Classic NKH is caused by genetic variants (mutations) in the genes that encode the components of the glycine cleavage enzyme system. This enzyme system is responsible for breaking down the amino acid glycine in the body. When it is not working properly, glycine accumulates in the body, resulting in the symptoms associated with NKH.
The glycine cleavage enzyme system is composed of 4 proteins, the P-protein encoded by the GLDC gene, the H-protein encoded by the GCSH gene, the T-protein encoded by the AMT gene, and the L-protein. Mutations in GLDC or AMT cause classic NKH. The majority of individuals with classic NKH have mutations within the GLDC gene. No mutations have been identified in the GCSH gene.
Individuals with deficient enzyme activity, but no mutation in GLDC or AMT, have variant NKH. Many genes have been described in variant NKH including LIAS, BOLA3, GLRX5, NFU1, ISCA2, IBA56, LIPT1 and LIPT2.
NKH is inherited in an autosomal recessive inheritance pattern, meaning that an individual must have pathogenic variants in both copies of the causative gene in order to be affected. Individuals with a pathogenic variant in only one copy of the gene are carriers for the disorder, and are not affected themselves, but could potentially have an affected child if their partner is also a carrier. If both parents are carriers for NKH, then there is a 1 in 4 chance, with each pregnancy, of the child being affected with NKH.