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Reviewed July 2009

What is the official name of the HADHB gene?

The official name of this gene is “hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase (trifunctional protein), beta subunit.”

HADHB is the gene's official symbol. The HADHB gene is also known by other names, listed below.

Read more about gene names and symbols on the About page.

What is the normal function of the HADHB gene?

The HADHB gene provides instructions for making part of an enzyme complex called mitochondrial trifunctional protein. This enzyme complex functions in mitochondria, the energy-producing centers within cells. Mitochondrial trifunctional protein is made of eight parts (subunits). Four alpha subunits are produced from the HADHA gene, and four beta subunits are produced from the HADHB gene. As the name suggests, mitochondrial trifunctional protein contains three enzymes that each perform a different function. The beta subunits contain one of the enzymes, known as long-chain 3-keto-acyl-CoA thiolase. The alpha subunits contain the other two enzymes. These enzymes are essential for fatty acid oxidation, which is the multistep process that breaks down (metabolizes) fats and converts them to energy.

Mitochondrial trifunctional protein is required to metabolize a group of fats called long-chain fatty acids. Long-chain fatty acids are found in foods such as milk and certain oils. These fatty acids are stored in the body's fat tissues. Fatty acids are a major source of energy for the heart and muscles. During periods of fasting, fatty acids are also an important energy source for the liver and other tissues.

How are changes in the HADHB gene related to health conditions?

mitochondrial trifunctional protein deficiency - caused by mutations in the HADHB gene

Researchers have identified at least 26 mutations in the HADHB gene that cause mitochondrial trifunctional protein deficiency. These mutations reduce all three enzyme activities of mitochondrial trifunctional protein. Most mutations change one of the protein building blocks (amino acids) used to make the beta subunit. A change in amino acids probably alters the subunit's structure, which disrupts all three activities of the enzyme complex. Some mutations produce abnormally short, nonfunctional beta subunits and lead to decreased levels of mitochondrial trifunctional protein.

With a loss of mitochondrial trifunctional protein activity, long-chain fatty acids cannot be metabolized and processed. As a result, these fatty acids are not converted to energy, which can lead to some features of this disorder, such as a lack of energy (lethargy) and low blood sugar (hypoglycemia). Long-chain fatty acids or partially metabolized fatty acids may also build up and damage the liver, heart, and muscles. This abnormal buildup causes the other signs and symptoms of mitochondrial trifunctional protein deficiency.

other disorders - associated with the HADHB gene

A few mutations in the HADHB gene have been found to decrease only the long-chain 3-keto-acyl-CoA thiolase enzyme activity of mitochondrial trifunctional protein. These mutations change single amino acids used to make the beta subunit. The signs and symptoms of isolated long-chain 3-keto-acyl-CoA thiolase deficiency are similar to those of mitochondrial trifunctional protein deficiency. These features include feeding difficulties, lethargy, hypoglycemia, weak muscle tone (hypotonia), and liver problems. Infants with this disorder are also at high risk for serious heart problems, breathing difficulties, coma, and sudden death.

HADHB mutations appear to increase a woman's risk of developing two serious liver disorders during pregnancy, known as acute fatty liver of pregnancy (AFLP) and HELLP syndrome. AFLP begins with abdominal pain and can rapidly progress to liver failure. HELLP stands for hemolysis (the breakdown of red blood cells), elevated liver enzyme levels, and low platelets (cell fragments involved with blood clotting). A small percentage of women who have a mutation in one copy of the HADHB gene and carry a fetus with mutations in both copies of the HADHB gene develop one of these maternal liver diseases. Little is known about the relationship between HADHB mutations and liver problems in the mother during pregnancy. One possibility is that partially metabolized long-chain fatty acids produced by the fetus or placenta accumulate in the mother and are toxic to the liver.

Where is the HADHB gene located?

Cytogenetic Location: 2p23

Molecular Location on chromosome 2: base pairs 26,244,748 to 26,290,465

(Homo sapiens Annotation Release 107, GRCh38.p2) (NCBIThis link leads to a site outside Genetics Home Reference.)

The HADHB gene is located on the short (p) arm of chromosome 2 at position 23.

The HADHB gene is located on the short (p) arm of chromosome 2 at position 23.

More precisely, the HADHB gene is located from base pair 26,244,748 to base pair 26,290,465 on chromosome 2.

See How do geneticists indicate the location of a gene? in the Handbook.

Where can I find additional information about HADHB?

You and your healthcare professional may find the following resources about HADHB helpful.

You may also be interested in these resources, which are designed for genetics professionals and researchers.

What other names do people use for the HADHB gene or gene products?

  • HADH
  • hydroxyacyl-Coenzyme A dehydrogenase/3-ketoacyl-Coenzyme A thiolase/enoyl-Coenzyme A hydratase (trifunctional protein), beta subunit
  • hydroxyacyl dehydrogenase, subunit B
  • MTPB
  • TFPB
  • TP-beta

Where can I find general information about genes?

The Handbook provides basic information about genetics in clear language.

These links provide additional genetics resources that may be useful.

What glossary definitions help with understanding HADHB?

acids ; acute ; blood clotting ; breakdown ; cell ; clotting ; CoA ; coenzyme A ; coma ; deficiency ; dehydrogenase ; enzyme ; fasting ; fatty acids ; fatty liver ; fetus ; gene ; hemolysis ; hypoglycemia ; hypotonia ; lethargy ; liver failure ; maternal ; mitochondria ; muscle tone ; mutation ; oxidation ; placenta ; platelets ; protein ; subunit ; syndrome ; toxic

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.

See also Understanding Medical Terminology.

References (14 links)


The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.

Reviewed: July 2009
Published: February 8, 2016