“My short ‘elevator summary’ of porphyrias is that they are a group of rare diseases that result from defects in one of the eight enzymes that catalyze heme biosynthesis,” said Elizabeth L. Frank, PhD, a biochemist and pathologist at the University of Utah and ARUP Laboratories, a reference laboratory for hospitals and health centers, based in Salt Lake City, Utah.
Heme is the iron-containing porphyrin class molecule that binds iron and makes up the non-protein portion of hemoglobin, as well as other molecules found within cells. Heme is an essential molecule, as are the eight enzymes necessary to synthesize it.
Each porphyria subtype is due to a deficiency, but not a complete absence, of one of the heme biosynthesis enzymes. The result is a buildup of one of the intermediate substrates in the body, including porphyrin precursors or porphyrins themselves. Depending on the type of porphyria, the excess intermediates accumulate in the liver or erythroid cells in the bone marrow or distribute systemically into tissues, after which the molecules are excreted in the urine or bile.1 The urine of an individual with porphyria may be tinged by a red-purple color due to the presence of excess porphyrins. The color gives porphyrias their name, as porphyrus is the ancient Greek word for purple.
“These diseases typically present acutely with neurovisceral symptoms, such as nausea, vomiting, constipation, and pain, or as a painful sun sensitivity,” said Thomas G. DeLoughery, MD, a professor of medicine at the Oregon Health & Science University (OHSU) School of Medicine who runs a porphyria clinic at the OHSU Knight Cancer Institute in Portland.
ASH Clinical News spoke with Drs. Frank and DeLoughery, as well as Herbert Bonkovsky, MD, a gastroenterologist and professor of medicine at Wake Forest School of Medicine, who specializes in caring for patients with porphyria, about the laboratory testing used to diagnose individuals with porphyria.
From Presentation to Initial Laboratory Testing
In the setting of a suspected acute hepatic porphyria, “some patients have chronic abdomen pain or intermittent, recurring abdomen pain, and these individuals may have limb weakness, paralysis, and sometimes seizures or paralysis and mental illness,” explained Dr. DeLoughery. “These symptoms are all consistent with the acute hepatic porphyria subtypes.”
There are four acute hepatic porphyrias: acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), variegate porphyria (VP), and aminolevulinic acid (ALA)-dehydratase deficiency (ALAD) porphyria. AIP is the most common type of acute porphyria.
Dr. Bonkovsky noted that there are generally three categories of patients who should be tested for these conditions. “First, the typical patient with an acute hepatic porphyria is a young woman between approximately age 15 and age 50, as the severity of the acute attacks and their frequency tend to decrease after menopause,” he said. The key symptom for an acute hepatic porphyria diagnosis is abdominal pain. “Frequently, these women have been to the emergency room and may have seen a gastroenterologist and had an endoscopy but still have no clear diagnosis. They are suffering from severe abdominal pain, and often with constipation and sometimes tachycardia,” he added.
“The second category of patients are those in their 30s, or even 40s or 50s, who have blistering on sun-exposed skin and scar formation. For these individuals, testing for cutaneous porphyria is appropriate,” said Dr. DeLoughery.
“Third, a child who has been out in the sun, or even had sun exposure through a window, and has acute pain and prickling of the skin makes me think of one of the cutaneous porphyrias, such as erythropoietic protoporphyria (EPP),” Dr. DeLoughery said.
Besides EPP, there are three additional cutaneous porphyrias, porphyria cutanea tarda (PCT), congenital erythropoietic porphyria (CEP), and X-linked protoporphyria (XLP). PCT is the most common type of porphyria among the fewer than 200,000 individuals living with porphyria in the U.S.
Individuals with a clinical suspicion of porphyria due to abdominal pain and neurological symptoms should be tested for acute porphyria, particularly AIP, HCP, and VP, with an initial porphobilinogen (PBG) urine test.
“For the neurovisceral hepatic porphyria, screening the urine for excess porphobilinogen is critical,” said Dr. DeLoughery. “An individual with an acute hepatic porphyria will have an elevated porphobilinogen level of >10 mg/g of creatinine, or about three to 10 times normal levels.” Drs. DeLoughery and Bonkovsky both stressed that a urine porphyria screening is not appropriate for these individuals, as this analysis does not actually test for PBG, which is a porphyrin precursor rather than a porphyrin.
“The single biggest error that is made is testing only for porphyrins, not PBG, which is a porphyrin precursor,” said Dr. Bonkovsky. “If there is just one important take-home message on testing for porphyria, that is it.”
He added that there is “no other cause that will lead to a high elevation in urinary PBG other than acute hepatic porphyria, so this test is highly specific and sensitive.” For individuals who have had an acute attack of acute porphyria in the past, porphobilinogen levels typically remain elevated for months or years.
“The misconception is that a clinician will miss a transient elevation in porphobilinogen levels unless the test is taken within a few days of an acute attack,” said Dr. Bonkovsky. “This is generally not true. In our experience, most patients referred to our porphyria clinic for acute hepatic porphyria do not have high porphobilinogen levels and have been misdiagnosed because a clinician believed that they missed the porphobilinogen level elevation during an acute attack. Most of these patients actually have just a mild increase in porphobilinogen due to another cause.”
An ALA urine test can also be ordered for individuals with abdominal pain to test for ALAD porphyria, but this form of porphyria is extremely rare, with only a handful of such individuals identified worldwide. “All of the documented ALAD porphyria cases have, interestingly, been found in young men and not in women,” noted Dr. Bonkovsky.
If a patient has neurologic and abdominal symptoms plus cutaneous photosensitivity, both porphyrin and porphobilinogen levels should be tested in the urine. “If a patient has PCT, there will be strikingly abnormal results with all sorts of porphyrin intermediates present,” said Dr. DeLoughery.
Traditionally, a 24-hour urine sample was used, but now a simple spot urine test is done. If the test is positive for high levels of porphobilinogen, a confirmatory stool sample test for porphyrins is completed, with the option to perform testing for porphyrins in plasma or serum.
“Biochemical testing has been conducted for decades and works reasonably well,” said Dr. Frank. “The testing is relatively inexpensive, and the specimens are easy to get.” She added that the results are not ambiguous. “Healthy individuals have very little of these porphyrin intermediates in their body fluids, so we are not differentiating between small changes, but seeing large concentrations of intermediates in the samples.”
For those patients with skin photosensitivity including burning, itching, and redness, EPP or XLP could be the cause. In these cases, erythrocyte porphyrin and a whole blood analysis should be ordered.
Follow-up Testing
“Everything about porphyria is complicated,” said Dr. DeLoughery, referring to the genetics of individuals with porphyria. “A clinical and biochemical diagnosis is essential, but now that genetic testing is more prevalent and accessible, it also can help to screen family members.”
However, genetic testing should only be performed following a positive urine or blood biochemical test. Mayo Clinic Laboratories has developed a porphyria testing algorithm that outlines the initial urine testing (or erythrocyte porphyrin whole blood testing in the case of suspected EPP or XLP, to genetic testing for variants in porphyria-associated genes FIGURE).2
Mutations in the genes that code for the enzymes in the heme biosynthesis pathway are much more common in the population than a diagnosis of porphyria because the penetrance of these mutations is relatively low. “As many as 80% of individuals with a mutation may have never had an attack,” said Dr. Frank.
“This means that symptoms should lead to testing because many individuals have the same disease-causing mutations in one of the genes but are completely asymptomatic,” said Dr. DeLoughery. “This is because there may be genetic modifiers or environmental factors that play a major role. There are likely other factors that we don’t yet understand or know about.”
In particular, PCT is an environmental disease. Most individuals diagnosed with this subtype have an acquired, rather than inherited, mutation due to hepatic iron overload or potentially too much alcohol consumption.3
“The advantage of testing family members is that a sibling or other family member with only mild symptoms could receive early intervention,” said Dr. DeLoughery. In November 2019, the U.S. Food and Drug Administration approved the small interfering RNA (siRNA) therapeutic givosiran for the treatment of adults with acute hepatic porphyria. It targets the ALA synthase 1 messenger RNA for degradation, which leads to reduced circulating levels of both neurotoxic intermediates ALA and PBG.
Drs. DeLoughery and Bonkovsky cautioned that individuals with no symptoms who have a mutation in one of the heme biosynthesis enzymes may never develop symptoms and do not need to be treated.
Over-Interpretation Versus Over-Testing
Patient samples collected for biochemical and genetic testing are typically sent to a specialized reference laboratory. “This is not testing that’s available in your regular routine hospital laboratory,” said Dr. Frank.
A clinician with little or no experience with porphyria can order the initial biochemical testing, said Dr. DeLoughery, but the patient should seek care at a specialized porphyria clinic if the test indicates porphyria.
Over-ordering tests is not a major problem, according to Drs. DeLoughery and Bonkovsky. Rather, the interpretation of the results can be an issue.
“A test result can come back with a slightly high copper porphyrin level, which could be due to iron deficiency or fasting,” Dr. DeLoughery said. “Good reference labs can help a clinician correctly interpret the findings, making the point that small changes in intermediate levels are common and do not indicate that a patient has porphyria. Instead of over-testing, the major issue is over-interpretation and not getting the right test.”
“There are drugs, other liver diseases, anemia that can be associated with these mild fluctuations in porphyrins and their intermediates and these levels are not diagnostic of any form of porphyria,” Dr. Bonkovsky cautioned.
Clearly established algorithms can aid a clinician in conducting the initial workup for a patient who has a suspected porphyria. The key is to know the major symptoms of porphyria and to order the correct initial tests.
“Most clinicians have never seen a case of porphyria and have not thought about these disorders since medical school. They should know that these disorders do occur and there are well-established ways to test for these disorders,” said Dr. DeLoughery. “This is particularly important now that we have specific and new therapies that can have a dramatic impact on patients’ health and quality of life.” —By Anna Azvolinsky
References
- American Porphyria Foundation. About Porphyria. Accessed November 10, 2021. https://porphyriafoundation.org/for-patients/about-porphyria/.
- Mayo Clinic Laboratories. Porphyria (Acute) Testing Algorithm. Accessed November 10, 2021. https://www.mayocliniclabs.com/it-mmfiles/Porphyria__Acute__Testing_Algorithm.pdf.
- Yasuda M, Chen B, Desnick RJ, et al. Recent advances on porphyria genetics: inheritance, penetrance & molecular heterogeneity, including new modifying/causative Genes.” Mol Genet Metab. 2019;128(3):320-331.
