The Biology of Histidinemia and How It Is Diagnosed

While your baby looks and acts like any other newborn, their body is processing one specific part of their diet a little differently. To understand what is happening on the inside, we have to look at the “instruction manual” for the body: the HAL gene.

The Instruction Manual: The HAL Gene

The HAL gene is like a blueprint that tells the body how to build an enzyme called histidase (also known as histidine ammonia-lyase) ^[1][2]. Think of an enzyme as a tiny biological machine designed to do one specific job. The job of the histidase machine is to take an amino acid (a building block of protein) called histidine and break it down into something else ^[1].

In histidinemia, the HAL gene has a variation that causes the histidase machine to be either missing or not working effectively ^[3]. Because the histidine can’t be broken down, it starts to back up in the blood and urine, similar to how water backs up in a sink with a clogged pipe ^[4].

Confirming the Diagnosis: The Initial Screen vs. Follow-up

While the initial newborn screen used a simple heel prick (dried blood spot), the confirmatory test will likely involve a traditional liquid blood draw (checking plasma amino acids) or a urine collection ^[4]. This helps doctors get a highly accurate measurement of exactly how much histidine is in your baby’s system.

The Diagnostic Clue: Urocanic Acid

When the histidase machine is working correctly, it converts histidine into a substance called urocanic acid ^[1]. This is a crucial detail for doctors when they are confirming a diagnosis.

Because histidase is normally active in both the liver and the skin, children with histidinemia will have:

• High Histidine: High levels of the “starting material” in their blood and urine ^[3].
• Low Urocanic Acid: Very low or completely absent levels of the “finished product” in their blood, urine, or sweat ^[1].

Testing for low urocanic acid—often through a simple skin or sweat sample—is a traditional way to confirm the condition. The tests for urocanic acid are non-invasive and completely painless for your baby ^[1].

Important Note on Sun Protection: Urocanic acid acts as a natural UV protectant—a sort of physiological sunscreen for the skin ^[1]. Because children with histidinemia have very little or no urocanic acid, they may be slightly more susceptible to sunburns. Be diligent with standard sun protection, like using baby-safe sunscreen, hats, and seeking shade.

Confirming with DNA Testing

Today, many families choose HAL gene sequencing for a definitive answer ^[5]. This is a genetic test that looks directly at the DNA to find the specific variations in the HAL gene. This provides a record of the exact genetic cause without needing to measure biochemicals ^[3].

Inheriting the Condition

Histidinemia follows an autosomal recessive inheritance pattern. This means it takes “two to tango” ^[3]:

1. Carriers: Most parents of children with histidinemia are “carriers.” This means they have one working HAL gene and one non-working HAL gene. Because the working gene does enough of the job, carriers have no symptoms ^[6].
2. The Match: For a child to have histidinemia, they must inherit the non-working gene from both parents.

Since your child has histidinemia, they inherited the non-working gene from both of you. With each future pregnancy, there is a:

• 25% chance the child will have histidinemia (inherits two non-working genes).
• 50% chance the child will be a carrier (inherits one non-working gene).
• 25% chance the child will not have the gene variation at all (inherits two working genes).

Knowing that this is a genetic trait you were born with—and that it is a common biological variation—can help take away some of the mystery and fear surrounding the diagnosis ^[6].

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