IDENTIFICATION OF CRYPTIC REARRANGEMENTS IN PATIENTS WITH 18Q- DELETION SYNDROME

The majority of patients with 18q- syndrome appear cytogenetically to have a terminal deletion of the long arm of chromosome 18. These 18q- patients are diagnosed by use of standard cytogenetic banding techniqu es, which have resolution insufficient for precise genotyping. In our effort to obtain a thorough genotype, we have analyzed the DNA from 35 patients who originally were diagnosed as having de novo terminal del etions of chromosome 18. Molecular analysis was performed with polymor phic markers throughout the 18q-region. Cytogenetic FISH was performed with two human 18q telomeric probes, a chromosome 18-specific alpha-s atellite probe, and whole chromosome 18-specific paint. Of 35 patients previously reported to have terminal deletions of 18q, we found that 5 (14%) have more-complex cryptic rearrangements and that 3 (9%) retai n the most distal portion of 18q, consistent with an interstitial rath er than a terminal deletion. These findings indicate that a standard k aryotype can lead to insufficient characterization in 18q- syndrome. T his has important ramifications for phenotype mapping of this syndrome , as well as for proper prognosis.