Adenine Phosphoribosyltransferase Deficiency
What else is it called?
- 2,8-Dihydroxyadenine Urolithiasis
- APRT Deficiency Disorder
- APRT deficiency
- DHA Crystalline Nephropathy
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What causes it?
APRT deficiency is caused by mutations (changes) in the APRT gene. This gene codes for an enzyme called adenine Phosphoribosyltransferase, which catalyses the conversion of adenine to adenosine monophosphate (AMP). Adenine is one of the building blocks of DNA and gets converted to AMP when the cells need energy. Mutations (changes) in the APRT gene, damages the enzyme function. Instead of being used to make AMP molecules, adenine gets converted to 2,8- dihydroxyadenine (2,8-DHA). 2,8-DHA is toxic to the kidneys and crystallizes in the urine. This causes stones to be formed and results in the other symptoms associated with this disorder.
How common is it?
APRT deficiency is reported to occur mainly in some Japanese, Icelandic and European populations. The prevalence in Japan is estimated to be 1 in 27,000 individuals. In Iceland an estimated 1 in 15,000 individuals are affected.
The condition is shown to be rarer in Caucasian Europeans, with a prevalence of one in every 50,000-100,000 people being affected. However, the prevalence of carriers of the condition in Caucasian Europeans is 1 in every 100-250 individuals.
APRT deficiency can affect both male and female genders.
What are the signs and symptoms?
The onset of symptoms can occur anytime between childhood and adulthood. Symptoms have been observed in patients as young as 5 months old.
The main symptoms of the condition include:
- Reddish brown stain in the baby’s diapers due to the baby passing kidney stones in the urine
In childhood and adulthood:
- Affected kidney function
- Repeated kidney stone formations
- Pain during urination
- Abdominal pain
- Difficulty releasing urine
- Frequent urinary tract infections
- Haematuria (blood in the urine)
- 2,8-DHA nephropathy (a progressive worsening of kidney function)
About 15-20% of individuals with the condition show no symptoms. The severity of the condition can also vary amongst different patients, even patients belonging to the same family.
How is it diagnosed?
Diagnosis can be made through kidney stone analysis or looking for crystals in the urine.
Where kidney failure cannot be explained, an ultrasound or CT scan may identify kidney stones. Diagnosis can be confirmed via a specialised blood test measuring APRT enzyme activity in red blood cells and by genetic testing.
Many medical conditions have similar signs and symptoms and kidney stones are relatively common in the general population due to diet and lifestyle which may delay recognition of this disorder.
Can it be treated?
Treatment aims at preventing further formation of 2,8-DNA. This can be achieved through daily allupurinol treatment which is effective alongside a high fluid intake and a low purine diet. Dietary management will be overseen by a specialist dietitian.
Even if no symptoms are present, individuals should still take allupurinol to prevent kidney problems from occurring. Febuxostat may be used as an alternative to allupurinol if it is not tolerated.
With early and an accurate diagnosis, followed by proper ongoing treatment, the prognosis for APRT deficiency is good. Without a proper diagnosis or treatment, kidney malfunction or kidney failure may occur which may require transplantation.
Do my family need to be tested?
APRT deficiency is an inherited condition. Humans have chromosomes made up of DNA. Genes are pieces of DNA that carry the genetic information. Each chromosome may have several thousand genes. We inherit chromosomes from the egg of the mother and sperm of the father. The genes on those chromosomes carry the instructions that determine a person’s characteristics, which are a combination of the parents.
The pattern of inheritance of APRT deficiency is autosomal recessive. This means that carriers of the condition usually do not have the disorder because the other gene of this pair is working normally. Parents of children with APRT deficiency are carriers.
When both parents are carriers, the risk to the baby in each pregnancy is
- 25% chance (1 in 4) of developing the condition
- 50% chance (1 in 2) for the baby to be a carrier of the condition
- 25% chance (1 in 4) for the baby to have two working genes and neither have the condition nor be a carrier
Genetic counselling can be requested to get a full explanation.