Cancers are potent inducers of abnormal blood clotting, which usually heralds a more aggressive course of the disease and poor prognosis1. Venous thromboembolism (VTE) occurring in the peripheral circulation is known not only to lead to troublesome symptoms in cancer patients, but may also result in a potentially lethal lung condition known as pulmonary embolism (PE)2. In addition, activated clotting factors and platelets are increasingly regarded not only as biological mediators of blood clotting, but also as a part of the pro-inflammatory milieu that promotes cancer growth and metastasis3. For these reasons the nature and mechanisms of cancer associated thrombosis (CAT) remain a long-standing research priority with hopes to identify central regulators and biomarkers of impending vascular deterioration4.
Grappling with mechanisms of CAT has been an arduous and tumultuous affair. While various research groups have been zooming in on the components of the coagulation system in blood, or on the properties of the tissue microenvironments in cancer patients, others cast their eyes on cancer cells themselves and their molecular properties that could potentially provoke clotting. Over the past two decades this effort concentrated on cancer-associated clotting proteins, such as tissue factor (TF), plasminogen activator inhibitor 1 (PAI-1), podoplanin (PDPN), factor VII and many others. Other studies implicated the role of extracellular chromatin, microparticles or other carriers of procoagulant activity. The exchange of molecular information between cancer cells, their stromal neighbors and the hemostatic system resulting in activation of clotting processes has also been investigated to some extent2. As oncogenic mechanisms of cancer progression became more understood and large datasets more accessible it also became clear that cancer-specific mutations may orchestrate the pro-coagulant phenotype of cancer cells. For example, in a highly pro-thrombotic type of brain tumours known as glioblastoma multiforme, different subgroups of tumours (proneural, neural, classical and mesenchymal) are known to be driven by specific oncogenic mutations, and this pattern seems to translate into distinctive, subgroup-specific profiles in expression of coagulation related genes 5.
With the advent of single cell sequencing (SCS) this emerging order has been disturbed once again. Unlike traditional analysis of messenger RNA profiles in bulk samples of tumour tissue, SCS enables cataloguing thousands of genes in individual cancer (and normal) cells6. This approach revealed with a glaring clarity the long-known property of all cancers, namely their vast, but orderly diversity. Once again, glioblastoma may serve as case in point. In this case, on the basis of bulk analysis of mRNA each tumour could be assigned to a specific aforementioned molecular subgroup with a silent assumption that cells within each tumour exhibit a more or less similar gene expression profile. This assumption was demolished by the results of SCS, which suggested that each tumour is highly heterogeneous and composed of cellular populations resembling all other subgroups but mixed in different stable proportions and held in multicellular equilibrium by presently unknown factors, possibly networks of cell-cell interactions. For instance, mesenchymal glioblastomas were found to contain the majority of mesenchymal-like cells intermingled with cells with proneural, classical or intermediate properties.
What are the consequences of a such cancer cell mosaic for the mechanisms of CAT? In the recent article in Seminars of Thrombosis and Hemostasis we argued that cancer heterogeneity may generate not one but multiple clotting mechanisms co-existing within a given tumour. Indeed, the analysis of SCS datasets from glioblastoma tumours suggested that while certain types of these tumours exhibit a generally low expression of coagulation inducing genes, such as TF or PDPN, within these tumours there are rare cells that are high expressors7. This observation may have several implications for the future analysis, diagnosis and mitigation of CAT. For example, while present prevention and treatment of CAT involves agents acting on the common coagulation pathway, such as vitamin K antagonists (VKAs), low molecular weight heparin (LMWH) or direct oral anticoagulants (DOACs), a more targeted therapy could be envisaged on the basis of the coagulant profile of a given tumour. For example, in cancers with a prominent involvement of platelets it may be possible to include antiplatelet agents as a part of care. However, if tumours are heterogeneous it would be important to understand what are the constituent mechanisms of CAT that are expressed by cellular subpopulations to ensure their effective inhibition. It should also be known what coagulant profile predominates among cells and what is the threshold for various cell populations (e.g. TF-expressing or PDPN-expressing) to influence the coagulation system responses. While a single biomarker of CAT may reflect one aspect of the tumour mosaic, a panel of such markers reflective of the disease and cellular heterogeneity may be needed to make prediction as to the risk and complex nature of VTE.
There is still much to learn about CAT in different cancer contexts and the work is ongoing. Perhaps one of the most fascinating aspects in this area of research, though, is the increasing interdisciplinary ‘cross-pollination’ between hematology, genetics and cancer biology, collectively leading to new insights and paving the way to better outcomes.
- Timp, J. F., Braekkan, S. K., Versteeg, H. H. & Cannegieter, S. C. Epidemiology of cancer-associated venous thrombosis. Blood 122, 1712-1723, (2013).
- Hisada, Y. & Mackman, N. Cancer-associated pathways and biomarkers of venous thrombosis. Blood 130, 1499-1506, (2017).
- Versteeg, H. H. et al. Inhibition of tissue factor signaling suppresses tumor growth. Blood 111, 190-199 (2008).
- Falanga, A. & Marchetti, M. Hemostatic biomarkers in cancer progression. Thrombosis research 164 Suppl 1, S54-s61, (2018).
- Magnus, N., Gerges, N., Jabado, N. & Rak, J. Coagulation-related gene expression profile in glioblastoma is defined by molecular disease subtype. Journal of thrombosis and haemostasis : JTH 11, 1197-1200, (2013).
- Patel, A. P. et al. Single-cell RNA-seq highlights intratumoral heterogeneity in primary glioblastoma. Science 20;344, 1396-1401 (2014).
- Tawil, N., Bassawon, R. & Rak, J. Oncogenes and Clotting Factors: The Emerging Role of Tumor Cell Genome and Epigenome in Cancer-Associated Thrombosis. Seminars in thrombosis and hemostasis 45, 373-384, (2019).