Hemophilia A is caused by mutations in a single gene: the F8 gene, located on the X chromosome. This gene provides the instructions your body needs to produce coagulation factor VIII, a protein essential for forming blood clots. When the F8 gene is altered, the body either makes too little factor VIII or produces a version that doesn’t work properly, leading to prolonged bleeding after injuries, surgeries, or sometimes spontaneously.
What the F8 Gene Does
The F8 gene codes for a protein called coagulation factor VIII, which is one of several clotting factors that work together in a chain reaction to stop bleeding. Factor VIII circulates in your bloodstream in an inactive form, bound to a carrier molecule called von Willebrand factor. When a blood vessel is damaged, factor VIII activates, separates from its carrier, and partners with another clotting protein called factor IX. That interaction triggers a cascade of chemical reactions that ultimately produces a stable blood clot to seal the wound.
For decades, scientists assumed the liver’s main cells (hepatocytes) produced factor VIII. Recent research published in Blood Advances tells a different story. Factor VIII originates primarily from liver sinusoidal endothelial cells, which are specialized cells lining the tiny blood vessels inside the liver, not from the liver’s bulk tissue. A smaller amount also comes from endothelial cells in the kidneys. This distinction matters for gene therapy, since delivering a corrective gene to the right cell type is key to producing enough functional factor VIII.
Types of F8 Gene Mutations
Over 3,000 different mutations in the F8 gene have been identified in people with hemophilia A. These mutations fall into several categories, and the type of mutation plays a major role in how severe the condition is and how treatment works.
Intron 22 Inversion
The single most common mutation in severe hemophilia A is the intron 22 inversion. This involves a large section of the gene flipping orientation, which completely disrupts factor VIII production. About 44% of people with severe hemophilia A carry this inversion. A similar but less common rearrangement, the intron 1 inversion, accounts for fewer than 5% of severe cases.
Nonsense and Frameshift Mutations
Nonsense mutations insert a premature “stop sign” into the gene’s instructions, cutting the protein short before it can be completed. These almost always cause severe disease. Frameshift mutations, where small insertions or deletions of DNA throw off the entire reading sequence downstream, behave similarly. In studies cataloging dozens of these mutations, the overwhelming majority result in severe hemophilia with factor VIII levels below 1% of normal.
Missense Mutations
Missense mutations swap a single building block of the protein for a different one. Their effects are far more variable. Some missense mutations cause severe disease because the substitution lands in a critical region of the protein. Others cause only mild or moderate hemophilia because the protein still partially functions. This variability makes missense mutations harder to predict from genetic testing alone.
Large Deletions
Some people are missing entire sections of the F8 gene. Large deletions (greater than 50 base pairs) almost always cause severe disease because so much of the protein’s blueprint is gone that the body cannot produce a functional version.
How Mutations Determine Severity
Hemophilia A is classified into three levels based on how much working factor VIII remains in the blood. Severe hemophilia means less than 1% of normal factor VIII activity, moderate means 1% to 5%, and mild means greater than 5% but less than 40%. A person with normal clotting has 50% to 150% activity.
The pattern is straightforward: mutations that completely eliminate factor VIII production, like the intron 22 inversion, nonsense mutations, and large deletions, typically cause severe hemophilia. Mutations that allow some functional protein to be made, particularly certain missense mutations, tend to produce moderate or mild disease. But exceptions exist. Some frameshift mutations have been documented in people with moderate rather than severe hemophilia, and some missense mutations cause surprisingly severe disease depending on where in the protein the change occurs.
Why It Runs in Families the Way It Does
Because the F8 gene sits on the X chromosome, hemophilia A follows an X-linked inheritance pattern. Males have one X and one Y chromosome, so a single defective copy of F8 is enough to cause the disease. Females have two X chromosomes, meaning a working copy on the second X typically compensates for a mutated one.
If a mother carries one mutated F8 gene, each son has a 50% chance of inheriting hemophilia, and each daughter has a 50% chance of becoming a carrier. If a father has hemophilia, none of his sons will be affected (they get his Y chromosome), but all of his daughters will be carriers (they get his X chromosome with the mutated gene).
Not every case traces back through the family tree. A significant number of cases arise from new, spontaneous mutations. In one large study of 143 patients with severe hemophilia A, only about 31% had a known family history, and when researchers verified pedigrees with genetic testing, just 12.6% had confirmed de novo (brand new) mutations. The rest likely had carrier mothers who were unaware of their status.
When Female Carriers Have Symptoms
Female carriers aren’t always symptom-free. Early in embryonic development, every cell in a female’s body randomly shuts down one of its two X chromosomes, a process called X-chromosome inactivation. Normally this is roughly balanced, so enough cells use the working copy of F8 to maintain adequate factor VIII levels. But when this process is skewed, meaning the healthy X is silenced in most cells while the mutated X stays active, a carrier can end up with low factor VIII and real bleeding symptoms.
A case study published in Frontiers in Genetics illustrates this clearly. A woman who was a heterozygous carrier of the intron 22 inversion had a severe bleeding phenotype. Her mother and identical twin sister carried the same mutation but had normal factor VIII activity and no symptoms. The difference was X-chromosome inactivation: in the affected woman, the healthy X was preferentially silenced, leaving the mutated copy running the show. Skewed X-inactivation combined with an F8 mutation on the active chromosome is considered the most common reason female carriers develop hemophilia symptoms.
How Mutation Type Affects Treatment Response
One of the most clinically important consequences of mutation type is the risk of developing inhibitors. Inhibitors are antibodies the immune system produces against replacement factor VIII during treatment, essentially recognizing it as foreign and neutralizing it. This makes standard treatment ineffective and is the most serious complication of hemophilia A care.
In a study of 1,202 children with severe hemophilia A from the PedNet cohort, about 33% developed inhibitors. The risk varied dramatically by mutation type. The intron 22 inversion was found in 62% of patients who developed inhibitors, compared to 41% of those who did not. Large deletions also carried elevated risk, appearing in 4.5% of inhibitor patients versus 2% of non-inhibitor patients. Nonsense mutations were particularly likely to trigger high-responding inhibitors, the more dangerous type that spike aggressively when re-exposed to factor VIII.
Missense mutations carried the lowest inhibitor risk overall, at just 6.7%. This makes sense biologically: when the body produces some version of factor VIII, even a partially broken one, the immune system is less likely to treat replacement factor VIII as completely foreign.
Gene Therapy Targeting F8
Because hemophilia A traces to a single gene, it has long been considered an ideal candidate for gene therapy. In June 2023, the FDA approved the first gene therapy for severe hemophilia A in adults. Given as a one-time intravenous infusion, it uses a harmless viral shell to deliver a working copy of the F8 gene to liver cells, enabling the body to produce its own factor VIII. The therapy is designed for men 18 and older with severe hemophilia A who have not developed inhibitors. While it represents a major step, long-term durability of factor VIII production after treatment is still being tracked in ongoing follow-up studies.