Paroxysmal Nocturnal Haemoglobinuria

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Pathophysiology

 

X-linked PIG-A gene mutation à deficiency of GPI-anchored proteins (CD55 & CD59)

Normal role of CD55 is to block action of C3 convertases on red cells

Lack of CD55/CD59 à red cells vulnerable to MAC à chronic haemolysis

 

Clinical Features

 

Intravascular Haemolysis

à Large amounts of free Hb à consumption of NO à abdo pain, oesophageal spasm, erectile dysfunction and pulmonary hypertension

 

Thrombosis

-       Most common cause of death in PNH, occurs in 40% of patients, often unusual sites.

-       PNH is the highest risk thrombophilia

 

Panyctopenia

-       Associated with aplastic anaemia

 

Renal Failure

-       Renal vein thrombosis, ATN, siderosis of kidney

 

Who to Test (from PNH website)

 

-       Haemoglobinuria

-       DAT negative intravascular haemolysis

-       Unexplained haemolysis

-       Unexplained thrombosis

o   Either at unusual site – e.g. Budd-Chiari / intra-abdo, cerebral, dermal

o   Or in presence of haemolysis or cytopenias

-       MDS

-       Aplastic Anaemia

-       Other unexplained cytopenia

 

Diagnosis

 

Flow cytometry of peripheral blood for GPI-anchor deficiency

-       First test white cells to quantify the clone

o   2 Stages

§  Use markers to identify two WBC populations (not GPI-linked)

·      Neut – CD45 / CD15

·      Mono – CD45 / CD64

§  Then use 2 markers to identify deficient clone (GPI-linked)

·      Neut – FLAER  / CD24 / CD66b

·      Mono – FLAER / CD14

o   FLAER technique – modified bacterial protein that binds GPI anchors – i.e. quantifies the non-PNH cells.

-       Then if PNH clone found, test red cells to identify nature of deficiency (Type 2 vs 3)

o   2 Stages

§  Use CD235a (Glycophorin A) to identify red cell pop. (not GPI-linked)

§  Then use GPI-linked marker to find deficient pop. (CD59 preferred to CD55)

o   Results

§  Type 1 RBC – normal

§  Type 2 RBC – partial deficiency, may merge into the type 1 plots

§  Type 3 RBC – full deficiency

o   WBC used to identify the clone as false negatives occur with red cell tests due to short lifespan of the PNH cells, or recent transfusion of normal RBC’s.

 

Eculizumab

 

Mechanism

-       Anti-C5 antibody, prevents C5 cleavage into C5a and C5b

-       Prevents MAC from causing intravascular red cell haemolysis

-       Prevents thrombosis & haemolysis, prolongs survival, prevents LT complications

-       However, C3 continues to be produced à ongoing long-term extravascular haemolysis

-       Does not affect the underlying disease process of clonal haematopoeisis

 

Indications (From National PNH Service Website)

-       Symptomatic Haemolysis (LDH >1.5x ULN with Hb <90)

-       Complications of haemolysis (Renal impairment, Pul Hypertension)

-       Transfusion dependence (>4 transfusion in last 12 months)

-       Thrombosis related to PNH

-       Pregnancy (and up to 3 months post-partum)

-       Exceptional cases agreed by the joint service

 

Dose

-       600mg weekly for first 5 weeks (900mg on the fifth dose)

-       Then 900mg fortnightly long-term

 

Prior to use

-       Must give meningococcal vaccination prior to drug being released for use

 

Side Effects

-       Hyposplenic infection risks

-       If stopped suddenly à severe drop in Hb. The PNH cells that had been surviving suddenly at risk of simultaneous breakdown.

 

TRIUMPH 2006

-       2 weekly infusion

-       50% patients became transfusion independent (versus 0% of placebo group)

-       LDH improved

 

Other ‘Complementopathies’

 

Atypical Haemolytic Uraemic Syndrome (aHUS)

-       Acts around alternative pathway

-       Caused by mutations/deficiencies of proteins that inhibit the alternative pathway

-       Results in constant firing of the complement cascade

-       Extremely difficult to differentiate from TTP.

-       33% of patients die or develop ESRF after 1st episode

 

Cold Agglutinin Disease (CHAD)

-       Classical pathway, starts with C1

-       Recognised as a clonal LPD

-       Unresponsive to steroids/splenectomy (as haemolysis is occurring in the liver)

-       Rx: Rituximab

-       Anti-C1 antibodies in development.