FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a severe condition that begins in infancy and affects multiple body systems. It is primarily associated with brain dysfunction combined with muscle weakness (encephalomyopathy).

Infants with FBXL4-related encephalomyopathic mtDNA depletion syndrome have weak muscle tone (hypotonia) and difficulty growing and gaining weight at the expected rate (faltering weight). Children with FBXL4-related encephalomyopathic mtDNA depletion syndrome have delayed development of mental and motor skills and severely impaired speech development. Many affected individuals have seizures, movement abnormalities, and an unusually small head size (microcephaly) with a loss of nerve cells in the brain (cerebral atrophy).

All individuals with FBXL4-related encephalomyopathic mtDNA depletion syndrome have a buildup of a chemical called lactic acid in the body (lactic acidosis), and about half of individuals have an accumulation of ammonia in the blood. Buildup of these substances can be life-threatening. Many affected individuals also have heart abnormalities, such as congenital heart defects or heart rhythm abnormalities (arrhythmias). In addition, individuals with this condition can have vision problems, hearing loss, liver abnormalities (hepatopathy), and immune deficiency due to a decrease in white blood cells. Many children with FBXL4-related encephalomyopathic mtDNA depletion syndrome have distinctive facial features that can include thick eyebrows; outside corners of the eyes that point upward (upslanting palpebral fissures); a broad nasal bridge and tip; and a long, smooth space between the upper lip and nose (philtrum).

Because the encephalomyopathy and other signs and symptoms are so severe, people with FBXL4-related encephalomyopathic mtDNA depletion syndrome usually live only into early childhood.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

As its name suggests, FBXL4-related encephalomyopathic mtDNA depletion syndrome is caused by mutations in the FBXL4 gene. This gene provides instructions for producing a protein that is found within cell structures called mitochondria. Mitochondria are involved in a wide variety of cellular activities, including energy production, chemical signaling, and regulation of cell growth and division (proliferation) and cell death (apoptosis). Mitochondria contain their own DNA, known as mitochondrial DNA (mtDNA), which is essential for the normal function of these structures. The FBXL4 protein is likely involved in the maintenance of mtDNA. Having an adequate amount of mtDNA is essential for normal energy production within cells.

FBXL4 gene mutations that cause FBXL4-related encephalomyopathic mtDNA depletion syndrome lead to a loss of FBXL4 protein function. A lack of this protein's activity leads to problems with the maintenance of mtDNA, which can reduce the amount of mtDNA in cells (known as mtDNA depletion). Depletion of mtDNA impairs mitochondrial function in many of the body's cells and tissues. Reduced mitochondrial function eventually leads to cell dysfunction, most noticeably affecting the brain, muscles, and other tissues that have high-energy requirements. This cell dysfunction leads to encephalomyopathy and other features of FBXL4-related encephalomyopathic mtDNA depletion syndrome.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

FBXL4-related encephalomyopathic mtDNA depletion syndrome is a rare condition; the exact prevalence is unknown. At least 50 affected individuals have been described in the medical literature.

Which doctor should you see?

The suggested department for discussing FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome is Neurology, with a neurologist as the relevant type of clinician. General physician / Family Medicine; paediatrician for children. Referral depends on symptoms.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

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.

• Which nervous-system findings help explain the symptoms?
• Would an assessment of walking, communication or daily function be helpful?
• Are rehabilitation or other specialist services relevant?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for FBXL4-related encephalomyopathic mitochondrial DNA depletion 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.