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Page 12 of 20 Ottewell et al. J Cancer Metastasis Treat 2021;7:11 https://dx.doi.org/10.20517/2394-4722.2021.14

for experimental purposes, further adding time and expense to these experiments. Therefore, although
human-to-human bone metastasis models provide useful, clinically relevant tools for investigating tumour
cell-bone cell interactions and the metastatic process in a human-specific environment, their use is unlikely
to supersede the cheaper, more commonly used intra-cardiac injection models of metastases to mouse bone.

Immune competent models
Transgenic models
Modelling breast cancer bone metastasis in immunocompetent models enables the researcher to assess how
the immune system interacts with various stages of the metastatic process as well as interactions with anti-
cancer agents. Although there are multiple transgenic models in which genetic manipulation of genes
including P53, TGFα, Myc, Wnt1, β-Catenin, NOTCH4, Cox2 ErbB-1 (neu) and SV40 results in
spontaneously develop mammary cancers, from which metastasis to lung is commonplace, spontaneous
metastasis to bone appears to be an extremely rare event and is almost never reported in the literature
[55]
(reviewed in ). A possible exception to this is a transgenic mouse deficient in p53 and E-Cadherin in the
F/F
F/F
[56]
mammary gland . This Wcre;Cdk1 ;Trp53 mouse shows pleomorphic similarities to the invasive lobular
carcinoma seen in humans. Multifocal tumours spontaneously develop in ~74% of mice in several
mammary glands, and, although the majority of mammary cells from these mice are ER-negative,
occasional weak expression of ER was identified in low-grade tumours with ER status being inversely
correlated to tumour stage. In this model, female mice whose primary tumours reached ~1 cm in diameter
presented with extensive local invasion and metastasis to draining lymph nodes. Metastases were also
detected in the skin, lungs, liver, gastrointestinal tract, pancreas and spleen. Interestingly, the authors also
F/F
F/F
reported bone metastasis from several Wcre;Cdk1 ;Trp53 mice; however, no indications of metastatic
frequency or duration to detection of tumour cells in this site were reported, making it difficult to establish
the usefulness of this model for studying bone metastasis per se .
[56]
Syngeneic models
Because bone metastases are rare from transgenic models, researchers have developed syngeneic models in
which mouse mammary cancer cells are injected into recipient mice of a complementary background to
those in which the cells were originally generated. These syngeneic models, however, do not permit use of
human breast cancer cells or PDX. As the aetiologies of human and mouse mammary cancer metastases
differ fundamentally, in that human breast cancer primarily metastasises to bone, whereas mouse mammary
cancer preferentially metastasises to lung and almost never to bone, data from these models should be
interpreted accordingly. These low rates of spontaneous bone metastasis may, in part, be due to biological
differences between humans and mice: mouse cells have enhanced metabolic activity and longer telomerases
[57]
compared to human cells, influencing oncogenesis and phenotypic differences . Mammary tumours also
develop from different cell lineages with mouse tumours originating from mesenchymal tissue and human
[57]
tumours arising primarily from epithelial cells . In addition, mouse mammary tumours are hormone
independent, whereas the majority of human tumours that metastasise to bone are hormone responsive
requiring higher concentrations of oestrogen to support their growth .
[58]
To address the issue of low rates of bone metastasis, researchers have produced bone trophic sublines of
mouse mammary cancer cells via repeated in vivo passaging through bone. Orthotopic injection of some of
these cell lines results in spontaneous metastasis to the mouse skeleton to varying degrees (20% from 4T1
cells, 40%-60% from 4T1-2, 50% from KEP and 60%-80% from E0771) [6,33,59,60] . Primary tumours grow rapidly
in these models and often need to be surgically removed to allow time for metastatic outgrowth to be
detected in bone. Because all of these syngeneic cell lines also spontaneously metastases to lung and the lung
metastases develop more rapidly than the bone metastases, bone metastatic deposits are often very small
and difficult to detect on histological sections, complicating downstream analysis.