Clinical Characteristics and Prognostic Implications of NPM1 Mutations in Acute Myeloid Leukemia.

Nucleophosmin (NPM) is a nucleolar protein with multi-functions including centromere duplication, nuclear-cytoplasmic shuttling, ribosomal biogenesis, p53 stability. NPM1 mutations were found in a large number of patients with acute myeloid leukemia (AML) especially with normal karyotype. The mutations lead to the aberrant subcellular localization of NPM protein. However, their impacts on clinical outcome remain controversial. We screened the mutations of NPM1 in 257 AML patients and analyzed the clinical significance. NPM1 mutations were found in 64 of 257 patients (24.9%). Seven types of mutations, including four novel mutations, were identified. NPM1 mutations were associated with normal karyotype, FLT3 mutations (both FLT3-ITD and D835 mutation) but not with other gene alterations such as N-RAS, p53 mutations and partial tandem duplication of the MLL gene. In 190 patients except the M3 subgroup, who were treated according to the protocol of Japan Adult Leukemia Study Group, the multivariate analysis revealed that NPM1 mutation was a favorable factor for achieving complete remission, but significantly associated with relapse. A sequential analysis, using paired samples obtained at diagnosis and relapse in 39 patients, revealed that NPM1 mutations were lost at relapse in 2 of the seventeen patients who had NPM1 mutations at diagnosis and none of the patients, who did not have NPM1 mutations at diagnosis, gained NPM1 mutations at relapse.

Our results suggest that NPM-mutated AML should be a distinct subgroup with specific clinical characteristics and outcome. Loss of NPM mutations at relapse implies that NPM mutation is not necessarily a primary genetic alteration and that these leukemic clones could be sensitive to chemotherapy.