Abstract 790 Mechanisms in Hereditary Disease Platform, Tuesday, 5/4

Nephropathic cystinosis is a lysosomal storage disease due to defective transport of the disulfide amino acid, cystine, across the lysosomal membrane. The cystine carrier is presumably encoded by the cystinosis gene, CTNS, located on chromosome 17p. CTNS contains 12 exons distributed across 23 kb of genomic DNA and predicted to produce a 367 amino acid peptide with 7 transmembrane domains and 8 potential glycosylation sites.

Although 31 different CTNS mutations have been identified, the most common is a 65-kb deletion whose 3′ border cuts exon 10 and which apparently arose in Northern Europeans. Homozygotes for this deletion are detectable by the absence of polymorphic marker D17S829, but no method exists to identify heterozygotes. We determined the breakpoints of the 65-kb deletion and used PCR primers flanking the deletion to amplify a 423-bp fragment present only in the deletion alleles. Using this technique, we ascertained that 121 of 216 (56%) of the alleles in our American population of cystinosis patients bore the 65-kb deletion. No patient of non-European descent carried the deletion, and the deletion size and breakpoint appeared identical in all patients studied, supporting the concept of a founder effect and obviating the need to invoke one of the mechanisms of repeated gene rearrangements.

Using the deletion-flanking primers, combined with the use of D17S829 primers, we created a multiplex PCR system useful for diagnosing cystinosis patients homozygous and heterozygous for the 65-kb deletion. This should prove useful for prenatal and carrier diagnosis of cystinosis on a molecular basis.