HNSCC typically originates in deeper tissues and is usually harder to see or palpate. For that reason, it is often discovered too late, i.e., when it has already progressed locally or metastasised to the cervical lymph nodes. Approximately two-thirds of patients with HNSCC are diagnosed at an advanced stage, and more than half of them experience a recurrence at least once, 90% of them within the first two years
[2]. This contributes to the high burden of HNSCC; prompt diagnosis is therefore crucial
[3][4][5][6][7][8].
When it comes to diagnostics, some exosomal molecules’ unique and consistent expression patterns make them a promising biomarker in some diseases of ORL-HNS. Since EVs are ubiquitous, they can be isolated from tumour liquid biopsy samples or non-invasively collected body fluids, e.g., saliva, plasma, urine. Although using EVs in these settings is becoming increasingly popular in diagnosis, their use is still limited due to the overlapping cellular structure and composition of the tumour and normal cells
[9].
Recently, the most often studied potential EV-related biomarkers from different samples, including the above-mentioned diagnostic methods, have been EV levels.
A study by Hoshino et al. (2020) provided a proteomic analysis of EV and particles (EVPs) from 426 human samples, identifying pan-EVP markers, and biomarkers for EVP isolation, cancer detection and cancer type. In addition, a study of EVP proteomes was run to identify universal EVP markers, improve the isolation of human EVPs and offer a resource for early cancer detection with liquid biopsies. Among the exosome markers investigated, HSPA8 (Heat Shock Protein Family A (Hsp70) Member 8), HSP90AB1 (Heat Shock Protein 90 Alpha Family Class B Member 1), CD9, and ALIX (apoptosis-linked gene 2-interacting protein X), isolated from cells, tissues, and most biofluids, were found to be the most prominent and represent the so-called pan-EVP markers
[10].
Tumour tissue biopsies and fine-needle aspiration biopsies are the most commonly used diagnostic methods in ORL-HNS. However, due to their invasive nature and inadequate representation of tumoral heterogeneity, liquid biopsies are being explored as an alternative to track the dynamics of the disease. Liquid biopsy uses a non-solid tissue sample, such as blood, saliva, urine or cerebrospinal fluid, for the same purposes as traditional biopsy. In addition, samples are investigated for specific biomarkers related to the disease in question, most often circulating tumour cells (CTC), circulation tumour DNA (ctDNA) or exosomes, which can confirm the diagnosis and enable a further follow-up
[11].
Diagnosis Based on DNA Content in EVs
Little research has been done on EV DNA and its relation to HNC and cancer other than HNC. Some studies suggest that EVs may contain tumour-specific DNA
[17][18], and others have additionally shown that EVs are loaded with DNA fragments without any apparent selectivity
[19]. In relation to HNC, more research is yet to be done on the topic, and should focus on detecting tumour and human papillomavirus (HPV)-specific DNA, which plays a vital role in HNSCC pathogenesis.
Diagnosis Based on RNA Content in EVs
More recently, the ability of EVs to transport different types of RNAs, such as messenger RNAs (mRNAs), micro RNAs (miRNAs or miR), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), to target cells has become an essential topic of research. So far, the most frequently studied have been miRNAs, which are small, non-coding RNAs that take part in RNA silencing and the post-transcriptional regulation of gene expression. miRNAs are therefore involved in dysregulations, which lead to the development and invasion of many types of cancer. For that reason, EV-derived miRNAs have been studied as potential biomarkers in different types of tumours, including HNSCC
[20]. In a recent systemic review, specific exosomal miRNAs from saliva that showed tremendous potential for use as oral and oropharyngeal cancer biomarkers included miR-10b-5p, miR-486-5p, miR-24-3p and miR-200a. All of these are involved in sustaining a favourable microenvironment for tumour growth, whether through modulation of the immune response, tumour cell cycle and proliferation, growth and migration, or invasion and metastasis
[21].
Another critical aspect in HNC diagnosis that has been considered in EV profiling is HPV status. HPV+ oropharyngeal cancer shows different molecular, histopathologic and clinical characteristics, affects a different group of patients, and may require less aggressive therapy, with a better outcome than HPV- cancer
[22]. An analysis of exosomes produced by HNSCC showed a distinction between HPV+ and HPV- cancer cells. In terms of their immune response, only HPV- cell-derived exosomes suppressed the maturation of dendritic cells and the expression of proteins involved in antigen processing machinery. Further investigations of exosomal miRNA revealed the overexpression of specific miRNAs, such as miR-1972 in HPV- cells and miR-205-5p in HPV+ cells, which may indicate their role in the alteration of antigen processing machinery and tumour immune responses
[23].
Another study, by Ramayanti et al. (2019), performed exosomal miRNA sequencing in patients with nasopharyngeal cancer, which is proven to be associated with EBV (Epstein–Barr virus) infection. The study identified a crucial positive correlation between higher circulating EV BART13-3p miRNA levels and nasopharyngeal cancer compared to healthy individuals or those with asymptomatic EBV infection. miR-BART13-3p promotes the migration of tumour cells and metastasis by driving epithelial–mesenchymal transition via the downregulation of the tumour suppressor AB12
[24]. Compared to healthy individuals, its detection appeared to be most specific and selective for nasopharyngeal cancer status in the early and late stages of the disease. This EV biomarker seems to outperform the more classic EBV-DNA load or EBV IgA serology, since it showed higher sensitivity and specificity than the other two more traditional methods
[25].
circRNA consists of large, non-coding RNA, which plays a vital role in gene expression regulation by inhibiting miRNAs. Due to its more excellent stability because of its circular structure, it has been thought to have more potential use as a biomarker than linear RNA. The upregulation of specific types of circRNA has been related to laryngeal cancer
[26] and the staging of oral cancer
[27].
Diagnosis Based on Protein Content in EVs
Since EVs function as carriers for signal molecules, most of which are proteins, several studies have investigated this type of cargo as a potential biomarker. So far, some of the potential candidates with overexpression in HNSCC-derived EVs are EGFR (epidermal growth factor receptor), PD-L1 (programmed death-ligand 1) and CD44, promoting increased proliferation, migration and metastatic potential in tumoral cells, as well as a poor prognosis
[9]. Another example was the isolation of EV proteins from metastatic oral cancer cells, whereby the molecular chaperones HSP90 (heat shock protein 90), TRAP1(TNF receptor associated protein 1) and HSP105 (heat shock protein 105) were shown to be the most promising metastatic and prognostic biomarkers. They play an essential role in protein-folding, stabilising growth factor receptors and regulating anti-apoptotic pathways, with their increased expression coinciding with a worse prognosis
[28].
A study by Qu et al. (2021) has identified a set of EV proteins, dysregulated in EVs, isolated from the plasma of patients with oral tongue squamous cell cancer with or without neck lymph node metastasis. Thus, out of many potential candidates, a lot of them have been established as a potential biomarker for oral tongue squamous cell cancer, some of them (such as platelet factor 4 variant, tubulin beta-4A chain, histone H2B type 2-E, and collagen alpha-1) being informative of nodal status as well
[29].