Clinical applications of circulating biomarkers in non-small cell lung cancer

Abstract

Despite recent advances in cancer diagnostics and treatment, the mortality associated with lung cancer is still the highest in the world. Late-stage diagnosis, often accompanied by metastasis, is a major contributor to the high mortality rates, emphasizing the urgent need for reliable and readily accessible diagnostic tools that can detect biomarkers unique to lung cancer. Circulating factors, such as circulating tumor DNA and extracellular vesicles, from liquid biopsy have been recognized as diagnostic or prognostic markers in lung cancer. Numerous clinical studies are currently underway to investigate the potential of circulating tumor DNA, circulating tumor RNA, exosomes, and exosomal microRNA within the context of lung cancer. Those clinical studies aim to address the poor diagnostics and limited treatment options for lung cancer, with the ultimate goal of developing clinical markers and personalized therapies. In this review, we discuss the roles of each circulating factor, its current research status, and ongoing clinical studies of circulating factors in non-small cell lung cancer. Additionally, we discuss the circulating factors specifically found in lung cancer stem cells and examine approved diagnostic assays designed to detect circulating biomarkers in lung cancer patients.

Keywords: cancer stem cells; circulating factors; ctDNA; diagnostic tools; exosome; microRNA; non-small cell lung cancer.

FIGURE 1

Clinical statistics of non-small cell lung cancer (NSCLC) patients. (A) Proportion of NSCLC and small cell lung cancer (SCLC) among total lung cancer cases. (B) Proportion of three different types of NSCLC and SCLC. (C) The 5-year survival rate of lung cancer patients.

FIGURE 2

Clinical applications of circulating factors in NSCLC. (A) A liquid biopsy method using blood samples (plasma/serum) to detect and monitor circulating factors in NSCLC. (B) Circulating tumor DNAs (ctDNA/cfDNA), microRNAs, exosomes, and circulating tumor cells can be detected in the plasma of NSCLC patients. (C) Circulating stem cell–derived exosomal microRNAs and extracellular vesicles (CD133⁺CD326^-). (D) Diagnostic assays are being used to detect circulating liquid biomarkers in NSCLC patients.

FIGURE 3

Circulating factors in NSCLC. (A) ctDNAs are released into the bloodstream through several physiological processes including necrosis, apoptosis, and active secretion. (B) In plasma, the extracellular RNAs (mRNA, miRNA, long non-coding RNA, and other types of RNA) are secreted into plasma through protein–mRNA complexes, exosomes, tumor-educated platelets, or apoptotic bodies. (C) Exosomes are small vesicles released through exocytosis and can be secreted into body fluids. Exosomes derived from tumor cells can transport oncogenic ctDNA and miRNA. (D) CTCs are tumor cells from metastatic cancer cells that detach from the primary site, invade the neighboring extracellular matrix, enter the bloodstream, and circulate in the vascular system until they extravasate and form secondary tumors.

FIGURE 4

Schematic representation of circulating factors. (A) ctDNA may carry genetic mutation or DNA methylation changes in genes such as SHOX2, RARB2, and RASSF1A. (B) Exosomes are secreted from cancer cells, transporting miRNA or ctDNA into blood vessels. (C) Representative CTC markers of advanced NSCLC including EGFR, HER2, PIK3CA, AKT2, TWIST, and ALDH1 with the CellSearch system. In the CellSearch system, a CTC is characterized as positivity for EpCAM, cytokeratins, and DAPI and negativity for CD45. In subpopulations of NSCLC patients with chemoresistance, HER3 and MET are expressed in CTCs. As biomarkers of CTCs, PD-L1, EpCAM, CK8, CK18, CK19, vimentin, and TWIST expressions were also tested.