CD56 and PGP expression in acute myeloid leukemia: impact on clinical outcome

Overexpression of P-glycoprotein (PGP), a multidrug-related (MDR) protein, is one of the most important factors responsible for reduced drug sensitivity in acute myeloid leukemia (AML). Recently, we demonstrated that the presence of CD56 antigen, an isoform of the neural adhesion molecule, in AML cells is a negative independent prognostic factor for the achievement of complete remission (CR) and correlates with shorter survival. Since in our previous report we observed a more frequent PGP expression in CD56+ patients, we hypothesized that the reduced response to chemotherapy in this group of patients was due to increased PGP-mediated drug efflux. To confirm this hypothesis in this study PGP and CD56 expression on AML cells was correlated with other clinical and biological features and treatment response.

Immunophenotypic analysis, including evaluation of CD56 and PGP expression, was performed using multiparameter flow cytometry on fresh and/or cryopreserved blast cells, obtained after informed consent, from bone marrow and/or peripheral blood of 143 consecutive newly diagnosed AML cases at the time of diagnosis. Samples expressing CD56 in at least 15% or more cells were considered as positive (CD56+). PGP expression was expressed as a mean fluorescence index (MFI) i.e. as the ratio of sample mean fluorescence channel and the isotypic control mean fluorescence channel.

Overall results showed that 67/143 cases were PGP-/CD56-, 23/143 were PGP+ /CD56+, 40/143 were PGP+/CD56- and the remaining 13/143 were PGP-/CD56+. CD56+ and PGP+ on AML cells significantly reduced the CR rate (83% in the PGP-/CD56- group vs 60% in the PGP-/CD56+ group, 46% in the PGP+/CD56- group and 58% in the PGP+/CD56+ group, p = 0.002). In addition we observed a significantly higher proportion of total failures in patients expressing PGP or CD56 compared to in the group not expressing either (73% vs 27%, respectively; p = 0.0001). CD56 and PGP overexpression influenced the overall survival: in fact, the median survival of CD56+ and PGP+ patients ranged from 10 to 23 months, while the actuarial survival of CD56-/PGP- patients at 5 years is 52% (p = 0.023).

Our data underline the independent negative prognostic role of PGP and CD56 expression in acute myeloid leukemia. Since the mechanism by which CD56 reduces drug sensitivity is still unknown, further investigations are required.