The protein variant that refuses to turn off
In short: Most people carry two normal copies of the F5 gene, but a single variant can change everything. Known as Factor V Leiden, this mutation makes a vital clotting protein resistant to the body's natural brakes, significantly increasing the risk of dangerous blood clots.
Factor V Leiden is an inherited condition caused by a specific variant of the protein Factor V, which is essential for blood clotting. In a healthy system, proteins like protein C and protein S act as checks to stop excessive clotting. However, the Leiden variant features a structural change that makes the protein resist these regulatory proteins, meaning it is harder to 'turn off' than normal Factor V.
The condition follows an autosomal dominant inheritance pattern, meaning only one parent needs to pass the variant on for a child to inherit it. People can be heterozygotes, carrying one abnormal gene and one normal gene, or homozygotes, carrying two abnormal genes. Approximately 1 in 25 people are heterozygotes, while fewer than 1 in 1,000 are homozygotes. While many people with the condition will never experience thrombosis, the risks are significant: having one gene increases blood clot risk by 5 times, while having two increases it by 80 times.
The most common complications are Deep Vein Thrombosis (DVT), often presenting as swelling and pain in the leg, and Pulmonary Embolism (PE), which can cause sudden shortness of breath and chest pain. While the condition itself is asymptomatic, certain factors can escalate the danger. For instance, the risk of thrombosis is four times higher for women on the combined pill who carry the gene, and hormone replacement therapy can approximately double the risk. Because the gene is present from birth, it can be identified through a blood sample.
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Sources
- Summary by Intellect · source: Factor V Leiden Information - Milton Keynes University Hospital
- Also: What Is Factor V Leiden Thrombophilia?
- Also: Factor 5
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