Colon cancer stem cells.

Colorectal cancer (CRC) is the third most common form of cancer and the second cause of cancer-related death in the Western world, leading to 655,000 deaths worldwide per year (Jemal et al. in CA Cancer J Clin 56:106-130, 2006). Despite the emergence of new targeted agents and the use of various therapeutic combinations, none of the treatment options available is curative in patients with advanced cancer. A growing body of evidence is increasingly supporting the idea that human cancers can be considered as a stem cell disease. According to the cancer stem cell model, malignancies originate from a small fraction of cancer cells that show self-renewal and pluripotency and are capable of initiating and sustaining tumor growth (Boman and Wicha in J Clin Oncol 26:2795-2799, 2008). The cancer-initiating cells or "cancer stem cells" were first identified in hematologic malignancies and most recently in several solid tumors, including CRC. The hypothesis of stem cell-driven tumorigenesis in colon cancer raises questions as to whether current treatments are able to efficiently target the tumorigenic cell population that is responsible for tumor growth and maintenance. This review will focus on the different aspects of stem cell biology in the context of CRC, which might help to understand the mechanisms that give rise to tumor development and resistance to therapy. First, we will briefly revise the knowledge available on normal intestinal stem cells and recent advances in understanding crypt biology, which have led to new theory on the origins of colon adenomas and cancers. Then, we will summarize the evidence and current status on colon cancer stem cells, focusing on their relevance and promises for the treatment of colorectal carcinoma.