Hemophilia A via the F8 Gene

Hemophilia A is the most common inherited bleeding disorder worldwide with 1 in 5,000 males being affected. Symptoms include bleeding into joints, blood into the urine, bruising, nosebleeds, prolonged bleeding from cuts, and gastrointestinal tract or urinary tract hemorrhaging. In affected individuals, impaired F8 protein function leads to a diminished clotting potential. Severity ranges from mild to moderate to severe and is directly correlated to the degree of impaired F8 function. The frequency of mild, moderate, and severe forms are 50%, 10%, and 40% respectively (Graw et al. 2005). Many mild cases go unnoticed until later in life when an individual incurs surgery or trauma. Recurrent bleeding in joints becomes painful and is indicative of chronic synovitis (Konkle et al. 2011). F8 is produced by liver sinusoidal cells and endothelial cells outside the liver. This protein circulates bound to von Willebrand Factor (VWF) in an inactive state. In response to injury, F8 is released from VWF and interacts other coagulation factors to form clots. Hemophilia A and von Willebrand Disease (Test #449) are phenotypically similar due to the cooperation between F8 and von Willebrand Factor to promote clotting. Hemophilia A may be treated with desmopressin or recombinant F8 to help with clotting. These treatments are particularly important for individuals with Hemophilia A undergoing dental or medical surgeries to prevent excessive bleeding (Konkle et al. 2013). Hemophilia A is clinically similar to Hemophilia B, but treatment for each condition requires transfusion of different proteins to restore clotting functions. Genetic testing can further be used to distinguish between acquired and inherited forms of Hemophilia A (Keeney et al. 2011).

Hemophilia A is an X-linked recessive disorder characterized by many different mutations in the F8 gene. Males are primarily affected, but homozygous females for F8 mutations have also been shown to exhibit clinical features. To date, over two thousand mutations in the F8 gene have been described in patients with Hemophilia A (Kemball-Cook et al. 1998) suggesting each family bears its own mutation (Lakich et al. 1993). Point and small deletions or insertions account for about 60% of all cases, while inversions at intron 1 or 22 lead to 2-3% and 45% of cases respectively. A third of patients have no family history of Hemophilia with onset due to de novo mutations (Konkle et al. 2011). F8 protein circulates bound to von Willebrand Factor (VWF) in an inactive state. In response to injury F8 is released from VWF and interacts with other coagulation factors to form clots. Causative mutations in the F8 gene lead to a loss of protein clotting function (Graw et al. 2005).

Mutations in the F8 gene are the only known cause of Hemophilia A. Analytical sensitivity is ~60% as inversions are not detected by this sequencing method and are the causative mutations for Hemophilia A in ~40% of cases.

This test provides full coverage of all coding exons of the F8 gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.

This testing will NOT detect commonly found inversions in intron 1 and 22.