MOLECULAR AND CYTOGENETIC CHARACTERIZATION OF 9P-ABNORMALITIES

We report on 2 girls with terminal deletion of the short arm of chromo some 9 with concurrent duplication unrecognizable by routine chromosom e studies. The phenotype of the patients was not specifically suggesti ve of the 9p- syndrome in the absence of trigonocephaly and long philt rum as cardinal manifestations. In addition to psychomotor retardation , their manifestations were mild and include upward slant of palpebral fissures and dolichomesophalangy which are characteristic of del(9p). Chromosome abnormalities were de novo in both cases. The two rearrang ed chromosomes 9 exhibit similar G-banding patterns and suggested the possible duplication of distal 7p. Fluorescence in situ hybridization (FISH) with a chromosome-7 specific library probe indeed identified th at one derivative chromosome 9 was the result of a translocation betwe en chromosomes 7 and 9 [der(9)t(7;9)(p15.3;p24] but failed to detect a signal on the other derivative 9. In the second case, the concurrent abnormality was an inverted duplication of proximal 9p and deletion of distal 9p [inv dup(9)(p13-->p22::p22-->qter)] confirmed by FISH using a chromosome 9 specific library probe. FISH clearly identified the or igin of these 2 abnormal chromosomes 9 and provided crucial informatio n for clinical evaluation. We emphasize the importance of utilizing up dated cytogenetic and molecular techniques in the precise delineation of subtle or complex abnormalities where there are no useful phenotypi c clues.