T h e n e w e ng l a n d j o u r na l o f m e dic i n e

Cl inic a l I m pl ic a t ions of B a sic R e se a rch
Elizabeth G. Phimister, Ph.D., Editor

Timing of Metastasis in Breast Cancer
Robert S. Schwartz, M.D., and John K. Erban, M.D.
In 2015, almost 232,000 women in the UnitedStates received a diagnosis of invasive breast
cancer, and approximately 40,000 women diedof metastatic breast cancer. The leading cause of
these deaths was metastatic spread. The timing
and distribution of breast-cancer metastases varyconsiderably. In approximately 5% of womenwith breast cancer, metastases are clinically evident at the time of diagnosis. In other women,
metastases become apparent years or even decades
after the initial diagnosis. Moreover, the numberof metastases varies considerably.
The mechanisms that account for the wide
variability in the propensity of breast cancer tometastasize are unknown, although this much isclear: metastatic spread from a primary breast
tumor can occur at an early, presymptomaticstage, and disseminated cells often settle in thebone marrow where they can lie dormant foryears before becoming clinically evident. In two
recent studies, Hosseini et al.1 and Harper et al.2
provide insights into the metastatic process ofbreast-cancer cells that successfully establish
themselves in the bone marrow (Fig. 1).
Mechanisms that influence the formation ofmetastases in breast cancer include the expression of the cell-surface protein human epidermal
growth factor receptor 2 (HER2), a transmembrane tyrosine kinase receptor. HER2 augments
the metastatic potential of breast-cancer cells;cells that express HER2 are more likely than
HER2-negative cells to propagate metastases.3
There is a long-held view that the increase inmalignant behavior during the course of a neoplasm is related to the stepwise acquisition bycancer cells of variants that predispose to theacquisition of additional variants that confer
malignant and metastatic behavior.4 This “cascading” model may pertain to some tumors, but it
has been challenged in the case of breast cancer.5
The model predicts that a subset of mutations in
the primary tumor will also be found in meta2486n engl j med 376;25

static cells. Metastatic cells, however, can acquire mutations independent of mutations in theprimary tumor — a phenomenon known as parallel progression. One may therefore ask whether
samples from the primary tumor, blood, or metastases should be used for molecular analysis toguide therapy.
The reports by Hosseini et al. and Harper et al.
together make a strong case for increased scrutiny of cells of the early lesion and of the establishment and proliferation of these cells at thesite of metastasis. Hosseini et al. performed experiments using mice that were engineered to express high levels of the rat orthologue of HER2
in mammary epithelial cells. These mice areprone to breast cancer and are therefore used asa model of HER2-positive breast cancer. The authors observed that 80% of the mice had bonemarrow metastases arising from cells that had
disseminated from an early neoplastic lesion inbreast epithelium soon after induction of HER2
expression. Moreover, Wnt4 and receptor activator of nuclear factor-κβ ligand (RANKL) signals
secreted by early tumor cells that express bothHER2 and the progesterone receptor were foundto promote the dissemination of progesteronereceptor–negative cells from the lesion. Wnt4
supports cell proliferation, survival, and differentiation as well as carcinogenesis; RANKL supports the survival of metastatic cells. Hosseini
et al. found that, although cells in early lesions
are prone to metastasize, cells in late tumors aremuch less likely to disseminate from the tumor
and are more prone to remain in situ and proliferate; these cells robustly express HER2 and lackexpression of progesterone receptor. The authors
also found that suppression of the progesterone
receptor in late tumors is mediated by the highexpression of HER2.
In the article by Hosseini et al., an analysis ofdisseminated cancer cells that were isolated
from women who had received a diagnosis ofnejm.org

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Clinical Implications of Basic ResearchAB
Epithelialcells

MILK DUCT
Early lesion withloosely packed cells

DNASomatic geneticchanges specificto cells of theearly lesion

PGR
HER2

RANKL
Wnt4

HER2DNA
Established tumor
with densely
packed cells

PGR signaling increases
migrating potentialof PGR-negative,
HER2-expressing cells

HER2DisseminationDNA
DISTANT SITE

Accrual of additional
genetic changes results
in proliferation ofdisseminated cells toform a metastasis

DNA
DISTANT SITE

TimeTimeDNA

DNA

Figure 1. A Model of Parallel Progression after Early Dissemination.
Recent experiments with a mouse model of human epidermal growth factor receptor 2 (HER2)–positive breast cancer1,2
have uncovered events critical to the dissemination of cancer cells from the early lesion (Panel A) and cells of theestablished tumor (Panel B). The cells in the early lesion are loosely arranged; some of them express the progesterone
receptor (PGR) in addition to HER2. These PGR-positive cells secrete soluble factors, such as receptor activator ofnuclear factor-κβ ligand (RANKL) and Wnt4, that induce the migration of PGR-negative cells from the lesion. Thecells in the established tumor (Panel B) are packed more tightly, typically do not express the PGR, and are less likely
to metastasize. The cells that disseminate from established tumors to the bone marrow carry many of the genetic
variants, such as loss of chromosome 8p, that are found in established tumors, whereas those that disseminate fromearly lesions do not and instead evolve in parallel with the primary tumor and other disseminated cells.

n engl j med 376;25

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Clinical Implications of Basic Researchbreast cancer (both women with and women
without metastases) showed that disseminatedcells isolated from women without metastases
did not harbor chromosomal changes that were
found in the primary tumor, such as loss ofchromosome 8p. In contrast, an analysis of thedisseminated cancer cells isolated from womenwith metastases showed that they had a genetic
makeup similar to that of the primary tumor.
This finding supports the hypothesis that cancerous cells that are disseminated from the earlylesion follow an independent, or parallel, pathof genetic evolution with respect to cells in theprimary tumor.
Using a mouse model similar to the one usedby Hosseini et al. (in which HER2 is expressedin the mammary ducts), Harper et al. found that
HER2-positive cells examined early in the evolution of breast cancer in mice activate a programof Wnt-dependent dissemination. Both Harperet al. and Hosseini et al. concluded that progesterone-dependent signaling pathways may activate
HER2, which activates processes (such as theweakening of cell adhesion) that favor metastasis. Hosseini et al. also found that the density ofcells (the number of cells per unit volume, which
was high in established tumors vs. low in earlylesions) influences the expression of progesterone
receptors and the propensity to metastasize.
What in the microenvironment of the bonemarrow determines whether the metastatic cellremains dormant or becomes activated? Myeloid
cells in the vascular niches of the marrow express
RANK, the receptor of RANKL. Opportunity certainly exists for interactions between metastasized breast-cancer cells and the cells native tothe bone marrow. And what (if anything) in thebone marrow microenvironment influences theacquisition of mutations in cancer cells independently of mutations that accrue in the primarytumor? Mesenchymal cells also reside within thevascular niches of the bone marrow and regulate
stem-cell renewal and proliferation, usually bymeans of secreted cytokines. These mesenchymal
cells may regulate the dormancy and activationof metastatic cells, although there is much to belearned about the mechanisms that govern thesestates. Stromal cells in the bone marrow can also
activate or suppress immune cells, including B
cells, T cells, and macrophages — cell types thathave an important role in the defense against
cancer. Perhaps an improved understanding of thestromal-cell–immune-cell axis will yield therapeutic targets in the same way that the work by
Hosseini et al. and Harper et al. has yielded insights into the process of metastasis.1,2,4
Disclosure forms provided by the authors are available withthe full text of this article at NEJM.org.
From Tufts University, Boston.
1. Hosseini H, Obradović MM, Hoffmann M, et al. Early dis-semination seeds metastasis in breast cancer. Nature 2016 December 14 (Epub ahead of print).
2. Harper KL, Sosa MS, Entenberg D, et al. Mechanism of early
dissemination and metastasis in Her2+ mammary cancer. Nature
2016 December 14 (Epub ahead of print).
3. Slamon DJ, Clark GM, Wong SG, Levin WJ, Ullrich A, McGuire
WL. Human breast cancer: correlation of relapse and survivalwith amplification of the HER-2/neu oncogene. Science 1987;​235:​177-82.
4. Markowitz SD, Bertagnolli MM. Molecular basis of colorectal cancer. N Engl J Med 2009;​361:​2449-60.
5. Klein CA. Parallel progression of primary tumours and metastases. Nat Rev Cancer 2009;​9:​302-12.
DOI: 10.1056/NEJMcibr1701388
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