This week we profile a recent publication in PNAS from the laboratory of Dr. Wei Wei (pictured) at the Institute for Systems Biology.
Can you provide a brief overview of your lab’s current research focus?
My lab is focusing on cross-disciplinary areas of single-cell analysis, cancer epigenetic dysregulation, and systems biomedicine. Specifically, 1) Treatment-induced adaptive cell state transition, epigenetic plasticity & dysregulation, and signaling network dynamics associated with the development of non-genetic adaptive resistance in cancer, with a particular focus on how external perturbations (drug, TME, etc.) interact and modulate epigenetic states of tumor cells. 2) Development and implementation of multi-omics single-cell tools and systems-level computational approaches for interrogating the heterogeneous tumor cells and immune cells across multiple bimolecular layers; 3) Liquid biopsy-based rare tumor cell analysis for predictive cancer diagnostics and immunotherapy.
What is the significance of the findings in this publication?
Our study addressed a long-term puzzle in the CTC field, which is why CTCs are much less frequently detected in NSCLC compared to other major cancer types, such as breast, prostate, and colorectal cancer, despite the aggressive and highly metastatic nature of NSCLC. It’s because that conventional CTC detection methods, as exemplified by the FDA-cleared CellSearch system, normally rely on the use of a family of proteins called cytokeratins (CKs) that are typically found in epithelial tissues. As roughly 90 percent of human cancers arise in epithelial tissues and express CKs, these methods work very well in many major cancer types. But this is not the case for NSCLC. Our study found that while most NSCLC patients are bearing CK-positive primary tumors, their CTCs can be CK-negative. CK-negative CTCs are actually a prevalent subtype in 50% of NSCLC patients analyzed. One-third of patients only had CK-negative CTCs circulating in their blood. These CTCs are not accessible to and normally overlooked by conventional epithelial marker-based CTC detection methods.
We addressed this challenge by exploiting a common feature of a wide range of cancer cells – elevated glucose consumption driven by the high level of Hexokinase-2 (HK2). The use of HK2 as a biomarker allowed us to develop metabolic activity-based methods to cover a greater spectrum for CTCs, including both CK-positive and the novel CK-negative CTC subtypes, which significantly increased the CTC detection sensitivity for NSCLC.
In the meantime, we found that CK-negative CTCs have consistent genomic profiles with their CK-positive counterparts. This suggested that the CK-negative CTCs are not a distinct genetic clone in the primary tumor, but something derived from the CK-positive cells. In other words, the CK-positive tumor cells tend to transition to CK-negative phenotype when they detach from the primary sites and shed into the bloodstream in these NSCLC patients. Such a transition is regulated beyond the genetic level, possibly through epigenetic/transcriptomic reprogramming.
Why they need such a transition? Sequencing analysis revealed that these CK-negative CTCs are enriched with metastasis and drug-resistance molecular signatures. So, it’s possible that CTCs in the blood tend to transition to a CK-negative subtype to acquire a more metastatic signature, and, consequently, seed distant organs. However, we found that this CK-positive to CK-negative transition is independent of the well-known epithelial-to-mesenchymal transition (EMT) process, challenging the long-standing association between CK expression and EMT.
Consistently, patients with prevalent CK-negative CTCs in blood were found to have poorer therapy response, shorter progression-free survival, and a higher chance for metastasis. We also found that the CK-negative CTCs have a clear genotype association. Patients with the EGFRL858R mutation – a common driver mutation of NSCLC – are more likely to have CK-negative CTCs circulating in their blood compared to patients bearing EGFR19Del mutation. This finding actually partially explains a long-standing clinical observation, namely the suboptimal therapeutic efficacy of first-line EGFR inhibitors in EGFRL858R -mutant tumors.
What are the next steps for this research?
From the cancer biology perspective, it would be fascinating to interrogate the underlying molecular mechanism of phenotypic transition between CK-positive and CK-negative CTCs, and the association between CK-negative CTCs and EGFRL858R genotype. Such a study could help reveal the regulators and associated pathways that could be exploited therapeutically to arrest this unfavorable transition. Interrogating the vulnerabilities of CK-negative CTCs could also inform us of the potential drug targets.
From the clinical standpoint, it would be interesting to see how general the CK-negative CTCs are. Our preliminary data show that they are not specific to NSCLC but exist in other tumor types as well. So, it would be fantastic to know the molecular features and clinical implications of CK-negative CTCs in other tumor types. This would eventually allow us to use CK-negative CTCs as a clinical diagnostic biomarker to foretell the patient therapy response/prognosis before the onset of therapy.
This work was funded by:
This work was supported by Andy Hill CARE Fund and Washington Research Foundation Technology Development Grant, as well as other funding to our collaborators.
