Review Article
Circulating Tumor DNA as a Liquid Biopsy in Cancer
Rui Zhang1 and Wenjun Yang1,2*
1Key Laboratory of Fertility Preservation and Maintenance (Ministry of Education), Medical Oncology Department of the General Hospital, Ningxia Medical University, Yinchuan, Ningxia, 750004, P. R. China
2Department of Genetics, Medical College, South China University of Technology, Guangzhou 510006, P. R. China
*Corresponding author: Wenjun Yang, Key Laboratory of Fertility Preservation and Maintenance (Ministry of Education), Department of Medical Oncology, Ningxia Medical University, Yinchuan, Ningxia, 750004, P. R. China,
Published: 24 Apr, 2017
Cite this article as: Zhang R, Yang W. Circulating Tumor
DNA as a Liquid Biopsy in Cancer. Clin
Oncol. 2017; 2: 1265.
Abstract
Circulating tumor DNA represents a promising biomarker for non-invasive assessment of cancer progression and evolution. Clinical management of cancer patients could be improved through the development of noninvasive approaches for diagnostic detection, prognosis and recurrent tumors. Biopsies are invasive, costly and provide only a snapshot of the mutations present at a given time and location. For some applications, mutation detection in plasma DNA as a liquid biopsy could potentially replace invasive biopsies as a means to assess tumor genetic characteristics. In this review, we summarize the development and clinical results using circulating tumor DNA and discuss how future studies involving both scientists and clinicians could help to further develop this tool for the benefit of cancer patients.
Introduction
Cancer is a leading cause of death worldwide and requires appropriate diagnostic and prognostic
methods [1]. Current tumor diagnosis depends on pathological examination. However, with the
continuous emerging of tumor-specific molecules, tissue biopsies cannot always be reflective of the
current tumor dynamics or response to therapies. To overcome this problem, the concept of “liquid
biopsies” has been proposed, and clinical studies have proven that circulating biomarkers, such as
tumor-relevant protein molecules, circulating RNA or microRNAs that can be used to help guide
patient management will be available for only a minority of patients [2-4]. However, the sensitivity
and specificity of these biomarkers remain suboptimal [5,6]. Therefore, the identification of new,
highly sensitive and specific tumor biomarkers is particularly important and will also provide
insights into cancer biology that can form the basis of further research.
The history of ctDNA
In 1948, circulating free DNA (cfDNA) and RNA were found in the human blood plasma by
Mandel and Métais [7]. The clinical utility of cfDNA in the plasma has been an area of active research
in many disciplines of medicine [8]. The most successful area has been the evaluation of fetal DNA
in the blood of expecting mothers [9,10]. In patients with cancer, a fraction of the cfDNA is from
tumor cells and is referred to as circulating tumor DNA (ctDNA) [11]. Tumor DNA can be released
from primary tumors, micrometastasis, or overt metastasis into the blood of patients with cancer12.
However, the majority of such ctDNA is derived from apoptotic and necrotic tumor cells that release
their fragmented DNA into the circulation [13-16]. cfDNA in the circulation is typically fragmented
to 160 - 180 bp in length, and this DNA is derived mostly from apoptotic cells [17]. The ladder
pattern is frequently considered to be evidence that apoptosis may be the source of the observed
DNA fragments in the plasma when the circulating DNA is subjected to electrophoresis [18,19].
In terms of radiotherapy, chemotherapy and other cancer treatments, cell death by apoptosis and
less circulating DNA are found in cancer patients after treatment than before treatment, possibly
because of the inhibitory effect of treatment on the proliferation of cancer cells [20].
The blood is considered as a “reservoir” where the alterations can be detected in the form of
point mutations, copy number variations or chromosomal rearrangements. However, detection of
ctDNA derived from tumors carries some challenges, largely because the number of circulating
mutant gene fragments is small compared to the number of normal circulating DNA fragments and
is sometimes less than 0.01% [21]. The detectable levels of ctDNA are related to the stage of disease
and are lower in the localized tumors (49-78%) of patients than in metastatic tumors (86-100%) [6].
Recently, there have been numerous methods reported that involve the quantification of the
level of multiple tumor and genetic mutations present in the blood, with several reports suggesting
that there are many approaches that have high sensitivity, like droplet digital polymerase chain
reaction (ddPCR) [22], beads, emulsion, amplification and magnetics (BEAMing) [23], or
pyrophosphorolysis-activated polymerization (PAP) [24]. Tumorderived
ctDNA reflects the genomic alterations of tumors, but it
has a variable half-life in the circulation, ranging from 15 minutes
to several hours; consequently, ctDNA can be used as a dynamic
biomarker providing an accurate monitoring of tumor masses
in real-time [21,25]. Simultaneously, some studies have revealed
the correlation of ctDNA with many cancers, such as renal cancer
[26], bladder cancer [27], gastric cancer [28], colorectal cancer [15], Esophageal Squamous Cell Carcinoma (ESCC) [29] and non-small
cell lung cancer [30].
The correlation between tissues and ctDNA
Although tumor tissue is the gold standard for clinical study, major
barriers exist in obtaining tissue biopsies, including the discomfort
suffered by the patients, inherent clinical risks to the patient, potential
surgical complications and economic considerations [31]. Sampling
ctDNA from the blood overcomes the tumor heterogeneity and
accessibility problems [32].
Indeed, several reports have shown a high correlation between
ctDNA mutations and matched tumor biopsy mutations. The first
demonstrations of a correlation between tissue and ctDNA evaluated
PIK3CA mutations in tissue biopsies and plasma tumor DNA
(ptDNA) from breast cancer patients. This group demonstrated that
PIK3CA mutations in the tissue and blood were 100% in agreement
[33]. In non-small cell lung cancer (NSCLC), Xu “et al.” [34] found
that driver mutations in EGFR, KRAS, PIK3CA, and TP53 were
correlated between tumor DNA and plasma ctDNA in 32 of 42 (76%)
matched samples. In a prospective study, Rothe “et al.” [35] sequenced
tumor samples and matched the serial plasma samples from metastatic
breast cancer patients using NGS from a commercially available 50-
gene panel. This study also showed that all mutations detected in a
patient’s tumor sample were present in the ptDNA, and the ptDNA
of 2 of 17 patients contained a mutation that was not detected in
their synchronous tumor samples, thereby supporting the concept
that ptDNA may provide a more comprehensive mutational profile
[36]. Chu “et al.” [37] found that in both groups of patients, ESR1
mutations were always detected in the blood if they were present in
the tissue, and additional ESR1 mutations that were not detected in
the tissue were also present in the ptDNA of some patients. Similar
results have been seen in studies involving multiple different cancers
[6]. Recently, Schmiegel “et al.” [38] found a high concordance of
plasma and tissue results. This also demonstrated that blood-based
RAS mutation testing is a viable alternative to tissue-based RAS testing
using BEAMing. ctDNA is also believed to be released from all the
tumor deposits in a given patient and is less impacted by intratumor
heterogeneity than a signal tumor specimen [8]. Gerlinger “et al.” [39]
observed the mutational intratumor heterogeneity in multiple tumor
suppressor genes, which can lead to the underestimation of the tumor
genomic landscape from a single tumor-biopsy sample. In most cases,
only biopsies are available, and treatment decisions depend on the
results from a single tumor biopsy [40].
Detection methods of ctDNA
The quantity of ctDNA detectable in the blood is not only good
for studying cancer pathogenesis but also beneficial in the clinical
management of cancer. Though tumors are rarely observed, the
development of non-invasive methods to detect and monitor tumors
continues to be a principle challenge in oncology. Recently, PCRbased
approaches have reached high levels of sensitivity, ranging
from 0.1 to 0.01%, making it possible to detect to 1 mutated allele out
of 10000 normal alleles [41].
Now, there are several methods to detect ctDNA. Originally,
researchers used Sanger sequencing to detect plasma ctDNA.
However, there are many shortcomings for Sanger-based ctDNA
detection, such as low-throughput, laborious protocols, high cost,
and the potential bias introduced by PCR methodology [42]. Some
studies have developed a technique called BEAMing [23,43] to detect
ctDNA in the blood [21]. Using this method, Bettegowda reported
a sensitivity of 87.2% and a specificity of 99.2% to detect KRAS
mutations in colorectal cancer [6]. Forshew “et al.” [44] reported that
they applied tagged-amplicon deep sequencing (Tam-Seq) to directly
identify mutations in the plasma of cancer patients. The authors
conducted a proof-of-concept experiment by tracking the ctDNA
from an ovarian patient, while they also re-sequenced the tumor
tissue from a right oophorectomy specimen and identified a TP53
mutation [1]. Using this method, they identified cancer mutations
present in the circulating DNA at allele frequencies as low as 2%,
with sensitivity and specificity of >97% [44]. Newman “et al.” [42]
developed another new technique called cancer personalized profiling
by deep sequencing (CAPP-seq) for quantifying ctDNA. They use a
multi-phase bioinformatics approach consisting of biotinylated DNA
oligonucleotides for low DNA input masses that target recurrently
mutated regions in the cancer of interest. CAPP-Seq achieved a
maximum sensitively and specificity of 85% and 96%, respectively.
In contrast to these approaches targeting hotspot mutations, a
study used Whole-Genome Sequencing (WGS) analysis from plasma
DNA, and this approach suggested that tumor DNA concentrations
at levels >10% can be detected with a sensitivity of >80% and
specificity of >80% [45]. Murtaza “et al.” [46] used Whole-Exome
Sequencing (WES) in a proof-of-concept study involving 6 patients
with metastatic tumors. Compared to WES, WGS can screen a larger
spectrum of the genome. However, it is currently too expensive for
routine use to detect SNVs, whereas WES approaches allow more indepth
interrogation of multiple regions [1]. Recently, technology was
developed using a high-throughput Droplet Digital PCR (ddPCR)
system that can provide absolute quantitation of DNA copy number
[22]. A study developed a targeted 23-amplicon Next-Generation
Sequencing (NGS) panel for detection of mutations in ESR1, PIK3CA,
TP53, FGFR1 and FGFR2 in 48 patients with estrogen receptor-α-
positive metastatic breast cancer, and the selected mutations were
validated using droplet digital PCR (ddPCR) [47]. As validation,
they analyzed the common ESR1 p.D538G mutation in the baseline
cfDNA samples by ddPC, confirmed all 3 positive samples and
detected 6 additional cfDNA samples with this mutation at <1%. This
means that ddPCR is more sensitive than NGS technology.
Comparing ctDNA and CTCs
The first report of Circulating Tumor Cells (CTCs) was shown
for the first time in 1869 [48]. CTCs are extremely rare and are lost
in a large number of normal blood cells. Only 1.43% of patients
with progressive breast cancer had > 500 CTCs per 7.5 ml of blood
[49,50]. CTCs may become cloaked by platelets or by coagulation
factors, thereby shielding them from the immune system and making
it difficult to detect them [51]. The potential clinical value of CTCs
is clear in early detection, ultimately targeting the process of bloodborne
metastasis and in using CTC analyses as a readout of tumor
status therapeutically [52].
Circulating Tumor Cells (CTCs) and circulating tumor DNA
(ctDNA) are promising sources for biomarker tests and useful tools
for the management of patients with cancer. Although the current
Food and Drug Administration (FDA) has approved liquid biopsy
measures for intact CTCs to give a prognosis of overall survival, the
potential predictive value of ctDNA is much more exciting. ctDNA
allows us to understand specifically what type of molecular changes
are happening in the tumor in real time, which is a very big step
beyond CTCs in clinical terms [53]. Several investigational studies
have shown that ctDNAs could have more sensitivity than CTCs. A
study [54] compared the radiographic imaging of tumors with the
ctDNA and CTC assay in 30 women with metastatic breast cancer.
They found that ctDNA was successfully detected in 29 of the 30
women (97%) in whom somatic genomic alterations were identified;
however, CTCs were detected in 26 of 30 women (87%). Their result
showed that circulating tumor DNA levels had a large dynamic range
and a greater correlation with changes in the tumor burden than
circulating tumor cells. Furthermore, another study used droplet
digital PCR to assess the BRAF-V600E mutations in both circulating
tumor DNA and DNA extracted from CTCs in lung adenocarcinoma
[55]. The results showed that ctDNA seemed to be much more
sensitive than CTCs. In a comprehensive study by Punnoose “et al.”
[56], forty-one patients were enrolled in a single-arm clinical trial
of erlotinib and pertuzumab, and peripheral blood was analyzed
for oncogenic mutations in CTCs and ctDNA. They found greater
sensitivity for ctDNA than CTCs in mutation detection and reported
that the detected mutations were strongly concordant with mutation
status in the matched tumor. This proof-of-concept analysis showed
that circulating tumor DNA is a promising, informative, inherently
specific, and highly sensitive cancer biomarker [54].
Liquid Biopsies in the Clinic
We have presented the technologic considerations of ctDNA. However, with rapidly evolving sequencing platforms, we can provide only general guidelines about their comparative utility in clinical oncology [52]. For the clinical oncologist, it has become increasingly urgent to have access to accurate and sensitive methods for quantifying the response to cancer therapy, making a prognostic assessment, and predicting recurrence [57,58]. The development of cancer detection biomarkers will be propelled by technological improvements in how biomarkers are objectively measured. These biomarkers should be surrogate indicators for warning about possible recurrence, as well as disease progression or death, and should indicate if a specific treatment will reduce that risk. CtDNA might be a prognosis marker in cancer patients and, in the future, might be especially helpful for the selection of those patients who are at a higher risk of relapse and who might benefit from adjuvant therapies [59]. As such, ctDNA may be rapid, economical, and reliable for clinical applications. The steps required for the use of ctDNA in clinical oncology must be taken with great care, using well-designed, prospective clinical studies to statistically demonstrate clinical validation and clinical utility [36].
Assessment of Diagnosis and Prognosis
Cancer diagnosis and prognosis, especially through the use of a
noninvasive blood test, is of great interest to researchers and patients
alike. Obtaining a blood sample is less risky and noninvasive than
a tumor or metastatic lesion biopsy [2]. Assessing the prognosis of
an individual patient involves a combination of clinical observations
and stages and the bimolecular characterization of different tumor
types [31]. This information, which is derived from imaging, biopsy
specimens and cancer biomarkers, could offer prognostic value
for clinical oncology. However, previous tumor biomarkers like
cancer antigen 15-3, CEA, as well as CTCs, provide only limited
sensitivity and specificity and therefore cannot always meet the
clinical requirements. However, now there is a plasma biomarkerbased
approach that can evaluate tumor occurrence, progression and
recurrence. A number of studies have already shown the ability to
use ctDNA as a genetic-based biomarker for cancer detection, though
mostly in metastatic disease [60,61].
For example, Shu “et al”. [62] implemented NGS with a
gene panel of 382 cancer-relevant genes on 605 ctDNA samples,
indicating that ctDNA might be a suitable approach to guide
treatment decisions in multiple cancers. A study from Lecomte “et
al”. [63] focused on hotspot KRAS mutations and cyclin-dependent
kinase inhibitor 2A (CDKN2A) hypermethylation in patients with
CRC. They demonstrated that the 2-year survival rate was 100% in
patients with no evidence of ctDNA who possess KRAS mutations
or CDKN2A gene promoter hypermethylation, which can be found
in 40% or 20%–50% of CRC patients, respectively, suggesting a
prognostic value for these markers. Therefore, the presence of ctDNA
in plasma seems to be a relevant prognostic marker for patients
with CRC and may be used to identify patients with a high risk of
recurrence. Furthermore, it was shown that high concentrations of
cfDNA and mutant KRAS were clear indicators of a poor outcome
for metastatic CRC patients [64]. Therefore, ctDNA has the potential
to be used for the evaluation of tumor prognosis. Using ctDNA, the
detection rates among patients with stage I, II, III, and IV cancer were
47%, 55%, 69%, and 82%, indicating that ctDNA levels increase with
cancer progression6. Gene methylation patterns in tumor tissue can
be indicative of tumor aggressiveness and likelihood of recurrence
[65]. The methylated genes TIMP3 [66], GSTP1, MINT2 [67], SOX17
[68] and RARb2 [69] were present in the serum or plama and are in
turn linked to prognosis. Cell immunotherapy is another promising
immunotherapeutic approach in cancer. In a study of the response
indicators to T-cell transfer immunotherapy in metastatic melanoma
using ctDNA, their results show that ctDNA levels can be used to
rapidly identify patients who are responding from those that are not
[70].
In summary, liquid biopsies based on ctDNA analysis might
represent the next generation of tumor prognostic testing on account
of their high accuracy and sensitivity [71]. In a multivariate analysis,
KRAS mutations present in the plasma of 246 patients with advanced
stage Non-Small-Cell Lung Cancer (NSCLC) were shown to predict
poor prognosis in patients receiving first-line chemotherapy [72].
However, a parallel study, conducted in 308 patients with advancedstage
NSCLC, showed no correlation between prognosis and KRAS
mutations in the plasma [73]. BRAF mutations, as assessed in serum
samples, have also been shown to effectively stratify 103 patients
with melanoma into both early stage and advanced-stages [74].
Nevertheless, results remain contradictory in these small patient
populations. Accurate molecular diagnostics are essential for
personalized therapies and precise clinical decisions to assess patient
candidacy for other aspects of therapy based on drug-sensitizing
genetic tumor alterations [75].
Prediction of Recurrence
CtDNA could be used to monitor relapse status, resulting in a
10-month lead-time on the detection of relapse compared with the
conventional follow-up [76]. CtDNA could be a prognosis marker
in cancer patients and, in the future, for the patients at a higher
risk of recurrence who might benefit from adjuvant therapies [59].
Theoretically, the tumor burden is reflected by the ctDNA changes
and should correspond with the stage of the tumor. This was also
confirmed by a study from Diehl “et al.” [21], in which patients
who had detectable ctDNA after surgery generally relapsed within 1
year. Garcia-Murillas “et al.” [77] found that noninvasive mutation
tracking in plasma DNA can detect Minimal Residual Disease (MRD),
which the standard treatment has failed to eradicate, and thus identify
patients at high risk of recurrence. Schiavon provided two controls to
suggest that this is not the main explanation. The subset of patients
on Aromatase Inhibitors (AIs) with a ctDNA sample taken at the time
of relapse had a very low level of ESR1 mutation detection, and they
observed no mutation in the independent series of tumor biopsies
taken at relapse on AI therapy [78]. Additionally, Roschewski “et al.”
[79] found that the surveillance of circulating tumor DNA identified a
risk of recurrence before clinical evidence of disease in most patients,
with a higher sensitivity than CT imaging, resulting in a reduced
disease burden at relapse. Another study found similar results [80].
Similarly, Forshew “et al.” [44] found that targeted deep sequencing
of cancer-related genes was carried out on cfDNA in a patient who
had previously undergone surgery to resect synchronous cancers of
the bowel and ovary. It was shown that, on relapse, the metastasis
was derived from the original ovarian cancer. Additionally, Diehl
“et al.” [21] found that the majority of patients had significantly
decreased or absent ctDNA levels after surgery. Further followup
studies suggested that the patients with detectable ctDNA after
surgery all relapsed, while those without detectable ctDNA after
surgery remained in remission. Without elevated CEA, Sarah found
that ctDNA could detect recurrence prior to clinical detection [81].
Thus, there is great interest in whether the use of ctDNA for
monitoring disease can reliably predict patients that have responded
to therapies and were cured versus those that will ultimately have
recurrence [36]. Recently, a study reporting a better selection of
patients for whom liquid biopsy could be a good surrogate for a
tumor biopsy and for whom the cfDNA NGS analysis will contribute
to the sensitivity prediction score [82].
Monitoring Tumor Burden
Measuring the treatment response in patients with cancer is
usually done by serial clinical evaluation of symptoms and estimates
of tumor burden. However, serial radiographic imaging and tumor
biopsy are expensive and may fail to detect changes in tumor burden
[83]. Given these challenges, tumor biopsy may not represent an ideal
source for the genetic characterization of the cancer. Circulating
tumor DNA offers a “real time” tool for serial monitoring of cancer
tumor genomes in a non-invasive manner that provides accessible
genetics biomarkers [84].
Quantifying the disease burden to monitor the response to
cancer therapy has a direct intuitive appeal, with many potential
applications for individualizing treatment choices [57]. Circulating
tumor DNA (ctDNA) may also represent a promising biomarker for
noninvasive assessment of cancer burden, especially in circumstances
where imaging delivers indeterminate results [8]. Indeed, the specific
detection of tumor-derived cfDNA has been shown to correlate with
a change in the tumor burdens in response to treatment or surgery
[85]. Serum markers, such as PSA for prostate cancer, Cancer
Antigen (CA) 19-9 and Carcinoembryonic Antigen (CEA), can be
helpful but are not available for many tumor types; they frequently
lack specificity and may be elevated as a result of clinical situations
not related to tumor growth or progression [8]. Immunological
detection of circulating tumor cells identifies only cells present in the
blood and can result in false positive results due to nonmalignant cells
expressing the marker of interest [86]. Furthermore, the half-life of
ctDNA is less than 2 hours [21], whereas most protein biomarkers
persist in circulation for several weeks, thereby only allowing accurate
assessment over weeks to months [8]. The investigators observed that
mutant ctDNA concentrations showed a greater dynamic range and
greater correlation with changes in tumor burden than did CA15–3
[54]. A study used the known tumor burdens and pre-treatment
ctDNA levels measured in patients who harbored KRAS mutations in
their tumors prior to therapy [85], as well as data obtained in patients
with previous metastatic disease [21]. At a median follow-up of 507
days in patients with detectable ctDNA (5 of 6, 88%) compared with
undetectable ctDNA (5 of 72, 7%), recurrence rates were >10-fold
higher than the postoperative rates [87]. Tumor heterogeneity might
be limited in advanced disease, in which case radiographic imaging
should be involved [88].
However, compared with the tissue-based approach, prospective
studies will be needed to illustrate that treatment strategies guided by
the unique information from ctDNA yield superior clinical outcomes
[87].
Monitoring of Molecular Resistance
Serial analysis of ctDNA during treatment can provide a dynamic
picture of molecular disease changes, suggesting that this noninvasive
method could also be used to monitor the development of
secondary resistance of tumor cells that develop during the course of
treatment [53]. One major barrier to testing any hypothesis about the
nature of acquired resistance to anti-EGFR antibodies is the limited
access to post-treatment tumor tissue [85]. Repeated biopsies to study
genomic evolution as a result of therapy are difficult, invasive and
may be confounded by intra-tumor heterogeneity [39,89]. Molecular
tools, such as WES, can be implemented to find genetic differences
between the tissue collected before and after therapy. This will offer
a snapshot of the predominant resistant clones in a portion of the
lesion under examination [8]. Recently, a cancer exomes sequencing
report of the serial plasma samples was created to track the genomic
evolution of metastatic cancers in response to therapy [46]. Their
results showed that the CNVs and somatic mutations identified in the
ctDNA are widely representative of the tumor genome and provide
an alternative method of tumor sampling that can overcome the
limitations of repeated biopsies [46]. A case reported that ctDNA can
be used to dynamically monitor the onset of secondary resistance to
anti-EGFR therapy [90].
Acquisition of the T790M substitution in the membrane receptor
EGFR confers resistance to gefitinib and erlotinib in lung cancer in
approximately 50% of patients [91-93]. The T790M mutation was
first observed in relapsed patients and later confirmed through the
non-invasive analysis of plasma samples, suggesting that resistance to
targeted therapies can be monitored in the blood [31]. More recently
genomic heterogeneity of T790M-mediated resistance may explain
the reduced specificity observed for plasma-based detection of
T790M mutations versus tissue94. Analogously, secondary resistance
to anti-EGFR antibodies cetuximab and panitumumab is associated
with the emergence of KRAS mutations in colorectal cancer [95].
The detection of KRAS variants in the blood during treatment with
cetuximab or panitumumab demonstrated that it is possible to
detect the emergence of resistant alleles up to several months before
radiologic examination [85]. For metastatic breast cancer, targeted
NGS of cfDNA has potential clinical utility to detect biomarkers,
which are corrective with HER2-targeted therapies [96].
Parallel analyses of tumor biopsies and serial ctDNA monitoring
showed that lesion-specific radiographic responses to subsequent
targeted therapies can be driven by distinct resistance mechanisms
arising within separate tumor lesions in the same patient [97]. At
present, this more sensitive approach for detecting cancer exomes
in plasma is readily applicable to patients with high systemic tumor
burden, enabling detailed and comprehensive evaluation of the clonal
genomic evolution associated with treatment response and resistance
[46].
Discussion
To date, the prospects for using ctDNA as a liquid biopsy for
tumor management has generated a lot of excitement as this strategy
provides the opportunity for the noninvasive detection of human
cancers. As discussed, the study provides proof of the concept that
ctDNA represents a sensitive biomarker for tumor burden, diagnosis
and monitoring of drug resistance. Additionally, ctDNA may have
broad clinical applications because it is non-invasive and convenient,
and these assays can be repeatedly performed [98]. However, some
studies miss the majority of somatic alterations in cancer that would
require sufficient genomic coverage to identify in a tumor with
multiple molecular markers, rather than simply recognizing the
existing alterations [52].
The US Food and Drug Administration (FDA) has approved the
cobas® EGFR mutation Test v2, which is the first “liquid biopsy” blood
test for detecting epidermal growth factor receptor (EGFR) gene
mutations in Non-Small Cell Lung Cancer (NSCLC). The ability to
both isolate and genetically interrogate tumor DNA from a simple,
minimally invasive test that can subsequently inform treatment
decisions is a win for both the physician and patient. Currently,
there are no consistent guidelines regarding the choice of analyte, the
standardization of platforms or how the results are reporteded [36].
Likewise, the development of more accurate detection methods based
on ctDNA could benefit the cancer patients and may, as a result,
improve the clinical outcome in the near future [1]. There has been a
study on the application of sequencing, liquid biopsies, and patientderived
xenografts for personalized medicine in melanoma [99].
Together, different technologies are likely to be synergistic, rather
than strictly competitive in their clinical oncology applications [52].
Only in this way will their results optimize care for the majority of
patients.
Acknowledgements and Funding
This work was supported National Natural Science Foundation of China (81160249, 81460434). Thanks for the editing service of Wiley.
Statement of Conflict of Interest
The authors have declared that no conflict of interest exists.
Authors’ Contributions
Conceived and designed the paper: WY and RZ. Wrote the paper: RZ and WY.
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