Disorders of Creatine and Ornithine Metabolism
Creatine is synthesized mainly in the liver and pancreas by the action of arginine:glycine amidinotransferase (AGAT) and guanidinoace-tate methyltransferase (GAMT) with arginine, glycine, and S-adenosylmethionine as essential substrates. Creatine reaches muscle and brain via an active transmembrane creatine transport system (CRTR). Creatine is then utilized in the cellular pool of creatine/creatine-phosphate, which together with creatine kinase and ATP/ADP provides a high energy phosphate buffering system. Intracellular creatine and creatine phosphate are nonenzymatically converted to creatinine, with a 1.5% constant daily turnover rate of body creatine. Creati-nine is excreted in urine, and the daily urinary creatinine excretion is directly proportional to total body creatine.
Creatine deficiency syndromes represent a group of inborn errors of metabolism, including disorders of creatine synthesis (AGAT) and GAMT deficiency, and disorders of creatine transport, including the X-linked transmembrane creatine transporter (X-CRTR) deficiency.1 Inheritance of GAMT and AGAT deficiency is autosomal recessive, whereas the gene for X-CRTR deficiency (SLC6A8) is located on the X chromosome. GAMT and AGAT deficiency seem to be rare disorders, whereas X-CRTR deficiency has been diagnosed in up to 2% of patients who have X-linked mental retardation.
The main clinical symptoms observed in all three creatine deficiency syndromes are mental retardation with pronounced speech delay, autistic behavior, and seizures. Patients with GAMT deficiency may exhibit a more complex clinical phenotype, including intractable epilepsy, extrapyramidal movement disorder, and abnormal signal intensities of the basal ganglia.2 The clinical phenotype of X-CRTR deficiency varies from mild to severe mental retardation associated with speech delay and epileptic seizures. Heterozygote females may have learning difficulties. The brain is the major site of creatine depletion in all these disorders. Patients with GAMT deficiency have accumulation of guanidinoacetate in addition to creatine deficiency.
The typical biochemical abnormality of creatine deficiency syndromes is cerebral creatine deficiency, which is demonstrated by in vivo proton magnetic resonance spectroscopy.
Guanidinoacetate accumulates in GAMT deficiency and is deficient in AGAT deficiency. Thus, measurement of guanidinoacetate in body fluids may discriminate GAMT deficiency (high concentration) from AGAT deficiency (low concentration). In patients with X-CRTR deficiency, the urinary creatine excretion relative to the creatinine excretion is elevated, and an elevated urinary-creatine-to-creatinine ratio can be used as a first biochemical diagnostic marker for this disease. The diagnosis of all these disorders is confirmed by molecular genetic analysis of the respective genes and by studies of enzyme activities and creatine uptake, respectively.
Cerebral creatine deficiency, as caused by disorders of creatine synthesis (GAMT and AGAT deficiency), can be corrected by oral supplementation of creatine monohydrate (400 mg/kg body weight/day). Treatment of GAMT deficiency also requires therapeutic measures to reduce the accumulation of guanidinoacetate. This is mainly achieved by dietary restriction of arginine, which is the rate-limiting substrate to the synthesis of guanidinoacetate. Treatment leads to substantial clinical benefit, including significant developmental progress in AGAT deficiency and improvement of epilepsy and movement disorder in GAMT deficiency. Unfortunately, in X-CRTR deficiency, oral creatine substitution does not result in an increase of brain creatine levels, and no alternative causal treatment is available for this disorder.
Ornithine is a nonprotein amino acid that has various functions. First, it is a key intermediate in the urea cycle. Second, ornithine is transported via the ornithine transporter (ORNT1 or SLC25A15 gene) into the mitochondrium, where it serves as an essential substrate for ornithine transcarbamylase (OTC) in the intramitochondrial part of the urea cycle. Third, intramitochondrial ornithine is also substrate to ornithine aminotransferase (OAT), which catalyzes the conversion of ornithine to Δ1-pyrroline-5-carboxylate (P5C), which is an essential substrate for proline synthesis. Fourth, extramitochondrial ornithine is the key substrate for the formation of polyamines, spermine, and spermidine, which are involved in DNA packaging. Fifth, ornithine is derived from a reaction that arginine undergoes with glycine in order to form guanidinoacetate, an essential intermediate in creatine synthesis.
Three disorders affecting ornithine metabolism are known in humans.5 (1) ornithine aminotransferase (OAT) deficiency, which causes gyrate atrophy of the choroid and retina (MIM 258870); (2) ornithine transporter (ORNT1 or SLC25A15 gene) deficiency, and (3) Δ1-pyrro-line-5-carboxylate (P5C) synthase deficiency, which affects the synthesis of ornithine and proline.
OAT deficiency is a rare autosomal recessive disorder characterized by progressive retinopathy.
Pathophysiologically, OAT catalyzes the conversion of ornithine to Δ1-pyrroline-5-carboxylate (P5C). Normally the reaction equilibrium is directed toward the production of P5C. Thus, in OAT deficiency, hyperornithinemia is the biochemical hallmark. Interestingly, in neonates the reaction equilibrium of OAT is directed conversely, namely toward synthesis of ornithine from P5C. Thus, neonates with OAT deficiency have low levels of ornithine rather than hyperornithinemia.
OAT is diagnosed by demonstration of high plasma ornithine levels, which typically are 5 to 20 times higher than the normal range. Diagnosis is confirmed by measuring OAT activity in fibroblasts and by mutational analysis of the OAT gene.
Treatment aims to correct the amino acid abnormalities. Pyridoxal phosphate is a cofactor of OAT, and some patients respond to high dosages of pyridoxine with fair correction of the ornithine levels. In nonresponders, hyperornithinemia is partially corrected by dietary arginine restriction through a low-protein diet supplemented with arginine-free amino acid mixtures. The goal of treatment is to slow the progression of retinopathy. As neonates with OAT deficiency have decreased levels of ornithine and arginine, an arginine-restricted diet is contraindicated in this period of life.
This is a rare syndrome of hyperammonemia, hyperornithinemia and homocitrullinemia; it is autosomal recessive disorder of ornithine transport across the mitochondrial membrane (ORNT1, SLC25A15 gene).
Clinically, patients develop intermittent hyper-ammonemia associated with vomiting, ataxia, lethargy, irritability, confusion, and other psychiatric manifestations. In the long term, patients may develop chronic neurological deficits (eg, progressive spastic paraparesis), developmental delay, and mental retardation.
Diagnosis of HHH syndrome is established by the characteristic combination of elevated ornithine levels in blood and elevated excretion of homocitrulline (which arises from alternative metabolism of carbamyl phosphate via lysine) in urine along with intermittent hyper-ammonemia. Confirmation of the diagnosis is possible by ornithine incorporation assays in fibroblasts and by mutational analysis of the SLC25A15 gene.
Treatment is directed toward preventing increased ammonia production through a protein-restricted diet, avoidance of catabolic states and supplementation of citrulline.
P5C SYNTHASE DEFICIENCY
Only two patients from one family have been described with P5C synthase deficiency.4,6 These patients’ clinical signs were indicative of involvement of both the nervous system and the connective tissue. Hypoprolinemia and hypoornithemia are characteristic diagnostic features.
See references on DVD.