Mitochondrial DNA depletion syndrome is a group of rare genetic disorders that affect how cells produce energy, leading to severe health problems that typically begin in infancy or early childhood, though symptoms can sometimes appear later in life.

Understanding the Condition

Mitochondrial DNA depletion syndrome, sometimes called MDS or MDDS, represents a collection of genetic conditions that share a common problem: a significant drop in the amount of mitochondrial DNA (the genetic material inside mitochondria, the energy-producing structures in cells) in affected body tissues. To understand this condition, it helps to know that mitochondria are often called the powerhouses of cells because they create more than ninety percent of the energy our bodies need to function properly. When these structures don’t have enough DNA to work with, they cannot produce adequate energy, causing cells and organs to struggle or fail.^[1][2]

This is not a single disease but rather a group of related disorders that can affect different parts of the body in different ways. Some forms primarily impact muscles, others mainly affect the brain and muscles together, while still others target the liver and brain. Despite their differences, all these conditions stem from the same underlying problem: cells in affected organs do not have enough mitochondrial DNA to maintain normal energy production.^[3]

How Common Is This Condition?

Mitochondrial DNA depletion syndromes are very rare disorders. While exact numbers are difficult to determine, estimates suggest that approximately one in five thousand people has some form of genetic mitochondrial disease, though this number may be underestimated due to frequent misdiagnosis. Among mitochondrial diseases, the depletion syndromes represent a subset of these already uncommon conditions. Only about forty individuals with certain specific forms of the syndrome have been documented in medical literature, highlighting just how uncommon these conditions are.^[4][8]

The rarity of these conditions means that many families may never have heard of them before their child receives a diagnosis. Because symptoms can vary widely and affect multiple organ systems, healthcare providers sometimes struggle to recognize these syndromes, which can lead to delays in diagnosis or initial misdiagnosis.^[5]

What Causes Mitochondrial DNA Depletion?

These syndromes are caused by mistakes, called mutations, in genes that provide instructions for proteins essential to maintaining mitochondrial DNA. Importantly, the genetic problems are not in the mitochondrial DNA itself, but rather in genes located in the cell nucleus. These nuclear genes encode proteins that either help copy mitochondrial DNA, maintain the building blocks needed to make mitochondrial DNA, or support other functions crucial for keeping mitochondria healthy.^[2][7]

Scientists have identified mutations in several specific genes that can cause these syndromes. Genes like TK2, SUCLA2, SUCLG1, RRM2B, DGUOK, MPV17, POLG, and C10orf2 all play important roles in maintaining adequate amounts of mitochondrial DNA in cells. When any of these genes contains mutations that disrupt the function of the protein it encodes, the result can be a reduction in mitochondrial DNA content in certain tissues. This reduction impairs the ability of mitochondria to produce energy through a process called oxidative phosphorylation, which is the primary way cells generate the molecule ATP that powers most cellular activities.^[2][5]

How These Conditions Are Inherited

Mitochondrial DNA depletion syndromes follow an autosomal recessive inheritance pattern. This means that for a child to develop the condition, they must inherit two copies of the mutated gene, one from each parent. Typically, each parent carries one copy of the faulty gene but remains healthy because they also have one normal copy that provides sufficient function. When both parents are carriers, they have a one in four chance with each pregnancy of having a child who inherits both mutated copies and develops the syndrome.^[1][4]

Because carriers do not show symptoms and often have no family history of the disease, many parents are unaware they carry a genetic mutation until their child is diagnosed. This pattern of inheritance also means that siblings of an affected child have a higher risk of being affected or being carriers themselves.^[6]

Risk Factors

Unlike many health conditions where lifestyle choices or environmental exposures increase risk, mitochondrial DNA depletion syndromes are primarily determined by genetics. The main risk factor is having parents who both carry mutations in the same gene responsible for one of these syndromes. There are no known environmental triggers, dietary factors, or behaviors during pregnancy that can cause or prevent these conditions when the genetic mutations are present.^[5]

The age at which symptoms begin varies depending on which specific form of the syndrome a person has. For the form affecting the liver and brain, symptoms often appear very early, sometimes from birth to six months of age. For the muscle-affecting form, symptoms typically emerge between birth and two years. Some forms have been reported to affect older children, adolescents, or even young adults, though this is less common.^[1][5]

Signs and Symptoms

The symptoms of mitochondrial DNA depletion syndromes are extremely varied, reflecting which organs and tissues are most affected by the energy shortage. Because organs with high energy demands are most vulnerable, the nervous system, muscles, liver, and heart are frequently involved. Symptoms can appear at different ages depending on the specific type of syndrome, and even within the same type, the severity and exact combination of symptoms can differ greatly from one person to another.^[1][3]

In forms associated with mutations in the TK2 gene, which primarily affect muscles, infants often develop normally at first. Around two years of age, symptoms begin to emerge, including general muscle weakness called hypotonia (reduced muscle tone that makes the body feel floppy), tiredness, lack of stamina, and difficulty feeding. Some toddlers may start losing control of muscles in their face, mouth, and throat, making swallowing difficult. Motor skills that had been learned might be lost, though generally cognitive function and thinking ability remain unaffected.^[1]

Forms linked to SUCLA2 or SUCLG1 mutations, which affect both brain and muscle, typically show hypotonia very early, often before six months of age. Affected infants experience muscle wasting, delays in learning basic skills like walking and talking (called psychomotor delays), and may develop curved spine deformities. Additional problems can include abnormal involuntary movements, difficulty feeding, acid reflux, hearing loss, stunted growth, and breathing difficulties that increase the risk of lung infections. Seizures may also develop.^[1][4]

When RRM2B gene mutations are involved, symptoms again appear in early months and include hypotonia, signs of lactic acidosis (a buildup of lactic acid causing nausea, vomiting, and rapid deep breathing), failure to grow properly including unusually small head size, developmental delays or regression, and hearing loss. Many body systems can be affected simultaneously.^[1]

The liver and brain forms, associated with DGUOK or MPV17 mutations, can present in two ways. An early-onset form produces symptoms within the first week of life, affecting many organs and causing lactic acidosis and low blood sugar. Within weeks, liver failure can develop, leading to jaundice (yellowing of skin and eyes) and abdominal swelling. Neurological problems include developmental delays and regression, and uncontrolled eye movements. Rarely, symptoms affecting only the liver may emerge later in infancy or childhood.^[1][5]

Common symptoms across different forms include developmental regression (loss of previously acquired skills), muscle weakness, seizures, epilepsy, difficulty feeding, and problems with liver function. The wide range and severity of symptoms reflect the fact that energy failure can impact virtually any tissue in the body, with the most dramatic effects seen in organs that require the most energy to function.^[3][16]

Understanding How the Body Is Affected

The fundamental problem in mitochondrial DNA depletion syndromes is that affected tissues do not have enough mitochondrial DNA to support normal energy production. Mitochondrial DNA contains critical genes that encode essential components of the energy-producing machinery within mitochondria. When the amount of mitochondrial DNA drops significantly below normal levels, cells cannot produce sufficient quantities of the proteins needed for oxidative phosphorylation, the process that generates most cellular energy.^[2][7]

This energy shortage has cascading effects throughout affected organs. In muscle tissue, inadequate energy leads to weakness and wasting because muscle cells require enormous amounts of ATP to contract properly. In the nervous system, energy deficiency impairs the function of neurons, which are among the most energy-demanding cells in the body, leading to developmental problems, seizures, and loss of previously acquired skills. In the liver, energy shortage disrupts the organ’s many complex metabolic functions, potentially leading to liver failure.^[6][7]

The depletion of mitochondrial DNA occurs because nuclear genes that are mutated in these syndromes normally help maintain the pool of building blocks needed to synthesize mitochondrial DNA, or they help copy the mitochondrial DNA itself. When these genes don’t work properly, the cell cannot keep up with the normal turnover of mitochondrial DNA, leading to progressive depletion. Different tissues may be affected to different degrees depending on which specific gene is mutated and how that tissue’s energy demands and mitochondrial turnover rates interact with the genetic defect.^[7]

In addition to direct energy failure, the malfunctioning mitochondria can produce harmful byproducts. For example, many affected individuals accumulate lactic acid, which normally would be processed efficiently by properly functioning mitochondria. This buildup of lactic acid contributes to symptoms like nausea, rapid breathing, and metabolic disturbances. Similarly, other metabolic intermediates may accumulate, such as methylmalonic acid in some forms of the syndrome, causing additional complications.^[4][6]

Prevention Possibilities

When the genetic mutations causing mitochondrial DNA depletion syndrome are present, there is currently nothing that can be taken during pregnancy or given to an infant that will prevent the condition from occurring. The genetic defects are present from conception and begin affecting cellular function as development proceeds.^[14]

However, once a genetic mutation has been identified in a family, there are reproductive options available to help prevent the syndrome in future pregnancies. These options are particularly relevant for couples who have already had an affected child and know they are both carriers of mutations in the same gene. Prenatal testing during pregnancy can determine whether the fetus has inherited both mutated copies of the gene. This testing is typically performed through procedures like amniocentesis or chorionic villus sampling, which allow genetic testing of fetal cells. This information can help families make informed decisions, though it requires willingness to consider pregnancy termination if the fetus is found to be affected.^[14]

Another option increasingly available is preimplantation genetic diagnosis, which involves creating embryos through in vitro fertilization and testing them before implantation to select only embryos without the disease-causing mutations. This approach allows couples to have unaffected children without facing decisions about pregnancy termination. Genetic counseling is strongly recommended for families affected by these conditions to understand their options and the risks for future pregnancies.^[20]