Our results demonstrate that overexpression of PODXL in CRC is associated with unfavourable clinicopathological characteristics and an independent factor of poor prognosis. This is to our knowledge the first report on the prognostic value of PODXL expression in human CRC and the results are based on a large number of patients (n=536) from a prospective cohort study.
In our study, 13% of the tumours were denoted as having high expression of PODXL. In a previous study on breast cancer, using a somewhat different cutoff, high expression of PODXL was found in 6% of the tumours (Somasiri et al, 2004
). These data, together with the results from the antibody-based screening in the Human Protein Atlas, demonstrate that, in most cancer types, while upregulated compared with the corresponding normal tissue, PODXL is overexpressed in a relatively small proportion of cases.
In the vast majority of tumours with high PODXL expression, defined as a distinct cytoplasmic/membranous staining pattern, this was predominantly observed in a subset of scattered and infiltrating cells at the invasive tumour front. These findings are substantiated by previous studies demonstrating that overexpression of PODXL enhances migratory and invasive properties of tumour cells (Casey et al, 2006
; Nielsen et al, 2007
). Notably, the only patient in this study with a tumour displaying PODXL expression in >50% of the cells had generalised disease and died within a month of diagnosis, further supporting that PODXL expression is associated with an invasive and metastatic cellular phenotype in CRC in vivo
. Moreover, the upregulation of PODXL expression observed at the infiltrating edge of the tumour corresponds well to the morphological term ‘tumour budding', which has been demonstrated to be of prognostic importance (Hase et al, 1993
; Prall, 2007
), and biologically closely related to EMT (Prall, 2007
). Studies have demonstrated that cancer cells at the invasive front of primary tumours can convert to a motile mesenchymal phenotype (Yang and Weinberg, 2008
) and these cells are considered to be the cells with metastatic potential. Along this line, a high expression of PODXL has been recorded in TGFβ
-induced EMT (Meng et al, 2011
) indicating that PODXL has an integral role in cancer progression.
Moreover, studies have illustrated a link between EMT and the gain of stem cell-like properties such as mobility, anti-apoptosis circuitry and low level of differentiation, which are traits of great value for a malignant tumour (Mani et al, 2008
). The PODXL has previously been suggested as a marker for embryonal carcinoma stem cells and haematopoietic stem cells (Schopperle et al, 2003
) and found to be upregulated in tumour stem cells in glioblastoma (He et al, 2010
In the light of these results, the development of therapies targeting PODXL has been proposed as a new treatment strategy (Chen et al, 2004
; Somasiri et al, 2004
). Our results, however, indicate that colorectal tumours with high expression of PODXL also respond well to standard chemotherapy. Adjuvant chemotherapy reduces the risk of relapse but it also has side effects and can be difficult to tolerate, especially for older patients. In CRC Stage III, the recurrence-free survival increases with 10–20% by adding adjuvant chemotherapy and in stage II the incremental gain is only around 5%. Thus, only a minority of patients receiving adjuvant chemotherapy have any benefit from it. Therefore, it would be of great clinical value to find predictive tools for a better selection of patients to such treatment.
The present study indicates that PODXL might be used as a predictive marker for adjuvant treatment of CRC. In stage III, patients with high tumour PODXL expression were the ones who benefited from adjuvant chemotherapy, whereas patients having tumours with low PODXL expression did not seem to have any effect from the treatment. Whether the same is true in stage II could not be determined in our study, as the number of stage II patients that received adjuvant chemotherapy was too low for statistical analyses.
Even though the positive impact of chemotherapy in the tumours with high PODXL expression remained significant in multivariate analysis after correction for other prognostic factors, our findings must be interpreted with caution as this was an observational cohort study in which a potential selection bias compared with the general population must be taken into consideration (Berglund et al, 1993
). The distribution of clinical stages at diagnosis was, however, in line with the expected.
It is notable that only ~half of the patients with stage III tumours in our study received adjuvant treatment, which is on the low side compared with modern therapeutic guidelines. The main reason for this is most likely that this study started already in the mid 90s, during which time adjuvant chemotherapy was not yet standard of care in Sweden.
In this study, CRC-specific survival and OS were used as endpoints as this information had been recorded prospectively and therefore was available for all patients. In future studies, the impact of PODXL expression on recurrence free survival should also be assessed, preferably in cohorts where this information has been recorded prospectively. It will also be of value to correlate PODXL expression to other relevant molecular characteristics of CRC, for example, microsatellite instability status, KRAS, BRAF and PIK3C mutations.
In summary, the results from this large cohort study suggest that PODXL expression is independently associated with a poor prognosis in CRC. Our findings also suggest that adjuvant chemotherapy should be given to patients with tumours expressing high levels of PODXL, whereas patients with low PODXL-expressing tumours could be spared such treatment. Future prospective studies are needed to confirm this association as well as to elucidate the exact mechanisms by which PODXL affects tumour behaviour.