Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome

Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome (HHH) is a condition that prevents a person's body from being able to process and remove a waste product called ammonia. Ammonia is produced by the breakdown of food. When the body’s process for removing waste is disrupted, dangerous amounts of ammonia begin to build up in the blood. If left untreated, this can result in developmental delay, learning disabilities, or stiffness caused by abnormal tensing of muscles (spasticity). Early detection through newborn screening and treatment can prevent many of the serious symptoms and outcomes of HHH.

Symptoms of HHH may become apparent in the first few days of life while other individuasl affected by HHH have onset in infancy, childhood, or adulthood. Individuals with later onset typically have symptoms that are less severe than those of individuals with the infantile form.

Quick facts about Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome
Genes: SLC25A15
Inheritance: Autosomal Recessive

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 Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome inherited?

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is known as an autosomal recessive condition. For autosomal recessive conditions, if a person has a variation in one copy of their gene, they are a carrier. This means that they are healthy because they also have a working copy of the gene. But, they can still pass their non-working copy to their child.

If the other parent also happens to be a carrier of the same gene, there is a 25% (1 in 4) chance that they both pass this gene variation on to their child — and as such, have a child affected by the disease.

If both parents are carriers of Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, there’s a one in four chance that their children could develop symptoms.
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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 Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, 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 Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome before or early in pregnancy?

The biggest benefit of screening for Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome 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.

Since 90% of children that have a recessive genetic disease like Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome had no previous family history of it, it often feels completely out of the blue for the parents. Getting screened is a way to know this risk in advance, which can help familes manage or even prevent the disease in the first place.

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