The imino acids, proline and hydroxyproline, share a renal tubular reabsorptive mechanism with glycine. Iminoglycinuria (IG; 242600), 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) (summary by Broer et al., 2008). A phenotype of combined glucosuria and glycinuria has been described (see 138070).

Kleta (2006) reviewed aspects of renal stone disease. Nephrolithiasis and urolithiasis remain major public health problems of largely unknown cause. While disorders such as cystinuria (220100) and primary hyperoxaluria (see 259900) that have nephrolithiasis as a major feature have advanced understanding of the metabolic and physiologic processes of stone formation in general, they have not addressed the etiology of calcium oxalate stone formation, responsible for approximately 75% of urolithiasis cases in humans. Men are affected twice as often as women, but children show no such gender bias. The recurrence rate is also high. In populations of European ancestry, 5 to 10% of adults experience the painful precipitation of calcium oxalate in their urinary tracts. Thorleifsson et al. (2009) noted that between 35 and 65% of hypercalciuric stone formers and up to 70% of subjects with hypercalciuria have relatives with nephrolithiasis, and twin studies have estimated the heritability of kidney stones to be 56%. Genetic Heterogeneity of Calcium Oxalate Nephrolithiasis See also CAON2 (620374), caused by mutation in the OXGR1 gene (606922) on chromosome 13q32.

Medium-chain acyl-coenzyme A dehydrogenase (MCAD) is one of the enzymes involved in mitochondrial fatty acid ß-oxidation. Fatty acid ß-oxidation fuels hepatic ketogenesis, which provides a major source of energy once hepatic glycogen stores become depleted during prolonged fasting and periods of higher energy demands. MCAD deficiency is the most common disorder of fatty acid ß-oxidation and one of the most common inborn errors of metabolism. Most children are now diagnosed through newborn screening. Clinical symptoms in a previously apparently healthy child with MCAD deficiency include hypoketotic hypoglycemia and vomiting that may progress to lethargy, seizures, and coma triggered by a common illness. Hepatomegaly and liver disease are often present during an acute episode. Children appear normal at birth and – if not identified through newborn screening – typically present between age three and 24 months, although presentation even as late as adulthood is possible. The prognosis is excellent once the diagnosis is established and frequent feedings are instituted to avoid any prolonged periods of fasting.

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.

Isovaleric acidemia (IVA) is an inborn error of leucine metabolism caused by a deficiency of isovaleryl-CoA dehydrogenase. It can present with severe neonatal ketoacidosis leading to death, but in milder cases recurrent episodes of ketoacidosis of varying degree occur later in infancy and childhood (summary by Vockley et al., 1991).

The spectrum of propionic acidemia (PA) ranges from neonatal-onset to late-onset disease. Neonatal-onset PA, the most common form, is characterized by a healthy newborn with poor feeding and decreased arousal in the first few days of life, followed by progressive encephalopathy of unexplained origin. Without prompt diagnosis and management, this is followed by progressive encephalopathy manifesting as lethargy, seizures, or coma that can result in death. It is frequently accompanied by metabolic acidosis with anion gap, lactic acidosis, ketonuria, hypoglycemia, hyperammonemia, and cytopenias. Individuals with late-onset PA may remain asymptomatic and suffer a metabolic crisis under catabolic stress (e.g., illness, surgery, fasting) or may experience a more insidious onset with the development of multiorgan complications including vomiting, protein intolerance, failure to thrive, hypotonia, developmental delays or regression, movement disorders, or cardiomyopathy. Isolated cardiomyopathy can be observed on rare occasion in the absence of clinical metabolic decompensation or neurocognitive deficits. Manifestations of neonatal and late-onset PA over time can include growth impairment, intellectual disability, seizures, basal ganglia lesions, pancreatitis, and cardiomyopathy. Other rarely reported complications include optic atrophy, hearing loss, premature ovarian insufficiency, and chronic renal failure.

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).

The imino acids, proline and hydroxyproline, share a renal tubular reabsorptive mechanism with glycine. Iminoglycinuria (IG; 242600), 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) (summary by Broer et al., 2008). A phenotype of combined glucosuria and glycinuria has been described (see 138070).

3-Methylcrotonylglycinuria is an autosomal recessive disorder of leucine catabolism. The clinical phenotype is highly variable, ranging from neonatal onset with severe neurologic involvement to asymptomatic adults. There is a characteristic organic aciduria with massive excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine, usually in combination with a severe secondary carnitine deficiency. MCC activity in extracts of cultured fibroblasts of patients is usually less than 2% of control (summary by Baumgartner et al., 2001). Also see 3-methylcrotonylglycinuria I (MCC1D; 210200), caused by mutation in the alpha subunit of 3-methylcrotonyl-CoA carboxylase (MCCC1; 609010).

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.

Multiple mitochondrial dysfunctions syndrome is a severe autosomal recessive disorder of systemic energy metabolism, resulting in weakness, respiratory failure, lack of neurologic development, lactic acidosis, and early death (summary by Seyda et al., 2001). Genetic Heterogeneity of Multiple Mitochondrial Dysfunctions Syndrome See also MMDS2 (614299), caused by mutation in the BOLA3 gene (613183) on chromosome 2p13; MMDS3 (615330), caused by mutation in the IBA57 gene (615316) on chromosome 1q42; MMDS4 (616370), caused by mutation in the ISCA2 gene (615317) on chromosome 14q24; MMDS5 (617613), caused by mutation in the ISCA1 gene (611006) on chromosome 9q21; MMDS6 (617954), caused by mutation in the PMPCB gene (603131) on chromosome 7q22; and MMDS7 (620423), caused by mutation in the GCSH gene (238330) on chromosome 16q23.