Therefore, each parent of an individual with an autosomal recessive disorder has at least one defective copy of the gene. With autosomal recessive disorders, individuals with only one copy of a defective gene (heterozygotes) are considered "carriers" for the disorder. Carriers usually do not show signs or symptoms of the disorder. Pathophysiology
Glycine is the simplest amino acid, having no stereoisomers. It can act as a neurotransmitter in the brain, act as an inhibitor in the spinal cord and brain stem, while having excitatory effects in the cortex of the brain. Glycine is metabolized to final end products of ammonia and carbon dioxide through the glycine cleavage system (GCS), an enzyme complex made up of four protein subunits. Defects in these subunits can cause glycine encephalopathy, although some causes of the disease are still unknown. Normally, GCS shows its highest enzymatic activity in liver, brain and placental tissue. One of its main functions is to maintain normal glycine levels in the brain. Defects in GCS cause an increase of glycine concentration in blood plasma and cerebrospinal fluid. The exact pathophysiology of the disorder is not known, but it is considered likely that buildup of glycine in the brain is responsible for the symptoms.All forms of glycine encephalopathy show elevated levels of glycine in the plasma, as well as in cerebral spinal fluid (CSF).
: 811 With the classical, or neonatal presentation of glycine encephalopathy, the infant is born after an unremarkable pregnancy, but presents with lethargy, hypotonia, apneic seizures and myoclonic jerks, which can progress to apnea requiring artificial ventilation, and often death. Apneic patients can regain spontaneous respiration in their second to third week of life. After recovery from the initial episode, patients have intractable seizures and profound intellectual disability, remaining developmentally delayed. Some mothers comment retrospectively that they noticed fetal rhythmic "hiccuping" episodes during pregnancy, most likely reflecting seizure episodes in utero. Patients with the infantile form of glycine encephalopathy do not show lethargy and coma in the neonatal period, but often have a history of hypotonia. They often have seizures, which can range in severity and responsiveness to treatment, and they are typically developmentally delayed. Glycine encephalopathy can also present as a milder form with episodic seizures, ataxia, movement disorders, and gaze palsy during febrile illness. These patients are also developmentally delayed, to varying degrees. In the later onset form, patients typically have normal intellectual function, but present with spastic diplegia and optic atrophy. The mild form of the disorder corresponds to greatly reduced but not fully absent GCS activity.Transient neonatal hyperglycinemia has been described in a very small number of cases. Initially, these patients present with the same symptoms and laboratory results that are seen in the classical presentation.