Figure 1. Simplified model of the arrangement of cells along a hierarchy in the mammary gland, with adult breast stem cells sitting at t...
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Figure
1. Simplified model of the arrangement
of cells along a hierarchy in the mammary gland, with adult breast stem cells
sitting at the apex of the hierarchy. Gene expression correlations have pointed
to possible relationships between normal breast epithelial cells and the major
subtypes of breast cancer. For example, the luminal progenitor daughter cell is
the likely target cell for aggressive basal-like cancers. The image is taken
from Visvader, Genes and Development,
2009.
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Breast cancer is the
end result of a step-wise accumulation of genetic mutations that first arise in
normal breast tissue. In many ways, breast tumours should be viewed as a caricature
of normal breast development that has gone awry. Breast cancer is now known to
be a highly heterogeneous disease, likely reflecting its origins in the
different cell types that make up normal ductal tissue in the breast. From this
perspective, tumour heterogeneity represents abortive attempts by cancer cells
to undergo normal growth and maturation. In order to tackle the question of this
heterogeneity and to design more effective ‘personalized’ therapies, it is
essential to understand the normal cell types that constitute breast ductal
tissue and to understand how specific genetic mishaps give rise to cancer.
Integral to the
development of every organ is a hierarchy of cells that produce the full
repertoire of mature cells in that organ. In breast tissue, there are three
distinct cell types that constitute the mammary ductal tree: the inner layer of
luminal cells, alveolar cells (milk-producing cells in lactation) and an outer
sleeve of myoepithelial cells. In contrast to other organs that form in utero, mammary gland
development is largely a post-natal event and hence the majority of these cells
are produced during puberty and adulthood. During puberty, ductal branching and
elongation result in a complex network of ducts that fill the entire mammary
fat pad. Massive expansion of the tissue also accompanies pregnancy, when milk-secretory
alveolar structures develop and differentiate to produce milk for lactation. The
capacity to physically isolate distinct cell subpopulations from both normal tissue
and breast cancers (a relatively new capability) has enabled researchers to tackle
the issue of cancer heterogeneity and to specifically ask which ductal cell
type is the initial target for transformation into a cancer cell. Stem cells
lie at the apex of the normal tissue hierarchy and give rise to a variety of daughter
cells, which then yield mature ductal cells (Figure 1). Stem cells represent
strong candidates for ‘cells of origin’ in breast cancer since they are
long-lived and are capable of extensive self-renewal (making perfect copies of
themselves). Therefore, these cells have greater potential to acquire the
multiple genetic mishaps that characterise cancer. Daughter cells may also
serve as ‘cells of origin’ in cancer as their highly proliferative nature would
impart potent growth properties to a rogue cancer cell.
The breast stem cell hierarchy
Over the past
decade, our team has identified and isolated breast stem cells from mouse and
human mammary tissue through flow cytometry combined with transplantation
studies. Importantly, analogous findings were made by a Canadian group. Further
fractionation studies led to the isolation of distinct types of daughter cells
(the progeny of stem cells). Striking functional and molecular similarities (based
on gene expression signatures) were found between stem cells and their progeny
in human and mouse tissue, with a number of conserved pathways unveiled. In
parallel studies, we defined master transcriptional regulators of self-renewal,
cell-fate and differentiation, that act at specific phases of breast
development. Many of these molecules are deregulated in cancer, thus lending
support to the notion that perturbation of key developmental regulators can
lead to cancer. In exciting recent work that employs novel 3D imaging
technology to visualise large portions of intact breast tissue, we have
performed lineage tracing studies to track stem and progenitor cells in situ
(Figure 2). Based on this technology, stem cells were found to be very
long-lived, suggesting that they are prime candidates for breast cancers that can
arise many decades after incurring a genetic mutation.
The
purification strategy has not only made it possible for us to view breast
cancer through the prism of stem cell biology, but has shed light on how female
hormones regulate breast stem cells and cancer risk. The mammary epithelium is
highly sensitive to the effects of the ovarian steroid hormones oestrogen and
progesterone, which are integral to puberty, oestrus cycling and pregnancy. The
receptors for these nuclear steroid hormones (ERα and PR) remain critical
prognostic markers in breast cancer. To understand potential cellular
mechanisms that contribute to the strong epidemiological link between sustained
hormone exposure and increased breast cancer risk, we turned to analysis of the
cellular hierarchy. Our studies revealed that MaSCs were exquisitely responsive
to steroid hormones, despite the lack of expression of ERα and PR on these
cells, thus invoking an indirect (paracrine) mechanism of action. RANK ligand
was identified as a paracrine regulator of signalling from progesterone
responsive ductal cells to breast stem cells. These findings highlighted the
RANK pathway as a potential target for breast cancer prevention.
Breast tumours and the ‘cells of origin’ of cancer
Breast cancer
is a very heterogeneous disease at both the histological and molecular levels.
At least six distinct subtypes have been described on the basis of gene
expression profiling, with the most important determinants of these subtypes
being the presence or absence of expression of the oestrogen receptor (ER),
progesterone receptor (PR) or the amplification/overexpression of the
HER2/ERBB2 locus (Figure 1). Despite the ability of these subtypes to predict
outcome, patient response to chemotherapy or targeted therapy remains variable.
The prevailing concept in the field has been that these different subtypes
originate in distinct breast epithelial cells that serve as the ‘cell of
origin’. A better understanding of breast tumour heterogeneity and the nature
of tumour-propagating cells requires definition of epithelial cells in normal breast tissue. This is particularly relevant
to women at high risk of developing breast cancer such as those carrying
germ-line mutations in specific tumour suppressor genes.
Through
analysis of precancerous tissue isolated from women that carry a mutation in
the critical BRCA1 tumour suppressor
gene (the ‘Angelina Jolie gene’) and who have an increased risk of developing
aggressive breast cancers, we discovered that an aberrant population of daughter
cells is highly prone to carcinogenesis. The identification of cells in which
cancer originates should help highlight key genetic vulnerabilities of the
cancer cell that could be used to develop more refined therapeutic targets to
tailor therapy to distinct tumour subtypes. A number of genetic pathways have
now been identified for exploring new treatment strategies aimed at switching
off breast tumour growth.
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Figure 2. 3D confocal wholemount images of mammary
glands before puberty (upper panel) and in the adult (lower panel). The top
panel shows the primitive ductal tree evident before puberty (inset shows
elongated cells in the outer layer), while the lower panel depicts clones of
cells (along the ductal tree) that have likely arisen from single stem cells
that expanded in vivo for 8 weeks.
Note the yellow and red domains that depict clonal areas. Images are taken from
Rios et al Nature 2014.
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