Absence of tumor-specific over-expression of Polo-like kinase 1 (Plk1) in major non-Hodgkin lymphoma and relatively low expression of Plk1 in nasal NK/T cell lymphoma

Based on the presence of the tumor-specific over-expression of Plk1 (polo-like kinases) in various malignancies, we examined Plk1 expression in nine cases of reactive follicular hyperplasia (RFH), 42 of diffuse large B cell lymphoma (DLBCL), 16 of follicular lymphoma (FL), and 10 of nasal NK/T lymphoma. There was no significant difference in the Plk1-positive cell percentage between RFH and DLBCL. The Plk1-positive cell percentage ranged from 6 to 20% with a median of 12.9% in DLBCL. In FL, Plk1-positivity was at most 7%. Plk1-positivity in nasal NK/T cell lymphoma (4.7–14.1% with a median of 9.2%) was significantly higher than that of FL and tended to be lower than DLBCL (p < 0.001, p = 0.05, respectively). Although a strong correlation between positive cell percentages for Plk1 and Ki-67 in these three lymphomas specified Plk1 as a proliferation marker (r = 0.83–0.91), the Plk1-positive cell percentage relative to the other proliferation markers tended to be particularly low in nasal NK/T cell lymphoma. In 41 cases of DLBCL, the positive cell percentages of Plk1 and Ki-67 were both correlated with overall survival. The 4-year overall survival rates by Kaplan–Meier analysis for Plk1-negative and positive patients were 80 and 38%, respectively (p = 0.02).