Expansion of repetitive DNA into cytogenetically visible elements

Two cases of amplified repetitive elements accidentally identified in cance r samples are reported. In both cases, repeated DNA that is normally not vi sible by traditional chromosome banding had increased in amount to become c ytogenetically visible. In one case, an addition to the short arm of chromo some 1 was originally diagnosed. However, upon molecular analysis the diagn osis could be corrected to an amplification of the D1Z2 repeat. In the seco nd case, a strongly DAPI-positive band was visible at the top of the short arm of chromosome 22, and the original diagnosis was add(22). Staining for telomeric repeats revealed their presence inside the DAPI-positive element, thus confirming that the element in question was truly added to the end of the chromosome. Curiously, no telomeric repeats could be detected distal t o the DAPI-positive element. The identity of the DAPI-positive element coul d not be established, as it was not stained by any of the specific probes a pplied, nor in a scanning hybridization with labeled Cot-1 DNA. It thus see ms to represent an expansion from some lowly repetitive AT-rich DNA translo cated to the tip of chromosome 22. Copyright (C) 2001 S. Karger AG, Basel.