The Porphobilinogen Deaminase, Whole Blood test is used to a) confirm a diagnosis of acute intermittent porphyria (AIP) and/or b) to evaluate disease risk in family members of an individual with a confirmed diagnosis of AIP.
Porphobilinogen deaminase, also known as uroporphyrinogen I synthase, is commonly confused with uroporphyrinogen III synthase, the enzyme deficient in congenital erythropoietic porphyria (CEP).
The porphyrias are a group of inherited disorders resulting from enzyme defects in the heme biosynthetic pathway. Acute intermittent porphyria (AIP) is caused by diminished erythrocyte activity of porphobilinogen deaminase (PBGD), also known as uroporphyrinogen I synthase or hydroxymethylbilane synthase (HMBS).
Onset of AIP typically occurs during puberty or later. Individuals may experience acute episodes of neuropathic symptoms.
What is heme?
Heme is an iron-containing pigment that is a part of hemoglobin and a number of their proteins. It consists of an organic portion (protoporphyrin) bound to an iron atom. The synthesis of heme is a step-by-step process that requires the action of eight different enzymes. If there is a deficiency in one of these enzymes, the process is disrupted and intermediate porphyrins such as uroporphyrin, coproporphyrin, and protoporphyrin build up in the body's fluid and tissues. The precursors that accumulate depend on which enzyme is deficient, and they can exert toxic effects on the body.
Common symptoms include:
- severe abdominal pain,
- peripheral neuropathy,
and psychiatric symptoms.
Crises may be precipitated by:
- a broad range of medications (including barbiturates and sulfa drugs),
- alcohol,
- infection,
- starvation,
- heavy metals,
- and hormonal changes.
AIP is inherited in an autosomal dominant manner. At-risk family members of patients with a biochemical diagnosis of AIP should undergo appropriate testing. Timely diagnosis is important as acute episodes of AIP can be fatal. Treatment of AIP includes the prevention of symptoms through avoidance of precipitating factors. More than 80% of individuals with a deficiency variant in the HMBS gene remain asymptomatic throughout their lives.
The biochemical diagnosis of AIP is made by demonstrating increased urinary excretion of porphobilinogen (PBG) and is most accurate during an acute episode. In addition, the diagnosis of AIP can be confirmed through the measurement of PBGD enzyme activity in erythrocytes, although 5% to 10% of affected individuals exhibit normal erythrocyte PBGD activity. In addition, molecular genetic confirmation is available on a clinical basis and can be particularly helpful in identifying asymptomatic family members at risk of acute symptoms.
Reference ranges:
≥7.0 nmol/L/sec
6.0-6.9 nmol/L/sec (indeterminate)
<6.0 nmol/L/sec (diminished)
Notes:
- Reference ranges have not been established for patients who are younger than 16 years of age.
- A normal result does not rule-out acute intermittent porphyria; 5% to 10% of affected individuals will have normal erythrocyte porphobilinogen deaminase activity.
- Enzyme activity may be increased during an acute attack; therefore, the enzyme level should be assessed when the patient is asymptomatic.
References:
https://porphyriafoundation.org/for-patients/types-of-porphyria/aip/
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What does it mean if your Porphobilinogen Deaminase, Whole Blood result is too low?
Levels are usually decreased by up to 50% in patients with acute intermittent porphyria. However, some patients may have values within the reference interval.
Not all persons with the genetic defect have a history of clinical disease.
Acute Intermittent Porphyria (AIP) is a rare metabolic disorder that is characterized by deficiency of the enzyme hydroxymethylbilane synthase (HMBS), also known as porphobilinogen deaminase (PBGD). This enzyme deficiency can result in the accumulation of toxic porphyrin precursors in the body. However, the deficiency by itself is not sufficient to produce symptoms of the disease and most individuals with a HMBS gene mutation do not develop symptoms of AIP. Additional factors such as endocrine influences (e.g. hormonal changes), the use of certain drugs, excess alcohol consumption, infections, and fasting or dietary changes are required to trigger the appearance of symptoms.
The PBGD enzyme deficiency is caused by a mutation in the HMBS gene which is inherited as an autosomal dominant trait (only one HMBS gene copy is affected). However, the majority of people with a mutation in this gene do not develop symptoms of AIP; additional factors, often called “triggers” are also required to cause symptomatic acute Porphyria. These factors are not necessarily the same for each individual, and susceptibility to specific triggers may vary during a patient’s lifetime. Most of these triggers are believed to stimulate increased heme production (synthesis) in the liver and include certain drugs, excessive alcohol consumption, fasting or dieting (e.g. caloric restriction), stress, infections or certain hormonal (endocrine) factors.
Signs and Symptoms:
AIP manifests after puberty, especially in women (due to hormonal influences). Symptoms usually come as discrete attacks that develop over two or more days. Abdominal pain, which is associated with nausea, can be severe and occurs in most cases.
Other symptoms may include:
- vomiting
- constipation
- pain in the back, arms and legs
- muscle weakness (due to effects on nerves supplying the muscles)
- urinary retention
- palpitations (due to a rapid heart rate and often accompanied by increased blood pressure)
- confusion, hallucinations and seizures
- insomnia
Sometimes the level of salt (sodium and chloride) in the blood decreases markedly and contributes to some of these symptoms. The skin is not affected in AIP.
AIP can be associated with a range of symptoms and physical findings that can potentially involve multiple organ systems of the body. The course and severity of attacks is highly variable from one person to another. In some cases, particularly those without proper diagnosis and treatment, the disorder can potentially cause life-threatening complications. It is important to note the highly variable nature of AIP and that affected individuals may not have all of the symptoms discussed above. Affected individuals and parents of affected children should talk to their physician and medical team about their specific case, associated symptoms and overall prognosis.
The symptoms of AIP usually occur as episodes or “attacks” that develop over course of several hours or a few days. Affected individuals usually recover from an attack within days. However, if an acute attack is not diagnosed and treated promptly recovery can take much longer, even weeks or months. Most affected individuals do not exhibit any symptoms in between episodes. Onset of attacks usually occurs in the 20s or 30s but may occur at or just after puberty. Onset before puberty is extremely rare. Attacks are much more common in women than men, probably because of the menstrual cycle hormones. Approximately 3%-5% of affected individuals, predominately women, experience recurrent attacks, which are defined as more than 4 per year, for a period of many years.
Most people who inherit the gene for AIP never develop symptoms. However, experts recommend that all relatives of someone with AIP obtain testing, to determine who has the genetic trait and who does not. Those who test positive for the trait should be educated as to measures that will help avoid attacks. Prevention and avoidance of unsafe medications is essential to good management.
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