Lack of Bcl-2 expression in follicular lymphoma may be caused by mutations in the BCL2 gene or by absence of the t(14;18) translocation.

Follicular lymphoma (FL), except grade 3B, is characterized by the chromosomal translocation t(14;18)(q32;q21), which results in over-expression of the Bcl-2 protein. Ten per-cent of all FLs, however, do not show Bcl-2 protein expression with standard immunohistochemistry using a monoclonal Bcl-2 antibody against residues 41-54 of the Bcl-2 protein. In this study, the biological background of 18 Bcl-2-negative FL cases grade I, II, or IIIa was investigated by immunohistochemical staining and western blot analysis with alternative antibodies. Bcl-2 protein was demonstrated in five of the 18 cases and all of these carried the t(14;18) translocation. Of the 13 cases that were Bcl-2 negative with alternative antibodies, 12 lacked the t(14;18) translocation. PCR and subsequent sequence analysis of cDNA demonstrated that three cases with a t(14;18) contained somatic mutations in the translocated BCL2 gene, resulting in amino acid replacements in the region of the epitope recognized by the antibody. In conclusion, the majority of Bcl-2-negative FL lack a t(14;18) but a significant subset of these tumours are false negative due to mutations in the BCL2 gene. These findings may have consequences for the use of Bcl-2 immunohistochemistry for diagnostic purposes.