What is glycine, and why is too much glycine harmful?

Glycine is one of the simplest amino acids, the building blocks of protein. It's in almost everything we eat, and the body makes it on its own too. Normally that's fine, because a group of enzymes called the glycine cleavage system breaks down extra glycine when it is no longer needed.

Glycine also works as a neurotransmitter, a messenger that passes signals between brain cells. In the right amount it helps the brain function. In NKH, the cleanup system is broken, so glycine accumulates in the blood and in the fluid around the brain and spinal cord. Too much glycine disrupts brain signaling and can lead to seizures, low muscle tone, breathing problems, and developmental injury.

Because glycine is in so many foods, families can't simply avoid it. That's part of what makes NKH so relentless. In Drake, the glycine cleavage system has been described as almost completely wiped out, which is why his and Vivian's daily care is so intensive.

The gene therapy the family is fighting for aims to address the problem at its source by giving cells a working copy of the gene they're missing. Learn more about EmetAbba, the therapy the family named "Faithful Father."

Sources: MedlinePlus Genetics: Nonketotic hyperglycinemia · GeneReviews (NCBI): Nonketotic Hyperglycinemia