The pathophysiology of paroxysmal nocturnal haemoglobinuria

Paroxysmal nocturnal haemoglobinuria (PNH) is a rare, life-threatening condition caused
by an expansion of a clone of haemopoietic stem cells (HSC) harboring a somatic
mutation of the PIG-A gene. PNH blood cells are deficient in glycophosphatidylinositollinked
(GPI) proteins rendering PNH erythrocytes susceptible to complement attack
which leads to intravascular haemolysis and an increased tendency to develop
thromboses.
The survival of 79 consecutive patients treated with eculizumab was compared to both
age and sex matched normal controls and to 30 similar patients managed in the era
immediately prior to anti-complement therapy. The survival of those treated with
eculizumab was no different that of the control group and was significantly better than
the group with PNH that did not receive eculizumab (p=0.3). Transfusion requirements
and documented thromboses were reduced with eculizumab. Sixty-six percent of
transfusion dependent patients became transfusion independent and thrombotic events
reduced from 5.6 to 0.8 events per 100 years.
Eleven women were monitored through 15 pregnancies whilst on eculizumab. There was
1 first trimester miscarriage and 15 healthy babies born. There were no maternal deaths
observed and no thrombotic events occurred in the pregnancy or the postpartum period.
Eculizumab did not appear to cross the placenta or be expressed in breast milk.
Thirty-five patients were evaluated for PIG-M mutations to see if this mutation was
prevalent in PNH. No PIG-M promoter mutations were identified. Two genes were
evaluated to see if secondary mutations affecting them could account for clonal
expansion in PNH. Thirty-six patients underwent JAK2 V617F mutation analysis with 1
patient shown to have a JAK2 mutation. Forty-two patients were evaluated for increased
HMGA2 levels by 2 different PCR methods. There was an overall reduction in HMGA2
expression in PNH patients as compared to normal controls.
An in vitro model of the bone marrow in PNH was developed and 18 PNH bone marrow
samples were evaluated using this model. Colony forming assays (CFA) showed an
increase in colony formation when T-cells were removed from the PNH bone marrow
samples. This improvement was reversed when the T-cells were added back to the
experiments. This work supports an immune mechanism for the expansion of the PNH
clone.