We all inherit traits from our parents through DNA, such as hair color or eye color. Our DNA carries all the information about how we will look and how our bodies will grow and work. All of that information is carried on parts of our DNA called genes. We have 2 copies of every gene: 1 from our mother and 1 from our father. When we inherit a gene that doesn’t work right, it can cause health problems like a UCD.
Urea cycle disorders (UCDs) can be passed down when one or both parents pass a copy of a gene that doesn’t work right on to their child. The most common type of UCD is called ornithine transcarbamylase deficiency or OTC deficiency. OTC deficiency is an X-linked condition. This means it is usually passed on from a mother to her child on her X chromosome. In rare cases, OTC can also be passed down from a father to his daughter.
A carrier is a person who has 1 normal copy of a gene and 1 copy that does not work right. Genetic testing can discover if a person is or isn’t a carrier and determine the chances of passing on a UCD gene.
There Are 3 Ways You Can Get a UCD
1
X-Linked
With OTC deficiency, one parent (typically the mother) passes on the gene that doesn't work right via the X chromosome
- Men who inherit the gene that doesn’t work right will very likely have symptoms. Women who inherit the gene may or may not have symptoms. They are often called "carriers" and can pass it on to their children
2
Autosomal Recessive
Both parents pass on a copy of the gene that doesn't work right via any chromosome other than the X or Y chromosomes
- People who inherit two copies of the gene that causes an autosomal recessive urea cycle disorder will have UCD symptoms
3
Random Mutation
Also called "de novo", a mutation that happened for the first time when the cells were forming, creating a gene that doesn't work right and causes a urea cycle disorder
Urea cycle disorder subtypes
Different genes cause different types of UCDs. There are 8 different types of UCDs, often called "subtypes":
- Ornithine transcarbamylase deficiency (OTC) — the most common UCD
- Carbamoyl-Phosphate Synthetase 1 (CPS1) Deficiency
- Citrullinemia I (CTLN1)
- Argininosuccinic lyase (ASL)
- Arginase deficiency (hyperargininemia, ARG)
- Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome
- Citrullinemia II (CTLN2)
- N-acetylglutamate synthase (NAGS) deficiency
The name of the subtype tells you which enzyme or transporter in the urea cycle doesn’t work. For example, if you have ornithine transcarbamylase deficiency, it means your ornithine transcarbamylase enzyme is missing or not working.
