Transient neonatal cystinuria.
BACKGROUND
Cystinuria is an inherited disorder of luminal reabsorptive transport for cystine and dibasic amino acids in the renal proximal tubule. Two cystinuria genes have been identified. Mutations of SLC7A9, which encodes the luminal transport channel itself, tend to be dominant and mutations of SLC3A1 (rBAT), which encodes a transporter subunit, are always recessive. Patients who inherit two recessive mutations or two dominant mutations have equally severe forms of cystinuria. Heterozygotes excrete cystine in the normal (type I), moderate (type III), or high stone-forming (type II) range.


METHODS
Infants with cystinuria were identified via the Quebec Newborn Urinary Screening Program. In a subgroup of these infants, cystinuria was severe in the first months of life, but partially resolved by 2 to 4 years postnatally. We assigned each patient a final cystinuria phenotype at 3 to 4 years. In addition, we characterized SLC3A1 gene expression in fetal and postnatal human kidney.


RESULTS
Most infants with transient neonatal cystinuria are eventually classified as type III heterozygotes. All infants with mutant cystinuria genes have exaggerated neonatal cystine excretion except those who inherit two SLC3A1 mutations (type I/I cystinuria); these children have persistent severe cystinuria, implying that wildtype SLC3A1 is required for the maturational effect. Expression of SLC3A1 mRNA was found to be tenfold higher in postnatal vs. fetal kidney; SLC3A1 expression is doubled by the proximal tubule transcription factor, PAX8. rBAT is expressed in the proximal convoluted and straight tubules in both fetal and adult kidney.


CONCLUSIONS
Maturation of SLC3A1 gene expression between midgestation and 4.5 years postnatal age may account for transient neonatal cystinuria.
Boutros
Marylise
M
Department of Human Genetics, McGill University, Montreal, Quebec, Canada.
Vicanek
Caroline
C
Rozen
Rima
R
Goodyer
Paul
P
D026923:Amino Acid Transport Systems, Basic / Q000235:genetics*; D026921:Amino Acid Transport Systems, Neutral / Q000235:genetics*; D015152:Blotting, Northern; D003553:Cystine / Q000378:metabolism; D003555:Cystinuria / Q000209:etiology / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics; D006801:Humans; D007231:Infant, Newborn; D007668:Kidney / Q000378:metabolism; D009687:Nuclear Proteins / Q000235:genetics; D000072636:PAX8 Transcription Factor; D051761:Paired Box Transcription Factors; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000032:analysis; D015534:Trans-Activators / Q000235:genetics
D026923:Amino Acid Transport Systems, Basic; D026921:Amino Acid Transport Systems, Neutral; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D000072636:PAX8 Transcription Factor; C083325:PAX8 protein, human; D051761:Paired Box Transcription Factors; D012333:RNA, Messenger; C489857:SLC3A1 protein, human; C499489:SLC7A9 protein, human; D015534:Trans-Activators; D003553:Cystine