Large duplications at reciprocal translocation breakpoints that might be the counterpart of large deletions and could arise from stalled replication bubbles.

Reciprocal chromosome translocations are often not exactly reciprocal. Most familiar are deletions at the breakpoints, up to megabases in extent. We describe here the opposite phenomenon-duplication of tens or hundreds of kilobases at the breakpoint junction, so that the same sequence is present on both products of a translocation. When the products of the translocation are mapped on the genome, they overlap. We report several of these "overlapping-breakpoint" duplications in breast cancer cell lines HCC1187, HCC1806, and DU4475. These lines also had deletions and essentially balanced translocations. In HCC1187 and HCC1806, we identified five cases of duplication ranging between 46 kb and 200 kb, with the partner chromosome showing deletions between 29 bp and 31 Mb. DU4475 had a duplication of at least 200 kb. Breakpoints were mapped using array painting, i.e., hybridization of chromosomes isolated by flow cytometry to custom oligonucleotide microarrays. Duplications were verified by fluorescent in situ hybridization (FISH), PCR on isolated chromosomes, and cloning of breakpoints. We propose that these duplications are the counterpart of deletions and that they are produced at a replication bubble, comprising two replication forks with the duplicated sequence in between. Both copies of the duplicated sequence would go to one daughter cell, on different products of the translocation, while the other daughter cell would show deletion. These duplications may have been overlooked because they may be missed by FISH and array-CGH and may be interpreted as insertions by paired-end sequencing. Such duplications may therefore be quite frequent.