Microdissection and reverse painting reveals a microdeletion 6(q26qter) in a de novo r(6) chromosome

Ring chromosomes 6 are rare constitutional abnormalities with inconsistent phenotypic and clinical features. One of the reasons for this variability is the cytogenetically undetectable loss of chromosomal material from the telomeric segments at 6p or 6q. We have therefore used fluorescence in situ hybridization (FISH) to analyse a ring chromosome 6 that was detected in a newborn boy with dysmorphic features. Reverse painting of the microdissected ring chromosome onto normal metaphase spreads revealed a small deletion of the terminal region of the long arm, 6(q26qter). Moreover, the simple all-telomeric sequence (TTAGG)n was lost, whereas the p-specific subtelomeric sequence was still present. Our findings confirm that microdeletions occur during the formation of r(6) chromosomes and, therefore, are an important determinator of the associated phenotype.