Mitochondrial DNA Depletion

Mitochondrial DNA depletion syndrome is a rare genetic condition that dramatically reduces the amount of vital DNA inside the power generators of cells, leading to severe energy failure in affected organs and tissues.

Table of contents

• What Is Mitochondrial DNA Depletion Syndrome?
• What Causes This Condition?
• How Is It Inherited?
• Signs and Symptoms
• Different Forms of the Condition
• How Is It Diagnosed?
• Outlook and Life Expectancy
• Treatment and Management

What Is Mitochondrial DNA Depletion Syndrome?

Mitochondrial DNA depletion syndrome, also known as MDS or MDDS, is a group of genetic conditions that cause a severe reduction in the amount of mitochondrial DNA (mtDNA) in affected tissues^[1][2]. Mitochondria are the structures inside cells that act as energy factories, creating more than 90% of the energy our bodies need to function^[3]. These tiny structures have their own DNA, separate from the DNA in the cell’s nucleus, which is essential for producing energy.

In this condition, the number of copies of mitochondrial DNA becomes significantly reduced in certain tissues, even though the mtDNA itself has no mutations^[5]. This depletion means that affected cells cannot produce enough energy to work properly. Because energy production is impaired, organs and tissues with high energy demands—such as the brain, muscles, liver, and heart—are particularly vulnerable to damage^[2].

mtDNA depletion syndrome, Alper’s disease, mitochondrial DNA depletion myopathy

What Causes This Condition?

Mitochondrial DNA depletion syndrome is caused by changes (mutations) in genes found in the nucleus of cells—not in the mitochondria themselves^[2]. These nuclear genes provide instructions for making proteins that are essential for maintaining mitochondrial DNA. Some of these proteins help produce the building blocks needed to make new mtDNA, while others are directly involved in copying mtDNA^[2].

Scientists have identified mutations in several specific genes that can cause this condition. These include genes involved in making the chemical building blocks of DNA (TK2, SUCLA2, SUCLG1, RRM2B, DGUOK, and TYMP) and genes involved in copying mtDNA (POLG and C10orf2)^[2]. Additional genes such as MPV17 and FBXL4 have also been linked to the condition^[1][6].

When any of these genes don’t work properly due to mutations, the maintenance of mtDNA becomes disrupted. This leads to a shortage of mtDNA in affected tissues, which means mitochondria cannot produce enough energy. The resulting energy failure causes cells to malfunction or die, leading to problems in various organs throughout the body^[7].

How Is It Inherited?

Mitochondrial DNA depletion syndrome is inherited in an autosomal recessive pattern^[1][4]. This means that a person must inherit two copies of a mutated gene—one from each parent—to develop the condition.

Parents who each carry one copy of a mutated gene are called carriers. Carriers typically do not show signs or symptoms of the condition because they also have one normal copy of the gene, which is usually enough to maintain health^[4]. When both parents are carriers, there is a 1 in 4 (25%) chance with each pregnancy that their child will inherit both mutated genes and develop the condition.

Some genetic mutations can also develop newly (occur de novo) in the early embryo, meaning there may be no family history of the condition. This can catch families by surprise when they receive a diagnosis^[3].

Signs and Symptoms

The symptoms of mitochondrial DNA depletion syndrome are very diverse and can appear at different ages, from newborns to adults, depending on which specific form of the condition a person has^[1]. However, symptoms typically appear early in life, usually during infancy or in the first two years^[3].

Common symptoms across different forms include^[1][3]:

• Weak muscle tone (hypotonia)—muscles feel floppy or lack normal tension
• Muscle weakness and wasting
• Difficulty feeding and swallowing
• Developmental delays or loss of skills already learned
• Seizures and epilepsy
• Problems with liver function, including liver failure
• Hearing loss
• Vision problems
• Breathing difficulties
• Heart problems, including thickening of the heart muscle

Many children with this condition also experience a buildup of lactic acid in the blood and body tissues, which can cause nausea, vomiting, and rapid, deep breathing^[1]. Some may have abnormal movements such as uncontrolled jerking or twisting, curvature of the spine, and an abnormally small head size^[1][6].

Different Forms of the Condition

Mitochondrial DNA depletion syndrome is classified into several different forms based on which organs are primarily affected^[2]:

Myopathic Form (Affecting Muscles)

This form, usually caused by mutations in the TK2 gene, primarily affects the muscles^[2]. Infants generally develop normally at first, but around two years of age, symptoms of general muscle weakness, tiredness, lack of stamina, and difficulty feeding begin to appear^[1]. Some children may lose control of facial and throat muscles, making swallowing difficult. Motor skills that had been learned may be lost, though thinking and brain function are generally not affected^[1].

Encephalomyopathic Form (Affecting Brain and Muscles)

This form, caused by mutations in genes such as SUCLA2, SUCLG1, RRM2B, or FBXL4, affects both the brain and muscles^[2][6]. Symptoms typically appear during infancy with weak muscle tone, muscle wasting, and delays in learning basic skills like walking, talking, and coordinated movement^[1]. Additional features may include abnormal movements, curvature of the spine, hearing loss, stunted growth, seizures, and breathing problems that can lead to frequent lung infections^[1][4].

The SUCLG1-related form is particularly severe, with most affected individuals developing brain dysfunction and muscle weakness soon after birth^[4]. A substance called methylmalonic acid typically builds up in the blood and urine in this form.

Hepatocerebral Form (Affecting Liver and Brain)

This form, caused by mutations in genes such as DGUOK, MPV17, POLG, or C10orf2, primarily affects the liver and brain^[2]. There is an early-onset form where symptoms appear in the first week of life, including problems with lactic acid buildup, low blood sugar, and issues in many organs^[1]. Within weeks of birth, affected infants can develop liver failure with jaundice (yellowing of the skin) and abdominal swelling, along with neurological problems including developmental delays and uncontrolled eye movements.

One specific form called Alpers syndrome is part of this group and is often caused by mutations in the POLG gene^[14]. The three major features of Alpers syndrome are severe epilepsy, loss of developmental skills, and liver failure.

How Is It Diagnosed?

Diagnosis of mitochondrial DNA depletion syndrome is primarily made through genetic testing, often using a blood sample^[3]. Historically, a muscle or liver biopsy was required to confirm the diagnosis by directly measuring the amount of mitochondrial DNA in the tissue. Today, genetic testing is usually the first step, and tissue biopsy may only be done to confirm a genetic diagnosis or may not be needed at all if the genetic cause is clearly identified^[3].

Healthcare providers may suspect this condition based on the pattern of symptoms, particularly when multiple organ systems are affected at the same time. Blood tests may show elevated levels of lactic acid or other metabolic abnormalities. Additional testing might include imaging studies of the brain, heart function tests, and assessments of developmental progress.

Outlook and Life Expectancy

Mitochondrial DNA depletion syndrome is generally a severe condition with a poor outlook for the majority of affected individuals^[2]. The condition is typically fatal in infancy and early childhood^[1].

However, the prognosis varies significantly depending on the specific form:

• In the hepatocerebral form, onset is typically from birth to six months, and death often occurs in the first year due to liver failure^[5].
• In the myopathic form, onset is from birth to two years, and death typically occurs in infancy or childhood due to respiratory failure, though some patients with this form have survived into their teenage years^[1][5].
• Some individuals with the SUCLA2 encephalomyopathic variant have survived into adulthood^[1].
• The SUCLG1-related form typically results in death in childhood, with some infants dying only a few days after birth if they develop a particularly severe form called fatal infantile lactic acidosis^[4].
• Children with FBXL4-related encephalomyopathic mtDNA depletion syndrome usually live only into early childhood due to the severity of their symptoms^[6].

Treatment and Management

Currently, there is no cure for any form of mitochondrial DNA depletion syndrome, and no highly effective therapy is available for these conditions^[2][4]. However, some preliminary treatments have shown a reduction in symptoms^[1].

Treatment is directed mainly toward managing symptoms and preventing complications^[2]. Affected individuals should receive a comprehensive evaluation to assess how different body systems are involved. Management approaches may include:

• Seizure control with anticonvulsant medications—though it’s crucial to note that sodium valproate should be avoided in forms like Alpers syndrome, as this medication can trigger liver failure^[14]
• Feeding support, including feeding tubes when necessary
• Physical therapy to maintain muscle function
• Respiratory support for breathing difficulties
• Nutritional modifications and vitamin or cofactor supplementation, which may provide some benefit^[2]
• Management of heart, liver, and other organ complications

Liver transplantation remains controversial and has proved unsuccessful in patients with certain forms, such as Alpers syndrome^[2][14]. Stem cell transplantation has shown promising results in one specific related condition called MNGIE disease^[2].

For families with an identified genetic mutation, reproductive options may be available for future pregnancies, including prenatal testing and preimplantation genetic diagnosis. Genetic counseling can provide important information and support for affected families^[14].