How Nucleophosmin Mutation Causes Acute Myeloid Leukemia

Nucleophosmin (NPM1) is the most frequently mutated gene in de novo acute myeloid leukemia (AML). But just how this mutation causes leukemia has been unknown, until now.

In a study recently published in the Journal of Clinical Investigation, authors revealed how NPM1 mutation disrupts the master circuit that makes granulocytes and monocytes, thereby stalling myeloid precursor cells at inherently proliferative points in their maturation course.

“We are most excited because we show how we can turn this information into nontoxic treatment that reverses the mechanism of leukemogenesis. By understanding how myeloid differentiation is blocked, we can unblock it,” says the study’s team lead Yogen Saunthararajah, MD, of Cleveland Clinic Cancer Center. “Our research was done in test tubes and mice, but the drug molecules we used to treat the mice are available for use in clinical trials, and we hope to move forward with such trials soon.”

What happens in myeloid cells with mutant NPM1

Using proteomic techniques including mass spectrometry, researchers identified the molecular machinery within myeloid cells in which NPM1 participates. They found:

• NPM1 is a co-factor for PU.1 — the master transcription factor commander of monocyte lineage fates — and when NPM1 is mutated, it drags PU.1 into cytoplasm with it.1 is notable because it commands other transcription factors and hundreds of genes to determine cell fate. This “master transcription factor” drives the production of monocytes and contributes to the production of granulocytes. The dislocation of PU.1 from the nucleus to the cytoplasm causes it to malfunction.
• Without PU.1 in the nucleus, its partner master transcription factors CEBPA and RUNX1 are unable to activate granulocyte lineage programs. Large amounts of the master transcription factors CEBPA and RUNX1 are also present in AML cells (in the nucleus). These proteins collaborate with PU.1 to drive granulomonocytic differentiation. However, without PU.1, they repress (turn off) instead of activate (turn on) hundreds of granulocyte program genes.

“In brief, we discovered that mutant NPM1 disrupts the PU.1/CEBPA/RUNX1 master circuit to repress instead of activate granulomonocyte lineage programs,” says Dr. Saunthararajah. “Maturation is the usual cue to stop replicating. Because these cells don’t mature, they continue to replicate, causing AML.”

Reversing leukemogenic actions

Can these leukemogenic actions of mutant NPM1 somehow be reversed? That was the next step for the research team. Using in vitro and in vivo models, they discovered that:

• Mutant NPM1 causes PU.1 to be dislocated to the cytoplasm, but moving mutant NPM1 and PU.1 back to the nucleus activates the genes that trigger terminal monocyte differentiation.
• Selinexor, which inhibits nuclear export, effectively locks mutant NPM1 and PU.1 in the nucleus, activating terminal monocyte differentiation. Mice treated with low, non-cytotoxic doses of selinexor that were readily administered for several months, had significantly lower bone marrow and especially spleen (extra-medullary) AML burden than mice not receiving selinexor.
• Decitabine, which depletes the corepressor (cofactor that represses rather than activates genes) DNA methyltransferase 1 from the interactomes of CEBPA and RUNX1 that remained in the nucleus, activated terminal granulocyte differentiation.
• The concentrations or doses of selinexor and decitabine used did not terminate the growth of normal bone marrow cells, and normal blood counts were not decreased by several months of these treatments.

“When used together, the clinical small molecules selinexor and decitabine extended survival of mice with leukemia by more than 160 days, compared to mice that didn’t receive the drugs,” says Dr. Saunthararajah.

Precision medicine could bring new hope

These findings open the door to noncytotoxic differentiation-restoring treatments for patients with NPM1-mutated AML, says Dr. Saunthararajah.

NPM1 mutation is present in approximately 30 percent of AML cases. With current antimetabolite/cytotoxic treatments, only about 50 percent of these patients have long-term survival, he notes.

“There have been no targeted therapies for NPM1-mutated AML because, until now, we didn’t fully understand how mutant NPM1 was leukemogenic,” says Dr. Saunthararajah. “The results of our study can bring new hope of targeted or precision non-cytotoxic treatments for the many patients with chemorefractory, NPM1-mutated AML.”