Malonyl-CoA decarboxylase deficiency

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Malonyl-CoA decarboxylase deficiency is a condition that prevents the body from converting certain fats into energy.

The signs and symptoms of malonyl-CoA decarboxylase deficiency can begin in infancy or childhood and often include:

If malonyl-CoA decarboxylase deficiency is diagnosed during newborn screening, early treatment may help prevent or lessen many of the serious health problems that are associated with this condition.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Genetic changes that cause disease are called pathogenic variants. Pathogenic variants in the MLYCD gene cause malonyl-CoA decarboxylase deficiency. The MLYCD gene provides instructions for making an enzyme called malonyl-CoA decarboxylase. Malonyl-CoA decarboxylase is responsible for the chemical reaction that converts a molecule called malonyl-CoA into a molecule called acetyl-CoA, which is then used to make new fatty acids. Many tissues, including the heart and brain, use fatty acids as a major source of energy. When malonyl-CoA decarboxylase is not active, malonyl-CoA stops other enzymes from breaking down fatty acids.

Pathogenic variants in the MLYCD gene prevent malonyl-CoA decarboxylase from functioning properly. A shortage of functional malonyl-CoA decarboxylase disrupts the normal balance of fatty acid formation and breakdown in the body. Without enough functional enzyme, malonyl-CoA cannot be converted to acetyl-CoA, which leads to a buildup of malonyl-CoA. When too much malonyl-CoA is present, fatty acids are not converted to energy. This seems to have a particular impact on the brain and heart, although it is unclear how the specific features of malonyl-CoA decarboxylase deficiency, such as cardiomyopathy and developmental delays, occur.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

Malonyl-CoA decarboxylase deficiency is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a pathogenic variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Malonyl-CoA decarboxylase deficiency is very rare; only about 60 cases have been reported in the scientific literature.

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.

Delayed speech and language development · Very frequent (99-80%)

A degree of language development that is significantly below the norm for a child of a specified age.

Dystonia · Very frequent (99-80%)

An abnormally increased muscular tone that causes fixed abnormal postures. There is a slow, intermittent twisting motion that leads to exaggerated turning and posture of the extremities and trunk.

Elevated urine malonic acid level · Very frequent (99-80%)

The concentration of malonic acid in the urine, normalized for urine concentration, is above the upper limit of normal.

Methylmalonic aciduria · Very frequent (99-80%)

Increased concentration of methylmalonic acid in the urine.

Cardiomyopathy · Frequent (79-30%)

A myocardial disorder in which the heart muscle is structurally and functionally abnormal, in the absence of coronary artery disease, hypertension, valvular disease and congenital heart disease sufficient to cause the observed myocardial abnormality.

Failure to thrive · Frequent (79-30%)

Failure to thrive (FTT) refers to a child whose physical growth is substantially below the norm.

Feeding difficulties · Frequent (79-30%)

Impaired ability to eat related to problems gathering food and getting ready to suck, chew, or swallow it.

Global developmental delay · Frequent (79-30%)

A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age.

Growth delay · Frequent (79-30%)

A deficiency or slowing down of growth pre- and postnatally.

Hypoglycemia · Frequent (79-30%)

A decreased concentration of glucose in the blood.

Metabolic acidosis · Frequent (79-30%)

Metabolic acidosis (MA) is characterized by a fall in blood pH due to a reduction of serum bicarbonate concentration. This can occur as a result of either the accumulation of acids (high anion gap MA) or the loss of bicarbonate from the gastrointestinal tract or the kidney (hyperchloremic MA). By definition, MA is not due to a respirary cause.

Microcephaly · Frequent (79-30%)

Head circumference below 2 standard deviations below the mean for age and gender.

Reduced malonyl-CoA decarboxylase activity in cultured fibroblasts · Frequent (79-30%)

Activity of malonyl-CoA decarboxylase (EC 4.1.1.9) below the lower limit of normal in cultured fibroblasts.

Seizure · Frequent (79-30%)

A seizure is an intermittent abnormality of nervous system physiology characterized by a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain.

Other findings in the same source

From: Orphanet

Additional reported features include Diarrhea (Occasional (29-5%)); Hypotonia (Occasional (29-5%)); Ketosis (Occasional (29-5%)); Lethargy (Occasional (29-5%)); Vomiting (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Malonyl-CoA decarboxylase deficiency is Metabolic Medicine, with a metabolic specialist / clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with metabolic specialist / 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.

• Is a biochemical or molecular result needed to clarify the diagnosis?
• Does this condition require an individual plan for illness or reduced food intake?
• Should nutrition advice come from a specialist metabolic dietitian?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Malonyl-CoA decarboxylase deficiency. 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.