Iminoglycinuria is an autosomal recessive disorder of renal tubular transport affecting reabsorption of the amino acid glycine, and the imino acids proline and hydroxyproline. This results in excess urinary excretion of all three acids (-uria denotes "in the urine").Iminoglycinuria is a rare and complex disorder, associated with a number of genetic mutations that cause defects in both renal and intestinal transport systems of glycine and imino acids.Imino acids typically contain an imine functional group, instead of the amino group found in amino acids. Proline is considered and usually referred to as an amino acid, but unlike others, it has a secondary amine. This feature, unique to proline, identifies proline also as an imino acid. Hydroxyproline is another imino acid, made from the naturally occurring hydroxylation of proline. Presentation
The primary characteristic of iminoglycinuria is the presence of glycine and imino acids in the urine. Otherwise, it is thought to be a relatively benign disorder, although symptoms associated with disruptions of proline and glycine metabolism caused by malabsorption may be present with iminoglycinuria. These include encephalopathy, mental retardation, deafness, blindness, kidney stones, hypertension and gyrate atrophy.Gyrate atrophy is an inherited degenerative disorder of the retina and choroid, sometimes accompanying the metabolic condition hyperornithinemia. The presence of gyrate atrophy with iminoglycinuria stems from a deficiency of proline in chorioretinal tissues, while processes behind hyperornithinemia disrupt the metabolic pathway from ornithine to proline, which alters the catabolism of ornithine, and also results in reduced levels of proline.
Thus, gyrate atrophy can be found with either disorder, with proline deficiency as an underlying feature.Hyperglycinuria is another disorder affecting reabsorption of glycine and imino acids, similar to iminoglycinuria and considered to be a heterozygous form. When accompanied by a specific type of kidney stone (nephrolithiasis), it is sometimes referred to as "iminoglycinuria, type II". Genetics
Iminoglycinuria is believed to be inherited in an autosomal recessive manner. This means a defective gene responsible for the disorder is located on an autosome, and inheritance requires two copies of the defective gene—one from each parent. Parents of an individual with an autosomal recessive disorder both carry one copy of the defective gene, but usually do not experience any signs or symptoms of the disorder.A non-inherited cause of excess urinary excretion of proline and glycine, similar to that found in iminoglycinuria, is quite common to newborn infants younger than six months. Sometimes referred to as neonatal iminoglycinuria, it is due to underdevelopment of high-affinity transport mechanisms within the renal circuit, specifically PAT2, SIT1 and SLC6A18. The condition corrects itself with age. In cases where this persists beyond childhood, however, inherited hyperglycinuria or iminoglycinuria may be suspected. Pathophysiology
Glycine, proline and hydroxyproline share common renal tubular mechanisms of reabsorption, a function specific to the proximal tubule. Both reabsorption or absorption of glycine and imino acids takes place respectively at the proximal tubule or intestinal brush border epithelium.