NPM Mutations in Adult AML with Normal Karyotype: A Retrospective Study of the Acute Leukemia French Association (ALFA).

NPM mutation (NPMm) has been recently reported as the most frequent mutation in AML, especially in the presence of a normal karyotype. Association of NPMm with an increased complete remission (CR) rate has been suggested, but the long-term prognosis is not known. The abnormal mutated NPM protein shows an aberrant cytoplasmic localization that allows an immunohistochemical detection of NPM status. However, all mutations reported are insertion/deletion of exon 12 resulting in a global insertion of 4 nucleotides. We therefore developed a NPMm detection test based on DNA PCR amplification and fragment length analysis. As previously reported, we observed a higher frequency of NPMm in AML with normal karyotype (47%, 50/106) than in CBF AML (0%, 0/7) or in AML with poor risk cytogenetic (13%, 4/32). We further evaluated the clinical profile and the prognosis of NPM mutations in a retrospective cohort of 106 patients with normal karyotype treated according to the ALFA-9000 or ALFA-9802 protocols between 1990 and 2004. In these patients aged 17–65 years, NPMm was significantly associated with a high white blood cell count (69 vs 18 G/L, p<.001) and an involvement of the monocytic lineage (FAB AML-M4/M5, p=.009). NPMm was also associated with a low rate of CEBPA mutation (CEBPAm, 10% vs 27%, p=.05) and a higher rate of FLT3 internal tandem duplication (FLT3/ITD, 38% vs 25%, p=NS). We did not find any correlation between NMPm and CR rate (86% vs 88%, p=NS). Long-term outcome did not differ between NPMm and NPM wild-type (NPMwt) patients neither in univariate analysis (6y-OS, 43% vs 37%; 6y-RFS, 47% vs 34%, p=NS) nor in multivariate analysis after adjustment on covariates significantly associated with prognosis in univariate analysis (age, CEBPAm, FLT3/ITD). NPM mutational status was available for 15 patients at relapse time. Ten patients out of 15 had NPM mutations at diagnosis and still displayed NPM mutations at relapse time. None of the 5 NPMwt patients acquired NPM mutation at relapse. Recently, we developed a RQ PCR technique to study minimal residual disease. Fifteen patients were evaluated at diagnosis and follow-up. In the majority of the cases a sensitivity of 10⁻⁴ was obtained. The high frequency, the stability, and the homogeneity of NPMm provide a promising minimal residual disease marker that we are prospectively evaluating. Prospective studies are also needed to confirm the definitive role of NPM mutation in the prognosis of AML patients.