Diffuse large B‐cell lymphoma carrying t(9;14)(p13;q32)/ PAX5 ‐immunoglobulin heavy chain gene is characterized by nuclear positivity of MUM1 and PAX5 by immunohistochemistry

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Abstract We described four patients with diffuse large B‐cell lymphoma (DLBCL) carrying t(9;14)(p13;q32) that places the PAX5 adjacent to the immunoglobulin heavy chain ( IGH ) gene. Ages ranged between 63 and 80, and three were female. One developed a nodal disease, and the other three involved extranodal organs. The lymphoma cells were CD10 − /BCL6 − /MUM1 + in three and CD10 + /BCL6 + /MUM1 + in one. BCL2 was weak or negative. All had t(9;14)(p13;q32), and three had additional 14q32/ IGH translocations or +der(14)t(9;14)(p13;q32). Fluorescence in situ hybridization using the PAX5 break‐apart probe showed that the locus was disrupted between the 5′ and 3′ probes or within the 5′ probe. Immunohistochemistry (IHC) using a monoclonal antibody against PAX5 showed strong nuclear positivity in all four patients. Cell block IHC of a CD30 + DLBCL cell line, KIS‐1, which carried the t(9;14)(p13;q32) and PAX5 ‐ IGH fusion gene, reproduced the CD10 − /BCL6 − /MUM1 + immunophenotype, low‐level BCL2, and strong nuclear PAX5. Uniform nuclear positivity of MUM1 in all four cases and KIS‐1 cells suggest that these lymphomas arose at a late stage of B‐cell differentiation, where expression of PAX5 physiologically becomes downregulated. It is therefore possible that high‐level PAX5 resulting from t(9;14)(p13;q32) at this stage of differentiation perturbs the plasma cell differentiation program initiated by PAX5 repression, thereby contributing to the development of a fraction of DLBCL.

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