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Galletti et al. Using CTCs in prostate cancer

patients with mCRPC (19 of whom had previously Detection of TMPRSS2-ERG fusion and ERG
received or were being treated with AR signaling expression in CTCs
inhibitors), they found that detectable expression ERG is a transcription factor, which is over-expressed
of ARv7 was significantly higher in patients with in approximately half of primary PCs, following the
radiographic progression compared to patients with gene rearrangement that fuses the AR-regulated
only PSA progression or PSA response to AR signaling TMPRSS2 promoter with the coding region of ERG
inhibitors (71% vs. 5%, P = 0.007). gene . ERG rearrangements and over-expression
[72]
define the largest of the molecular classes of PC
To address the role of ARv7 as marker of response based on molecular oncogenic drivers and have
to AR targeting drugs and taxanes in CRPC, been repeatedly associated with accelerated disease
Scher et al. [69] adopted a protein-based CTC assay progression and worse prognosis [73,74] . More recently,
in which the isolated cells were fluorescently stained many reports have correlated response to treatment
for the presence of the variant by using the Epic to ERG expression, which became a compelling
Sciences™ HD-CTC platform. The authors observed biomarker to investigate clinically.
that the frequency of ARv7-positive CTC detection
increased with subsequent therapies ranging from The use of CTCs for ERG assessment has been
3% prior to first-line treatment to 31% prior to third extensively explored in both gene-targeted approaches
or subsequent lines. This protein-based approach (e.g. FISH, RT-PCR) and broad-spectrum techniques
confirmed that patients with ARv7-positive CTCs (aCGH) [56,75] . To date, several studies have focused on
were more resistant to AR signaling inhibitors, with the role of CTC-ERG as predictive marker to treatment
no significant difference for biochemical response response in PC, with conflicting data correlating ERG
and PFS after taxane treatment, thus supporting the expression and outcome after AR signaling inhibitors.
results of the other CTC-based correlative studies. Attard et al. investigated ERG fusion by FISH in
[76]
Even though probably less sensitive and specific CellSearch -isolated CTCs from therapy-naïve PC
®
than RT-PCR-based methods, this protein-based patients and found a significant correlation between
methodology provides information of the AR variants’ ERG rearrangement and PSA decline after abiraterone
status at a single CTC resolution and could potentially therapy. Subsequently, ERG rearrangement status, in
offer the possibility to perform CTC subpopulation particular the duplication of the fusion of TMPRSS2
analysis based on other informative molecular and to ERG sequences (2+ Edel), was associated with
phenotype characteristics, such as EMT markers. a better clinical response to abiraterone treatment,
supporting ERG assessment in CTCs as predictive
Taken together, these studies strongly encourage biomarker . In contrast, Danila et al. could not find
[78]
[77]
the use of CTC-based approaches to profile AR any correlation between TMPRSS2-ERG status and
splice variants and support the development of more clinical outcome after abiraterone therapy, by using
sensitive methodologies to detect and quantify these a RT-PCR-based approach to detect ERG fusions in
tumor-related biomarkers. CTCs from docetaxel-resistant CRPC patients. The
discrepancies in the observed results concerning ERG
Recently, other assays have been proposed to in CTCs and treatment outcome could be attributed to
identify and quantify AR splicing variants from significant differences in the methodologies adopted to
CTCs. Liu et al. [70] developed a CTC-based RT-PCR evaluate the biomarker and in the cohorts of patients
platform to assess both clinically relevant alternative analyzed. Further prospective evaluation of the role of
transcripts, ARv7 and ARv567 and tested it in 73 ERG expression in predicting response to AR-targeted
whole blood samples from 46 CRPC patients. The therapies is currently ongoing .
[79]
authors found ARv7 and ARv567 in 68% and 32%
of the samples, respectively, and detected a strong ERG fusion status also showed to correlate with
association of higher AR variants’ expression levels lack of response to taxane treatment . Reig et al.
[80]
[81]
with patients who had received hormonal therapies measured TMPRSS2-ERG mRNA expression by RT-
compared to hormone-naïve men. Qu et al. [71] PCR in PBMCs isolated from 50 docetaxel-treated
developed a highly sensitive and specific digital CRPC patients by gradient medium separation and
droplet PCR assay and quantified ARv7 expression showed that TMPRSS2-ERG significantly correlated
levels in PBMCs of 132 PC patients; ARv7 transcripts with lower biochemical response, radiological PFS
could be detected in more than 95% of the subjects and overall survival. Even though performed without
and a significant correlation was found between high any form of enrichment for epithelial CTCs, these
ARv7 levels and worse outcome after treatment with findings support the clinical role of ERG assessment
AR signaling inhibitors. to tailor treatment strategies.
200 Journal of Cancer Metastasis and Treatment ¦ Volume 3 ¦ September 27, 2017