Nucleophosmin/B23(NPM) As a New Prognostic Predictor in Acute Lymphoblastic Leukemia

Nucleophosmin (NPM, B23) is a multi-functional nucleolus protein. It takes part in lots of important cellular activities and is over-expressed in many kinds of solid tumor. We have found that an obvious up-regulation of NPM in many leukemia cell lines, especially in 3 resistant leukemia cell lines. It has been shown that NPM is mutated in around 30%-50% of adult acute myelogenous leukemia (AML) with a normal karyotype. These mutations may be related to favorable prognosis of AML. However, prognostic value of NPM in acute lymphoblastic leukemia (ALL) is unclear yet. Here we analyzed expression and mutation of NPM in ALL patients to investigate correlation between NPM and prognosis in ALL, and to provide new roles of NPM in ALL. In this study,Real-time quantitative PCR, direct sequencing and western blot were used for accessing NPM mRNA expression level, NPM mutation and protein expression level. 199 cases of ALL patients were analyzed, including 62 cases of newly diagnosed patients, 87 cases at complete remission (CR) patients and 50 cases refractory or relapsed patients.NPM mutations were screened among ALL samples, but no mutation was noted in all ALL samples. Compared to healthy normal controls, which was only 2.5% NPM positive (NPM⁺) (1/40), ALL patients showed higher positivity of expression of NPM, 32.3% (20/62) de novo ALL patients, 5.8% (5/87) CR patients and 56.0% (28/50) refractory/relapsed ALL, respectively. Expressions of NPM in refractory/relapsed patients were higher than in de novo patients (p =0.01) and CR patients (p <0.0001). Meanwhile, correlation between expressions of NPM and prognosis were also assessed. In follow-up time (median 17.5 months, 0.13-82.8 months), for those 54 de novo patients who received standard first-line chemotherapy and whose treatment outcome could be evaluated, patients with negative NPM expression (NPM^-) had 93.9% CR rate (31/33) , much higher than NPM⁺ patients (66.7%,14/21; p =0.02). Two groups were comparable in other potential prognostic factors, such as age, cytogenetics, WBC count, blast percentage. Furthermore, NPM^- patients seemed to have higher median survival time than NPM⁺, 13 months vs. 9 months. NPM positivity was significantly correlated with lower overall survival (OS) and relapse-free survival (RFS). Two-year OS of de novo patients with NPM⁺ were 10.0%, and patients with NPM^- were 37.0% (P=0.03). Compared with NPM^-, patients with NPM⁺ showed lower RFS, 14% in NPM⁺ vs. 42% in NPM^- of RFS at two-year (p =0.03). The median RFS were 5.7months and 10months, respectively. The two-year cumulative survival was 8% for the NPM⁺ group versus 23% for the NPM^- group of relapsed/refractory ALL patients (p=0.035). Multivariable analysis identified age of older than 40 years,NPM positivity and WBC count ≥ 100×10^9/L as independent significantly prognostic factors for OS. In conclusion,NPM positivity was significantly correlated with shorter OS and RFS in ALL. Over-expressions, but not mutation of NPM may become a new useful predictor in assessing the prognosis of ALL.