Abstract
High expression of cancer stem cell (CSC) marker CD133 has been used as a predictor for prognosis in colorectal cancer (CRC), suggesting that enumeration of CSCs, using CD133, is predictive for disease progression. However, we showed recently that both CD133 mRNA and protein are not downregulated during differentiation of colon CSCs, pointing to an alternative reason for the prognostic value of CD133. We therefore set out to delineate the relation between CD133 expression and prognosis. A CRC patient series was studied for expression of CD133 and other CSC markers by microarray and quantitative PCR analysis. In addition, several common mutations were analyzed to determine the relation with CD133 expression. CD133 mRNA expression predicted relapse-free survival in our patient series, whereas several other CSC markers could not. Moreover, no correlation was found between expression of other CSC markers and CD133. Interestingly, high CD133 expression was related to mutations in K-Ras and B-Raf, and inhibition of mutant K-Ras or downstream mitogen-activated protein kinase kinase (MEK) signaling decreases CD133 expression. In addition, an activated K-Ras gene expression signature could predict CD133 expression in our patient set as well as data sets of other tumor types. CD133 expression is upregulated in CRC tumors that have a hyperactivated Ras-Raf-MEK-ERK pathway and is therefore related to mutations in K-Ras or B-Raf. As mutations in either gene have been related to poor prognosis, we conclude that CD133 expression is not indicative for CSC numbers but rather related to the mutation or activity status of the Ras-Raf pathway.
Highlights
To develop a better strategy for defining which patient with colorectal cancer (CRC) should receive adjuvant chemotherapy after surgery, reliable markers for predicting relapse have been studied extensively
High CD133 expression was related to mutations in K-Ras and B-Raf, and inhibition of mutant K-Ras or downstream mitogen-activated protein kinase kinase (MEK) signaling decreases CD133 expression
CD133 expression is upregulated in CRC tumors that have a hyperactivated Ras– Raf–MEK–ERK pathway and is related to mutations in K-Ras or B-Raf
Summary
To develop a better strategy for defining which patient with colorectal cancer (CRC) should receive adjuvant chemotherapy after surgery, reliable markers for predicting relapse have been studied extensively (reviewed in ref. 1). Mutations in signaling pathways are used as markers for predicting prognosis, and expression levels of proteins that are suggested to identify the cancer stem cell (CSC) fraction of the tumor [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. CSCs, the driving force behind tumor initiation, growth, and metastasis, are hypothesized to be crucial for patient prognosis. It was confirmed that N-glycosylation of CD133 affects the cell surface recognition of AC133 [28] This suggests that the prognostic value of CD133 in CRC is not due to an enumeration of CSCs
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