra Syndrome
The regression does not happen to every person with the syndrome, but it occurs frequently enough to warrant close monitoring as individuals approach puberty. Families and clinicians who are aware of the possibility can watch for early warning signs and intervene with environmental and psychiatric support before the regression deepens. Whether this reflects a vulnerability in brain circuits that were marginally compensated during childhood, or a sensitivity to the hormonal and social upheaval of adolescence, or some combination, remains an open question.
Sleep Disruption
Sleep problems are a near-universal complaint among families affected by Kleefstra syndrome. The pattern tends to include frequent nighttime awakenings, irritability during the night, and sometimes ritualistic behaviors such as pinching, kicking, or repetitive drink requests that can make it extremely difficult for the rest of the household to get adequate rest. Sleep disruption in turn worsens daytime behavior and learning capacity, setting up a cycle that can be hard to break.13Journal of Neurology, Neurological Science and Disorders. Kleefstra syndrome and sleep disorders: An Italian case report
Melatonin supplementation is often tried as a first-line approach, since the circadian system seems to be part of the problem. Behavioral sleep hygiene strategies, including consistent bedtime routines, reduced screen time before bed, and a cool dark sleeping environment, can help but rarely solve the problem entirely. For families, sleep deprivation is often the single largest source of daily stress, and it compounds the already heavy demands of caregiving.
A Broader Epigenetic Network
One of the more intriguing findings in Kleefstra syndrome research is that EHMT1 does not work in isolation. Investigators studying individuals who shared the clinical features of the syndrome but had no EHMT1 mutations discovered de novo mutations in four other genes: MBD5, MLL3 (now known as KMT2C), SMARCB1, and NR1I3. All four encode proteins involved in the same broad category of work that EHMT1 performs, chemically modifying chromatin to regulate gene activity.14American Journal of Human Genetics. Identification of Chromatin-Modifying Enzyme Genes Linked to Kleefstra Syndrome
Experiments in fruit flies confirmed that these genes cooperate functionally. Flies with reduced activity in the fly version of KMT2C and those lacking the fly version of EHMT1 showed a significant overlap in which genes were disrupted, and the affected gene sets pointed to the same biological processes, especially synaptic plasticity, the ability of neural connections to strengthen or weaken with experience.15PLOS Genetics. Functional convergence of histone methyltransferases EHMT1 and KMT2C involved in intellectual disability and autism spectrum disorder This convergence helps explain why mutations in several different genes can produce overlapping clinical pictures, and it suggests that the intellectual disability and autism features associated with these conditions may trace back to a shared molecular pathway rather than being entirely independent problems.
Research Into Potential Treatments
There is no cure for Kleefstra syndrome today, but research in animal models has produced results that would have been considered wildly optimistic a decade ago. In a mouse model of the syndrome, researchers engineered a system to switch on production of the EHMT1 protein in neurons starting at three weeks of age, well after the initial period of brain development. This postnatal protein supply restored the number of dendritic spines, the tiny protrusions on neurons where synapses form, to normal levels. It also corrected an abnormal increase in inflammatory cells called microglia and normalized levels of an inflammatory enzyme that had been elevated in the model mice’s brains.16iScience. Postnatal GLP/EHMT1 supply improves cognitive and neurological phenotypes in a mouse model of Kleefstra syndrome
These findings are important for two reasons. First, they suggest that the brain damage caused by EHMT1 deficiency is not entirely locked in during prenatal development, which means there may be a window for intervention after birth. Second, the neuroinflammatory component hints at a possible drug target, since anti-inflammatory compounds are far easier to develop than gene therapies. Neither approach is anywhere near the clinic yet, but the direction of travel is encouraging.
On the diagnostic and modeling front, researchers have built a pipeline using CRISPR gene-editing technology to introduce Kleefstra-associated mutations into human stem cells, grow them into neurons, and study the resulting changes in gene activity across the entire genome. This system allows rapid assessment of whether a newly identified genetic variant is likely to cause disease and helps identify potential molecular targets for treatment.17PubMed Central. CRISPR single base editing, neuronal disease modelling and functional genomics for genetic variant analysis: pipeline validation using Kleefstra syndrome EHMT1 haploinsufficiency
The Caregiving Reality
The medical and developmental challenges of Kleefstra syndrome inevitably place enormous demands on families. Qualitative research interviewing mothers of children with the condition has found that caregiving stress is compounded by the rarity of the diagnosis itself. Parents often spend years seeking an answer before receiving a diagnosis, and once they have it, they may find that their local clinicians have never encountered the syndrome. The isolation of managing a condition that most people, including many doctors, have never heard of adds a layer of emotional burden on top of the practical demands of round-the-clock care, medical appointments, therapy sessions, and managing sleep disruption and behavioral challenges.18Scholars Crossing. An Interpretative Phenomenological Study Exploring the Lived Experiences of Mothers of Children With Kleefstra Syndrome
Online communities and syndrome-specific organizations have become lifelines for many families, providing both emotional support and practical information that would be difficult to assemble independently. The development of international clinical guidelines represents a significant step toward reducing the variation in care that families experience depending on where they live and which specialists they happen to see. For a condition this rare, having a structured set of recommendations that a local pediatrician can follow makes a tangible difference in how quickly problems are identified and addressed.
Diagnosis and Getting Tested
Kleefstra syndrome is usually confirmed through chromosomal microarray analysis, which can detect the large deletions on 9q34.3, or through exome or whole-genome sequencing, which can identify smaller point mutations within EHMT1. Microarray was historically the first-line test, and it remains good at catching deletions, but it misses the cases caused by single-nucleotide variants or small insertions and deletions. For children presenting with developmental delay, hypotonia, and characteristic facial features, broader genomic testing via exome sequencing is increasingly recommended to ensure nothing is missed.19PubMed Central. Clinical characteristics and genetic analysis of four pediatric patients with Kleefstra syndrome
Early diagnosis matters not because any current treatment can reverse the underlying genetic change, but because it opens the door to targeted surveillance and early intervention. Knowing a child has Kleefstra syndrome means their clinicians will screen for heart defects, monitor kidney function, keep a close eye on seizure risk, and begin speech and occupational therapy earlier. It also means families can be warned about the possibility of adolescent regression and prepared with strategies before it happens, rather than scrambling to respond after skills have already been lost.