Porphyrins and Porphyrias Flashcards
What are Haem Proteins?
- Haemoglobin
- Catalase
- Myoglobin
- Peroxidases
- Cyclooxygenase
- Tryptophan Pyrrolase
- Nitric Oxide Synthase
- Soluble Guanylate Cyclase
- Mitochondrial Cytochromes
- Microsomal Cytochrome P450s
What are Porphyrias?
- Due to partial deficiency of enzymes in the haem biosynthetic pathway. Defined by a unique biochemical pattern of accumulation and excretion of porphyrins and porphyrin precursors
- 8 main types; rare, mostly inherited disorders with variable penetrance
- Acute “porphyria attacks” or light sensitive skin disease or BOTH
What are the Acute Porphyrias?
Also Called Hepatic Porphyrias
- Acute intermittent porphyria (AIP): AIP does not have skin manifestations but all other porphyrias do.
- Variegate porphyria (VP)
- Hereditary coproporphyria (HCP)
- ALA Dehydratase porphyria (ADP)
What are the Non-Acute Porphyrias?
- Porphyria cutanea tarda (PCT): PCT is mainly sporadic, all others are inherited. It is Hepatic
- Erythropoietic protoporphyria (EPP)
- X-linked EPP (XLEPP)
- Congenital Erythropoietic Porphyria (CEP, syn. Günther’ s Disease)
What is the age of onset and inheritance of Congenital Erythropoietic Porphyria (CEP; Günther’ s)?
- Age of onset: Birth
- Inheritance: Autosomal recessive
What is the age of onset and inheritance of Erythropoietic Protoporphyria (EPP/XLEPP)?
- Age of onset: Birth
- Inheritance: Complex; several types
What is the age of onset and inheritance of Acute Intermittent Porphyria (AIP), Variegate Porphyria (VP) and Hereditary Coproporphyria (HCP)?
- Age of onset: 15-35y
- Inheritance: Autosomal Dominant
What is the age of onset and inheritance of ALA dehydratase Porphyria (ADP)?
- Age of Onset: Teens (but few kindreds described
- Inheritance: Autosomal Recessive
What is the age of onset and inheritance of Porphyria Cutanea Tarda (PCT)?
- Age of Onset: 35-55y
- Inheritance: ~90% sporadic;10% familial
How does Haem affect ALA Synthase?
- If increased requirement for production of Haem then ALA synthase is upregulated.
- Haem has negative feedback of ALA synthase
What are differential tissue regulation for Haem Biosynthesis?
Bone marrow:
- >80% haem biosynthesis (haemoglobin)
- Erythroid specific promotors and isoforms for key enzymes (ALAS2)
- Regulated by erythropoietin, iron supply and coupled to globin chain synthesis
Liver:
- ∼15% haem biosynthesis
- Haem exerts classical negative feedback on rate-limiting enzyme, ALA synthase (ALAS1)
What is the prevelance of Acute Hepatic Porphyrias?
Low penetrance: Only 10-20% of people “at risk” will develop overt, symptomatic disease with acute porphyria attacks
What are Features of Overt Symptomatic Acute Porphyria?
- Attacks are exceedingly rare before puberty
- More common in females
- Most people will experience only one attack and make a full recovery, BUT 5-10% patients (mostly young females) develop recurrent attacks
- Serious permanent, neurological morbidity and deaths still occur due to acute attacks
What are the clinical sign/symptoms of Acute Porphyria attacks?
- Severe abdominal pain
- Nausea
- Vomiting
- Constipation
- Progressive weakness
- Tachycardia
- Hypertension
- Hyponatraemia
- Convulsions, neuropathy, encephalopathy,
- Red urine: Urine darken on standing due to oxidation of colourless porphyrinogens to coloured porphyrins
What are triggers for Acute Porphyria Attacks?
- Drugs (many prescription drugs UNSAFE): increase in production of cytochrome enzymes
- Hormones (pre-menstrual phase): Women to avoid depo, cocp or implants
- Infection
- Calorie restriction,
- Alcohol
- Stress
- Smoking
What are the biochemical Hallmarks of Acute Porphyria Attack?
Accumulation of Porphyrin Precursors
- ALA
- PBG
What are the pathogenesis of Acute Attacks?
Haem pathway is stressed so ALAS1 is up-regulated and accumulation of ALA and PBG occurs.
The cause of damage by the compounds has a few Hypotheses:
- Direct neurotoxicity of ALA and/or PBG
- Relative / absolute haem deficiency leads to widespread intracellular dysfunction
How is the confirmation and identfication of the type of Porphyria conducted?
URINE:
- Quantitation of urinary ALA and PBG, following isolation by column chromatography
- Quantitation and assessment of pattern of urinary porphyrins by HPLC
BLOOD (EDTA):
- Plasma spectrofluorimetric scan
FAECAL porphyrin analysis (HPLC) in selected cases
Wha are the investigations for the Acute Porphyria Attack?
- Urinary PBG typically is elevated at least 5 fold during an acute attack (except in the extremely rare ADP). Urinary PBG is elevated in no other disorder, unlike urinary porphyrins, which are also elevated in other more common conditions.
- Quantitative assay needed; result expressed as creatinine ratio. Spot urine used and no need for 24hr urine
- Pitfall: Older qualitative tests prone to false positive and false negative results
What is the management of Acute Attacks?
- Withdraw unsafe, precipitating drugs
- Treat pain, agitation, vomiting, raised BP and heart rate, infection, fits, constipation with SAFE drugs
- Carbohydrate support
- Fluid and electrolyte balance (NB hyponatraemia – avoid iv dextrose): IV Glucose avoided due to causing worsening of hyponatraemia
- Specific treatment to suppress up-regulation of haem pathway: IV Human Haemin
What is the IV Human Haemin?
- Haem Arginate (Normosang®) in Europe and worldwide,Panhematin® in USA
- Provides the end-product and thus downregulates rate-limiting enzyme (ALAS1) in hepatic haem synthesis
- Improves symptoms and shortens duration of acute attacks (NB does not reverse established neuropathy so most effective if given early)
- In UK: National Acute Porphyria Service (NAPS)
What is the Advise for the New Cases of Acute Porphyria?
- Patient and family education vital
- Offer mutation analysis (with counselling and consent) so at risk family members can be tested
- Referral to specialist clinic advisable
What are Cutaneous Porphyrias?
Porphyrias causing accumulation of porphyinogens/porphyrinsleading to Photosensitivity
What are tpes of Cutaneous Porphyrias?
Immediate (acute), non-blistering
- Burning +/- oedema, redness within minutes of sun exposure:
- Erythropoietic protoporphyria (Classical EPP and XLEPP)
Chronic, blistering (“bullous”)
- Skin fragility, blisters, hypertrichosis, scarring, pigmentation (PCT, VP*, HCP*, CEP)
What Pathogenesis of Photosensitivity in Cutaneous Porphyrias?
- Porphyrin structure contains “delocalised” electrons. Light (400-410nm) absorption causes transition from ground state to excited state
- Energy can be transferred to molecular O₂ to produce reactive singlet O₂·
- Clinical effects depend on site of accumulation of porphyrins in skin (determined by water solubility)
What are features of Immediate (“acute”) photosensitivity: Erythropoietic protoporphyria?
- Symptoms present from infancy
- Intense burning, stinging in skin within minutes of sun exposure
- Few physical signs apart from redness, oedema of face, hands. Blistering rare. Linear depressed scars may occur
- Delayed clinical diagnosis is common
What happens in the Biochemistry of a patient with Immediate (“acute”) photosensitivity: Erythropoietic protoporphyria?
Ferrochelatase (FECH) catalyses incorporation of iron into Haem
- Deficiency of FECH leads to accumulation of Protoporphyrin IX in red cells, plasma, skin and liver
- Skin → photoactivation of protoporphyrin generates reactive oxygen species
What is revealed on investigations of Immediate (“acute”) photosensitivity: Erythropoietic protoporphyria?
If EPP suspected, key sample is EDTA blood
- Plasma fluorescence absorption peak at 626-634nm
- Raised red cell protoporphyrin (free, instead of zinc bound form)
- Anaemia raises zinc bound and free raised in prophyrias
Raised faecal protoporphyrin in ∼60% of patients (protoporphyrin is lipophilic; excreted via biliary route, not in urine)
Urinary porphyrin precursors and porphyrins normal (unless complicated by liver disease → coproporphyrinuria)
What are complications of Immediate (“acute”) photosensitivity: Erythropoietic protoporphyria?
- Socially very disabling
- Vitamin D deficiency – almost all patients
- Mild anaemia and/or haemolysis ∼25%
- Increased risk of gallstones (15-20%)
- Most serious and unpredictable complication is progression to severe, cholestatic liver disease and liver failure (<2%)
What is the Management for Erythropoietic protoporphyria (EPP)?
Essential for all:
- Sun protection measures, protective clothing,
- Prescribe effective barrier sunscreens
- Treat vitamin D deficiency
- Education, advice about inheritance risks
- Regular monitoring
Options
- Courses of narrow-band UV therapy
- Beta-carotene supplements
- α-MSH analogue (Afamelanotide) implant (awaiting decision by NICE)
What are features of Porphyrias causing chronic, blistering photosensitivity?
- Commonest porphyria in UK (~1:25,000)
- 80-90% sporadic: 10-20% familial (AD)
- Often occurs in the setting of chronic liver disease
What are triggers to development of skin lesions in sporadic PCT?
- Alcohol
- Hepatitis C / HIV infection
- Iron overload (NB hereditary haemochromatosis)
- Oestrogens
- Chemicals e.g. hexachlorobenzene
How can Liver disease, Uroporphyrinogen decarboxylase and iron leads to Porphyrin accumulation?
- Specific activity of hepatic UROD reduced in sporadic PCT
- Intracellular pool of stored iron may be released by “trigger” (alcohol, HCV) to promote formation of reactive oxygen species
- This may lead to generation of a specific inhibitor (?uroporphomethene)
- Leads to progressive inactivation of UROD catalytic site and accumulation of porphyrins
What is the clinical presentation of Porphyria Cutanea Tarda?
- Excessive skin fragility
- Blistering, erosions, scarring and milia on sun-exposed skin (seasonal variation)
- Secondary infection
- Hypertrichosis on cheeks and forehead
- Hyperpigmentation (face and other sun-exposed areas)
- Sclerodermoid / morphoea-like changes
- Patient may notice dark/red urine
What are differential diagnosis in Suspected PCT?
- Two acute porphyrias: VP and HCP
- Mild phenotype CEP
- Pseudoporphyria
Long list of other non-porphyria skin conditions!
What are the investigations in patient presenting with blisters on sun exposed skin?
Specimens: Urine and plasma
- Raised total urine porphyrin alone is insufficient – need HPLC fractionation for accurate diagnosis
- Urinary excretion pattern on HPLC usually characteristic but should always be confirmed with plasma spectrofluorimetric scan
- Plasma emission peak at 623-626nm is highly specific for variagate porphyria
- Faeces may be needed in selected cases
What are features of Congenital Erythropoietic Porphyria?
- Severe multi-system disorder
- Autosomal recessive with genotype-phenotype correlation (some mild forms)
- May be suspected at birth because of pink /red urine, staining of nappies
- Results in severe disfigurement unless rigorous photo-protection always observed
- Anaemia, skeletal and other complications
- May become transfusion dependent
- Bone marrow transplant is an option for some
What are the Biochemical Findings of Congenital Erythropoietic Porphyria?
- Urine: Uroporphyrin I and Coproporphyrin I x100-1000 ULN (pink urine)
- Red cells: Uro I, copro I and (in some patients) protoporphyrin
- Plasma peak 615-620nm
- Faeces: Copro I
