Hemophilia B Leyden (HBL) is a sub-type of hemophilia B, a genetic bleeding disorder characterized by a lack of a blood-clotting protein called factor IX. HBL accounts for approximately 3% of all hemophilia B cases.
HBL is distinct from other forms of hemophilia because, while it is caused by very low levels of clotting factor IX early in life, over time, patients begin to produce factor IX. This is because mutations causing HBL occur in the promoter region of the gene for clotting factor IX. The promoter region is an area in the DNA that controls when a certain gene is turned off or on. In HBL, one activator region is disrupted by mutations, but the promoter region that responds to activation by hormones (estrogen and testosterone) is unaffected.
The increase in these hormones around puberty turns on gene expression to a certain degree. By midlife, patients have levels of factor IX on the low end of the normal range, and may no longer need treatment for bleeding.
Like most other types of hemophilia, HBL is characterized by the inability of blood to clot. This means that even small injuries (like cuts and bruises) can be very dangerous for patients because they continue to bleed.
Like other forms of hemophilia B, HBL is an X-linked disorder, meaning that the genetic mutation causing the disease is located on the X-chromosome, which is the female sex chromosome.
Because men have only one X-chromosome, if they inherit the disease-causing mutation, they will definitely develop HBL. Therefore, HBL is more common in men compared to women.
For women, the situation is more complex. Having two copies of the X-chromosome, if a woman inherits a disease-causing mutation on one of her X-chromosomes, she may or may not develop the disease. During development, one X-chromosome is randomly inactivated. If the disease-causing mutation is on the inactive X-chromosome, the woman is unlikely to develop the disease.
Hemophilia is diagnosed by performing a blood clotting test. A sample of blood is collected from the patient, and the physician measures how long it takes for the blood to clot. If it takes longer than normal, the physician may test for levels of specific clotting factors to determine whether there is a deficiency.
The patient’s family history is also taken into account to determine whether the condition could have been inherited.
Depending on the level of clotting factors, patients will be diagnosed with either mild, moderate, or severe hemophilia.
HBL can be difficult to distinguish from hemophilia type B since they are caused by the lack of the same clotting factor. Genetic sequencing can identify the mutation, but most often, patients are diagnosed with HBL when their physician notes that they are beginning to produce clotting factor IX again at puberty. The medical history of the family can provide clues that patients need to be tested again later in life if other family members have been diagnosed with HBL.
Like hemophilia B, HBL is treated by supplying the missing factor IX protein to the patient. This can be done to treat a specific bleeding episode, or patients can receive this treatment on a schedule at home to prevent bleeding episodes, depending on the severity of their condition.
Clotting factor replacement therapy can be prepared from donor blood, or be made synthetically.
Other treatments, used for all types of hemophilia, including HBL are summarized below.
- Clot-preserving medications, or antifibrinolytics. These medications help prevent clots from breaking down and include aprotinin, tranexamic acid, and aminocaproic acid.
- Fibrin sealants. These medications can be applied directly to wound sites to promote clotting and healing. Fibrin sealants are especially useful in dental therapy.
- Physical therapy can ease symptoms if internal bleeding has caused damage to joints.
- First aid for minor cuts. Using pressure and a bandage will generally stop the bleeding. For small areas of bleeding beneath the skin, an ice pack can be used. Ice pops can be used to slow minor bleeding in the mouth.
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