hypophosphatasia – Pseudohypophosphatasia
What else is it called?
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What causes it?
Since pseudo-HPP is a variant form of Hypophosphatasia. It is going to be caused by a problem in the ALPL gene which provides instructions to create the protein known as tissue non-specific alkaline phosphate (TNSALP), resulting in a non-functional form of the disease being made.
How common is it?
The true prevalence of this disease is unknown since this is an extremely rare version of hypophosphatasia. Affects males and females in equal numbers.
What are the signs and symptoms?
Pseudo-HPP typically develops during adulthood and the signs and symptoms are:
- Osteopathy of skull
- Long bones
- Decreased muscle tone
- High amounts of calcium in the blood (hypercalcinaemia)
- Wide head
How is it diagnosed?
Molecular genetic testing may be used in order to diagnose pseudo-HPP.
Can it be treated?
Currently all forms of hypophosphatasia can be managed by avoiding vitamin D and its metabolites. Oral phosphate supplements may be effective in some patients. Any other treatments are meant to treat the symptoms and support the family, in addition genetic counselling is advised for all families.
Do my family need to be tested?
Pseudo-HPP can either be an autosomal recessive or an autosomal dominant inherited disease. Humans are made up of DNA and the DNA is made up of chromosomes. Genes are sections of DNA which provide instructions to produce a protein that has a specific function. When there is an issue with a particular gene it causes the protein to be made incorrectly leading to disease. Humans inherit chromosomes from both their mother and father in the forms of eggs and sperm and these chromosomes determine the child’s characteristics. An inherited autosomal recessive disease is a disease that a child will inherit from their parents. Every human has two genes needed to make a functioning TNSALP and pseudo-HPP will develop when both of the genes are not working. For a child to develop pseudo-HPP they will need to inherit one non-working gene from each parent. The parents do not necessarily need to be diagnosed with pseudo-HPP to pass it on, they can also be carriers. a carrier is someone who possess’ one non-working gene but does not have the disease because the other gene is working correctly.
In the event that both of the parents are carriers of Pseudo-HPP. There is a:
- 25% chance that one of the children will be unaffected by pseudo-HPP and will not be a carrier.
- 25% chance that the child will inherit pseudo-HPP.
- 50% chance that the child will be unaffected by pseudo-HPP and will be a carrier.
For autosomal dominant We inherit particular chromosomes from the egg from the mother and sperm from the father. Every human has two genes that are needed in order to make the protein TNSALP. In an autosomal dominant disorder, a child only needs to inherit one of the non-working ALPL gene in order for the child to inherit it.
When one parent has pseudo-HPP, the chance of the baby getting pseudo-HPP a in each pregnancy is as follows:
- 50% unaffected if they receive two working genes
- 50% affected if they receive at least one fault gene.