Paroxysmal Nocturnal Haemoglobinuria (PNH)

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Pathophysiology

 

X-linked PIG-A gene mutation —> 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 —> red cells vulnerable to MAC —> chronic haemolysis

 

Clinical Features

 

Intravascular Haemolysis

—> Large amounts of free Hb —> consumption of NO —> 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 (national PNH website)

 

Haemoglobinuria

DAT negative intravascular haemolysis

Unexplained haemolysis

Unexplained thrombosis

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

  • 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

  • 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

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

  • 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)

  • Results

    • Type 1 RBC – normal

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

    • Type 3 RBC – full deficiency

(N.B. WBC are 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.)

(N.B. FLAER technique – modified bacterial protein that binds GPI anchors – i.e. quantifies the non-PNH cells.)

 

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 —> ongoing long-term extravascular haemolysis

  • Does not affect the underlying disease process of clonal haematopoeisis

 

Indications (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 —> 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.