Mutant NPM1 Maintains Leukemic State via HOX Expression in AML

Treatment of acute myeloid leukemia (AML) has remained a challenge, partly because of an insufficient understanding of the molecular mechanisms that promote and maintain the leukemic state of AML cells. Although it has long been recognized that mutations of the NPM1 gene play an important role in AML, it was not determined how the normal and the mutated forms of the NPM1 protein function.

“It is one of the biggest enigmas in AML,” said Margaret Goodell, PhD, of Baylor College of Medicine and senior author of a recent study published in Cancer Cell.¹

The most common mutations associated with AML occur within the NPM1 gene, but no mechanism of action has been uncovered to date, she noted. In AML cells, NPM1 mutations result in abnormal cytoplasmic localization of the mutant protein (NPM1c), but it has not been known whether NPM1c is required to maintain the leukemic state.

Using CRISPR technology and new strategies developed in Goodell’s lab to readily target and manipulate only the mutant form of the protein (NPM1c), leaving the rest of the cell intact and allowing investigators to unlock cell function, the research team was able to show that the loss of the mutant NPM1 gene from the cytoplasm results in immediate downregulation of homeobox (HOX) genes, followed by differentiation. HOX genes direct the formation of many body structures during early embryonic development.

They also showed that inhibition of XPO1 (exportin 1, a nuclear export protein that is overactive in AML and which mediates transport of growth-regulatory proteins, including tumor suppressors²) relocalizes NPM1c to the nucleus, promotes differentiation of AML cells, and prolongs survival of NPM1-mutated leukemic mice.

“We found that when the NPM1 protein is allowed to leave the nucleus, it activates a set of genes that drive growth of leukemia,” said study coauthor Lorenzo Brunetti, MD, PhD. “If NPM1c can be prevented from entering the cytoplasm, AML cells will differentiate or die and cease to repopulate as cancer cells.”

The researchers conclude that the “exquisite dependency” of NPM1-mutant AML cells on NPM1c, which they describe, provides a rationale for the use of nuclear export inhibitors in a large fraction of patients with AML. The research has profound implications, according to Brunetti. There is already a drug available that inhibits the export of multiple proteins, including NPM1, from the nucleus to the cytoplasm.

“This gives us reasonable evidence that we can treat this type of AML with an existing therapy,” he said.

“Demonstrating this dependency in NPM1-mutant AML cell lines of distinct genetic backgrounds, in primary AML samples, as well as in mice, definitively proves that differentiation upon loss of mutant NPM1 from the cytoplasm results in a programmed response independent of co-occurring mutations,” the researchers conclude.

The study also extends the use of a CRISPR knockin strategy that will be useful in other contexts, the researchers say. Although the molecular mechanisms remain to be elucidated, the study team demonstrated that NPM1c facilitates HOX/MEIS1 expression, and that HOX genes support the leukemic state in NPM1-mutant AML.

“The data strongly suggest that the further development of therapeutic agents that act through nuclear export inhibition in the context of AML with mutated NPM1 is warranted,” the researchers note, and further study with XPO1 inhibitors should be pursued, ideally in the context of AML patients with mutated NPM1.

References

1. Brunetti L, Gundry MC, Sorcini D, et al. Mutant NPM1 maintains the leukemic state through HOX expression. Cancer Cell. 2018;34(3):499-512. doi: 10.1016/j.ccell.2018.08.005

2. Hing ZA, Fung HYJ, Ranganathan P, et al. Next-generation XPO1 inhibitor shows improved efficacy and in vivo tolerability in hematologic malignancies. Leukemia. 2016;30(12):2364-2372. doi: 10.1038/leu.2016.136.