7- Cancer Stem Cells

• miscroscopic examination of solid/ liquid tumors > saw different cell types (looked heterogeneous) • transplantation experiments > 10^3-10^7 cells needed to efficiently transplant tumor into new host > suggests not all cells can regenerate/ create a tumor • teratocarcinoma analysis (benign/ derived from ESCs) > composed of both highly tumorigenic/ well-differentiated (non-tumorigenic) cells • H-thymidine DNA labelling experiments in leukemia patients > most tumor cells = post-mitotic/ continuously replenished by a small fraction (5%) that cycle rapidly

• tumors have hierarchical cell organization

self-renewing stem cell > transit-amplifying/ progenitor cells > post-mitotic/ differentiated cells

• generate a large # of cells from only 1 division • do not divide constantly, only periodically (quiescent) • do not represent the majority of proliferative cells in the tumor

• inability to eradicate CSCs • since therapies target dividing cells/ only divide periodically

Stochastic Model - tumors are biologically homogeneous/ functional heterogeneity caused by random influences at a given moment that affect the behaviour of individual cells within the tumor > intrinsic/ extrinsic Hierarchy Model - tumors are like normal tissue where cell hierarchies are maintained by stem cells > heterogeneity caused by CSCs (biologically distinct/ self-renewing)

TIC = tumor-initiating cell

Intrinsic- degree of cell signalling activation/ levels of TFs Extrinsic- host factors > microenvironment/ immune response

• ability to self-renew • able to recapitulate tumor heterogeneity through differentiation

• both predict that only a fraction of cells in tumor can initiate tumor growth/ neither predicts frequency of CSCs in a tumor

Stochastic Model - all cells in tumor are biologically similar in oncogenic capacity - any cell can become a TIC, given the right influences > therapy should be designed to target all tumor cells Hierarchy Model - CSCs are biologically distinct from majority of cells in tumor - CSCs responsible for generating other tumor cells > therapy should be designed to target CSCs

AML (acute myeloid leukemia)

• ability to purify subpopulations of tumors based on properties (surface antigen expression) • functional transplantation assay to test ability of cell populations to generate tumors in vivo

• accumulated knowledge on hematopoiesis • well-characterized hematopoietic cell surface antigens • availability of xenotransplantation assays > SCID mice • cell-sorting methods > FACS

FACS = Fluorescent-activated cell sorting - fluorescent antibodies label cell surface antigens - separate populations based on fluorescent intensity

• CSC cell surface markers (different)

CD44 high/ CD24 low

• stem cell compartment (of normal tissue) undergoes transforming mutation (already has stem cell characteristics) • progenitor compartment (of normal tissue) > transforming mutation must be strong enough to induce stem cell features

• no > why patients relapse • CSCs evade standard therapies (chemo/ radiation) • breast CSC population ↑/ higher mammosphere forming efficiency after treatment

• no > CSC heterogeneity is a problem for treatment • tumor evolution > ↑ heterogeneity (already to begin with) > different responses to therapy • CSCs can be generated de novo > EMT (tumor cell plasticity)

EMT (epithelial-mesenchymal transition) - new CSCs can be generated (promotes stem-like properties)

• ↑ plasticity enables bidirectional interconvertibility between CSCs/ non-CSCs (transit-amplifying/ progenitor cells) • CSCs = moving target

• quiescent (divide periodically, not constantly) • have activated anti-apoptotic programs • express drug resistance transporters (drug pumps)

• CSCs do not respond to conventional therapies • Hard to define CSC “unique” signalling pathways since they share many with normal stem cells • Moving target (replenished due to tumor cell plasticity/ EMT)