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What are the laboratory findings in cystinuria?
The key laboratory finding is a large amount of cystine in the urine. There are also increased urinary levels of 3 other amino acids with a similar structure -- lysine, arginine, and ornithine. The membrane transport for cystine is unique to cystine but serves to transport all 4 so-called dibasic amino acids -- lysine, arginine, orthnithine and, of course, cystine.
This excess of amino acids in the urine constitutes aminoaciduria. Some persons (heterozygotes) carrying just one gene for cystinuria have aminoaciduria, although milder. There are actually 3 types of cystinuria. Carriers of 2 of these types of cystinuria (types II and III) show abnormal amounts of cystine in their urine.
How is cystinuria inherited?
The genetics of cystinuria are complicated.
There are, as mentioned, 3 types of cystinuria. They are termed cystinuria type I (abbreviated CSNU1), cystinuria type II (CSNU3), and cystinuria type III (CSNU3). Each type of cystinuria can be inherited as an autosomal recessive trait. For example, in a family each parent may have one CSNU1 gene (and a normal gene paired with it) while their boy or girl may have the misfortune of receiving the CSNU1 gene from both of them, thereby acquiring cystinuria.
Cystinuria type I (CSNU1) is caused by a mutation (a change) in a gene (the SLC3A1 amino acid transporter gene) which is situated on chromosome 2p16.
Cystinuria type 2 (CSNU2) is, quite remarkably, also caused by a gene in 2p16.3. Studies of families with both cystinuria I and cystinuria II have shown that the genes for CSNU1 and 2 are at exactly the same spot in chromosome 2p16.3 and in fact both involve the same gene (the SLC3A1 amino acid transporter gene).
CSNU1 and 2 are therefore said to be allelic. The fact that CSNU1 and 2 are allelic means that genetic compounds can form, producing three clinical types -- persons with not just CSNU1/CSNU1 and CSNU2/CSNU2 but also the compound type CSNU1/CSNU2.
Cystinuria III (CSNU3) is due to a mutation at a separate location, namely at chromosome 19q13.1.