KBG syndrome is a rare genetic disorder that affects multiple body systems, causing distinctive facial features, unusually large front teeth, short stature, and developmental delays. The condition was named after the initials of the first three families diagnosed in 1975, and while more than 150 cases have been reported worldwide, doctors believe many more people remain undiagnosed because symptoms can be mild and easily overlooked.

Epidemiology

KBG syndrome is extremely rare, though the exact number of affected individuals worldwide remains unknown. According to medical literature, more than 150 cases have been documented to date, though experts strongly believe this number significantly underestimates the true prevalence of the condition.^[1][4] The actual number of people living with KBG syndrome is likely much higher because many individuals with mild symptoms may never receive a formal diagnosis or may be misdiagnosed with other conditions.

One particularly interesting pattern in KBG syndrome is that males appear to be affected more frequently and more severely than females. For unknown reasons that researchers are still working to understand, women with the condition tend to have milder symptoms than men.^[1][6] This difference in severity between sexes initially confused doctors about how the condition was inherited, with some thinking it might be passed down through the X chromosome rather than through regular chromosomes.

The condition affects people across all ethnic backgrounds and geographical regions, making it a pan-ethnic disorder that does not favor any particular population group.^[6] One of the main challenges in understanding how common KBG syndrome truly is comes from the fact that many affected individuals have very subtle features that can be easily missed during routine medical examinations, especially when symptoms are mild. Because of this, doctors suspect the syndrome is significantly underdiagnosed and underreported in medical literature.

Causes

KBG syndrome is caused by changes, called mutations, in a specific gene known as ANKRD11. A gene is a set of instructions in our cells that tells the body how to grow and function properly. The ANKRD11 gene provides instructions for making a protein that helps control the activity of other genes and allows different proteins to work together properly.^[1]

The ANKRD11 protein is especially important in nerve cells within the brain. It plays a crucial role in proper brain development and may be involved in something called plasticity, which is the brain’s ability to change and adapt over time. This adaptability is essential for learning and forming memories. The protein also appears to function in other cells throughout the body and seems to be involved in normal bone development, though researchers are still working to understand exactly how.^[1]

Most genetic changes that cause KBG syndrome result in an abnormally short ANKRD11 protein that has little or no function. When this protein doesn’t work properly, it leads to the various signs and symptoms seen in the condition. This reduction in protein function is called haploinsufficiency, meaning that having only one working copy of the gene instead of two is not enough for the body to function normally.^[4][8]

The genetic alterations that cause KBG syndrome can happen in two ways. The vast majority of identified mutations are what scientists call “loss of function” changes, which include nonsense variants, frameshift variants, and larger deletions at a specific location on chromosome 16 (position 16q24.3).^[6] These types of mutations essentially prevent the gene from producing a functional protein.

Because the ANKRD11 protein is so important for neurons and brain development, researchers believe that the partial loss of its function leads to the developmental delays and intellectual disability seen in KBG syndrome. However, scientists haven’t yet fully figured out the exact mechanism by which this happens. Similarly, it remains unclear exactly how the loss of ANKRD11 function leads to the skeletal features and other physical characteristics of the condition.^[1]

Risk Factors

Understanding risk factors for KBG syndrome is somewhat different from understanding risk factors for conditions like heart disease or diabetes. KBG syndrome is inherited in what doctors call an autosomal dominant pattern. This means that having just one copy of the altered ANKRD11 gene in each cell is sufficient to cause the disorder.^[1]

In approximately two-thirds of reported cases, the genetic change occurs for the first time in the affected person and is called a de novo mutation. This means it appears spontaneously, seemingly out of nowhere, in a family with no previous history of the condition.^[2] For deletions of the chromosome region 16q24.3 that includes the ANKRD11 gene, about 75 percent are de novo, while the remaining 25 percent are inherited from a parent.^[2]

When a person has KBG syndrome, each of their children has a 50 percent chance of inheriting the genetic change and developing the condition. However, because the syndrome shows such significant variability even within families, a child who inherits the mutation might have symptoms that are very different from their affected parent—either more severe or more mild.

Interestingly, there don’t appear to be traditional “risk factors” like lifestyle choices or environmental exposures that increase the likelihood of developing KBG syndrome. The de novo mutations that cause most cases appear to occur randomly during the formation of reproductive cells or in early embryonic development. Advanced parental age has been associated with increased rates of new mutations in some genetic conditions, but this specific connection hasn’t been clearly established for KBG syndrome in the available medical literature.

Symptoms

The symptoms of KBG syndrome can vary considerably from person to person, even among members of the same family. This variability makes the condition sometimes challenging to recognize. However, there are several characteristic features that doctors look for when considering a diagnosis.^[2]

One of the most distinctive features of KBG syndrome is macrodontia, which means unusually large teeth, especially the upper central incisors (the two front teeth on top). This hallmark feature is observed in about 80 percent of cases and is often one of the first clues that leads doctors to suspect KBG syndrome.^[4][8] Additional dental problems can include missing teeth, premature tooth loss in adults, and enamel hypoplasia, where the protective outer layer of the teeth doesn’t form properly.

Facial features in KBG syndrome tend to be quite characteristic, though they are prominent in only about half of affected individuals. These include a triangular-shaped face, a wide and short skull (called brachycephaly), widely spaced eyes (hypertelorism), and wide or bushy eyebrows that may grow together in the middle (known as synophrys).^[2][4] Other facial characteristics include a prominent nasal bridge with a bulbous tip, a long space between the nose and upper lip (called the philtrum), thin upper lip, and prominent ears.

Short stature is very common in KBG syndrome, with height below the 10th percentile observed in about two-thirds of cases. Most affected individuals are shorter than average from birth and remain below average throughout their lives.^[4][8] A particularly notable finding is delayed bone age, where bone development lags behind chronological age. For example, a 3-year-old child with KBG syndrome might have bones that look more like those of a 2-year-old.^[1]

Skeletal abnormalities beyond short stature are common. The most frequent include brachydactyly (unusually short fingers or toes) and fifth finger clinodactyly (curved pinky fingers).^[4][8] Other skeletal features can include abnormalities of the spine bones and ribs, flat feet, a large anterior fontanelle (soft spot on a baby’s head) with delayed closure, and sometimes scoliosis (curvature of the spine).

Developmental delays are almost always present in KBG syndrome. This includes delayed motor milestones and markedly delayed speech development.^[4][8] Most affected individuals learn to speak and walk later than typically developing children and have mild to moderate intellectual disability. However, the degree of intellectual impairment varies considerably among individuals.

Behavioral disturbances have been recognized as constant clinical features of KBG syndrome. These include hyperactivity, aggressiveness, attention deficit, and anxiety.^[4][8] Many people with this condition also have autism spectrum disorder, which is characterized by impaired communication and social interactions.

Less common but notable features include hearing loss (which can be conductive, mixed, or sensorineural), recurrent ear infections (otitis media), seizures with or without abnormalities on brain wave tests, feeding difficulties particularly in infancy, palatal abnormalities, premature puberty, and congenital heart defects.^[1][2][4] Some individuals may also have brain malformations visible on imaging studies.

Prevention

Because KBG syndrome is a genetic condition caused by mutations in the ANKRD11 gene, there is currently no way to prevent the condition from occurring when it arises as a new (de novo) mutation. These spontaneous genetic changes happen randomly during the formation of reproductive cells or early embryonic development and cannot be predicted or prevented by any known lifestyle modifications or medical interventions.

However, for families who already have a member diagnosed with KBG syndrome, genetic counseling can provide valuable information about the risk of the condition occurring in future children. When an affected person has the genetic alteration, each of their children has a 50 percent chance of inheriting the mutation because the condition follows an autosomal dominant inheritance pattern.^[1][2]

For families where a causative genetic alteration has been identified in an affected family member, prenatal testing and preimplantation genetic testing are possible options.^[2] Prenatal testing can detect the genetic change during pregnancy, while preimplantation genetic testing can be performed on embryos created through in vitro fertilization before pregnancy is established. These options allow families to make informed decisions about family planning based on their personal values and circumstances.

While prevention of the genetic condition itself isn’t possible, early diagnosis and intervention can significantly improve outcomes for individuals with KBG syndrome. Regular developmental screening and early identification of the condition can lead to prompt initiation of supportive therapies and interventions that help affected individuals reach their full potential.

For children diagnosed with KBG syndrome, preventing secondary complications is an important focus of medical care. This includes routine monitoring for hearing loss, which if detected early can be addressed with hearing aids or other interventions to prevent speech and language delays from worsening. Regular dental care is essential to maintain oral health given the characteristic large teeth and potential enamel problems. Vision screening should be conducted routinely to identify and correct any refractive errors or other eye problems early.

Because recurrent ear infections are common in KBG syndrome, prompt treatment of infections and consideration of pressure-equalizing tubes when appropriate can help prevent hearing loss.^[2] For individuals with seizures, appropriate medication management is crucial to prevent complications associated with uncontrolled seizure activity.

Growth monitoring throughout childhood and adolescence allows doctors to track whether a child’s growth pattern is appropriate and whether interventions like growth hormone therapy might be beneficial. Monitoring pubertal development in prepubertal individuals can identify premature puberty early, allowing for intervention if needed to maximize final adult height.^[2]

Pathophysiology

The pathophysiology of KBG syndrome—meaning how the condition causes changes in normal body functions—centers on the loss of function of the ANKRD11 protein. This protein, encoded by the ANKRD11 gene, acts as what scientists call a chromatin regulator, meaning it helps control how DNA is packaged and which genes are turned on or off in cells.^[4][8]

The ANKRD11 protein has been shown to be crucial for controlling histone acetylation and gene expression during neural development. Histones are proteins around which DNA wraps, and acetylation is a chemical modification that affects how tightly or loosely DNA is wound around these proteins. This in turn affects whether genes can be easily accessed and activated. By controlling this process, ANKRD11 essentially acts as a master regulator that influences the activity of many other genes during brain development.^[4][8]

When ANKRD11 function is reduced due to mutations, this carefully orchestrated control of gene activity goes awry. In nerve cells, where the protein is particularly abundant and important, this disruption appears to interfere with normal brain development and the brain’s ability to adapt and change over time—a property called neuroplasticity that is essential for learning and memory.^[1] While researchers understand that partial loss of ANKRD11 function leads to the developmental delays and intellectual disability characteristic of KBG syndrome, the precise molecular mechanisms by which this happens are still being investigated.

The skeletal abnormalities seen in KBG syndrome, including delayed bone age and various bone malformations, suggest that ANKRD11 also plays an important role in bone development and growth. The protein appears to function in cells beyond neurons, though exactly how it regulates skeletal development remains unclear. The consistent finding of delayed bone mineralization—where bones develop more slowly than expected for a child’s age—indicates that ANKRD11 is somehow involved in the complex processes that control how bones form and mature.^[1]

The wide variability in how severely different individuals are affected by KBG syndrome, even when they carry similar types of mutations, suggests that other genetic and environmental factors likely modify how the loss of ANKRD11 function manifests. Some people might have other genetic variations that partially compensate for the loss of ANKRD11, leading to milder symptoms, while others might lack such protective factors and therefore show more severe features.

The fact that males tend to be more severely affected than females is particularly intriguing and suggests possible differences in how the condition affects developing male versus female bodies. However, the biological basis for this sex difference remains unknown and is an active area of research interest.^[6]

Individuals with deletions of the chromosome region 16q24.3 that encompass ANKRD11 plus neighboring genes tend to have additional features not typically seen in those with mutations affecting only the ANKRD11 gene itself. These can include a higher incidence of brain anomalies, congenital heart defects, severe vision problems, blood platelet abnormalities, and potentially more severe behavioral problems.^[6] This observation suggests that loss of other genes in the deleted region contributes additional symptoms, helping scientists understand which features are directly attributable to ANKRD11 loss versus which result from loss of nearby genes.

Notably, ANKRD11 has been identified as one of the most frequently mutated genes in patients with neurodevelopmental disorders diagnosed through comprehensive genetic testing methods like whole exome sequencing.^[4] This highlights its critical importance in normal brain development and suggests that even subtle disruptions in its function can have significant consequences for cognitive development.