Deregulated BCL6 expression in DLBCL cases carrying BSE1 mutations.

(A) Northern blot analysis of BCL6 expression in various DLBCL-derived cell lines. Equal amounts (12 μg) of total RNA were loaded on a formaldehyde-agarose gel, blotted, and sequentially hybridized with a full-length BCL6 cDNA probe and with GAPDH as control for the amount and integrity of the RNA. Quantitative analysis of the data was performed using PhosphorImager (bottom panel). For each sample, the presence of 3q27 chromosomal translocations [t(3q27)], mutations affecting the BSE1 sites [M (BSE1)], and mutations located outside the BSE1 sites [M (others)] are indicated. The lymphoblastoid cell line CB33, which does not express BCL6, was included as a negative control. (B) Western blot analysis of BCL6 protein expression in the same cell lines, using the anti-BCL6 (N3) antibody and an anti–β-tubulin antibody as control for protein loading. (C) Nucleotide sequencing of RT-PCR products generated from the mutated lymphoma cell lines Ly1 and BJAB. Only the allele A, identified by the 257T>C mutation in the BSE1 site, but not the allele B (containing a normal sequence in exon 1) is expressed in Ly1, whereas BCL6 is biallelically expressed in the control line BJAB, heterozygous for nonderegulating mutations. Ramos cells are shown as a control for unmutated exon 1 sequences.