A large deletion on chromosome 11 in acute intermittent porphyria

Laboratory Diagnosis of Porphyria

Porphyrias are a group of diseases that are clinically and genetically heterogeneous and originate mostly from inherited dysfunctions of specific enzymes involved in heme biosynthesis. Such dysfunctions result in the excessive production and excretion of the intermediates of the heme biosynthesis pathway in the blood, urine, or feces, and these intermediates are responsible for specific clinical presentations. Porphyrias continue to be underdiagnosed, although laboratory diagnosis based on the measurement of metabolites could be utilized to support clinical suspicion in all symptomatic patients. Moreover, the measurement of enzymatic activities along with a molecular analysis may confirm the diagnosis and are, therefore, crucial for identifying pre-symptomatic carriers. The present review provides an overview of the laboratory assays used most commonly for establishing the diagnosis of porphyria. This would assist the clinicians in prescribing appropriate diagnostic testing and interpreting the testing results.

Porphyria Diagnostics-Part 1: A Brief Overview of the Porphyrias

Porphyria diseases are a group of metabolic disorders caused by abnormal functioning of heme biosynthesis enzymes and characterized by excessive accumulation and excretion of porphyrins and their precursors. Precisely which of these chemicals builds up depends on the type of porphyria. Porphyria is not a single disease but a group of nine disorders: acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), variegate porphyria (VP), δ-aminolevulinic acid dehydratase deficiency porphyria (ADP), porphyria cutanea tarda (PCT), hepatoerythropoietic porphyria (HEP), congenital erythropoietic porphyria (CEP), erythropoietic protoporphyria (EPP), and X-linked protoporphyria (XLP). Each porphyria results from overproduction of heme precursors secondary to partial deficiency or, in XLP, increased activity of one of the enzymes of heme biosynthesis. Taken together, all forms of porphyria afflict fewer than 200,000 people in the United States. Based on European studies, the most common porphyria, PCT, has a prevalence of 1 in 10,000, the most common acute porphyria, AlP, has a prevalence of ∼1 in 20,000, and the most common erythropoietic porphyria, EPP, is estimated at 1 in 50,000 to 75,000. CEP is extremely rare, with prevalence estimates of 1 in 1,000,000 or less. Only six cases of ADP are documented. The current porphyria literature is very exhaustive and a brief overview of porphyria diseases is essential in order for the reader to better appreciate the relevance of this area of research prior to undertaking biochemical diagnostics procedures. This unit summarizes the current knowledge on the classification, clinical features, etiology, pathogenesis, and genetics of porphyria diseases.

Role of genetic testing in the management of patients with inherited porphyria and their families

The porphyrias are a group of mainly inherited metabolic conditions that result from partial deficiency of individual enzymes in the haem biosynthesis pathway. Clinical presentation is either with acute neurovisceral attacks, skin photosensitivity or both, and is due to overproduction of pathway intermediates. The primary diagnosis in the proband is based on biochemical testing of appropriate samples, preferably during or soon after onset of symptoms. The role of genetic testing in the autosomal dominant acute porphyrias (acute intermittent porphyria, hereditary coproporphyria and variegate porphyria) is to identify presymptomatic carriers of the family specific pathogenic mutation so that they can be counselled on how to minimize their risk of suffering an acute attack. At present the additional genetic factors that influence penetrance are not known, and all patients are treated as equally at risk. Genetic testing in the erythropoietic porphyrias (erythropoietic protoporphyria, congenital erythropoietic porphyria and X-linked dominant protoporphyria) is focused on predictive and preconceptual counselling, prenatal testing and genotype-phenotype correlation. Recent advances in analytical technology have resulted in increased sensitivity of mutation detection with success rates of greater than 90% for most of the genes. The ethical and consent issues are discussed. Current research into genetic factors that affect penetrance is likely to lead to a more refined approach to counselling for presymptomatic gene carriers.

Partial protoporphyrinogen oxidase (PPOX) gene deletions, due to different Alu-mediated mechanisms, identified by MLPA analysis in patients with variegate porphyria

Variegate porphyria (VP) is an autosomal dominantly inherited hepatic porphyria. The genetic defect in the PPOX gene leads to a partial defect of protoporphyrinogen oxidase, the penultimate enzyme of heme biosynthesis. Affected individuals can develop cutaneous symptoms in sun-exposed areas of the skin and/or neuropsychiatric acute attacks. The identification of the genetic defect in VP families is of crucial importance to detect the carrier status which allows counseling to prevent potentially life threatening neurovisceral attacks, usually triggered by factors such as certain drugs, alcohol or fasting.In a total of 31 Swedish VP families sequence analysis had identified a genetic defect in 26. In the remaining five families an extended genetic investigation was necessary. After the development of a synthetic probe set, MLPA analysis to screen for single exon deletions/duplications was performed.We describe here, for the first time, two partial deletions within the PPOX gene detected by MLPA analysis. One deletion affects exon 5 and 6 (c.339-197_616+320del1099) and has been identified in four families, most probably after a founder effect. The other extends from exon 5 to exon 9 (c.339-350_987+229del2609) and was found in one family. We show that both deletions are mediated by Alu repeats.Our findings emphasize the usefulness of MLPA analysis as a complement to PPOX gene sequencing analysis for comprehensive genetic diagnostics in patients with VP.

Porphyrias at a glance: diagnosis and treatment

Porphyrias are a group of eight rare inherited metabolic disorders of heme biosynthesis pathway. Porphyrias are still underdiagnosed, although examinations of urine and plasma are first-line tests for detecting excess of porphyrins or heme precursors in suspected patients. Diagnosis, particularly for the acute forms, is essential to avoid precipitating factors and the use of triggering drugs. Mutation screening of family members is recommended to identify presymptomatic carriers and to prevent acute attacks. The therapeutic approach should be appropriate regarding specific forms of porphyria and treatment should be started promptly.

Diagnostic strategies for autosomal dominant acute porphyrias: retrospective analysis of 467 unrelated patients referred for mutational analysis of the HMBS, CPOX, or PPOX gene

BACKGROUND

Clinically indistinguishable attacks of acute porphyria occur in acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), and variegate porphyria (VP). There are few evidence-based diagnostic strategies for these disorders.

METHODS

The diagnostic sensitivity of mutation detection was determined by sequencing and gene-dosage analysis to search for mutations in 467 sequentially referred, unrelated patients. The diagnostic accuracy of plasma fluorescence scanning, fecal porphyrin analysis, and porphobilinogen deaminase (PBGD) assay was assessed in mutation-positive patients (AIP, 260 patients; VP, 152 patients; HCP, 31 patients).

RESULTS

Sensitivities (95% CI) for mutation detection were as follows: AIP, 98.1% (95.6%-99.2%); HCP, 96.9% (84.3%-99.5%); VP, 100% (95.7%-100%). We identified 5 large deletions in the HMBS gene (hydroxymethylbilane synthase) and one in the CPOX gene (coproporphyrinogen oxidase). The plasma fluorescence scan was positive more often in VP (99% of patients) than in AIP (68%) or HCP (29%). The wavelength of the fluorescence emission peak and the fecal coproporphyrin isomer ratio had high diagnostic specificity and sensitivity for differentiating between AIP, HCP, and VP. DNA analysis followed by PBGD assay in mutation-negative patients had greater diagnostic accuracy for AIP than either test alone.

CONCLUSIONS

When PBG excretion is increased, 2 investigations (plasma fluorescence scanning, the coproporphyrin isomer ratio) are sufficient, with rare exceptions, to identify the type of acute porphyria. When the results of PBG, 5-aminolevulinate, and porphyrin analyses are within reference intervals and clinical suspicion that a past illness was caused by an acute porphyria remains high, mutation analysis of the HMBS gene followed by PBGD assay is an effective strategy for diagnosis or exclusion of AIP.