It is widely believed that metastasis is a late event in cancer progression. This view is based on several clinical and experimental observations. First, most cancer patients die from metastasis and not the primary disease. Second, early surgery is often the only cure. Third, somatic genetic changes accumulate during local progression (Fearon and Vogelstein, 1990) which was extrapolated to systemic progression. Fourth, repeated rounds of in vivo selection led to cell lines with increased metastasis formation (Kang et. al 2003; Minn et. al 2005). However, in a recent study titled Systemic Spread is an Early Step in Breast Cancer, published in Cell in 2008, Hüsemann, Geigl, Schubert et. al report that tumor cells can disseminate systemically from the earliest epithelial alterations in HER-2 and PyMT transgenic mice and from ductal carcinoma in situ in women.
The mouse model they work with is the BALB/c mice transgenic for the activated rat HER-2/neu gene (BALB-Neu T mice). This model mimics progression and gene expression profiles of human breast cancer. Females hemizygous for the rat HER-2 gene under control of the MMTV (mouse mammary tumor virus) promoter develop invasive mammary cancer while their HER-2 negative siblings (wild-type BALB/c mice) remain tumor free. Typically, in the BALB-Neu T model, the mammary epithelia starts to express the oncogene at about weeks 3 to 4 (which coincides with the onset of puberty). Hyperplasia can be detected microscopically at weeks 7-9. Progress to in situ carcinomas occurs between weeks 14-18 and at weeks 23-30, invasive cancers become apparent (see Fig. 1a)
Fig. 1a; Left Panel: Whole mount of the mammary gland at week 9 showing absence of tumor in branching ductal tree; Middle Panel: Histological Section at week 9 showing side buds displaying the morphology of atypical ductal hyperplasia; Right Panel: Histology of invasive cancer at week 30 (Adapted from Hüsemann, Geigl, Schubert et. al (2008))
The principle goal was to determine when cells expressing the HER-2 transgene disseminate. The authors chose the lung and bone marrow as the sites to look out for metastasis since the HER-2 receptor is not expressed in either of these two organs. In addition to HER, they also used anti-cytokeratin (CK) antibodies to check whether disseminated cells were epithelial in origin. Surprisingly in BALB-Neu T mice, CK+ and HER-2+ cells became detectable 4-9 weeks when meticulous analysis could only detect atypical ductal hyperplasia (ADH) (see Fig. 1b)
Fig. 1b; Left Panel: Increase in tumor area at primary site. Triangles indicate mean value, whiskers indicate 95% confidence interval and solid line indicates best fitted curve; Right Panel: Number of CK+ cells (red dots) and HER+ cells (blue dots) per 5000 bone marrow cells. Triangles, whiskers and solid line indicate same. Note the presence of disseminated cells in the bone marrow even when the primary tumor area is zero.(Adapted from Hüsemann, Geigl, Schubert et. al (2008))
Further, lung micrometastasis from mammary tissue was confirmed by demonstration of mammary-specific alpha casein and lactalbumin transcripts (see Fig. 1c)
Fig. 1c; Left Panel: Lung micrometastasis at week 27, detected using anti-HER-2 antibody;Right Panel: Eight HER-2 + and two samples of normal lung tissues, analyzed for mammary gland specific transcripts. "+" indicates normal mammary gland and "-" indicates mock control (Adapted from Hüsemann, Geigl, Schubert et. al (2008))
Despite exponential growth at the primary tumor site, the number of CK+ cells and HER-2+cells in the bone marrow rose marginally over the course of time. Cells singly positive for HER-2 and CK were not congruent. Not all tumor cells expressed both markers (see Fig. 1d)
Fig. 1d; Left Panel: CK+, HER-2- cell; Middle Panel: CK-, HER-2+ cell; Right Panel: CK+, HER-2+ cell (Adapted from Hüsemann, Geigl, Schubert et. al (2008))
This suggests, either the existence of heterogeneous tumor cell populations that disseminate to distant sites or different cellular states of the disseminated tumor cells (DTCs)
More Evidence for Early Dissemination
Could the HER-2+, CK+ cells have disseminated from extra-mammary tissues expressing the transgene? The authors took mammary gland fragments from 3-12 week old transgenic mice (displaying only atypical ductal hyperplasia) and transplanted the them into 3 week old wild-type siblings. Wild type bone marrow was screened at different time points (see Fig. 2)
Fig. 2; Left Panel: Week 14 post-transplantation, CK+ cells in bone marrow of wild-type siblings; Right Panel: HER-2+ cells in the bone marrow (Adapted from Hüsemann, Geigl, Schubert et. al (2008))
Although CK+ and HER-2+ cells were observed in the bone marrow, the levels were lower than transgenic BALB-Neu T mice, but were definitely above rare false positives. Additionally, just as in the transgenic mice, there was no significant increase in disseminated cells from ADH stages to invasive cancer, in the wild-type siblings.
The malignant nature of these DTCs was established by Comparative Genomics Hybridization (see Fig. 3)
Fig. 3; Top Panel: Karyogram (left) and CGH profile (right) of sorted mouse metaphase of a single HER-2+ cell disseminated from a transplanted mammary gland and subsequently isolated from bone marrow of a recipient wild-type mouse; Bottom Panel: Sorting of mouse metaphase of a single leukocyte in a balanced CGH profile; Green and red bars indicate genomic gains and losses respectively. (Adapted from Hüsemann, Geigl, Schubert et. al (2008))
Evidence that early disseminated cells can grow into metastases
The onset of metastasis relative to primary tumor growth was assessed . Histological sections of the lungs were analyzed and micrometastases could be detected from weeks 20 to 21 onward, a time point at which mostly in-situ carcinomas are present at the primary sites (see Fig. 4)
Fig. 4; Progression of lung metastasis in BALB-Neu T mice, with and without the removal of the primary tumor. Left Panel: Increase in tumor area over time in BALB-Neu T mice (same as Fig. 1b); Right Panel: Size of the largest lung metastasis detected in individual mice. Blue squares indicate average size of metastases from non-operated mice at various time points and red triangles indicate average size from operated animals at 10-15 weeks after surgery. Whiskers indicate 95% confidence intervals (Adapted from Hüsemann, Geigl, Schubert et. al (2008))
Since metastases need time to grow, their increase in size paralleling that of the primary lesion supports the conclusion that, at least in some cases, founder cells of metastasis had disseminated earlier and had started to proliferate.
Some other experiments conducted by the group, demonstrate that bone marrow disseminated cells that do not grow into metastases can be released from growth arrest. Also, there does not appear to be an association between the number of disseminated cells and the stage of the tumor. Large tumors do not necessarily seed more.
Overall the paper presents some compelling evidence for the occurrence of dissemination, even before the primary tumor is established. This would indeed force the field to rethink it's current strategies in combating cancer.
Hüsemann, Y., Geigl, J., Schubert, F., Musiani, P., Meyer, M., Burghart, E., Forni, G., Eils, R., Fehm, T., & Riethmüller, G. (2008). Systemic Spread Is an Early Step in Breast Cancer Cancer Cell, 13 (1), 58-68 DOI: 10.1016/j.ccr.2007.12.003