Glycine encephalopathy with normal serum glycine

Glycine encephalopathy [DS:H00191], also known as nonketotic hyperglycinemia (NKH), is characterized by severe neurologic dysfunctions. Recently, some individuals with NKH-like symptoms but lacking the typical elevation of serum glycine have been reported. They carry mutations in the SLC6A9 gene, that encodes glycine transporter (GLYT1). GLYT1 is located predominantly on astrocytes and is essential for the clearance of glycine from the extracellular space and termination of glycinergic neurotransmission.