Biomedical Engineering Reference
In-Depth Information
an international meeting (Clarke et al., 2006). In analogy to normal stem cells, a
cancer stem cell should be able to self-renew and generate the more differentiated
cells present in a tumor. The practical translation of this consensus definition to
date is the capacity of a cell population to initiate a phenocopy of the original
malignancy upon injection into immuno-compromised mice (Clarke et al., 2006).
Importantly, this newly established tumor should contain a functional cancer
stem cell compartment. This is believed to address the issue of self-renewal and
can be experimentally confirmed by sequential propagation of the malignancy
into a secondary recipient. For example, in AML, purification of the CD34
+
/
CD38
fraction of cells results in enrichment for a population with the capacity
to induce an AML, including a rare CD34
+
/CD38
fraction, in SCID mice
(Bonnet and Dick, 1997; Lapidot et al., 1994). The more differentiated CD34
or CD38
+
cells do not have this leukemia-initiating capacity. This suggests that
the AML stem cells reside in the immature CD34
+
/CD38
fraction of cells.
For solid malignancies purification of specific populations of cells is pre-
ceded by enzymatic digestion of the tumor. Enzymes such as collagenase and
hyaluronidase are used to breakdown the extracellular matrix that surrounds
the tumor cells. This procedure results in a single cell suspension that allows for
selection of cells based on cell surface protein expression. Cells are purified by
binding to antibodies and subsequent fluorescent-activated cell sorting (FACS)
or magnetic bead separation.
4.2 Colon-Cancer Stem Cell Markers
Colorectal cancers are adenocarcinomas that form more or less differentiated
glandular structures surrounded by variable amounts of stroma. The epithelial
cells surrounding the crypts display clear heterogeneity in cell morphology and
marker expression. This is somewhat similar to the situation in normal colonic
epithelium where different cellular characteristics are associated with a variety
of differentiated cell types. Moreover the heterogeneity present in normal
colonic epithelium that results from a differential grade of differentiation can
also be demonstrated in malignant transformed colon tissue. For example
nuclear-localized b-catenin that is normally only detected at the crypt base
and is thus associated with the stem and progenitor compartment is detected
in a limited number of CRC cells (Fodde and Brabletz, 2007). In addition the
colon precursor cell-associated protein Musashi-1 is present in a low number of
cells in CRC tissue (Todaro et al., 2007b). So the question is whether these
different populations of cells differ in their capacity to fuel tumor growth and
their ability to transplant the malignancy to immune-deficient mice.
In the first reports describing isolation of a CRC cell population with the
capacity to exclusively induce a phenocopy of the original primary human
malignancy in mice, the surface marker CD133 was used for purification
(O'Brien et al., 2007; Ricci-Vitiani et al., 2007; Todaro et al., 2007a; Fig 1).
