Page 366 - Read Online

Anstine et al. J Cancer Metastasis Treat 2019;5:50 I http://dx.doi.org/10.20517/2394-4722.2019.24 Page 11 of 16

each tumor expressed profiles resembling a spectrum of epithelial differentiation statuses . It now can be
[80]
presumed that cells residing anywhere along the continuous differentiation trajectory may acquire tumor-
initiating mutations [Figure 2] and that the location of the cell within the differentiation hierarchy will
determine its transcriptional status and influence whether acquired mutations will be oncogenic. This is
most consistent with the inability to identify a single lineage that contributes to HER2/Neu-induced tumors
because amplification or overexpression of the ERBB2 gene may occur in an array of cell states rather than a
single cell of origin for all tumors [81,82] . Similar findings have been reported in leukemia, in which progenitor
cells rely on the activation of the β-catenin pathway for oncogenic transformation . Although it is noted
[83]
that some populations may be more susceptible to mutagenic events than others [84,85] , the possibility that
phenotypically similar tumors arise from cells residing in the exact same cellular state is slim. This notion
is supported by the transcriptional heterogeneity observed across breast cancer patient tumors within the
same subtype . Several genetic mutations have been associated with the stratification of breast cancer
[86]
subtypes. These include mutations in GATA3, RUNX1, or NCOR1 which are associated with the Luminal
A subtype; whereas mutations in the tumor suppressor retinoblastoma, RB1, or CDH1 have been associated
with luminal B tumors [10,87,88] . Although these mutations are associated with the acquisition of a specific
breast cancer subtype, it is likely the combination of the preexisting epigenetic and transcriptional status of
the cell along with the transforming mutations that explain tumor heterogeneity and give rise to the variable
therapeutic responses observed between patients.

Accumulating evidence suggests that a rare population of stem-like progenitor cells are responsible for tumor
initiation, progression, metastasis, and therapeutic resistance . These cells are referred to as cancer stem
[89]
cells (CSCs) or “tumor initiating cells” and share many characteristics with normal mammary epithelial stem
cells including the ability to self-renew and differentiate. Additionally, CSCs share gene expression profiles
that closely resemble those of primitive mammary epithelial cells, including the expression of stem and EMT
associated genes . This has led to many to posit that CSCs arise from mammary stem/progenitor cells,
[90]
however definitive evidence for the origin of CSCs in breast cancer is lacking . The extensive heterogeneity
[91]
within the mammary epithelium that has been revealed by scRNA-seq makes it reasonable to presume that
many cellular states have the potential to give rise to CSCs. Furthermore, the unique transcriptional and
epigenetic context of an epithelial cell prior to transformation likely determines the aggressive nature of
the arising tumor, including its ability to metastasize. Indeed, many studies have identified transcriptional
regulators of normal mammary development that also encourage aggressive behaviors during cancer. This is
the case for transcriptional drivers of stem cell and EMT programs including SOX9 , SLUG , and SOX10 .
[92]
[58]
[92]
In a study by Guo et al. , expression of both SOX9 and SLUG was found to be necessary for maintenance of
[92]
the MaSC phenotype. Moreover, coexpression of SOX9 and SLUG in breast cancer cell lines was sufficient to
induce stem cell-like properties, including activation of an EMT program and metastasis-seeding abilities.
Similar results were observed for SOX10 . Binding of SOX10 within regions of open chromatin that flank
[58]
genes associated with EMT or with embryonic neural crest cells (NCC) was found to promote migratory and
invasive behavior within mammary tumor cells. Thus, it is possible that epithelial cells expressing high levels
of transcriptional drivers of EMT and stem-like states at the time of transformation may be predisposed
to malignant phenotypes. Likewise, the chromatin accessibility of genes associated with processes such as
EMT and NCC may ‘prime’ cells for aggressive behaviors associated with CSC phenotypes.

The formation of CSCs and their ability to seed new tumors at distant sites is also highly dependent on the
surrounding microenvironment [93-95] . Notably, contextual stimuli such as exposure to TGFβ, can initiate
stem cell and EMT programs in normal mammary epithelial cells that then promote their transition to a
CSC state . The resulting CSCs remain highly susceptible to surrounding cues. This is especially relevant
[94]
in the context of metastasis, in which Paget’s “seed and soil” hypothesis proposes that a tumor-permissive
environment is required for disseminating tumor cells to engraft and metastasize into new tissue sites [93,96] .
As breast cancer arises from the normal epithelium, the contextual cues regulating normal epithelial