Cystinuria

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What is cystinuria?

Cystinuria is a condition characterized by the buildup of cystine crystals or stones in the kidneys and bladder. Cystine is an amino acid, one of the building blocks of proteins. Normally, cystine is reabsorbed back into the bloodstream after blood entering the kidneys is filtered to create urine. Only small amounts of cystine can stay dissolved in urine. People with cystinuria cannot properly reabsorb cystine into their bloodstream. As urine becomes more concentrated in the kidneys, the cystine cannot stay dissolved and forms crystals. As these crystals become larger, they form stones that may lodge in the kidneys or in the bladder. Sometimes cystine crystals combine with calcium in the kidneys to form large stones. Blockages in the urinary tract by these crystals and stones reduce the ability of the kidneys to eliminate wastes through urine. The stones also provide sites where bacteria may cause infections.

How common is cystinuria?

Cystinuria affects approximately 1 in 10,000 people.

What genes are related to cystinuria?

Mutations in the SLC3A1 and SLC7A9 genes cause cystinuria.

The SLC3A1 and SLC7A9 genes provide instructions for producing the two parts of a transporter protein that is made primarily in the kidneys. Normally this protein allows certain amino acids, including cystine, to be reabsorbed into the blood from the filtered fluid that will become urine. Mutations in either of these genes disrupt the ability of this transporter protein to reabsorb these amino acids, allowing them to become concentrated in the urine. As the levels of cystine in the urine increase, the crystals typical of cystinuria are able to form. The other amino acids that are not reabsorbed do not create crystals in urine.

How do people inherit cystinuria?

This condition is inherited in an autosomal recessive pattern, which means two copies of the gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder are carriers of one copy of the altered gene but do not show signs and symptoms of the disorder.

Source: National Institutes of Health

Findings from University of Wisconsin broaden understanding of cystinuria

"B vitamin supplementation supports the need for enzyme cofactors but cannot provide substrate in the form of methyl groups. l-Methionine is an essential amino acid and is required for protein synthesis, but it also plays a unique role in metabolism as S-adenosylmethionine, which is the primary methyl donor in metabolism. The observation that l-homocysteine, which is produced in the metabolism of l-methionine, is remethylated 2-4 times before it is destroyed is key to understanding the possibility of a methyl group deficiency. This suggests that the requirement for methyl groups (ie, S-adenosylmethionine) may be 2-4 times that for methionine in support of protein synthesis. l-Homocysteine can be remethylated to form l-methionine by betaine or N(5)-methyltetrahydrofolate," wrote N.J Benevenga and colleagues, University of Wisconsin.

The researchers concluded: "Betaine and one-carbon sources that lead to the production of N(5)-methyltetrahydrofolate and the remethylation of l-homocysteine to form l-methionine should be considered along with B vitamin supplementation in the treatment of homocystinuria and neural tube defects."

Benevenga and colleagues published their study in The American Journal of Clinical Nutrition (Consideration of betaine and one-carbon sources of N5-methyltetrahydrofolate for use in homocystinuria and neural tube defects. The American Journal of Clinical Nutrition, 2007;85(4):946-9).

For more information, contact N.J. Benevenga, University of Wisconsin-Madison, Dept. of Animal Sciences and Nutritional Sciences, Madison, WI USA.

Publisher contact information for the The American Journal of Clinical Nutrition is: American Society Clinical Nutrition, 9650 Rockville Pike, Subscriptions, RM L-3300, Bethesda, MD 20814-3998, USA.

Keywords: United States, Madison, Amino Acid, Cystinuria, Dietary Supplement, Genetics, Homocystinuria, L-Methionine, Metabolic Disease, Micronutrient, Neural Tube Defects.

This article was prepared by Biotech Business Week editors from staff and other reports. Copyright 2007, Biotech Business Week via NewsRx.com.