The FAH gene helps the liver and kidneys finish breaking down tyrosine, a part of protein from food. If it does not work, toxic substances build up and can cause tyrosinemia type 1, which may lead to liver disease, kidney problems, poor growth, and episodes of weakness, pain, or confusion. This gene plays an important role in keeping normal protein breakdown safe for the body.
What is a gene?
Genes are stretches of DNA that carry instructions your cells use to make molecules or regulate processes in the body. Some genes give the directions for making proteins, the parts of cells that do much of the work. Others mainly help determine when certain processes happen, where they take place, and how strongly they run. Together, genes influence how the body develops and functions, and they can shape how you respond to food, stress, or medications. To explore this topic more deeply, you can visit the LearnHub.
Overview
FAHGene function
What if the gene works
The FAH gene helps the body break down tyrosine, a protein building block from food. Its protein works mainly in the liver and kidneys. This helps prevent harmful chemical buildup.
Risk
What if the gene does not work
When the FAH gene does not work, fumarylacetoacetate builds up and causes tyrosinemia type 1, usually affecting the liver and kidneys. More severe variants often cause earlier, more pronounced illness, while milder variants may appear later and vary in features.
Treatment
What does that mean for your treatment
Pathogenic variants in both copies of the FAH gene make nitisinone a standard treatment, usually alongside a diet low in tyrosine and phenylalanine. If control remains poor or the liver is badly affected, liver transplantation can become part of management.
Conditions associated with FAH
Gene function
What if the gene works
Your body uses the FAH gene during the last step of tyrosine breakdown, a routine process that starts when you digest protein and when older proteins are recycled. Its protein is most active in the liver and kidneys. This is how the FAH gene helps break down tyrosine into smaller parts that fit into normal energy and cleanup pathways.
One of the roles of the FAH gene is to finish the last step of breaking down tyrosine, a building block of protein. Its protein cuts a leftover compound into smaller parts that can move on through normal body chemistry.
The smaller products made by the FAH protein feed into pathways that help cells handle energy. This supports routine metabolic work, especially after protein is processed in the liver.
FAH activity is highest in liver and kidney cells, where much of the body’s protein breakdown work takes place. In these tissues, it helps keep that process moving in an orderly, step by step way.
What is a variant?
A genetic variant is a small difference in the DNA sequence within a gene. These differences are common and help shape normal human diversity, such as variation in appearance, metabolism, or the way the body responds to environmental influences. Many variants do not have any noticeable effect and simply represent natural genetic variation. At times, though, a variant can change how a gene functions more substantially and may contribute to or cause a genetic condition.
Genetic variants
What if the gene does not work
Variants in the FAH gene can leave the FAH protein unstable, misshapen, or absent, which lowers or abolishes enzyme activity and lets harmful byproducts build up inside cells. What happens if the FAH gene does not work varies by the DNA change: milder variants may leave some activity, while more severe variants often stop the enzyme almost completely. In tyrosinemia type 1, less remaining activity is generally linked to earlier and more pronounced disease.
How variants are linked to conditions
Pathogenic variants in the FAH gene cause tyrosinemia type 1, a rare inherited condition that mainly affects the liver and kidneys. Many people first come to medical attention with poor growth, vomiting, jaundice, or episodes of weakness, while clinicians may find liver injury, kidney problems, and a higher risk of liver cancer over time. This FAH gene and tyrosinemia type 1 link is well established.
A variant is not necessarily a diagnosis
Genetic variants are common across the general population and are a normal feature of human biology. Most do not directly affect health and simply contribute to natural human diversity. For many genes, carrying a variant associated with a condition points to a possible link rather than a definitive diagnosis, particularly when several genes and environmental factors play a role. In some situations, though, one well-characterized variant in a particular gene is enough to confirm a diagnosis.
Are there things that can increase or change my risk?
For FAH, risk is mostly set by inheritance rather than lifestyle. A child is affected only if they inherit a changed FAH gene from both parents, so risk is higher when both parents are carriers, especially in families with a known FAH change or where blood relatives have children together.
Inheritance of FAH variants
FAH-related tyrosinemia type 1 follows an autosomal recessive inheritance pattern. This means a child usually inherits one FAH gene variant from each parent, and parents who carry one variant are typically unaffected carriers. When both parents are carriers, each pregnancy has a 25% chance of being affected, a 50% chance of producing a carrier child, and a 25% chance of producing a child with no familial FAH variant. In practical terms, how FAH gene variants are inherited depends on both parents passing on the altered copy.
What does my FAH result mean for my family?
FAH-related tyrosinemia type 1 is usually passed down in a recessive pattern, so close relatives may also be carriers even if they are healthy. Brothers and sisters of an affected child may also be affected or be carriers, and other family members may carry the same FAH change. This can matter for future pregnancies and for relatives who want to understand their own family planning risks.
Genetic information
Genetic testing
How you find out about variants
Genetic testing for FAH gene variants uses a blood or saliva sample, and the lab reads the DNA sequence in the FAH gene to look for changes. This can be done as single gene testing or as part of a larger panel, and the result can show whether a disease causing variant is present.
When should I consider getting a genetic test?
Consider FAH genetic testing if a baby or child has a newborn screen suggesting tyrosinemia type 1, a rare inherited condition that can harm the liver and kidneys, or has unexplained liver disease, poor growth, vomiting, or kidney problems. Testing also makes sense when this condition runs in the family or a relative is known to carry an FAH variant.
A genetic result is only one part of your health story
A genetic result can be informative, but what it can tell you varies with the condition and the kind of variant involved. Over time, lifestyle, environment, age, and medical history all affect how the body functions. For some conditions, especially those influenced by many genes, genetic information offers context rather than certainty and is best understood together with other parts of care. In other situations, a genetic result can provide a clear answer. In areas such as medication response, it can also help put individual experiences into context.
Pharmacogenetics
How variants shape your response to medicine
Genetic variation can affect how people respond to medicines. Small differences in genes can change how the body breaks down a drug, moves it through the bloodstream, or responds to it at the target tissue. This helps explain why the same medication may work well for one person, have little effect for another, or cause more side effects in someone else. These differences are common and are one part of why treatment response can vary from person to person.
What is Pharmacogenetics?
Pharmacogenetics studies how differences in genes affect the way people respond to medications. By looking at relationships between genes and drugs, it helps explain why the same treatment may work differently from one person to another. For many medicines, genes can influence how the body processes a drug and how strong its effects are. This information can support treatment decisions that are both better informed and more individualized. You can read more about this topic in the LearnHub.
Drugs influenced by the FAH gene
When should I seek medical advice?
There isn't yet a broadly confirmed role for this gene in predicting how someone will respond to medicines. Questions about side effects, starting a new medication, or how treatment fits with your overall health are best discussed with your doctor.
History
FAH entered medical genetics through research on hereditary tyrosinemia type 1, a rare inherited condition first recognized from its pattern in affected families and children with severe liver disease. Initially understood only through visible traits, later recognized as tied to a specific gene, FAH was identified in the late 1980s and early 1990s as the key gene involved in this condition. From these first observations, research expanded steadily, and scientists mapped disease causing variants in the FAH gene in families from several countries. This helped confirm the diagnosis at the DNA level and made family testing more precise. The FAH gene’s history is closely linked to major progress in newborn screening, earlier diagnosis, and better long term care for tyrosinemia type 1.