Delineating the mechanism of fragility at BCL6 breakpoint region associated with translocations in diffuse large B cell lymphoma

BCL6 translocation is one of the most common chromosomal translocations in cancer and results in its enhanced expression in germinal center B cells. It involves the fusion of BCL6 with any of its twenty-six Ig and non-Ig translocation partners associated with diffuse large B cell lymphoma (DLBCL). Despite being discovered long back, the mechanism of BCL6 fragility is largely unknown. Analysis of the translocation breakpoints in 5' UTR of BCL6 reveals the clustering of most of the breakpoints around a region termed Cluster II. In silico analysis of the breakpoint cluster sequence identified sequence motifs that could potentially fold into non-B DNA. Results revealed that the Cluster II sequence folded into overlapping hairpin structures and identified sequences that undergo base pairing at the stem region. Further, the formation of cruciform DNA blocked DNA replication. The sodium bisulfite modification assay revealed the single-strandedness of the region corresponding to hairpin DNA in both strands of the genome. Further, we report the formation of intramolecular parallel G4 and triplex DNA, at Cluster II. Taken together, our studies reveal that multiple non-canonical DNA structures exist at the BCL6 cluster II breakpoint region and contribute to the fragility leading to BCL6 translocation in DLBCL patients.