Dr Tim Bracey
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Introduction
“The journey is more important than the destination“; a phrase we all know well, but in cancer research the journey has been somewhat neglected in favour of studying the first part of a cancer cell’s journey and and the metastatic site destination. Every pathologist knows that cancer cells look different inside vessels. That is part of the reason why jobbing histopathologists are able to recognise lymphovascular invasion (LVI) , often even with their lowest power microscope objective.
The striking appearance of cancer cells inside blood vessels and lymphatics is one of those features pathologists might take for granted or underestimate as merely a prognostic factor to complete on cancer minimum datasets, but frequent morphological changes are rarely if ever insignificant when studying disease biology, as they are of course a direct manifestation of the underlying genetic and epigenetic changes.
Despite the fundamental nature of metastasis in the clinical progression and subsequent death of patients with cancer, the morphology of intravascular carcinoma has been rarely studied, even by histopathologists (those who see it most). Although there have been many studies involving histopathologists correlating LVI with prognosis, histopathological studies on intravascular carcinoma are rare in the literature and almost exclusively limited to our own group1-4.
Even in disseminated cancer, most carcinoma still resides outside of blood vessels, and not surprisingly this makes intravascular carcinoma difficult to study, despite being undoubtedly critical to the metastatic process.
Background
At the University Hospital Plymouth, being one of the largest UK specialist regional upper GI cancer centres, we have access to almost 100 cancer resection specimens per year of oesophagus and stomach cancer surgical resections in our OGSDMT database. Unfortunately (for the patient) the majority of these postoperative cases contain lymph node metastases; of those, a high proportion also contain intravascular carcinoma, in and around the primary tumour and the lymph node deposits. Given this, we thought gastric carcinomas would be an ideal group of tumours to continue to study the intravascular changes in cohesion, differentiation and adhesion molecule expression we had previously reported in other cancer types1-2,4.
EMT and MET
The first part of the metastatic process has been studied extensively and particularly in gastrointestinal cancers there has been considerable recent interest in the process of epithelial mesenchymal transition (EMT). Most GI pathologists will be familiar with tumour cell budding at the invasive front of colorectal adenocarcinoma (see figure 2) which is a manifestation of EMT. It can be so prominent that the budding cells are difficult to distinguish from mesenchymal cells such as fibroblasts.
EMT is a process hijacked by cancer cells from a developmental program allowing epithelial cells, which normally remain anchored to each other and the basement membrane, to escape their neighbours and acquire more motile, infiltrative characteristics. One of these characteristics is undoubtedly the ability to infiltrate (intravasate) blood vessels and lymphatic spaces.
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Gastric diffuse type (poorly cohesive) adenocarcinomas are the archetypal cancer showing EMT, in that its cells (often showing signet ring morphology) lose epithelial connections early in their journey from the mucosal primary site (see figure 3). One of the cancers most feared by histopathologists; it subtly infiltrates through the tissues, signet cells can be almost impossible to see in frozen sections, and surgeons can be shocked to find their margins positive when they seemed widely clear at the time of resection.
We noticed LVI in poorly cohesive, diffuse-type (including signet ring) adenocarcinoma was particularly eye-catching, as the intravascular tumour was remarkably more cohesive and better differentiated, compared with the stromal invasive cancer cells, being so dyscohesive and infiltrative as it invades. Unlike budding colorectal cancer which shows EMT only at its invasive front, diffuse type gastric cancers often show EMT in their entire invasive component (see figure 3,4).
Results and Discussion
We showed the intravascular component of our cohort of diffuse type adenocarcinomas had higher expression of cadherin cell adhesion molecules in the majority of cases, and became significantly better differentiated and more cohesive within the blood vessels compared with tumour outside the vessels3. We hypothesised that the reconstituted epithelioid intravascular morphology, along with increased epithelial marker expression represents a reversal of EMT within vessels, and therefore represents an intravascular “mesenchymal epithelial transition” (MET; see figure 5).
Given that many more tumour cells are in the intravascular compartment in lymph nodes and distant metastases compared with LVI at primary sites, this may (at least in part) explain the findings of other investigators who have shown that cadherins are overexpressed at the metastatic site compared with the primary site in gastric adenocarcinoma. The re-expression of cadherin at the metastatic site implies that the mechanism must be epigenetic rather than mutational. We think it is likely therefore that the phenotypic plasticity resulting from epigenetic changes in these intravascular morphological changes we have observed are of fundamental importance, not just in this peculiar subtype of gastric cancer, but perhaps in most types of cancer showing a propensity for vascular invasion. We did not correlate these changes with prognosis in this series, but since we specifically selected cases with florid LVI, and the phenomenon was seen most frequently in advanced cancers with lymph node metastasis, it seems likely to be a mechanism associated with metastatic dissemination.
Although we believe that reversal of EMT in intravascular carcinoma is an important paradigm, we do not claim that this concept explains all the morphological characteristics of intravascular carcinoma. Intravascular carcinoma may acquire other phenotypic characteristics specific to its intravascular location. An example of this is the reversal of glandular polarity, described in the intravascular components of invasive ductal breast carcinoma2, a change that brings previously luminal antigens to the external surface of intravascular tumour cell clumps . The fact that this intravascular reversal of glandular polarity can be seen in other diverse types of cancer (even mesothelioma4) suggests it may be another mechanism cancer cells use to potentiate metastatic dissemination.
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Potential theories for reversed intravascular glandular polarity include facilitating attachment of previously hidden luminal antigens to endothelium, and / or preventing programmed cell death caused by detachment of epithelial cells from local mesenchyme. Regardless of the underlying biological cause of these changes, they are so significant in our studies that we suggest that they must have profound biological importance in the process of local and distant metastatic dissemination. We would therefore like to bring other pathologists and researchers’ attention to these findings to encourage further research and correlate intravascular carcinoma phenotypic changes (I like to call it the “Cowley-Smith phenomenon”1) with prognosis. In addition, given that lymphovascular invasion is such a well-known prognostic factor for many cancer types, recognition and quantification of the intravascular Cowley-Smith phenomenon both by pathologists, and even potentially by trained AI algorithms, may increase the efficiency of LVI detection. Furthermore, a greater understanding of the phenotypic and epigenetic changes associated with this fundamental phenomenon may lead to targeted treatments to control and manage metastatic spread in different cancer types.
We hope that interested readers will be drawn to this fascinating, important, and much neglected area of cancer cell biology research and will be stimulated to join us studying this underappreciated journey.
Selected references
- Cowley, GP, Smith, MEF. Modulation of E-cadherin expression and morphological phenotype in the intravascular component of adenocarcinomas. Int J Cancer. 1995; 60:325-329.
- Adams SA, Smith MEF, Cowley GP, et al. Reversal of glandular polarity in the lymphovascular compartment of breast cancer. Journal of Clinical Pathology 2004; 57:1114-1117.
- Adams SA, Sherwood AJ, Smith MEF. Malignant mesothelioma: PAS–diastase positivity and inversion of polarity in intravascular tumour. Histopathology 2002; 41:1–3.
The full list of references and articles can be found at Dr Tim S Bracey’s researchgate webpage.