Glycine disorders, also known as glycine encephalopathy or nonketotic hyperglycinemia (NKH), are a group of rare neurometabolic disorders where glycine, an amino acid, accumulates in the body, particularly in the brain, due to a defect in glycine metabolism
  • Primary Glycine Disorder: Nonketotic hyperglycinemia (NKH), also known as glycine encephalopathy
    • Caused by deficiency of the glycine cleavage enzyme
    • Leads to accumulation of glycine in the body
    • All forms result in cerebral dysfunction
  • Function of the Glycine Cleavage Enzyme:
    • Breaks glycine into carbon dioxide and ammonia
    • Transfers a methyl group to tetrahydrofolate → forms methylenetetrahydrofolate
    • Consists of four subunits:
      • P-protein: Pyridoxal-phosphate dependent
      • H-protein: Hydrogen carrier, requires lipoylation
      • T-protein: Requires tetrahydrofolate
      • L-protein: Lipoamide dehydrogenase
  • Secondary Enzyme Deficiencies:
    • Disorders affecting pyridoxal-phosphate (e.g., PNPO deficiency) can cause secondary NKH
    • Disorders in protein lipoylation result in variant forms of NKH
  • Neonatal Presentation of NKH:
    • Typically occurs in the first week of life
    • Symptoms include:
      • Lethargy and poor feeding
      • Progression to coma
      • Frequent hiccupping and seizures
      • Severe hypotonia
      • Characteristic burst suppression pattern on EEG
    • Spontaneous recovery of respiratory function typically occurs within 3 weeks
  • Infantile Presentation of NKH:
    • Symptoms include:
      • Hypotonia
      • Lethargy
      • Seizures (spasms or myoclonic)
    • EEG findings:
      • Multifocal epilepsy
      • Hypsarrhythmia