curable early-stage malignancies, there are still several barriers for
cancer screening [33, 34]. Many tests are uncomfortable for
patients (colonoscopies, pap smears, mammograms) or expose
patients to radiation (CT scans, mammography). Some are unpleas-
ant to accomplish (stool studies). And almost all tests require a
schedule of additional healthcare visits. Thus, the optimal cancer
screening assay remains an area of active research.
Noninvasive technique is a hot spot of cancer screen research.
Two novel noninvasive techniques were recently approved by FDA
for CRC screening. In 2014, the FDA approved fecal DNA testing
with Cologuard (Exact Sciences Corporation, Madison, WI, USA).
Cologuard is the first noninvasive screening test for colorectal
cancer that analyzes both stool DNA and blood biomarkers. It
has been proven to find 92% of cancers and 69% of the most
advanced precancerous polyps in average-risk patients. In 2016,
the FDA approved Epi proColon (Epigenomics, Berlin, Germany).
It is a blood-based test that detects methylated Septin9 (SEPT9)
DNA to identify CRC patients who choose not to undergo colo-
noscopy and stool-based fecal immunochemical tests [35]. Other
assays for detecting early-stage GI malignancies are currently under
development.
Liquid biopsy, as a kind of noninvasive techniques, has been
widely applied to detect malignancies in clinical setting. In a study
of 125,426 women undergoing noninvasive prenatal testing
(NIPT) for fetal aneuploidy via Verifi Prenatal Test (Illumina, San
Diego, California, USA), 3757 women were positive for one or
more aneuploidies involving chromosome 13, 18, 21, X, or Y. Of
these women, 39 had multiple aneuploidies in which 7 (18%) were
diagnosed with occult malignancy [36, 37]. Three other women
with single aneuploidy were also diagnosed with malignancies. In
three of the ten cases with malignancy, the discordant NIPT test
prompted the evaluation of malignancy. One limitation of this
approach for cancer screening is that it requires a comparison
between two different types of tissue—in this case mother and
fetus. One approach to overcome this barrier is “nucleosome foot-
print”—ctDNA nucleosome spacing is unique with respect to the
nuclear architecture and gene expression. Thus, it may inform the
cell type [38].4.2 Surveillance for
Recurrent Disease
ctDNA is an effective biomarker for detecting tumor recurrence.
For example, ctDNA has been shown to be reliable in detecting
tumor progression or metastasis for breast cancer, often with clini-
cal lead time of up to almost 1 year [39]. Similar studies have found
that liquid biopsies can also be used for surveillance of CRC. For
CRC patients, the BEAMing and patient-specific somatic structural
variant (SSV) techniques have been investigated as tools to identify
cancer recurrence [40]. Based on both methods, ctDNA was foundThe Introduction and Clinical Application of Cell-Free Tumor DNA 51