Factor IXa
Pathway · Haemophilia B APTT Vitamin K dep.
Factor IX is activated by factor XIa on the intrinsic route and by TF-VIIa through the Josso loop. With its cofactor VIIIa, calcium and phospholipid it forms the intrinsic tenase complex, which activates factor X far more efficiently than TF-VIIa can — the reason haemophilia bleeds even though the extrinsic pathway is intact.
Traced from the start
Detail
- Vitamin K dependent
- Yes — its Gla domain binds Ca2+ and anionic phospholipid
- Activated by
- Factor XIa, and by the tissue factor-factor VIIa complex
- Forms
- Intrinsic tenase: IXa + VIIIa + Ca2+ + phospholipid, which activates factor X
- Gene
- F9 on Xq27 — X-linked recessive, so males are affected and females are usually carriers
- Measured by
- APTT and a factor IX activity assay; the APTT corrects on 50:50 mixing unless an inhibitor is present
When it goes wrong
Haemophilia B (Christmas disease) — X-linked recessive factor IX deficiency, about 1 in 30,000 male births
Clinically indistinguishable from haemophilia A: haemarthrosis of knees, ankles and elbows, muscle haematoma and delayed post-traumatic bleeding. Prolonged APTT with normal PT, platelet count and bleeding time; only a factor assay separates them. Treat with recombinant factor IX — desmopressin does not work
Severe (under 1% activity) versus mild (5-40%) disease
Severe disease bleeds spontaneously into joints from the time of crawling and progresses to haemophilic arthropathy; mild disease may only declare itself after surgery or dental extraction
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