Autosomal dominant hypocalcemia is characterized by low levels of calcium in the blood (hypocalcemia). Affected individuals can have an imbalance of other molecules in the blood as well, including too much phosphate (hyperphosphatemia) or too little magnesium (hypomagnesemia). Some people with autosomal dominant hypocalcemia also have low levels of a hormone called parathyroid hormone (hypoparathyroidism). This hormone is involved in the regulation of calcium levels in the blood. Abnormal levels of calcium and other molecules in the body can lead to a variety of signs and symptoms, although about half of affected individuals have no associated health problems.

The most common features of autosomal dominant hypocalcemia include muscle spasms in the hands and feet (carpopedal spasms) and muscle cramping, prickling or tingling sensations (paresthesias), or twitching of the nerves and muscles (neuromuscular irritability) in various parts of the body. More severely affected individuals develop seizures, usually in infancy or childhood. Sometimes, these symptoms occur only during episodes of illness or fever.

Some people with autosomal dominant hypocalcemia have high levels of calcium in their urine (hypercalciuria), which can lead to deposits of calcium in the kidneys (nephrocalcinosis) or the formation of kidney stones (nephrolithiasis). These conditions can damage the kidneys and impair their function. Sometimes, abnormal deposits of calcium form in the brain, typically in structures called basal ganglia, which help control movement.

A small percentage of severely affected individuals have features of a kidney disorder called Bartter syndrome in addition to hypocalcemia. These features can include a shortage of potassium (hypokalemia) and magnesium and a buildup of the hormone aldosterone (hyperaldosteronism) in the blood. The abnormal balance of molecules can raise the pH of the blood, which is known as metabolic alkalosis. The combination of features of these two conditions is sometimes referred to as autosomal dominant hypocalcemia with Bartter syndrome or Bartter syndrome type V.

There are two types of autosomal dominant hypocalcemia distinguished by their genetic cause. The signs and symptoms of the two types are generally the same.

Autosomal dominant hypocalcemia is primarily caused by mutations in the CASR gene; these cases are known as type 1. A small percentage of cases, known as type 2, are caused by mutations in the GNA11 gene. The proteins produced from these genes work together to regulate the amount of calcium in the blood.

The CASR gene provides instructions for making a protein called the calcium-sensing receptor (CaSR). Calcium molecules attach (bind) to the CaSR protein, which allows this protein to monitor and regulate the amount of calcium in the blood. Gα11, which is produced from the GNA11 gene, is one component of a signaling protein that works in conjunction with CaSR. When a certain concentration of calcium is reached, CaSR stimulates Gα11 to send signals to block processes that increase the amount of calcium in the blood.

Mutations in the CASR or GNA11 gene lead to overactivity of the respective protein. The altered CaSR protein is more sensitive to calcium, meaning even low levels of calcium can trigger it to stimulate Gα11 signaling. Similarly, the altered Gα11 protein continues to send signals to prevent calcium increases, even when levels in the blood are very low. As a result, calcium levels in the blood remain low, causing hypocalcemia. Calcium plays an important role in the control of muscle movement, and a shortage of this molecule can lead to cramping or twitching of the muscles. Impairment of the processes that increase calcium can also disrupt the normal regulation of other molecules, such as phosphate and magnesium, leading to other signs of autosomal dominant hypocalcemia. Studies show that the lower the amount of calcium in the blood, the more severe the symptoms of the condition are.

Normal Function

The CASR gene provides instructions for making a protein called the calcium-sensing receptor (CaSR). Calcium molecules attach (bind) to CaSR, which allows this protein to monitor and regulate the amount of calcium in the blood. The receptor is turned on (activated) when a certain concentration of calcium is reached, and the activated receptor sends signals to block processes that increase the amount of calcium in the blood.

The CaSR protein is found in abundance in cells of the parathyroid glands. The parathyroid glands produce and release a hormone called parathyroid hormone that works to increase the levels of calcium in the blood. When large amounts of calcium bind to CaSR in the parathyroid glands, the production of parathyroid hormone is blocked, which prevents the release of more calcium into the blood. CaSR signaling also blocks the growth and division (proliferation) of cells that make up the parathyroid glands.

The CaSR protein is also found in kidney cells. Kidneys filter fluid and waste products in the body and can reabsorb needed nutrients and release them back into the blood. Increased calcium binding to CaSR in kidney cells blocks the reabsorption of calcium from the filtered fluids.

Health Conditions Related to Genetic Changes

Autosomal dominant hypocalcemia

Mutations in the CASR gene can cause a condition called autosomal dominant hypocalcemia type 1, which is characterized by low levels of calcium in the blood (hypocalcemia). Some affected individuals also have a shortage of parathyroid hormone (hypoparathyroidism).

Most CASR gene mutations involved in this condition change single protein building blocks (amino acids) in the CaSR protein. The changes (called activating mutations) lead to an overactive CaSR that is more sensitive to calcium, meaning even low levels of calcium can trigger signaling. The overactive CaSR blocks the release of parathyroid hormone, which prevents the release of calcium into the blood. In addition, the overactive CaSR prevents reabsorption of calcium from the fluids filtered through the kidneys. Hypocalcemia can cause muscle cramping and seizures, although about half of people with this condition have no associated health problems.

Familial isolated hyperparathyroidism

Mutations in the CASR gene have been found in some people with familial isolated hyperparathyroidism, a condition characterized by overactivity of the parathyroid glands (primary hyperparathyroidism). Primary hyperparathyroidism disrupts the normal balance of calcium in the blood, which can lead to kidney stones, thinning of the bones (osteoporosis), nausea, vomiting, high blood pressure (hypertension), weakness, and fatigue in people with familial isolated hyperparathyroidism. This condition is caused by changes to one copy of the gene in each cell. The CASR gene mutations associated with this condition change single amino acids in the CaSR protein. These genetic changes are called inactivating mutations because the altered CaSR protein is less sensitive to calcium and therefore requires an abnormally high concentration of calcium to trigger signaling. As a result, parathyroid hormone is produced even when the concentration of calcium in the blood is elevated, allowing the calcium levels to continue to rise. In addition, parathyroid cells may proliferate without control, which occasionally causes enlargement of the parathyroid glands in people with familial isolated hyperparathyroidism. Overproduction of parathyroid hormone from these abnormal glands may further stimulate the release of calcium into the blood. The high levels of calcium cause the signs and symptoms of familial isolated hyperparathyroidism.

Some researchers believe that familial isolated hyperparathyroidism caused by CASR gene mutations is a more severe form of a similar condition called familial hypocalciuric hypercalcemia (described below).

Kidney stones

MedlinePlus Genetics provides information about Kidney stones

Other disorders

Mutations in the CASR gene are involved in several other conditions associated with abnormal calcium levels. Inactivating mutations that lead to a reduction in CaSR function can cause familial hypocalciuric hypercalcemia. This condition is characterized by high levels of calcium in the blood (hypercalcemia) and low levels of calcium in the urine (hypocalciuria), but affected individuals typically have no symptoms related to the condition. Rarely, affected individuals have enlarged parathyroid glands and slightly elevated levels of parathyroid hormone. Like familial isolated hyperparathyroidism, this condition is caused by mutation of a single copy of the CASR gene.

A more serious condition called neonatal severe hyperparathyroidism is caused by genetic mutations that lead to very little or no CaSR function. In people with this condition, both copies of the CASR gene are altered. Neonatal severe hyperparathyroidism is a potentially fatal condition that becomes apparent in infants under the age of 6 months. Affected babies often have overgrowth of one or more of their parathyroid glands (parathyroid hyperplasia), causing high levels of parathyroid hormone in their blood. The excess hormone abnormally stimulates the release of calcium into the blood, causing hypercalcemia. The calcium is often removed from bone, resulting in skeletal abnormalities. The extreme hypercalcemia in these individuals can lead to neurological problems because the excess calcium interferes with nerve signaling.

Other Names for This Gene

• calcium-sensing receptor
• CAR
• extracellular calcium-sensing receptor
• GPRC2A
• parathyroid Ca(2+)-sensing receptor 1
• parathyroid cell calcium-sensing receptor
• PCAR1

Genomic Location

Normal Function

The GNA11 gene provides instructions for making one component, the alpha (α) subunit, of a protein complex called a guanine nucleotide-binding protein (G protein). Each G protein is composed of three proteins called the alpha, beta, and gamma subunits. Specifically, the protein produced from the GNA11 gene, called Gα11, is the alpha subunit for a G protein called G11.

In a process called signal transduction, G proteins trigger a complex network of signaling pathways that ultimately influence many cell functions. The G11 protein plays many roles in cells. It works with another protein called the calcium-sensing receptor (CaSR) to affect processes that regulate calcium levels in the blood. CaSR proteins in kidney cells and cells of the parathyroid gland sense when a certain concentration of calcium in the blood is reached; the CaSR protein then stimulates the G11 subunits, including Gα11, to send signals that block processes that increase the amount of calcium in the blood. In particular, this signaling blocks the production and release of a hormone called parathyroid hormone. Parathyroid hormone enhances the release of calcium into the blood, so blocking this hormone prevents calcium release. In the kidneys, which filter fluid and waste products in the body and can reabsorb needed nutrients and release them back into the blood, G11 signaling blocks the reabsorption of calcium from the filtered fluids.

G11 signaling is also involved in the growth and division (proliferation) and self-destruction (apoptosis) of cells in tissues throughout the body, including those in the eyes, skin, heart, and brain.

Health Conditions Related to Genetic Changes

Autosomal dominant hypocalcemia

At least five mutations in the GNA11 gene have been found in individuals with autosomal dominant hypocalcemia type 2. This condition is characterized by low levels of calcium in the blood (hypocalcemia). The mutations involved in this condition change single protein building blocks (amino acids) in Gα11. These genetic changes are called activating mutations because the altered alpha subunit is overactive, sending signals to block a rise in calcium levels, even when levels are very low. As a result, calcium levels in the blood remain abnormally low, causing hypocalcemia. Hypocalcemia can cause muscle cramping and seizures, although about half of people with the condition have no associated health problems.

Because overactive Gα11 signaling blocks the production of parathyroid hormone, the levels of this hormone in the blood can be abnormally low, which is known as hypoparathyroidism. For this reason, autosomal dominant hypocalcemia is sometimes referred to as autosomal dominant hypoparathyroidism.

Other disorders

Germline mutations in the GNA11 gene are also involved in a different condition related to abnormal calcium concentrations. Genetic changes that impair Gα11 signaling cause familial hypocalciuric hypercalcemia type 2. In contrast to autosomal dominant hypocalcemia (described above), this condition is characterized by high levels of calcium in the blood (hypercalcemia) and low levels of calcium in the urine (hypocalciuria). Because Gα11 signaling to block parathyroid hormone release is impaired, calcium levels in the blood rise, leading to hypercalcemia. Affected individuals typically have no symptoms related to the condition.

Cancers

The gene mutations that cause autosomal dominant hypocalcemia (described above) are typically inherited and found in every cell in the body (known as germline mutations). However, some gene mutations are not inherited and are instead acquired during a person's lifetime. These changes, which are called somatic mutations, are present only in certain cells. Somatic mutations in the GNA11 gene have been found in cancerous tumors in the eye known as uveal melanomas. These tumors occur in the colored part of the eye (the iris) or in related tissues known as the ciliary body and the choroid (collectively, these tissues are called the uvea).

Less commonly, GNA11 gene mutations are associated with a type of skin tumor called a blue nevus, so named because of its characteristic bluish appearance. This type of growth is typically noncancerous. GNA11 gene mutations are activating, leading to production of an overactive Gα11 protein that stimulates uncontrolled proliferation of the pigment-producing cells (melanocytes) in the uvea or in the skin.

Individuals with uveal melanomas or blue nevi appear to have normal levels of calcium in their blood, and people with an inherited GNA11-related calcium concentration disorder do not seem to have an increased risk of developing one of these types of tumors.

Other Names for This Gene

• FBH
• FBH2
• FHH2
• guanine nucleotide binding protein (G protein), alpha 11 (Gq class)

Genomic Location

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.

In most cases, an affected person inherits the mutation from one affected parent. A small number of cases result from new mutations in the gene and occur in people with no history of the disorder in their family.

The prevalence of autosomal dominant hypocalcemia is unknown. The condition is likely underdiagnosed because it often causes no signs or symptoms.