Multiple mitochondrial dysfunctions syndrome is characterized by impairment of cellular structures called mitochondria, which are the energy-producing centers of cells. While certain mitochondrial disorders are caused by impairment of a single stage of energy production, individuals with multiple mitochondrial dysfunctions syndrome have reduced function of more than one stage. The signs and symptoms of this severe condition begin early in life, and affected individuals usually do not live past infancy.

Affected infants typically have severe brain dysfunction (encephalopathy), which can contribute to weak muscle tone (hypotonia), seizures, and delayed development of mental and movement abilities (psychomotor delay). These infants often have difficulty growing and gaining weight at the expected rate (failure to thrive). Most affected babies have a buildup of a chemical called lactic acid in the body (lactic acidosis), which can be life-threatening. They may also have high levels of a molecule called glycine (hyperglycinemia) or elevated levels of sugar (hyperglycemia) in the blood. Some babies with multiple mitochondrial dysfunctions syndrome have high blood pressure in the blood vessels that connect to the lungs (pulmonary hypertension) or weakening of the heart muscle (cardiomyopathy).

Multiple mitochondrial dysfunctions syndrome can be caused by mutations in the NFU1 or BOLA3 gene. The proteins produced from each of these genes appear to be involved in the formation of molecules called iron-sulfur (Fe-S) clusters or in the attachment of these clusters to other proteins. Certain proteins require attachment of Fe-S clusters to function properly.

The NFU-1 and BOLA3 proteins play an important role in mitochondria. In these structures, several proteins carry out a series of chemical steps to convert the energy in food into a form that cells can use. Many of the proteins involved in these steps require Fe-S clusters to function, including protein complexes called complex I, complex II, and complex III.

Fe-S clusters are also required for another mitochondrial protein to function; this protein is involved in the modification of additional proteins that aid in energy production in mitochondria, including the pyruvate dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex (also known as the oxoglutarate dehydrogenase complex). This modification is also critical to the function of the glycine cleavage system, a set of proteins that breaks down a protein building block (amino acid) called glycine when levels become too high.

Mutations in the NFU1 or BOLA3 gene reduce or eliminate production of the respective protein, which impairs Fe-S cluster formation. Consequently, proteins affected by the presence of Fe-S clusters, including those involved in energy production and glycine breakdown, cannot function normally. Reduced activity of complex I, II, or III, pyruvate dehydrogenase, or alpha-ketoglutarate dehydrogenase leads to potentially fatal lactic acidosis, encephalopathy, and other signs and symptoms of multiple mitochondrial dysfunctions syndrome. In some affected individuals, impairment of the glycine cleavage system leads to a buildup of glycine.

Normal Function

The NFU1 gene provides instructions for making a protein involved in the formation of molecules called iron-sulfur (Fe-S) clusters. These clusters are attached to certain other proteins and are required for their proper function.

Two versions (isoforms) of the NFU-1 protein are produced from the NFU1 gene. One version is found in cellular structures called mitochondria. Mitochondria are the energy-producing centers of cells. In these structures, several proteins carry out a series of chemical steps to convert the energy in food into a form that cells can use. Many of the proteins involved in this process require Fe-S clusters to function, including protein complexes called complex I, complex II, and complex III.

Fe-S clusters are also required for another mitochondrial protein to function; this protein is involved in the modification of additional proteins that aid in energy production in mitochondria, including the pyruvate dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex. This modification is also critical to the function of the glycine cleavage system, a set of proteins that breaks down a protein building block (amino acid) called glycine when levels become too high.

The other version of the NFU-1 protein is found in the fluid-filled space inside the cell (the cytoplasm). While this protein is likely involved in Fe-S cluster formation in the cytoplasm, the role of this isoform is not well understood.

Health Conditions Related to Genetic Changes

Multiple mitochondrial dysfunctions syndrome

Mutations in the NFU1 gene can cause multiple mitochondrial dysfunctions syndrome. This severe condition is characterized by impairment of more than one mitochondrial function, such as reduced activity of complex I, II, or III, pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, or the glycine cleavage system. Affected infants often have severe brain dysfunction (encephalopathy) and elevated levels of a chemical called lactic acid in the body (lactic acidosis). These babies usually do not survive past infancy.

NFU1 gene mutations lead to production of an altered NFU-1 protein that is likely broken down quickly. Although some mutations affect both isoforms of the NFU-1 protein, loss of the mitochondrial version appears to be responsible for the condition. The lack of mitochondrial NFU-1 protein impairs Fe-S cluster formation. Consequently, proteins affected by the presence of Fe-S clusters, including those involved in energy production and glycine breakdown, cannot function normally. Reduced activity of complex I, II, or III, pyruvate dehydrogenase, or alpha-ketoglutarate dehydrogenase leads to potentially fatal lactic acidosis, encephalopathy, and other signs and symptoms of multiple mitochondrial dysfunctions syndrome. In some affected individuals, impairment of the glycine cleavage system leads to a buildup of glycine (hyperglycinemia).

Other Names for This Gene

• CGI-33
• HIRA-interacting protein 5
• HIRIP
• HIRIP5
• iron-sulfur cluster scaffold protein
• MMDS1
• Nfu
• NFU1 iron-sulfur cluster scaffold homolog, mitochondrial
• NFU1 iron-sulfur cluster scaffold homolog, mitochondrial isoform 1
• NFU1 iron-sulfur cluster scaffold homolog, mitochondrial isoform 2
• NFU1 iron-sulfur cluster scaffold homolog, mitochondrial isoform 3
• NFU1_HUMAN
• NifU
• NifU-like C-terminal domain containing
• NIFUC

Genomic Location

Normal Function

The BOLA3 gene provides instructions for making a protein whose function is not well understood. The BOLA3 protein is thought to be involved in the formation of molecules called iron-sulfur (Fe-S) clusters or in the attachment of these clusters to other proteins. Certain proteins require attachment of Fe-S clusters to function properly.

Two versions (isoforms) of the BOLA3 protein are produced from the BOLA3 gene. One version is found in cellular structures called mitochondria. Mitochondria are the energy-producing centers of cells. In these structures, several proteins carry out a series of chemical steps to convert the energy in food into a form that cells can use. Many of the proteins involved in this process require Fe-S clusters to function, including protein complexes called complex I, complex II, and complex III.

Fe-S clusters are also required for another mitochondrial protein to function; this protein is involved in the modification of additional proteins that aid in energy production in mitochondria, including the pyruvate dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex. This modification is also critical to the function of the glycine cleavage system, a set of proteins that breaks down a protein building block (amino acid) called glycine when levels become too high.

The other version of the BOLA3 protein is found in the fluid-filled space inside the cell (the cytoplasm). While this protein is likely involved in Fe-S cluster formation in the cytoplasm, the role of this isoform is not well understood.

Health Conditions Related to Genetic Changes

Multiple mitochondrial dysfunctions syndrome

At least three mutations in the BOLA3 gene have been found to cause multiple mitochondrial dysfunctions syndrome. This severe condition is characterized by impairment of more than one mitochondrial function, such as reduced activity of complex I, II, or III, pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, or the glycine cleavage system. Affected infants often have severe brain dysfunction (encephalopathy) and elevated levels of a chemical called lactic acid in the body (lactic acidosis). These babies usually do not survive past infancy.

BOLA3 gene mutations lead to production of an altered protein that is likely broken down quickly. Although some mutations affect both isoforms of the BOLA3 protein, loss of the mitochondrial version appears to be responsible for the condition. The lack of mitochondrial BOLA3 protein impairs Fe-S cluster formation. Consequently, proteins affected by the presence of Fe-S clusters, including those involved in energy production and glycine breakdown, cannot function normally. Reduced activity of complex I, II, or III, pyruvate dehydrogenase, or alpha-ketoglutarate dehydrogenase leads to potentially fatal lactic acidosis, encephalopathy, and other signs and symptoms of multiple mitochondrial dysfunctions syndrome. In some affected individuals, impairment of the glycine cleavage system leads to a buildup of glycine (hyperglycinemia).

Other Names for This Gene

• bolA homolog 3
• bolA-like protein 3
• bolA-like protein 3 isoform 1
• bolA-like protein 3 isoform 2
• BOLA3_HUMAN
• MMDS2

Genomic Location

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.

Multiple mitochondrial dysfunctions syndrome is a rare condition; its prevalence is unknown. It is one of several conditions classified as mitochondrial disorders, which affect an estimated 1 in 5,000 people worldwide.