Isolated growth hormone deficiency is a condition caused by a severe shortage or absence of growth hormone. Growth hormone is a protein that is necessary for the normal growth of the body's bones and tissues. Because they do not have enough of this hormone, people with isolated growth hormone deficiency commonly experience a failure to grow at the expected rate and have unusually short stature. This condition is usually apparent by early childhood.

There are four types of isolated growth hormone deficiency differentiated by the severity of the condition, the gene involved, and the inheritance pattern.

Isolated growth hormone deficiency type IA is caused by an absence of growth hormone and is the most severe of all the types. In people with type IA, growth failure is evident in infancy as affected babies are shorter than normal at birth.

People with isolated growth hormone deficiency type IB produce very low levels of growth hormone. As a result, type IB is characterized by short stature, but this growth failure is typically not as severe as in type IA. Growth failure in people with type IB is usually apparent in early to mid-childhood.

Individuals with isolated growth hormone deficiency type II have very low levels of growth hormone and short stature that varies in severity. Growth failure in these individuals is usually evident in early to mid-childhood. It is estimated that nearly half of the individuals with type II have underdevelopment of the pituitary gland (pituitary hypoplasia). The pituitary gland is located at the base of the brain and produces many hormones, including growth hormone.

Isolated growth hormone deficiency type III is similar to type II in that affected individuals have very low levels of growth hormone and short stature that varies in severity. Growth failure in type III is usually evident in early to mid-childhood. People with type III may also have a weakened immune system and are prone to frequent infections. They produce very few B cells, which are specialized white blood cells that help protect the body against infection (agammaglobulinemia).

Isolated growth hormone deficiency is caused by mutations in one of at least three genes. Isolated growth hormone deficiency types IA and II are caused by mutations in the GH1 gene. Type IB is caused by mutations in either the GH1 or GHRHR gene. Type III is caused by mutations in the BTK gene.

The GH1 gene provides instructions for making the growth hormone protein. Growth hormone is produced in the pituitary gland and plays a major role in promoting the body's growth. Growth hormone also plays a role in various chemical reactions (metabolic processes) in the body. Mutations in the GH1 gene prevent or impair the production of growth hormone. Without sufficient growth hormone, the body fails to grow at its normal rate, resulting in slow growth and short stature as seen in isolated growth hormone deficiency types IA, IB, and II.

The GHRHR gene provides instructions for making a protein called the growth hormone releasing hormone receptor. This receptor attaches (binds) to a molecule called growth hormone releasing hormone. The binding of growth hormone releasing hormone to the receptor triggers the production of growth hormone and its release from the pituitary gland. Mutations in the GHRHR gene impair the production or release of growth hormone. The resulting shortage of growth hormone prevents the body from growing at the expected rate. Decreased growth hormone activity due to GHRHR gene mutations is responsible for many cases of isolated growth hormone deficiency type IB.

The BTK gene provides instructions for making a protein called Bruton tyrosine kinase (BTK), which is essential for the development and maturation of immune system cells called B cells. The BTK protein transmits important chemical signals that instruct B cells to mature and produce special proteins called antibodies. Antibodies attach to specific foreign particles and germs, marking them for destruction. It is unknown how mutations in the BTK gene contribute to short stature in people with isolated growth hormone deficiency type III.

Some people with isolated growth hormone deficiency do not have mutations in the GH1, GHRHR, or BTK genes. In these individuals, the cause of the condition is unknown. When this condition does not have an identified genetic cause, it is known as idiopathic isolated growth hormone deficiency.

Normal Function

The BTK gene provides instructions for making a protein called Bruton tyrosine kinase (BTK), which is essential for the development and maturation of B cells. B cells are specialized white blood cells that help protect the body against infection. These cells can mature into cells that produce special proteins called antibodies or immunoglobulins. Antibodies attach to specific foreign particles and germs, marking them for destruction. The BTK protein transmits important chemical signals that instruct B cells to mature and produce antibodies.

Health Conditions Related to Genetic Changes

Isolated growth hormone deficiency

A few mutations in the BTK gene have been found to cause isolated growth hormone deficiency type III, a condition characterized by slow growth, short stature, and a weakened immune system. Mutations that cause this condition lead to production of a nonfunctional version of the BTK protein. People with isolated growth hormone deficiency are prone to infections because they produce very few B cells and have a shortage of antibodies (agammaglobulinemia). A lack of the BTK protein is likely responsible for the immune system symptoms, but how a shortage of BTK protein causes short stature in affected individuals is unclear.

X-linked agammaglobulinemia

More than 600 different mutations in the BTK gene have been found to cause X-linked agammaglobulinemia (XLA). Most of these mutations result in the absence of the BTK protein. Other mutations change a single protein building block (amino acid), which probably leads to the production of an abnormal BTK protein that is quickly broken down in the cell. The absence of functional BTK protein blocks B cell development and leads to a lack of antibodies, causing an increased susceptibility to infections in people with XLA.

Some people with XLA have large DNA deletions that remove one end of the BTK gene and all of a neighboring gene known as TIMM8A. Mutations in TIMM8A cause deafness-dystonia-optic neuronopathy (DDON) syndrome, which is characterized by hearing loss, vision problems, a decline in intellectual function (dementia), and involuntary muscle tensing (dystonia) or difficulty coordinating movements (ataxia). Individuals with large DNA deletions that include the BTK gene and the TIMM8A gene have the signs and symptoms of both XLA and DDON syndrome.

Other Names for This Gene

• AGMX1
• AT
• ATK
• BPK
• Bruton agammaglobulinemia tyrosine kinase
• Bruton's tyrosine kinase
• BTK_HUMAN
• dominant-negative kinase-deficient Bruton's tyrosine kinase
• IMD1
• MGC126261
• MGC126262
• PSCTK1
• tyrosine-protein kinase BTK
• XLA

Genomic Location

Normal Function

The GH1 gene provides instructions for making the growth hormone protein. Growth hormone is produced in the growth-stimulating somatotropic cells of the pituitary gland, which is located at the base of the brain. Growth hormone is necessary for the normal growth of the body's bones and tissues. The production of growth hormone is triggered when two other hormones are turned on (activated): ghrelin, which is produced in the stomach; and growth hormone releasing hormone, which is produced in a part of the brain called the hypothalamus. Ghrelin and growth hormone releasing hormone also stimulate the release of growth hormone from the pituitary gland. The release of growth hormone into the body peaks during puberty and reaches a low point at about age 55.

Cells in the liver respond to growth hormone and trigger the production of a protein called insulin-like growth factor-I (IGF-I). This protein stimulates cell growth and cell maturation (differentiation) in many different tissues, including bone. The production of IGF-I by the actions of growth hormone is a major contributor to the promotion of growth.

Growth hormone also plays a role in many chemical reactions (metabolic processes) in the body. By acting on specific tissues, growth hormone is involved in protein production and the breakdown (metabolism) of fats and carbohydrates.

Health Conditions Related to Genetic Changes

Isolated growth hormone deficiency

More than 70 mutations in the GH1 gene have been found to cause isolated growth hormone deficiency, a condition characterized by slow growth and short stature. Mutations that prevent the production of growth hormone, such as a deletion of a large section of the gene, result in isolated growth hormone deficiency type IA and severe growth failure by age 6 months. GH1 gene mutations that cause type IB are found throughout the gene and allow some growth hormone to be produced.

Most GH1 gene mutations that cause isolated growth hormone deficiency type II occur in a part of the gene called intron 3. These mutations result in the production of growth hormone that is shorter than normal. This short version of growth hormone is not released from cells and is not available to work in the body. In addition, this shorter growth hormone interferes with the function of normal growth hormone, so there is very little functional growth hormone available for use. A decrease in usable growth hormone results in the signs and symptoms of isolated growth hormone deficiency type II.

Other Names for This Gene

• GH
• GH-N
• GHN
• hGH-N
• pituitary growth hormone
• SOMA_HUMAN
• somatotrophin (ST)
• somatotropin
• somatotropin (ST)

Genomic Location

Normal Function

The GHRHR gene provides instructions for making the growth hormone releasing hormone receptor. This receptor is active (expressed) on the growth-stimulating somatotropic cells in the pituitary gland. This gland is is located at the base of the brain and produces many hormones, including growth hormone. Growth hormone is necessary for the normal growth of the body's bones and tissues. The GHRHR receptor attaches (binds) to a molecule called growth hormone releasing hormone. This binding, along with the actions of other molecules, triggers the production of growth hormone and its release from the pituitary gland.

Health Conditions Related to Genetic Changes

Isolated growth hormone deficiency

More than 20 mutations in the GHRHR gene have been found to cause isolated growth hormone deficiency, a condition characterized by slow growth and short stature. Mutations in the GHRHR gene cause some cases of isolated growth hormone deficiency type IB. Most of these mutations replace single protein building blocks (amino acids) in the receptor protein, preventing the production of a functional receptor. A nonfunctional receptor cannot effectively trigger the production of growth hormone or signal its release from the pituitary gland. As a result, little growth hormone is produced or available in the body in people with GHRHR gene mutations, leading to isolated growth hormone deficiency type IB.

Other Names for This Gene

• GHRFR
• GHRH receptor
• GHRHR_HUMAN
• GRF receptor
• GRFR
• growth hormone-releasing factor receptor
• growth hormone-releasing hormone receptor

Genomic Location

Isolated growth hormone deficiency can have different inheritance patterns depending on the type of the condition.

Isolated growth hormone deficiency types IA and IB are inherited in an autosomal recessive pattern, which means both copies of the GH1 or GHRHR 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.

Isolated growth hormone deficiency type II can be inherited in an autosomal dominant pattern, which means a mutation in one copy of the GH1 gene in each cell is sufficient to cause the disorder. This condition can also result from new mutations in the GH1 gene and occur in people with no history of the disorder in their family.

Isolated growth hormone deficiency type III, caused by mutations in the BTK gene, is inherited in an X-linked recessive pattern. The BTK gene is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Treatment of growth hormone deficiency involves the administration of daily growth hormone injections at home. This is long-term and must be often done for several years.

During this period, continuous medical monitoring is required. Common side effects of the treatment include:

• Headache
• Muscle and joint aches
• Water retention
• Allergic reactions
• Fracture in the ball of the hip joints

The incidence of isolated growth hormone deficiency is estimated to be 1 in 4,000 to 10,000 individuals worldwide.