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Global Translational Medicine ctDNA in management of patients with NSCLC

focuses on silencing cancer suppressor genes by inactivating that can be used in cancer detection. Histone proteins can
specific chromatin structures. DNA methylation often be modified in a variety of ways, including acetylation,
happens on CpG islands (CGIs) around the 5’ untranslated methylation, and phosphorylation. Core histones’ N-terminal
regions (5gionsl of genes, which are always associated with tails protrude from nucleosomes and are embellished with
promoters and are rich in GC base-pairs. The methyl groups post-translational modifications. Based on the functions,
on CGIs are offered by S-adenosyl-methionine to the position the enzymes are categorized into four groups: Acetylation by
5 of cytosine and catalyzed by DNA methyltransferases, HATs, deacetylation by HDACs, methylation by HMTs, and
including DNA methyltransferase 1 (DNMT1), DNMT3A, demethylation by HDMs. Modifications on histones inactivate
and DNMT3B, among which DNMT1 plays the main part cancer suppressor genes, and posttranslational modifications
of maintaining the methylated status . Moreover, tumor’s complement by promoting or inhibiting cell metabolism .
[27]
[32]
genes are highly methylated and consequently occupies the However, posttranslational modifications on histones are
majority of DNMT1. As a result, the methylation landscape neither exclusive nor independent in tumor tissues, and there
in a patient with tumor burden turns to be highly methylated are numerous combinations that form modification crosstalk.
in CGIs with generally global demethylation . Figuring the tumor-specific modifications or combinations
[28]
[28]
For DNA methylation that has no nucleotide sequence are one of its greatest challenges .
changes, the way to detect methylation is quite different MicroRNA (miRNA), long ncRNA (lncRNA),
from the above. Three mainstream detection methods are circular RNA (circRNA), and PIWI interacting RNA
applied to differentiate methylated DNA spots. Bisulfite (piRNA) comprised the majority of ncRNA in liquid
sequencing first converts unmethylated cytosine into uracil biopsy. Single miRNA can target more than 400 targeted
under the effect of bisulfite, then transferred into thymine mRNA and it is believed that more than half of human’s
[33]
by PCR, with methylated nucleotide theoretically intact. genes are directly modulated by miRNA . Researchers
This bisulfite method is cheap and accurate, and honored realized the great diagnostic and therapeutic potential of
as a gold standard of methylation detection. However, its miRNA soon after the discovery and identification, and
default is also apparent, and the bisulfite transformation miRNA is widely used in various studies compared with
weakens the DNA double-chains’ stability, which may other ncRNA biomarkers. miRNA specifically binds to
cause bias to the result . The second detection tool is targeted mRNA by complementary sequence and forms
[29]
using methylation-sensitive restriction enzymes before RNA-induced silencing complex to silence or degrade
amplification, aiming to digest unmethylated CGIs before mRNA. Many investigations had confirmed that miR-126
gel electrophoresis or PCR sequencing across the restriction and miR-155 were oncogene in many kinds of cancers,
site. Besides, affinity-enrichment-based methods are while let-7 miRNA and miR-34a were regarded as tumor
considered in the methylation detection. Researchers also suppressors . miRNA can be best extracted from plasma
[34]
designed methyl-binding protein or antibodies specific for and the extraction methods can be grossly divided into
5 mC, intending to elevate the concentration of methylated three groups, namely, organic extraction methods, filter-
spots. By running sequencing techniques in a high density based methods, and magnetic particles-based methods.
of targeted DNA, this affinity-enrichment-based method When combined with other extraction methods, the
achieved high-sensitivity detection with less efforts [30,31] . accuracy of these methods can be further improved .
[35]
In comparison, methylation-sensitive restriction 2. Early detection and screening
enzyme method is limited to recognizable unmethylated
CpG loci out of the enzyme’s specificity range, but the 2.1. Status quo of lung cancer screening
bisulfite-based sequencing and affinity-enrichment-based As discussed above, lung cancer is a vital disease, but the
method are capable for the whole genome. However, prognosis is tightly associated with the stage when it was first
bisulfite-based technique has a large working load, and discovered. Low-dose CT (LDCT) among high-risk population
the accuracy of affinity-enrichment-based method is is one of the current lung cancer screening strategies, which
closely related to the protein-target binding. As for target is recommended as a screening guidance of lung cancer in
preference, bisulfite sequencing is repeatable and sensitive many countries . National Lung Screening Trial (NLST)
[36]
especially for single CpG resolution. Methylation-sensitive was the first to verify LDCT’s effect in reducing mortality in
restriction enzymes have high efficiency on low-CpG- large-scale trial. A multi-centered cohort comprising 53,454
density regions and affinity-enrichment-based method high-risk patients who were randomly subjected to regular
performs better on CpG-rich ones . LDCT or single-view posteroanterior chest radiography
[31]
Histone modification and the amount of special non- examination for 3 years was analyzed for the following lung
coding RNA (ncRNA) are additional epigenetic changes cancer-related death. LDCT group had a relative reduction

Volume 1 Issue 1 (2022) 5 https://doi.org/10.36922/gtm.v1i1.96