Hemophilia type A is a rare genetic disorder caused by missing or defective factor VIII (FVIII), a clotting protein in the blood. The disease is usually inherited, but in about one-third of known cases, it is caused by a spontaneous mutation. More than half of all patients eventually develop a severe form of the disease.
Genetics of hemophilia type A
Hemophilia A is inherited in an X-linked pattern, meaning that the disease-causing gene is located on the X-chromosome (one of two sex-determining chromosomes).
Females inherit two X-chromosomes, one from their mother and one from their father. Males inherit an X-chromosome from their mother and a Y-chromosome from their father. This means that if a man has inherited an X-chromosome from his mother that contains a disease-causing mutation, he will have hemophilia.
For women, the situation is slightly more complex because they have two X-chromosomes. During development, one of the two X-chromosomes is inactivated to prevent multiple copies of genes being active at the same time. If a woman inherits a single copy of the disease-causing mutation, but the healthy copy is inactivated during development, she can also develop hemophilia. If the chromosome carrying the disease-causing mutation is inactivated during development, she will most likely not develop hemophilia, but will be a carrier of the disease and risks passing the faulty gene to her children.
Patients with hemophilia A are divided into categories depending on how much clotting factor is present in their blood. Patients with mild hemophilia have 6–49% of normal levels of clotting FVIII in their blood. Patients with moderate disease have 1–5%, and severe hemophilia patients less than 1%.
This decrease in clotting factor means that hemophilia A patients might bleed longer than normal, internally or externally, depending on how much FVIII is in their blood.
Following a symptomatic event, patients may be diagnosed by testing for the time it takes for their blood to clot, levels of clotting factors in their blood, or by genetic testing.
Hemophilia type A is a treatable disease, and there are different ways it can be treated.
Replacement therapy — in which the missing clotting FVIII is replaced to prevent complications — generally works well. Replacement therapy is usually used as an “episodic therapy” to treat patients with mild or moderate hemophilia A after an injury.
Some patients with mild forms of the disease may also be treated with desmopressin, which raises the levels of all clotting factors in the blood.
Antifibrinolytics, which slow the breakdown of clotting factors in the blood, are used to treat milder forms of the disorder.
Patients with severe hemophilia A may need to receive periodic FVIII infusions to prevent bleeding, in an approach referred to as prophylactic therapy. Patients can administer these infusions at home themselves if they receive training. This is important for people with severe forms of the disease, because the infusion works best within one hour of a bleeding episode. The goal is to stop the bleeding as soon as possible.
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