Growth Hormone Receptor Deficiency

Growth and metabolism of the human body is regulated by a hormone secreted by the anterior pituitary gland. This hormone is a polypeptide chain of 191 amino acids and is known as the "Growth hormone". Growth hormone acts by binding to specific receptors in the body. These receptors are responsible for triggering several cellular responses when the hormone binds to them. Deficiency of growth hormone results in growth-related disorders. Sometimes a person with sufficient levels of growth hormone in the body may also show retarded growth, due to the deficiency of growth hormone receptors. Improper functioning of receptors may result in Laron syndrome, characterized by dwarfism, delayed bone growth, and insensitivity towards administered growth hormone.

The growth hormone receptor is a protein with a number of cytokine receptors and a prolactin receptor, and belongs to transmembrane proteins. There are two forms of growth hormone receptor structure -

• A 620 amino acids, membrane bound, protein structure with a single transmembrane region covering full length.
• A soluble form, known as the growth hormone binding protein that corresponds to extracellular domain of full length receptor.

Growth Hormone Receptor Deficiency

Mechanism of Activation
The growth hormone receptor is a constitutive dimer. Its activation occurs when the growth hormone binds to it. Due to binding, the catalytic domains of tyrosine-protein kinase JAK2, position themselves side by side below the membrane of the cell. This causes activation of JAK2 due to transphosphorylation, followed by phosphorylation of tyrosines of the receptor. This allows binding of target proteins and direct phosphorylation of target proteins.

Types
There are two types of growth hormone receptor deficiency -

• Type I - This type is characterized by defect in growth hormone receptor gene, preventing the binding of growth hormone to its receptor. This results in increased levels of hormone in the plasma as it is not utilized in the body.
• Type II - The growth hormone binds properly to the cell surface however, there are problems related to its processing.

Cause
This deficiency disease is rarely observed, and is inherited from the parents in an autosomal recessive manner.

Symptoms
Several symptoms indicate deficiency of growth hormone receptors. Some of them are -

• Severe retarded growth after birth
• Short hands and feet
• Short stature
• Increased level of growth hormone in the body
• Underdeveloped muscles and jaw
• Abnormal skull growth
• Thin skin
• Delayed puberty
• High cholesterol level in blood
• Low fasting blood glucose level
• Obesity

Treatment
Deficiency of growth hormone receptor is treated by the administration of recombinant human IGF - 1 or Insulin-like growth factor - 1. Apart from this, a child with this deficiency is given frequent meals during infancy to avoid hypoglycemia. Doctors keep a check on growth and development, blood glucose levels, and development of adverse effects due to IGF - 1. Exogenously administered IGF - 1 along with naturally occurring growth hormone result in increased amount of circulating growth hormone in the body. This may result in excessive activation of the receptors resulting in gigantism and acromegaly. If the receptors activate due to autocrine activation, it may result in cancer and cell transformation.

Complications
Few complications may arise in this condition -

• Bone fractures
• Osteopenia or reduced bone mass
• Seizures due to low blood glucose level

A person or a child showing growth hormone receptor deficiency symptoms may actually have the disorder. It is therefore essential to get the person examined by a specialist doctor to come to a particular conclusion about the underlying condition. If the test results are positive, the doctor will immediately start the treatment so that receptor deficiency does not hamper the growth, development and metabolic activity of the person.