Follicular lymphoma (FL) is characterized by the translocation t(14;18)(q32;q21) resulting in constitutive overexpression of BCL2. However, in 10% to 15% of FL grade 1/2, immunohistochemical staining for BCL2 remains negative. To analyze the incidence of BCL2 negativity and the underlying molecular mechanisms in FL grade 1/2, BCL2 expression was investigated with 3 antibodies (clones 100D5, E17, SP66). The presence of a break in the BCL2 locus was determined by fluorescence in situ hybridization. The region of the BCL2 gene where the epitope of the standard BCL2 antibody resides was sequenced. Twenty-two (9.2%) of 240 identified cases of FL grade 1/2 were negative with the standard BCL2 antibody. Of these, 12 cases (55%) carried a break in the BCL2 gene locus, which, in all but one case, correlated with BCL2 expression using the alternative antibodies E17 and SP66 and with missense mutations of BCL2. Ten (45%) of the 22 cases had an intact BCL2 gene locus; 2 cases carried a BCL6/IGH translocation. All 10 cases were negative for the E17/SP66 antibodies and showed a wild-type sequence of BCL2. Six of these showed an aberrant phenotype, with CD10 negativity (30%) or CD23 expression (30%). In summary, the alternative E17/SP66 antibodies identify 2 immunohistochemically and genetically distinct subgroups of BCL2-"negative" FL grade 1/2.
