Background: POKEMON (for POK, Erythroid, Myeloid ONtogenic factor) has recently been shown to be a novel proto-oncogene, playing a key role in cellular transformation and repression of ARF. Pokemon overexpression leads to overt oncogenic transformation both in vitro and in vivo in transgenic mice. Because these transgenic mice developed T-cell lymphoma we sought to expand our original screening analyses. We intially showed POKEMON expression in diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL) and co-expression of BCL6 and POKMEON showed higher proliferation and predicted for better overall survival in DLBCL (Nature 433, 278–285). Nodular lymphocyte predominant Hodgkin Lymphoma (NLPHL) is another tumor that expresses BCL6 and shows localization to L&H tumor cells. We therefore sought to compare POKEMON protein expression in NLPHL and Classical Hodgkin Lymphoma (cHL) cases.
Design: Stains for POKEMON were performed with the anti-POKEMON hamster monoclonal antibody (clone 13E9). Sections of reactive human tonsil were stained as controls showing localization to squamous epithelium and reactive germinal centers as well as paracortical regions. For NLPHL, the cohort comprised of 19 males, 2 females; for cHL 16 males, 14 females. 20 NLPHL and 30 cHL tissue biopsies were stained using whole sections. Intensity and percent positivity of malignant L&H cells and Reed-Sternberg (RS) cells as well as surrounding reactive lymphocytes was scored. Expression in T-cell lymphomas was expanded to include a total of 84 cases analyzed by tissue microarray (TMA) that included: 8 ALCL (Anaplastic Large Cell Lymphoma), 11 AILT (Angioimmunoblastic T-cell Lymphoma), 17 T-ALL (T Lymphoblastic Lymphoma), 43 PTCL (Peripheral T-cell Lymphoma), 3 T/NK (Nasal type T/NK cell Lymphoma), and 2 PTCL, NOS (Not Otherwise Subclassified). Tumor cells in T-cell lymphomas and HL were scored and defined as 0 (negative), 1 (scattered <50%, weak positive), and 2 (diffuse >50%, strong positive) with a nuclear localization pattern.
Results: For NLPHL, 18/20 (90%) cases showed diffuse strong positivity in >50% of malignant L&H tumor cells whereas in cHL cases, 6/30 (20%) cases showed diffuse strong positivity in >50% of malignant RS tumor cells for POKEMON protein expression. In T-cell lymphomas, POKEMON expression was strongest in ALCL (6/8, 75%) and AILD (7/11, 74%) compared to the other subtypes. In contrast, BCL6 protein expression was positive in 12/20 (60%) cases which showed weak, scattered to diffuse strong positivity in L&H tumor cells and 1/30 (3%) with scattered weak reactivity in cHL malignant RS cells
Conclusion: POKEMON is co-expressed with BCL6 in malignant L&H tumor cells in NLPHL but not in cHL. Pokemon’s critical role in cellular transformation makes it an attractive target for therapeutic intervention in NLPHL.