Hemophilia causes
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Overview
Causes
Haemophilia is a group of hereditary genetic disorders that impair the body's ability to control blood clotting, which is used to stop bleeding when a blood vessel is broken.
Haemophilia A
Hemophilia A is caused by an inherited X-linked recessive trait, with the defective gene located on the X chromosome. Females have two copies of the X chromosome. So if the factor VIII gene on one chromosome does not work, the gene on the other chromosome can do the job of making enough factor VIII.
Males have only one X chromosome. If the factor VIII gene is missing on a boy's X chromosome, he will have hemophilia A. For this reason, most people with hemophilia A are male.
If a woman has a defective factor VIII gene, she is considered a carrier. This means the defective gene can be passed down to her children. Boys born to such women have a 50% chance of having hemophilia A. Their daughters have a 50% chance of being a carrier. All female children of men with hemophilia carry the defective gene.
Haemophilia B
Hemophilia B is caused by an inherited X-linked recessive trait, with the defective gene located on the X chromosome.
Females have two copies of the X chromosome. If the factor IX gene on one chromosome does not work, the gene on the other chromosome can do the job of making enough factor IX.
Males have only one X chromosome. If the factor IX gene is missing on a boy's X chromosome, he will have Hemophilia B. For this reason, most people with hemophilia B are male.
If a woman has a defective factor IX gene, she is considered a carrier. This means the defective gene can be passed down to her children. Boys born to such women have a 50% chance of having hemophilia B. Their daughters have a 50% chance of being a carrier.
All female children of men with hemophilia carry the defective gene. In 1990, George Brownlee and Merlin Crossley showed that two sets of genetic mutations were preventing two key proteins from attaching to the DNA of people with a rare and unusual form of haemophilia B – haemophilia B Leyden – where sufferers experience episodes of excessive bleeding in childhood but have few bleeding problems after puberty. This lack of protein attachment to the DNA was thereby turning off the gene that produces clotting factor IX, which prevents excessive bleeding.
Haemophilia C
It is caused by a deficiency of coagulation factor XI and is distinguished from haemophilia A and B by the fact it does not lead to bleeding into the joints. Furthermore, it has autosomal inheritance, since the gene for factor XI is located on chromosome 4 (close to the prekallikrein gene); and it is not completely recessive, individuals who are heterozygous also show increased bleeding. Many mutations exist, and the bleeding risk is not always influenced by the severity of the deficiency.