Liquid biopsy: miRNA as a potential biomarker in oral cancer
Introduction
Head and neck squamous cell carcinoma (HNSCC), the ninth most common neoplasm across the globe, is a significant cause of cancer morbidity and mortality [1]. HNSCC mainly comprises neoplasms of the oral cavity, pharynx, larynx, paranasal synapses, nasal cavity and salivary glands. According to the recent GLOBOCAN 2018 report, cancers of the lip and oral cavity are the most frequent cancer in Melanesia and South Central Asia [2]. It is the leading cause of cancer death among men in India and Sri Lanka [2]. Oral squamous cell carcinoma (OSCC) is the major component of HNSCC, accounting for about 90% of all oral malignancies representing a major global burden with an estimated 300,373 cases per annum [1]. The recent GLOBOCAN 2018 report estimated 354,864 new cases of lip and oral cavity cancer per annum with around 1.8 million deaths predicted in 2018 [2]. Oral cancer consists of malignant neoplasms arising in the lips, hard palate, upper and lower alveolar ridges, sublingual region, buccal mucosa, anterior two-thirds of the tongue, retromolar trigone and floor of the mouth [3,4]. Variations in the incidence of oral cancer across the globe are generally attributed to differences in habits practised by different populations [1]. The habits of smoking, chewing tobacco, areca nut, betel quid, alcohol consumption, poor oral hygiene, chronic mechanical trauma and Human Papillomavirus (HPV) infections are considered to be the major factors for the higher incidence of OSCC in India [3,5]. Oral potentially malignant disorders (OPMD) are also reported to advance to malignancies, if left untreated [5].
One of the main reason for poor survival of oral cancer patients is late detection. Clinical examination of the oral cavity and biopsy of the suspected lesion followed by histological analysis are generally used for diagnosis. Biopsy is an invasive procedure in which a portion of the suspected malignant tissue is obtained and further subjected to specialized and sophisticated histopathology or cytology procedures. Biopsy is gold standard till date in detecting the histopathological type of a neoplasms and its degree of differentiation, and has been in practice since the 11th century [6,7]. Sometimes it becomes difficult to procure biopsy material due to inaccessibility of certain tumors, physical pain involved after the procedure, surgical complications, financial burden and lack of trained clinicians. Cell free nucleic acids (cfNAs), such as cell free DNA and RNA are present in body fluids of all individuals [8,9]. The expression profile of cfNAs varies between different disease states, so liquid biopsy has potential as a minimally invasive method for detection of disease, including malignant neoplasms [10,11].
Liquid biopsy first came on the scene in 1984 when Mandel and Metais referred to cfNAs as free floating nucleic acids in blood [8,9]. The source of cfNA in body fluids of cancer patients is considered to be apoptotic and necrotic cells of the neoplasm itself, and of other cells in the tumour microenvironment. Secretion has also been suggested as a potential source of cfNA [8]. Circulating tumor cells were first observed in the blood of a cancer patient in 1869, but it took more than a century before their entry into commerce. The clinical utility of liquid biopsy was first realized in 1994 when point mutations of a gene were successfully detected from the cfDNA derived from the plasma of acute myelogenous leukemia patients, whereas the DNA derived from the cellular component of blood did not show any mutations [12].
MiRNAs are usually secreted into the body fluids in membrane bound vesicles known as exosomes. Exosomes are 50–100 nm-sized membrane bound molecular carriers that play important roles in cell-cell interaction [13]. Release of miRNAs from exosomes is a significant mechanism of genetic exchange between cells [14]. Circulating miRNAs are extremely stable and can be conveniently used as informative biomarkers for complex diseases such as cancers [15]. Recent studies of circulating miRNAs in plasma, serum, and other body fluids show that miRNAs secreted from a particular cell type not only has a local action, but may also act at distant sites [16,17].
Section snippets
Serum/plasma miRNA as a biomarker in oral cancer
Circulating miRNAs have been demonstrated to show different levels in the body fluids of OSCC patients compared to healthy individuals [18,19]. MiRNAs are highly stable in the serum or plasma leading to rising hopes for their future use as cancer biomarkers [20]. Circulating miRNAs are not digested by RNase and are stable at high pH, after boiling and after multiple freeze-thaw cycles [21,22]. While bound to the argonaute protein, miRNAs are stable in the extracellular environment whether
Salivary miRNA as a biomarker in oral cancer
“Whole mouth fluid”(WMF) is a complex biological fluid containing saliva secreted by three paired major salivary glands, namely parotid, submandibular and sublingual, plus a large number of widely distributed minor salivary glands. There is a significant contribution from gingival crevicular fluid, an inflammatory serum and cellular exudates [35]. Moreover, WMF contains serum and desquamated epithelial cells from the oral mucosa, including some leucocytes, especially if there is ulceration or
Similar and dissimilar expression profiles of miRNAs in the saliva/WMF and serum/plasma of OSCC patients
Several miRNAs are reported to show similar concentrations in different body fluids of OSCC patients. For example, miR-24, miR-146a, miR-184, miR-31, miR-451 and miR-26a have been reported to be mostly uniformly expressed both in the saliva/WMF and serum or plasma of OSCC patients compared to healthy controls (Table 3). The presence of these miRNAs in different body fluids as well as among different populations suggests that they can be used as biomarkers for oral cancer detection irrespective
MiRNAs as prognostic biomarkers in oral cancer
The expression patterns of certain miRNAs have shown positive correlation with clinical stage, lymph node metastasis and patient survival, indicating that these miRNAs can act as prognostic predictors in OSCC. Higher expression levels of miR-372 is shown to induce nodal metastasis and poor prognosis of oral carcinoma [53], while miR-134 showed association with number of metastases in HNSCC [54]. MiR-146a is reported to show increased metastasis by down-regulating the expression of IRAK1, TRAF6
Therapeutic role of circulating miRNAs in oral cancer
The ability to manipulate miRNA expression and function by local and systemic delivery of miRNA inhibitors or miRNA mimics has recently gained immense interest as a novel therapeutic approach [68,69]. The advantage of miRNA based cancer therapy lies in the ability of miRNAs to concurrently target multiple effectors of pathways involved in cell proliferation, differentiation, and survival. However, the major challenges are in vivo delivery of these polyanionic oligonucleotides. Naked miRNAs
Conclusion
In oral cancer, the lesion is usually visible and easily accessible for procuring biopsy, therefore, liquid biopsy may sound redundant in oral cancer diagnostics. But, biopsy is an invasive procedure and involves physical pain to the patients. Hence, liquid biopsy plays a pivotal role in non-invasive detection of cancer at early stages of the disease.
Several miRNAs have been reported to be deregulated in the body fluids of OSCC patients compared to healthy controls suggesting that these miRNAs
Funding
This work was supported by the CSIR extramural (Grant no 27(0306)/14//EMR-II) to RC, JR and KC and ISI intramural funding to RC.
Conflict of interest
The authors declare no relevant conflicts of interest.
Acknowledgments
SM is thankful to CSIR for providing the CSIR-NET fellowship.
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Present address: Burdwan Dental College and Hospital, Power House Road, Shyamlal Colony, Khosbagan, Bardhaman, West Bengal, 713101, India.