5-Oxoprolinuria can be caused by genetic defects in either of 2 enzymes involved in the gamma-glutamyl cycle of glutathione metabolism: glutathione synthetase (GSS; 601002) or 5-oxoprolinase (OPLAH; 614243). GSS deficiency (266130) is best characterized as an inborn error of glutathione metabolism, but there is debate as to whether OPLAH deficiency represents a disorder or simply a biochemical condition with no adverse clinical effects because patients lack a consistent clinical picture apart from 5-oxoprolinuria (summary by Calpena et al., 2013).

Phang et al. (2001) noted that prospective studies of HPI probands identified through newborn screening as well as reports of several families have suggested that it is a metabolic disorder not clearly associated with clinical manifestations. Phang et al. (2001) concluded that HPI is a relatively benign condition in most individuals under most circumstances. However, other reports have suggested that some patients have a severe phenotype with neurologic manifestations, including epilepsy and mental retardation (Jacquet et al., 2003). Genetic Heterogeneity of Hyperprolinemia See also hyperprolinemia type II (HYRPRO2; 239510), which is caused by mutation in the gene encoding pyrroline-5-carboxylate dehydrogenase (P5CDH, ALDH4A1; 606811) on chromosome 1p36.

The imino acids, proline and hydroxyproline, share a renal tubular reabsorptive mechanism with glycine. Iminoglycinuria (IG), a benign inborn error of amino acid transport, is also a normal finding in neonates and infants under 6 months of age (Chesney, 2001). Early studies of families with iminoglycinuria suggested genetic complexity, with homozygotes developing IG and heterozygotes manifesting only hyperglycinuria (HG; 138500) (summary by Broer et al., 2008). Iminoglycinuria also occurs as part of the generalized amino aciduria of the Fanconi renotubular syndrome (134600).

Hyperprolinemia can also occur with other conditions, such as malnutrition or liver disease. In particular, individuals with conditions that cause elevated levels of a chemical called lactic acid in the blood (lactic acidosis) may have hyperprolinemia as well, because lactic acid stops (inhibits) the breakdown of proline.\n\nHyperprolinemia type II results in proline levels in the blood between 10 and 15 times higher than normal, and high levels of a related compound called pyrroline-5-carboxylate. This form of the disorder is more likely than type I to involve seizures or intellectual disability that vary in severity.\n\nPeople with hyperprolinemia type I often do not show any symptoms, although they have proline levels in their blood between 3 and 10 times the normal level. Some individuals with hyperprolinemia type I exhibit seizures, intellectual disability, or other neurological or psychiatric problems.\n\nHyperprolinemia is an excess of a particular protein building block (amino acid), called proline, in the blood. This condition generally occurs when proline is not broken down properly by the body. There are two forms of hyperprolinemia, called type I and type II.