Hypocalcemia, autosomal dominant

Other names:ADH autosomal dominant hypoparathyroidism familial hypercalciuric hypocalcemia familial hypocalcemia

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.

Types

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.

Epidemiology

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

Cause

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.

Inheritance

Autosomal dominant pattern, a 50/50 chance.

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.

Signs and symptoms

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.

Diagnosis

Diagnosis is made through analysis of calcium levels in the serum and urine and PTH levels, Molecular analysis of CaSR followed by GNA11 confirms diagnosis.

Differential diagnosis Differential diagnosis includes all other causes of hypoparathyroidism as well as BS in patients with renal salt wasting.

Antenatal diagnosis Antenatal diagnosis is possible.

Treatment

Treatment to normalize calcemia levels should be considered with caution, as any increase in calcium levels (even within the normal range) will be perceived by renal cells as hypercalcemia and lead to increased urinary calcium excretion, and possibly to nephrocalcinosis and renal failure.

Treatment should aim towards finding a balance between the clinical signs of hypocalcemia and maintenance of calcium homeostasis, without being iatrogenic. Urine calcium levels should be monitored in order to avoid hypercalciuria rather than adapting treatment towards hypocalcemia. In asymptomatic and mildly symptomatic patients, treatment may not be necessary.

Special care must be given to children as chronic hypocalcemia has deleterious effects on intellectual development. Treatment is based on administration of 1-alpha hydroxylated vitamin D (doses ranging from 0.5 to 1.5 micrograms/day in adults; higher doses are sometimes required in children). Careful monitoring of calciuria and regular kidney ultrasound are required. In cases where calcium homeostasis is difficult to achieve, exogenous PTH administered by infusion pump can be proposed.

Prognosis

The prognosis is variable, depending on the severity of the hypocalcemia and the possible consequences of inadequate treatment.

NIH genetic and rare disease info

• NIH info on Hypocalcemia, autosomal dominant

Hypocalcemia, autosomal dominant is a rare disease.

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