NPM1 alternative transcripts are upregulated in acute myeloid and lymphoblastic leukemia and their expression level affects patient outcome.

Background: Expression of the NPM1 gene, encoding nucleophosmin, is upregulated in cancers. Although more than ten NPM1 transcripts are known, the reports were usually limited to one predominant transcript. In leukemia, the NPM1 expression has not been widely studied so far. In acute myeloid leukemia (AML), the mutational status of the gene seems to play a pivotal role in carcinogenesis. Therefore, the aim of the study was to quantify alternative NPM1 transcripts in two types of acute leukemia, AML and ALL (acute lymphoblastic leukemia).

Methods: Using droplet digital PCR, we analyzed the levels of three protein-coding NPM1 transcripts in 66 samples collected from AML and ALL patients and 16 control samples. Using RNA-seq, we detected 8 additional NPM1 transcripts, including non-coding splice variants with retained introns. For data analysis, Welch two sample t-test, Pearson's correlation and Kaplan-Meier analysis were applied.

Results: The levels of the particular NPM1 transcripts were significantly different but highly correlated with each other in both leukemia and control samples. Transcript NPM1.1, encoding the longest protein (294 aa), had the highest level of accumulation and was one of the most abundant transcripts in the cell. Comparing to NPM1.1, the levels of the NPM1.2 and NPM1.3 transcripts, encoding a 265-aa and 259-aa proteins, were 30 and 3 times lower, respectively. All three NPM1 transcripts were proportionally upregulated in both types of leukemia compared to control samples. In AML, the levels of NPM1 transcripts decreased in complete remission and increased again with relapse of the disease. Low levels of NPM1.1 and NPM1.3 were associated with better prognosis. The contribution of non-coding transcripts to the total level of NPM1 gene seemed to be marginal, except for one short 5-end transcript accumulated at high levels in AML and control cells. Aberrant proportions of particular NPM1 splice variants could be linked to abnormal expression of genes encoding alternative splicing factors.

Conclusions: The levels of the studied NPM1 transcripts were different but highly correlated with each other. Their upregulation in AML and ALL, decrease after therapy and association with patient outcome suggests the involvement of elevated NPM1 expression in the acute leukemia pathogenesis.