NPM1 mutation is a stable marker for minimal residual disease monitoring in acute myeloid leukaemia patients with increased sensitivity compared to WT1 expression.

Mutation in the NPM1 gene occurs in 60% of acute myeloid leukaemia (AML) patients with normal karyotype. NPM1 mutation is potentially a superior minimal residual disease (MRD) marker compared to WT1 gene overexpression by being specific to the malignant clone, although experimental evidence published so far includes very limited numbers of relapsed cases. Also, the stability of the NPM1 mutation has been questioned by reports of the mutation being lost at relapse. In the present study we compared NPM1 mutation and WT1 overexpression as MRD markers in 20 cases of relapsed AML. The 20 patients experienced a total of 28 morphological relapses. Karyotypic evolution was detected in 56% of relapses. All relapses were accompanied by high levels of NPM1 mutation, along with high WT1 mRNA levels, thus demonstrating complete stability of both markers during relapse. Detectable NPM1 mutation following a period of morphological remission was accompanied by a morphological relapse in all cases. In contrast, WT1 expression was detected in 33% of the NPM1 mutation negative samples. This background WT1 expression produced by non-leukaemia cells was highly variable, both between and within patients, and limited the de facto sensitivity of the WT1 expression analysis. The present study therefore provides important experimental evidence demonstrating that NPM1 mutation is superior to WT1 overexpression as marker of MRD in NPM1-mutated AML, even in the presence of extensive karyotypic evolution.