Hot TopicCurrent applications and challenges of circulating tumor DNA (ctDNA) in squamous cell carcinoma of the head and neck (SCCHN)
Introduction
Squamous cell carcinoma of the head and neck (SCCHN) is the seventh most common malignancy and is responsible for 3.7% of cancer deaths worldwide [1]. The main risk factors are alcohol and tobacco. The human papillomavirus (HPV) is another risk factor for oropharyngeal cancer (OPC) and its incidence is rising [2].The 5-year survival rate of SCCHN is still poor, ranging from 25% to 60% depending on tumor staging and site [3]. Most patients present with locally-advanced disease, but despite multimodal treatment including chemoradiation or surgery followed by (chemo)radiation, >50% of patients will relapse. Once patients have incurable recurrent/metastatic (R/M) disease, the standard of care is palliative systemic treatment. Platinum-based chemotherapy, programmed cell-death 1 (PD-1) inhibitors and cetuximab improve survival, but the prognosis is dismal with an overall survival of 10–15 months [4], [5]. Many challenges remain to improve patient outcomes in SCCHN including better screening to diagnose more patients at an early stage, early detection of relapse after curative treatment, and the implementation of precision medicine.
Liquid biopsies (LB) are emerging in the oncology field with promising data as new diagnostic, prognostic and treatment-monitoring tools [6], [7], [8]. LB refers to the sampling and analysis of several biological fluids as opposed to solid biopsy which analyzes a tissue sample [9]. The most popular form of LB is the analysis of blood samples, but LB can also refer to samples of saliva, urine, ascites, cerebrospinal fluids and pleural effusions. Different composites are exploitable in a blood-based biopsy to characterize tumors, including circulating tumor cells (CTCs) [10], circulating tumor DNA (ctDNA), circulating cell-free RNA, circulating extracellular vesicles (EVs), proteins and metabolites. Besides genomics, including the mutational profile and copy number variations, LB can provide information regarding transcriptomics [11], epigenomics [12], proteomics [13] or metabolomics [14]. Currently, only assays analyzing CTCs and ctDNA have been approved by the US Food and Drug Administration (FDA). CTC enumeration (CellSearch®, Menarini Silicon Biosystems, Huntingdon Valley, Pennsylvania USA) is FDA-approved and has a prognostic value in metastatic breast [15], [16], colorectal [17], and prostate cancers [18], but its clinical utility is still to be demonstrated. The FDA also approved the cobas® EGFR Mutation Test v2 (Roche Molecular Diagnostics, Pleasanton, California, USA) to detect EGFR mutation in the cell-free DNA (cfDNA) of lung cancer patients [19], [20]; and Epi proColon® (Epigenomics, Berlin, Germany) to assess the methylation status of the SEPT9 promoter in cfDNA to screen patients for colorectal cancer [21], [22], [23].
The gold standard for cancer diagnosis remains tissue biopsy as, in contrast to liquid biopsy, it provides histological features of the tumor in addition to molecular characterization. However, obtaining a tissue biopsy can sometimes be challenging and technically difficult due to disease localization. LB is easily accessible, less invasive and more comfortable for the patient. At the molecular level, LB should theoretically be able to capture intra-patient tumor molecular heterogeneity as it allows ctDNA released from multiple tumor regions to be analyzed. Therefore, it should be able to reflect the different genomic alterations between the primary tumor and metastatic lesions [24]. In addition, LB may also address tumor heterogeneity over time [25] by enabling multiple samples to be taken over the course of the disease.
With ease of sampling making it clinically attractive, the field of LB is rapidly evolving. Awareness of its potential applications, technical limitations and supportive evidence is now useful knowledge for clinicians. In this review, we provide an overview of the potential use and technical challenges of ctDNA and then focus on the available data of this new diagnostic tool in squamous cell carcinoma of the head and neck.
Section snippets
Potential clinical applications of ctDNA
ctDNA is cell-free DNA (cfDNA) from tumoral origin that is essentially derived from apoptotic or necrotic tumour cells [26], [27], although active release is also implicated [28]. The analysis of ctDNA can provide information on tumor-specific genomic alterations including somatic mutations, copy number alterations, fusions or epigenetic alterations. Below, we briefly describe the potential clinical applications of ctDNA.
Technical aspects of ctDNA somatic mutation analysis
The optimal sequencing technique should consider the end goal(s) of ctDNA analysis. The main challenge with ctDNA analysis is that the proportion of ctDNA can sometimes be extremely low (<1%) [47]. Therefore, the detection of MRD, for example, will require a highly-sensitive technique that is able to detect ctDNA at very low proportions.
Table 1 provides an overview of ctDNA sequencing approaches. There are multiple techniques to analyse ctDNA and these vary in the number of genomic alterations
cfDNA in SCCHN
Some studies have explored LB as a diagnostic, prognostic or monitoring tool for SCCHN, but data are limited compared to other cancers and there is currently no clinical application. LB could, however, be of interest at different stages of SCCHN disease (Fig. 1). For instance, 40–50% of patients treated with curative intent will recur. The suspicion of local relapse is currently based on physical examination, including flexible head and neck fiberoptic endoscopy and imaging. The differential
Conclusion
Although the literature on cfDNA analysis for SCCHN is scarce compared to other tumors, preliminary results seem to hold promise for the future. Early evidence suggests that highly sensitive techniques on limited gene panels could be useful to detect ctDNA in a substantial proportion of patients with SCCHN. This is of particular interest to detect minimal residual disease, for which proof of concept has already been shown. The next step would be to evaluate the clinical utility of ctDNA
Funding
Rachel Galot is a research fellow supported by a grant from the Belgian National Research Fund (Télévie/FNRS 7650918F).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank Aileen Eiszele for writing and editing assistance. Fig. 1 has been created with Biorender.com.
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