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Page 2 of 7 Mizejewski. J Cancer Metastasis Treat 2018;4:27 I http://dx.doi.org/10.20517/2394-4722.2018.20
[1,2]
disease . As such, brain metastases are one of the leading causes of morbidity and mortality among cancer
[3]
patients with metastatic disease in the United States . Indeed, the vast majority of cancer deaths can be
attributed to tumor cell metastasis to distant sites rather than demise from the primary tumor mass itself.
Several past reports have demonstrated that the presence of early circulating tumor cells (CTCs) provide
a “feeder source” of break-away cells from the tumor mass cells that intravasate into the bloodstream to
[4-6]
circulate, aggregate, then eventually migrate to metastatic sites of chemo-attracted target organs . It may
take extended time periods for the circulating cancer cells to develop aggregates detectable by radio-imaging
[7]
as metastatic cell masses . The early stages of disseminated tumor cells are able to aggregate to form micro-
metastatic cell islets which further progress into macro-metastatic cell clusters. By the time the metastatic
cells have migrated and “nested” into target tissues such as bone marrow and brain, the cells are already
[8,9]
proliferating at exponential rates . This advanced growth state of the tumor cell greatly reduces therapeutic
options for the cancer patient. Therefore, it becomes crucial to detect and identify CTCs that represent very
early stages of cells which disseminate from the primary tumor mass.

A recent report has, in fact, described use of a cutting-edge mRNA technology to characterize such CTCs.
Hence, the first objective of the present commentary was to discuss the development of a biomarker mRNA
signature that could screen and identify CTCs utilizing human breast cancer as the model. A second
objective was to propose use of a potential therapeutic tool which could be directed against CTCs. These
novel agents are referred to as “protein-derived peptide fragments” as discussed below.

BACKGROUND STUDIES
It may be deduced from the above discussion that early screening and identification of CTCs associated with
metastasis could be beneficial for evaluation of treatment options and their responses to brain metastasis.
[10]
In a recent study using breast cancer (BC) cells, Boral et al. reasoned that mRNA characterization of
CTCs from BC derived cells could provide a means for early diagnosis of brain metastases; this knowledge
could aid in planning therapeutic strategies and determining the effectiveness of targeting metastatic cells.
Boral’s studies confirmed that patients with BC produce CTCs that express high levels of biomarkers that are
associated with regulation of cell growth, proliferation, and signal transduction pathways directly involved
with metastasis. Using a comprehensive analysis of CTC total mRNA transcriptomes, these investigators
derived a unique “transcriptome signature” (TS) that distinguished circulating BC cells from those of the
primary tumor mass. Even more intriguing, the derived TS revealed distinct signaling pathways inherent
to BC-derived circulating cells that could provide the means of metastases to the brain. The Boral’s report
concluded that the CTC biomarker profile and knowledge of their signaling pathways could be valuable as
screening tools for: (1) micro- and macro-metastatic cell characterization; (2) decision-making in treatment
modalities; and (3) monitoring post-treatment responses in patients with metastatic disease.

Previous literature reports have emerged which enumerated epithelial cell adhesion molecules (EpCAM)
present on CTCs, thus providing an estimate of the overall metastatic cell burden present in BC patients [11-13] .
[10]
Using previous EpCAM-related studies as a starting point, Boral et al. devised a computer-based genomic
and mRNA workflow strategy which revealed a higher cell surface frequency and expression of metastatic-
[10]
associated biomarkers in BC patient’s cells versus cells from healthy blood donors . Finally, these
investigators utilized parametric flow cytometry and MRI-proven metastasis brain scans to analyze their BC
patient’s cell populations.

COMPONENTS OF THE CTC SIGNATURE
The CTC signature derived from circulating BC cells was parsed down to 126 genes involved in metastatic
[10]
cell activities and signaling cascades . Overall results from the 126 genes demonstrated that mRNA from
73 genes in the BC-CTC group were up-regulated while 53 genes were down-regulated. All of the CTC genes