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Cystinuria is an inherited autosomal recessive[1] metabolic disorder that is characterized by the formation of cystine stones in the kidneys, ureter, and bladder.
Cystinuria is characterized by the inadequate reabsorption of cystine during the filtering process in the kidneys, thus resulting in an excessive concentration of this amino acid in the urine. Cystine may precipitate out of the urine, if the urine is neutral or acidic, and form crystals or stones in the kidneys, ureters, or bladder.
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. These defects prevent proper reabsorption of basic, or positively charged amino acids such as histidine, lysine, ornithine, arginine and cysteine.[2]
Normally this protein allows certain amino acids, including cysteine, 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 cysteine in the urine increase, the crystals typical of cystinuria are able to form, resulting in kidney stones. Cystine crystals form hexagonal-shaped crystals which can be viewed upon microscopic analysis of the urine. The other amino acids that are not reabsorbed do not create crystals in urine. The disorder affects 1 in 7,000 people, making it one of the most common inherited diseases, and the most common genetic error of amino acid transport. Cystinuria is inherited in an autosomal recessive pattern.
This disease is known to occur in only two mammalian species. These include humans, and canines like the Maned Wolf of South America. Newfoundland dogs are at an increased risk for cystinuria, compared with other breeds of dog.[citation needed]
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