A possible route to depleting leukemic stem cells while preserving healthy hematopoietic stem cells has opened up, with the discovery that cyclin-dependent kinase regulatory subunit 1 (CKS1) inhibition suppresses cancer stem cells and at the same time protects healthy stem cells from the toxic effects of chemotherapy.

Using a small-molecule CKS1 inhibitor, researchers at the Francis Crick Institute, London, demonstrated that depending on the type of stem cell, CKS1-dependent protein degradation has two very different outcomes. On one hand, in patient-derived acute myeloid leukemia (AML) blasts, inhibition depleted leukemic stem cells. But inhibition of CKS1 in normal hematopoietic stem cells improved their functionality and was chemoprotective.

Uncovering this vulnerability in the leukemic stem cells that drive the development of resistance and relapse suggests inhibiting CKS1 could improve treatment outcomes in AML patients, the researchers say.

"We were surprised to find divergent responses between healthy and malignant cells," said William Grey, visiting scientist at the Hematopoietic Stem Cell Laboratory at the Crick and co-author of a paper in Science Translational Medicine, June 22, 2022, describing the findings.

"We initially hypothesized that using this specific inhibitor against only one of the functions of CKS1 could also have a negative effect on healthy stem cell systems, hence testing this in our work. But, once we delved deeper into the underlying mechanism, the dependency of cancer stem cells on the specific function targeted explained why only cancer stem cells would be sensitive," Grey told BioWorld Science.

Leukemic stem cells are the source of relapse in AML and new approaches to targeting these cells are critical to improve the average 2-year survival of only 5-15% in older patients. Picking apart the biological differences between normal and cancer stem cells is seen as central to finding ways to selectively reduce leukemic stem cells while sparing healthy counterparts.

Overexpression of CKS1B, the protein encoded by the CKS1 gene, is known to be associated with poor prognosis in a variety of solid tumors, but to date the evidence in relation to AML is ambivalent, with available data showing expression varies significantly between both normal and malignant stem cells, and within different subtypes of hematopoietic cells.

The researchers hypothesized that in the case of CKS1B expression in AML cancer stem cells, there could be a selective susceptibility to CKS1-dependent protein degradation. Knockdown of CKS1B in AML cells resulted in a dose- and time-dependent reduction in their viability, and separating patients on the basis of CKS1B expression showed there was significantly increased drug sensitivity in high-CKSB1-expressing tumors.

In addition, a multivariate analysis of patient phenotypes found that only CKSB1 expression was correlated with CKS1 inhibition in vitro.

To investigate the effect of CKS1 on AML in vivo, the researchers engrafted five patient samples with a range of CKS1 expression levels into immunodeficient mice. A single course of CKS1 inhibitor significantly reduced the leukemia burden in mice engrafted with high-CKS1B-expressing AML. There was no immediate effect of inhibition on patient samples with the lowest CKS1B expression.

However, all of the xenograft mice showed a delay in AML bone marrow colonization over time, regardless of tumor reduction immediately after CKS1 inhibitor treatment.

There also was improved overall survival compared with untreated controls, indicating CKS1 inhibition has additional effects beyond acute reduction of leukemic burden in high-CKS1B-expressing AML mice.

Current chemotherapy treatment for AML does not target the quiescent leukemic stem cells that are the source of relapse. Given the effect on bone marrow colonization and improvement in overall survival, even in mice with low-CKS1B-expressing tumors, the researchers looked to see if CKS1 inhibition was having a specific effect on AML stem cells.

Across a range of experiments it was shown that these cells have a high concentration of CKS1, regardless of the overall level of expression in the tumor as a whole, and that CKS1 inhibition directly targets these quiescent cells.

"The reduction of leukemic stem cells by CKS1 inhibition indicates a clear route to combatting AML in all patients, independent of bulk CKS1B expression," the researchers say.

While AML leukemic stem cells and AML cell lines undergo apoptosis in response to CKS1 inhibition, healthy stem cells do not, leading researchers to posit that CKS1 inhibition could provide a chemoprotective effect during chemotherapy, shielding healthy hematopoietic cells from toxicity.

In engrafted mice, CKS1 inhibition prevented chemotherapy-induced cell death in normal stem cells, and there subsequently was an increase in their number. In other words, inhibiting CKS1 has the opposite effect in healthy stem cells and AML cancer stem cells.

Grey said the finding also could read across to other types of cancer. "We actually looked at healthy stem cells in the intestine and it has a similar effect there. Combine this with reports of CKS1 [being] overexpressed and correlating with poor prognosis in other cancer types and we think there is potential to use this approach across cancers," he said.

There are no currently approved inhibitors that specifically target only CKS1.

As to the biological cause underpinning the divergent responses to CKS1 inhibition of healthy and leukemic stem cells, the point of separation is the generation of reactive oxygen species.

"We demonstrated that inhibition of CKS1 actually puts healthy cells in a state of quiescence," Grey said. "This leads to lower metabolic activity and less production of reactive oxygen species. Whereas in cancer stem cells, there is such a reliance on CKS1-dependent protein degradation being chronically active that it tips the balance towards uncontrolled metabolism, leading to accumulation of reactive oxygen species and ultimately death."

The Crick Institute has an explicit brief to carry out projects that aim to promote translation of academic research. Grey said there will now be an effort to accelerate the finding on CKS1 inhibition to clinical trials, while continuing to use the insights to design more effective therapies for AML.