5.4.1 Transposable Element Expression in Breast Cancer Cell Lines
The use of MCF10A cell line and its derivations of cancer cell lines as a model for breast cancer progression has been extensively studied [17-20]. However, little is known as to its concordance of transposable element expression during MCF10A transformation and derived cancer lines with clinical samples of breast cancer progression. Therefore, it is imperative to determine the TE expression profile of the specific cell lines we use as models of oncogenic phenotypes during TE dysregulation and TE activity inhibition.
5.4.2 Activity of Transposable Elements
The use of pharmaceuticals within a dose range known to induce TE expression or inhibit TE activity to measure metastatic potential is necessary. However, it is imperative these studies be done in conjunction with studies that can measure activity of transposable elements. One such approach is the reverse transcriptase activity assay. Briefly, purified bacteriophage MS2 RNA are incubated with cell lysate aliquots. Mixtures are exposed to PCR amplification using MS2-specific primer pairs to determine if MS2 cDNA sequences were synthesized. The presence of amplified MS2 cDNA is a measure of reverse transcriptase activity. The assay is roughly based on the protocol established by Voisset et al., 2001 [21]. Versions of the assay have previously described endogenous reverse transcriptase activity in preimplantation embryos and cancer [22, 23].
5.4.3 Patient-Derived Xenograft Models
Patient-derived tumor xenograft (PDX) mouse models offer a unique translational prospect for breast cancer progression research. These models seem to be quite similar to
their donor, retaining important tumor histomorphology, imaging and gene expression characteristics of the donor [24-26]. Furthermore, they have been used to study predictive clinical outcomes with regards to drug efficacy, biomarker analysis, and patient outcomes, reviewed here [27], with high concordance between patients and their PDX models. However, there is a lack of published studies with PDX models recapitulating breast cancer progression from non-obligate in situ carcinomas to metastasis. Not only would these studies provide valuable insight into why only 1/3 of patients progress into invasive breast cancer they may also indicate biomarkers or factors for invasiveness. Furthermore, currently there are no studies revealing the transposable element or non-coding RNA expression profile for breast cancer PDX models.
5.5 Final Thoughts
The idea that TEs were fundamental and continue to contribute to the evolution of regulating gene networks started with the pioneering work of Dr. Barbara McClintock. The selfish replicative nature of TEs has predisposed them to the co-option of host gene regulation. However, this co-option of TEs is widely seen as a double-edged sword. As described in Chapter 3, there is a large body of evidence for the diversity of domesticated TE mechanisms employed by whole organism, tissue-specific, and cellular systems. Contrarily, strong evidence linking aberrant TE activity to disease states, such as, cancer, ageing, neurological disorders, and autoimmunity is increasing. The double-edge nature of parasitic elements becoming integral components of many host functions presses for more granular experimental evidence of TE activity at the individual element level. We are in the gene-targeted therapy revolution. Current technologies, such as CRISPR-Cas systems, are becoming increasingly more accurate at genetic and RNA manipulation. As
technologies advance toward functionally testing non-coding and repeat regions of the genome, it will provide greater, much needed, insight into the roles of TEs in disease states.
5.6 References
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