Variable breakpoints target PAX5 in patients with dicentric chromosomes: A model for the basis of unbalanced translocations in cancer

The search for target genes involved in unbalanced acquired chromosomal abnormalities has been largely unsuccessful, because the breakpoints of these rearrangements are too variable. Here, we use the example of dicentric chromosomes in B cell precursor acute lymphoblastic leukemia to show that, despite this heterogeneity, single genes are targeted through a variety of mechanisms. FISH showed that, although they were heterogeneous, breakpoints on 9p resulted in the partial or complete deletion of PAX5. Molecular copy number counting further delineated the breakpoints and facilitated cloning with long-distance inverse PCR. This approach identified 5 fusion gene partners with PAX5: LOC392027 (7p12.1), SLCO1B3 (12p12), ASXL1 (20q11.1), KIF3B (20q11.21), and C20orf112 (20q11.1). In each predicted fusion protein, the DNA-binding paired domain of PAX5 was present. Using quantitative PCR, we demonstrated that both the deletion and gene fusion events resulted in the same underexpression of PAX5, which extended to the differential expression of the PAX5 target genes, EBF1, ALDH1A1, ATP9A, and FLT3. Further molecular analysis showed deletion and mutation of the homologous PAX5 allele, providing further support for the key role of PAX5. Here, we show that specific gene loci may be the target of heterogeneous translocation breakpoints in human cancer, acting through a variety of mechanisms. This approach indicates an application for the identification of cancer genes in solid tumours, where unbalanced chromosomal rearrangements are particularly prevalent and few genes have been identified. It can be extrapolated that this strategy will reveal that the same mechanisms operate in cancer pathogenesis in general.