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Condition report · Metabolic

Hyperphosphataemic Familial Tumoral Calcinosis (GALNT3)

Whole genome sequencing reads the GALNT3 gene for inherited changes in both copies that cause high blood phosphate and hard calcium deposits around joints and in other tissues.

A condition report looks for rare variants known to cause the conditions it covers.

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About this condition

A rare inherited condition in which high phosphate levels lead to hard calcium deposits forming around joints and in other tissues. It is caused by changes in both copies of the GALNT3 gene.

Hyperphosphataemic familial tumoral calcinosis is a rare inherited condition in which high levels of phosphate in the blood lead to deposits of calcium and phosphate forming in the soft tissues. Deposits usually form near large joints such as the hips, shoulders and elbows, often from childhood or early adulthood. Despite the name, the large deposits are not tumours and are not cancerous. Painful bone swelling, dental changes and calcium in blood vessels are less common.

GALNT3 helps protect the hormone FGF23 from being broken down. FGF23 signals the kidneys to reabsorb less phosphate. Without enough GALNT3, too little active FGF23 is made and the kidneys keep too much phosphate. The same condition can also be caused by changes in FGF23 or KL, which this report does not cover, so a negative result does not rule it out.

Treatment aims to lower blood phosphate, using a low-phosphate diet and phosphate binders, although responses vary. Surgery to remove large deposits is generally reserved for people with significant pain or limited movement. Genetic counselling is recommended, and carrier testing is available for relatives.

The condition we analyse

Hyperphosphataemic Familial Tumoral Calcinosis 1

Autosomal recessive

Caused by changes in both copies of the GALNT3 gene. GALNT3 protects the hormone FGF23 from being broken down; without it, too little active FGF23 is made and the kidneys keep too much phosphate.

Signs and symptoms

Calcium deposits develop near large joints such as the hips, shoulders and elbows, often from childhood or early adulthood. Blood phosphate levels are high. Less often there are painful bone swelling (hyperostosis), dental changes, eye calcification or calcification of blood vessels.

What a diagnosis changes

Treatment aims to lower phosphate, with a low-phosphate diet and phosphate binders; other medicines such as acetazolamide are sometimes used. Large deposits may be removed surgically. Genetic counselling is recommended, and carrier testing is available for relatives.

Genes analysed

Also known as

calcinosis, tumoral, with hyperphosphatemia; HFTC; HFTC1; hyperostosis-hyperphosphatemia syndrome; Morbus Teutschlaender; Teutschlaender disease, familial; tumoral calcinosis, hyperphosphatemic, familial; tumoral calcinosis, HYPERPHOSPHATEMIC, familial, 1; tumoral calcinosis, primary Hyperphosphatemic

MONDO:0100252OMIM 211900MedGen 1642611Sources: [1][2][3]

Why whole genome sequencing

This condition is inherited in a recessive way, so relatives can be offered carrier testing once the family change is known. Whole genome sequencing reads the GALNT3 gene, one of three known genes that can cause it.

Relatives can be offered carrier testing

Each sibling of an affected person has a 1 in 4 chance of being affected and a 1 in 2 chance of being a carrier. Once the GALNT3 change is known, carrier testing for relatives and prenatal testing for a pregnancy at increased risk are possible.

Avoiding calcium and phosphate excess

The report advises avoiding calcium salts as phosphate binders, excessive dietary calcium, excessive vitamin D and supplements, and foods high in phosphorus. Evaluating apparently well siblings of an affected person is also advised, so that treatment and preventive measures can start early.

Limitations

  • The same condition can be caused by changes in FGF23 or KL, which are not included in this report's gene list, so a negative result does not rule out familial tumoral calcinosis.

References

  1. Ramnitz MS, Gafni RI, Collins MT, 2018. Hyperphosphatemic Familial Tumoral Calcinosis. GeneReviews
  2. Hyperphosphatemic familial tumoral calcinosis. MedlinePlus Genetics
  3. GALNT3 gene. MedlinePlus Genetics

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