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Ornithine transcarbamylase (OTC) deficiency

People affected by ornithine transcarbamylase (OTC) deficiency experience a build up of ammonia in the bloodstream, which damages the nervous system. Symptoms typically begin in the newborn period and include feeding difficulties, lack of energy (lethargy), and difficulty breathing and controlling body temperature. Without treatment, seizures, coma, liver damage, irreversible intellectual disability, or death may occur.

OTC deficiency is X-linked. Males are affected more often and more severely than females. Female carriers may also develop symptoms. Some individuals, both affected males and carrier females, may develop symptoms later in life, and their symptoms are typically, but not always, less severe.

These patients may experience episodes of altered mental status, headaches, vomiting, aversion to protein foods, and seizures. In all forms of OTC, episodes of dangerous ammonia buildup (hyperammonemic episodes) that can lead to coma and permanent cognitive impairment can be triggered by environmental stressors including a high-protein diet, pregnancy, and illness. Early initiation of treatment and therapies is recommended to maximize outcome.

Quick facts about ornithine transcarbamylase (OTC) deficiency
Genes: OTC
Inheritance: X-linked
Relevant resources for ornithine transcarbamylase (OTC) deficiency

A quick genetics rundown

As humans we have about 23,000 genes. These genes are like tiny instruction manuals that influence our health, growth and development. We inherit half of our genes from our biological mum and the other half from our biological dad. These genes are lined up on structures called chromosomes. Most of us have 23 pairs of chromosomes. The first 22 pairs are called autosomes and for the most part - these are the same among men and women. The 23rd pair determine our sex - two X chromosomes for a female and one X and one Y chromosome for males.

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How is ornithine transcarbamylase (OTC) deficiency inherited?

ornithine transcarbamylase (OTC) deficiency is an X-linked condition. X-linked conditions occur when there is a genetic variation on the X-chromosome. For these conditions, the sex of the parent who passes on the gene variation influences the risk for the children.

If the father is affected, all his daughters will have a variation on one of their X-chromosomes and be healthy carriers. Sons can only inherit these x-linked conditions from their mother and therefore, in this situation, will not be affected by the disease.

If the mother is a healthy carrier of an X-linked variation and has a son, there is a 50% chance that they could inherit this variant and be affected by the disease. If they have a daughter, there is a 50% chance that they will be a healthy carrier (just like her mum).

Another way of looking at this is that there is a 25% chance of having an affected son and a 75% chance that the child will not be affected by the disease.

What is carrier screening?

Carrier testing is like a checkup for your genes. It tests to see if you carry a gene variation that could cause a serious genetic disease in your child. Eugene offers an inclusive genetic carrier screening panel that includes ornithine transcarbamylase (OTC) deficiency, but there's a total 301 conditions that can be tested.

Eugene's carrier test is a clinical grade test that can be done from the comfort of your own home — it's just a saliva test. You're also paired with a genetic counsellor who provides mindful support and guidance every step of the way.

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Should parents screen for ornithine transcarbamylase (OTC) deficiency before or early in pregnancy?

The biggest benefit of screening for ornithine transcarbamylase (OTC) deficiency is that it can help future parents understand their reproductive risk so they can be ready and empowered to make more informed decisions. If neither partner are carriers, it provides reassurance and peace of mind that the risk of having a child with a genetic disease is low.