GeneticsTier 1Medical Sciences concept

Paroxysmal nocturnal haemoglobinuria (PNH) - mechanism (PIGA gene, CD55/CD59, complement-mediated haemolysis)

Core concept

Acquired somatic mutation in PIGA (X-linked, Xp22.1) in a haematopoietic stem cell
- PIGA makes the first step of the glycosylphosphatidylinositol (GPI) anchor
- -> no GPI anchor -> every GPI-anchored surface protein is lost, across all progeny lineages
- One hit is enough because PIGA is X-linked - a single mutation silences it in either sex
Two of those lost proteins are complement regulators
- CD55 (DAF) - accelerates decay of C3 and C5 convertases (C4b2a, C3bBb)
- CD59 (MIRL) - blocks C9 incorporation into C5b-8 -> prevents MAC assembly
-> unopposed alternative-pathway C3 tick-over on the red cell surface -> chronic intravascular haemolysis
- The complement defect is on the cell, not in the plasma - complement levels are normal
Clonal expansion needs a second event - PIGA mutation alone is found in normal people at low level
- Immune escape in the setting of aplastic anaemia / autoimmune marrow attack: GPI-anchor-negative cells evade the T-cell attack directed at a GPI-linked antigen
- -> the PNH-aplastic anaemia-MDS axis

Consequences of intravascular haemolysis

Free plasma haemoglobin scavenges nitric oxide -> smooth muscle dystonia
- -> abdominal pain, oesophageal spasm/dysphagia, erectile dysfunction, pulmonary hypertension, fatigue out of proportion to anaemia
Thrombosis - the leading cause of death (~40-67% of deaths)
- Venous >> arterial; unusual sites: hepatic vein (Budd-Chiari), portal, mesenteric, cerebral sinus, dermal
- Mechanisms: NO depletion, platelet GPI-anchor loss with microvesiculation, complement activation on platelets, impaired fibrinolysis (loss of GPI-anchored uPAR)
Haemoglobinuria in only ~25% at presentation; classically nocturnal (named for it, but the association with sleep is weak)
Iron deficiency from urinary loss of haemosiderin; LDH high, haptoglobin absent, unconjugated bilirubin up
Marrow failure - cytopenias beyond the haemolysis

Diagnosis

Flow cytometry is the gold standard - loss of GPI-anchored proteins on >=2 lineages using >=2 markers
- Granulocytes and monocytes give the true clone size (RBC estimate is falsely low - the affected cells are being lysed, and falsely low again after transfusion)
- FLAER (fluorescent aerolysin, binds GPI anchor directly) is the most sensitive reagent, plus CD24/CD14/CD157
- RBC: CD59 - type I (normal), type II (partial), type III (complete deficiency)
Ham's acid haemolysis test and sucrose lysis are obsolete
Test for PNH in: unexplained Coombs-negative intravascular haemolysis, aplastic anaemia and hypoplastic MDS at diagnosis and on follow-up, thrombosis at an unusual site, iron deficiency with haemolysis

Complement inhibition

Drug	Target	Route	Note
Eculizumab	C5	IV q2 weekly	First-in-class; also aHUS, refractory gMG, NMOSD, AMR
Ravulizumab	C5	IV q8 weekly	Engineered recycling, longer t1/2
Crovalimab	C5	SC q4 weekly	Recycling antibody
Pegcetacoplan	C3	SC infusion	Proximal - controls extravascular haemolysis too
Iptacopan	Factor B	Oral	Proximal, alternative pathway; monotherapy effective
Danicopan	Factor D	Oral	Add-on to a C5 inhibitor for residual extravascular haemolysis

Terminal C5 blockade leaves C3b on the cell -> extravascular haemolysis by splenic/hepatic macrophages in ~20% - the reason for proximal (C3/factor B/factor D) inhibitors
*All complement inhibitors -> Neisseria risk - meningococcal vaccination (ACWY + B) >=2 weeks before starting, plus antibiotic prophylaxis; vaccination does not remove the risk - patients need a warning card and a low threshold for treating fever*
- Proximal inhibitors additionally raise encapsulated-organism risk -> pneumococcal and Hib vaccination
Allogeneic stem cell transplant is the only cure - reserved for marrow failure or refractory disease
Anticoagulate for thrombosis; complement inhibition markedly reduces thrombotic risk
Breakthrough haemolysis occurs with infection, surgery, pregnancy - complement amplification overwhelms the block

3 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.