NKH is a genetic condition. It is caused by variants (mutations) in genes that build the glycine cleavage system. According to MedlinePlus Genetics, about 80 percent of cases involve the GLDC gene and about 20 percent involve the AMT gene.
NKH follows an autosomal recessive inheritance pattern. That means a child must inherit a non-working copy of the gene from both parents. The parents are usually healthy carriers with one working copy and one non-working copy, and most have no idea they carry it until a child is diagnosed. Nothing a parent did during pregnancy causes NKH.
How severe NKH is depends in part on how much activity the glycine cleavage system still has. Variants that eliminate it entirely tend to cause the severe form, while variants that leave some activity tend to cause the attenuated form.
Tarah and Eric O'Sullivan learned all of this the hard way, first with Drake in 2016 and again with Vivian in 2019. Questions about a specific family's genetics belong with a genetic counselor or a metabolic specialist. If you'd like to understand how NKH can affect two children in the same family, read can two siblings both have NKH?
Sources: MedlinePlus Genetics: Nonketotic hyperglycinemia · MedlinePlus Genetics: Inheritance patterns
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