It is not fully understood why people develop PNH. Inside the bone marrow, blood cells are made by stem cells (the seed cells of the marrow). PNH occurs due to a change (or mutation) in a gene in one of the stem cells in the bone marrow. This mutation happens in a gene called PIG-A and the cells made by this stem cell lack certain proteins on their surface making them ‘weaker’ than normal blood cells. The mutation is “acquired” meaning that it develops in the bone marrow stem cell whilst it is multiplying but is not present from birth. We have evidence that almost all people in the population will develop a PNH mutation at some time but as the resulting PNH cells are weak they rarely grow to a level that they can be routinely detected. Normal blood cells have proteins on the surface that protect the cells from part of the immune system. PNH cells don’t have these protective proteins, which is what makes them ‘weaker’ and causes them to be either destroyed (red cell haemolysis) or activated or stimulated (white blood cells and platelets).

Who gets PNH?

PNH is an extremely rare condition and is thought to affect around 16 people per million in the United Kingdom. It is not an inherited condition, meaning it cannot be passed on from parents to children. Equally, you cannot catch PNH from someone who has it. It can occur at any age. Men and women are affected equally.

PNH and bone marrow failure

PNH is associated with other blood conditions, particularly Aplastic Anaemia (AA) and Myelodysplasia (MDS). Both AA and MDS are types of bone marrow failure meaning the bone marrow is unable to make blood cells normally. In AA, the bone marrow cells look normal but there are just too few present to make enough blood cells whereas in MDS the marrow cells look abnormal and cannot make blood efficiently. In AA and MDS, patients usually have reduced levels of platelets, red and white blood cells. These are the different blood cells normally produced in the bone marrow. Treatment for patients with PNH may be directed at the bone marrow failure as well as PNH itself.