KBG syndrome

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

KBG syndrome is a rare disorder that affects several body systems. "KBG" represents the surname initials of the first families diagnosed with the disorder. Common signs and symptoms in individuals with this condition include unusual facial features, skeletal abnormalities, and intellectual disability.

A characteristic feature of KBG syndrome is unusually large upper front teeth (macrodontia). Other distinctive facial features include a wide, short skull (brachycephaly), a triangular face shape, widely spaced eyes (hypertelorism), wide eyebrows that may grow together in the middle (synophrys), a prominent nasal bridge, a long space between the nose and upper lip (long philtrum), and a thin upper lip.

A common skeletal abnormality in people with KBG syndrome is slowed mineralization of bones (delayed bone age); for example, an affected 3-year-old child may have bones more typical of a child of 2. In addition, affected individuals can have abnormalities of the bones of the spine (vertebrae) and ribs. They can also have abnormalities of the bones of the hands or feet, including unusually short or curved fifth (pinky) fingers (brachydactyly or clinodactyly, respectively) and flat feet (pes planus). Most affected individuals are shorter than average from birth.

Development of mental and movement abilities is also delayed in KBG syndrome. Most affected individuals learn to speak and walk later than normal and have mild to moderate intellectual disability. Most people with this condition have neurodevelopmental disorders, such as hyperactivity; anxiety; or autism spectrum disorder, which is characterized by impaired communication and social interactions.

Less common features of KBG syndrome include hearing loss, seizures, and heart defects.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

KBG syndrome is caused by mutations in the ANKRD11 gene. The protein produced from this gene enables other proteins to interact with each other and helps control gene activity. The ANKRD11 protein is found in nerve cells (neurons) in the brain. It plays a role in the proper development of the brain and may be involved in the ability of neurons to change and adapt over time (plasticity), which is important for learning and memory. ANKRD11 may function in other cells in the body and appears to be involved in normal bone development.

Most of the ANKRD11 gene mutations involved in KBG syndrome lead to an abnormally short ANKRD11 protein, which likely has little or no function. Reduction of this protein's function is thought to underlie the signs and symptoms of the condition. Because ANKRD11 is thought to play an important role in neurons and brain development, researchers speculate that a partial loss of its function may lead to developmental delay and intellectual disability in KBG syndrome. However, the mechanism is not fully known. It is also unclear how loss of ANKRD11 function leads to the skeletal features of the condition.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.

In some cases, an affected person inherits the mutation from one affected parent. Other cases result from new mutations in the gene and occur in people with no history of the disorder in their family.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

KBG syndrome is a rare disorder that has been reported in more than 150 individuals in the medical literature, though there are likely more who have not been recorded in the literature. For unknown reasons, males are affected more often than females. Doctors think the disorder is underdiagnosed because the signs and symptoms can be mild and may be attributed to other disorders.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Abnormal hair pattern · Frequent (79-30%)

An abnormality of the distribution of hair growth.

Abnormality of skeletal morphology · Frequent (79-30%)

An abnormality of the form, structure, or size of the skeletal system.

Anteverted nares · Frequent (79-30%)

Anteriorly-facing nostrils viewed with the head in the Frankfurt horizontal and the eyes of the observer level with the eyes of the subject. This gives the appearance of an upturned nose (upturned nasal tip).

Bilateral conductive hearing impairment · Frequent (79-30%)

A bilateral type of conductive hearing impairment.

Cleft palate · Frequent (79-30%)

Cleft palate is a developmental defect of the palate resulting from a failure of fusion of the palatine processes and manifesting as a separation of the roof of the mouth (soft and hard palate).

Cryptorchidism · Frequent (79-30%)

Testis in inguinal canal. That is, absence of one or both testes from the scrotum owing to failure of the testis or testes to descend through the inguinal canal to the scrotum.

Cutaneous syndactyly · Frequent (79-30%)

A soft tissue continuity in the A/P axis between two digits that extends distally to at least the level of the proximal interphalangeal joints, or a soft tissue continuity in the A/P axis between two digits that lies significantly distal to the flexion crease that overlies the metacarpophalangeal or metatarsophalangeal joint of the adjacent digits.

Delayed skeletal maturation · Frequent (79-30%)

A decreased rate of skeletal maturation. Delayed skeletal maturation can be diagnosed on the basis of an estimation of the bone age from radiographs of specific bones in the human body.

Other findings in the same source

From: Orphanet

Additional reported features include Feeding difficulties (Frequent (79-30%)); Global developmental delay (Frequent (79-30%)); Hypertelorism (Frequent (79-30%)); Long palpebral fissure (Frequent (79-30%)); Long philtrum (Frequent (79-30%)); Macrodontia (Frequent (79-30%)); Macrotia (Frequent (79-30%)); Microcephaly (Frequent (79-30%)); Oligodontia (Frequent (79-30%)); Prominent nasal bridge (Frequent (79-30%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing KBG syndrome is Clinical Genetics, with a clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with clinical geneticist referral.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Does the exact genetic or chromosome finding explain the observed features?
• Would a genetic counsellor help the family understand the result?
• Which organ-specific assessments are appropriate for this particular diagnosis?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for KBG syndrome. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.