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Bleeding disorder due to platelet prostaglandin H synthase‐1 (PGHS‐1) deficiency

Defective platelet prostaglandin H synthase (PGHS) activity has been recognized as a cause of bleeding disorders, but the defect has not been characterized. We evaluated three female patients aged 37, 48 and 55 who presented with a mild bleeding disorder due to platelet dysfunction. None of the patients had underlying diseases or reported use of aspirin or other nonsteroidal anti‐inflammatory drugs. Coagulation screening tests and platelet count were normal in each patient. Platelet aggregation in response to adenosine diphosphate (ADP), collagen and epinephrine were subnormal, characterized by an abnormal second‐wave aggregation and propensity for disaggregation. Arachidonate‐induced platelet aggregation was defective, whereas PGH2‐induced aggregation was normal. Platelet thromboxane A2 (TXA2) production in response to arachidonic acid was reduced in all three patients, i.e. 11.7, 4.6 and 4.4 ng TXB2/3  108 plt respectively (normal range was 49–81 ng/3 10 8 plt), whereas they were normal in response to exogenous PGH2, i.e. 71.4, 56.6 and 48.9 ng/3  108 plts, respectively (normal range 49–85 ng/3  108 plt). These results are consistent with a deficiency of platelet PGHS activity. The level of the constitutive platelet PGHS‐1 and TXA2 synthase (TXAS) proteins were determined on platelet microsomal fractions by Western blot analysis using affinity‐purified polyclonal antibodies highly specific for human PGHS‐1 and TXAS, respectively. In two patients the 70 kD PGHS‐1 protein was undetectable, whereas it was normal in the third patient. The 60 kD TXAS band was normal in all three patients. These findings indicate that human platelet PGHS‐1 deficiency is due to two types of enzyme defects: type 1 defect is manifested by an undetectable PGHS‐1 protein in platelets whereas the type 2 defect is manifested by a normal quantity of PGHS‐1 protein which has an impaired catalytic activity.