1. Introduction
Pancreatic cancer (PanC) is an aggressive malignancy of the digestive and endocrine system that develops in the head of the pancreas most commonly, as well as in the tail and body of the organ
[1]. The majority of PanC arises from exocrine glands of the pancreas, in which pancreatic ductal adenocarcinoma (PDAC) is the most common type, while the less common pancreatic tumours are of the endocrine type (e.g., pancreatic neuroendocrine tumour (PNET)
[2] (
Figure 1)).
Figure 1. Classification of pancreatic cancer.
The incidence of PanC has increased worldwide in recent decades and is expected to continually rise
[3][4][5][6][7]. Moreover, PanC is the seventh leading cause of mortality by cancer worldwide
[8]. Globally, the incidence and mortality rate of PanC is observed at 4.9% and 4.5%, respectively
[9]. In the Malaysian context, the incidence and mortality rate of PanC is rather low (2.2% and 3.6%, respectively) as compared to other Asian countries such as Singapore (3.4% and 6.2%, respectively) and Japan (4.3% and 9.6%, respectively) as well as worldwide
[9]. Regardless, its infamous reference as a silent killer steadfastly remains with its aggressive clinical symptoms only being presented at an advanced stage, thus posing a great challenge for intervention at the early stage of disease
[10]. Further, unlike cancers of the colorectum
[11], breast
[12] and lung
[13] which was reported to have a reduced mortality rate due to the advancement in their treatment modalities, limited options available for the treatment of patients with advanced PanC confer only a minimal effect to patients’ overall survival
[14]. Given this, a myriad of research is being conducted worldwide to develop effective biomarkers for PanC that aids in the early detection of the disease as well as to evaluate prognosis and monitor treatment response of PanC patients, which in turn could make room for unrestricted treatment and management options, in addition to improvement of its current dismal 5-year survival rate
[15][16][17][18].
The term ‘biomarker’ refers to the characteristics or signs that are measurable and as such could indicate the normal or pathogenic biological processes or responses to treatment
[19]. Along this line, a diagnostic biomarker detects and confirms the presence of disease while a prognostic/predictive biomarker identifies the outcomes of disease and/or tumour recurrence. On the other hand, treatment response biomarkers assess whether a particular treatment is beneficial to an individual.
There are numerous strategies exploited for the exploration and/or identification of biomarkers for PanC
[20]. There are those that are based on various DNA, mRNA, microRNA (miR), small nuclear RNA, long noncoding RNA, proteins, circulatory tumour cells, metabolites, and carbohydrates (glycans) (
Table 1) using different types of biological sample matrices. The development of cancer is a multistep process including various alterations of protein structures, functions, interactions, and expressions
[21]. In light of this, proteomics has emerged as one of the popular methodologies for PanC research
[22].
Table 1. Selected recently identified biomarkers for PanC.
| Target * |
Name |
Clinical Utility |
References |
| DNAs |
K-ras mutation |
Diagnosis |
[23] |
| Methylated ADAMTS1 and BNC1 |
Early diagnosis |
[24] |
| TP53 mutation |
Prognosis |
[25] |
| Mutations of BRCA2, EGFR, ERBB2 and KDR |
Monitoring treatment response |
[26] |
| Peritoneal lavage tumour DNA |
Prognosis/Monitoring tumour recurrence |
[27] |
| mRNAs |
WASF2 mRNA |
Early diagnosis |
[28] |
| EVL mRNA |
Prognosis |
[29] |
| FAM64A mRNA |
Prognosis |
[30] |
| MicroRNAs (miR) [31] ** |
miR-181c miR-210 |
Diagnosis |
[32] |
miR-10b miR-155 miR-216 |
Prognosis |
[33] |
| miR-196a |
Prognosis |
[34] |
| miR-21 |
Diagnosis/Prognosis/Monitoring treatment response |
[32][35][36] |
| miR-155 |
Monitoring treatment response |
[37] |
miR-142-5p miR-506 miR-509-5p miR-1243 |
Monitoring treatment response |
[36] |
| miR-451a |
Prognosis/Monitoring tumour recurrence |
[38] |
| Long noncoding RNAs |
SNHG15 |
Early diagnosis |
[39] |
HOTAIR MALAT-1 |
Prognosis |
[40] |
| LINC00460 |
Prognosis |
[41] |
| PVT1 |
Monitoring treatment response |
[42] |
| Circulating tumour cells |
|
Diagnosis |
[43] |
| |
Prognosis |
[44] |
| Vimentin (surface marker) |
Monitoring treatment response |
[45] |
| |
Monitoring tumour recurrence |
[46] |
| Metabolites |
Panel of acetylspermidine, diacetylspermine, indole-derivative and two lysophosphatidylcholines |
Early diagnosis |
[47] |
| Polyamines |
Diagnosis |
[48] |
| Ethanolamine |
Prognosis |
[49] |
Lactic acid L-Pyroglutamic acid |
Monitoring treatment response |
[50] |
| Carbohydrates (glycan) |
Alpha-2,6-linked sialylation and fucosylation of tri- and tetra-antennary N-glycans |
Diagnosis |
[51] |
| N-glycan branching: alpha-1,6-mannosylglycoprotein 6-beta-N-acetylglucosaminyltransferase A |
Early diagnosis |
[52] |
| β1,3-N-acetylglucosaminyltransferase 6 |
Prognosis |
[53] |
| Hyaluronan |
Monitoring treatment response |
[54] |
ADAMTS1—A disintegrin and metalloproteinase with thrombospondin motifs 1; BNC1—zinc finger protein basonuclin-1; BRCA2—Breast cancer susceptibility gene-2; EGFR—Epidermal growth factor receptor; ERBB2—Erb-b2 receptor tyrosine kinase 2; EVL—Ena/VASP-like; FAM64—Family with sequence similarity 64 member A; HOTAIR—HOX transcript antisense RNA; KDR—Kinase insert domain receptor; KRAS—Kirsten rat sarcoma viral oncogene homolog; LINC00460—Long intergenic non-protein coding RNA 460; LDLRAD3—Low density lipoprotein receptor class A domain containing 3; MALAT-1—Metastasis associated lung adenocarcinoma transcript 1; PVT1—Plasmacytoma variant translocation 1; RNU2-1—RNA U2 small nuclear 1; SNHG15—Small nucleolar RNA host gene 15; WASF-2—Wiskott–Aldrich syndrome protein family member 2. * Recently identified protein-based biomarkers for PDAC will be discussed in the subsequent section of this review. ** This has been previously extensively reviewed by Tesfaye et al. (2019).
1.1. PDAC: Risk Factors, Diagnosis, Staging and Treatment
PDAC typically appears as poorly defined masses with extensive fibrosis surrounding the tumour tissues known as desmoplasia. In PDAC, desmoplasia promotes the growth of the tumour and metastatic spread as well as inhibition of drug penetration and uptake
[55]. The presence of extensive desmoplastic stroma consisting of pancreatic stellate cells, proliferating fibroblasts, inflammatory cells and macrophages, marrow-derived stem cells and various other growth factors (e.g., epidermal growth factor, fibroblast growth factor, transforming growth factor-β), all of which are suitable for nourishing and facilitating invasive behaviour of the cancer cells, making it a distinguishing feature for PDAC
[56].
The development of PDAC is not de novo. It could originate from three types of precursor lesions which include mucinous cystic neoplasm, intraductal papillary mucinous neoplasm and pancreatic intraepithelial neoplasia (PanIN), with higher predominance
[57]. Individuals with chronic pancreatitis, diabetes, a family history of PDAC, genetic disorders (e.g., Lynch syndrome and Peutz–Jeghers syndrome) as well as poor lifestyle habits are generally at higher risk of developing this disease
[8].
At present, the gold standard for the diagnosis of PDAC is an endoscopic ultrasound (EUS)-guided fine-needle aspiration biopsy (FNAB)
[58] followed by histopathological examination (HPE) of the tumour tissue for microscopic characterisation and assessment
[59]. Further, EUS imaging supports the diagnosis of PDAC based on the visualization of tumour size (>2 cm), vascularity of the tumour mass (irregular arterial and/or absence of venous vasculature), irregular dilation of the pancreatic ducts, absence of cysts within the tumour mass and presence of lymphadenopathy
[58]. Other imaging modalities used in the diagnosis of PDAC include non-invasive imaging such as magnetic resonance imaging (MRI) and multidetector computed tomography (CT)
[60], and minimally invasive imaging such as endoscopic retrograde cholangiopancreatography
[61].
Despite its wide use in clinics, these imaging techniques are not without limitations
[62]. For example, EUS is not effective in differentiating malignant lesions from those of inflammatory masses, thus complicating treatment decisions particularly among the latter conditions
[63]. Further, FNAB is generally not recommended for body and tail tumours of the pancreas (PDAC) during EUS procedures due to needle tract seeding, thus posing a theoretical risk of spread through the biopsy needle
[64]. CT and MRI imaging, on the other hand, can easily overlook smaller lesions among patients with early-stage PDAC
[65] as well as occult and unsuspected tumour metastasis. In the latter case though, the staging laparoscopy is used to supplement the limitations of non-invasive imaging techniques
[66].
The designation of PDAC grading and staging are determined based on American Joint Committee Cancer (AJCC) Staging Manual
[67][68] that take both the histopathological grading (G) and TNM scores into consideration
[69][70]. The histopathological gradings (G1 to G3) are assigned based on the levels of glandular differentiation and pattern of tumour growth in the neoplastic pancreatic stroma on haematoxylin and eosin-stained tissue sections
[69]. On the other hand, the TNM system is an expression of the anatomic extent of the primary tumour (T), presence or absence of regional lymph node metastasis (N), and the presence or absence of distant metastasis (M) while the numerical subsets of the TNM components (T0, T1, T2… M1), indicate the progressive extent of the malignancy.
Pancreaticoduodenectomy or best known as the Whipple procedure remains the standard surgical treatment of care
[71], depending on the location of the PDAC. Unfortunately, since most PDAC cases are clinically presented at a very advanced stage, only less than 20% of patients qualify for surgical resection
[72]. Nevertheless, the 5-year survival rate of patients with PDAC who had undergone successful surgical resection was reported to range between 15% and 40% and despite advances in surgical techniques, the rate of tumour recurrence remains as high as 80%
[73]. On the other hand, patients diagnosed with locally advanced unresectable PDAC
[72] are otherwise subjected to palliative therapy/care (e.g., chemoradiotherapy or stereotactic body radiotherapy) and other appropriate disease management practice
[74].
Since 1997, gemcitabine, a drug that interferes with DNA synthesis, has been approved by the US Food and Drug Administration (FDA) and accepted as the first-line therapy for the alleviation of PanC symptoms in addition to exhibiting moderate improvement in patient survival rates
[75]. Given its favourable effect, combination drug therapies consisting of gemcitabine and other cytotoxic agents such as paclitaxel and docetaxel are now under clinical trials for PDAC
[76][77]. Aside from this, FOLFIRINOX, a multidrug containing fluorouracil, leucovorin, irinotecan, and oxaliplatin which was observed to improve the overall survival of PDAC patients
[78] is also under clinical trial but intended for the treatment of metastasised PDAC
[79]. Although both surgery and drug therapy persist as the standard treatment for PDAC, identification of novel biomarker(s) remains a necessity particularly in assessing the suitability of the selected treatment modalities, thus paving the way for personalised medicine and care of patients.
1.2. FDA-Approved Biomarkers for PDAC
The FDA plays an important role in the medical sector in which standards are established for the implementation of biomarkers into clinical practice
[80]. These biomarkers could either function as routine diagnostic tests, for evaluation of prognosis or to monitor treatment response and tumour recurrence in patients. In 1981, researchers discovered the over-production of cancer antigen 19-9 (CA 19-9) in patients with PDAC, apart from those with colon carcinoma
[81], which then later, was extensively studied for its potential use in the management of PDAC
[82]. Currently, CA 19-9 is the most routinely and widely applied biomarker for PDAC that has been approved by the FDA
[83][84]. The standard clinical threshold levels of CA 19-9 is at 37 U/mL
[82] and patients with increased levels are at high risk of developing PDAC
[81]. However, the utility of CA 19-9 in PDAC remains obscure owing to various interpretations of its applications in PDAC in the literature
[81]. Although commonly used as a diagnostic marker
[82], particularly in combination with imaging modalities such as MRI
[85], the clinical utility of CA 19-9 better serves to provide information on prognosis and overall survival
[82], monitor treatment responses
[86], predict post-operative recurrence and prognosis
[87], as well as to predict tumour stages and respectability in PDAC patients
[88]. Nevertheless, elevated CA 19-9 levels can also be caused by biliary obstruction, endocrinal, gynaecological, hepatic, pulmonary, and spleen diseases
[89] as well as other malignancies (e.g., colon, stomach, lung)
[90][91]. Furthermore, individuals with Lewis antigen-negative phenotype (lack of Lewis glycosyltransferase) do not express CA 19-9
[92], thus undermining the use of CA19-9 as a diagnostic biomarker for PDAC in this cohort.
Another FDA-approved tumour marker that has been reported in various studies in the context of PDAC is the use of carcinoembryonic antigen (CEA). The increased production of CEA was initially detected, and subsequently implemented for use as biomarker for colorectal cancer in the 1960s
[93]. However, it was later discovered that CEA levels were also elevated (>5 ng/mL)
[94] in approximately 30–60% of patients with PanC
[95] and significantly associated with poor prognosis and worse overall survival
[96]. However, CEA alone is deemed unsuitable for screening for PDAC due to its lower diagnostic accuracy
[97], but as a vital supplement to CA19-9
[82][95]. On a brighter note though, CEA can be potentially used as a diagnostic marker for Lewis antigen-negative individuals instead
[95].
2. Proteomics-Based PDAC Research: Techniques, Samples, and Samples Processing
The advancements in proteomic technology in recent years has enabled an in-depth exploration at cellular and molecular levels for a better understanding of complex diseases such as cancer
[98]. The wide range of proteomics tools and technologies allow systematic, comprehensive and/or targeted analyses of structure, function, expression, interactions, and modifications of proteins
[99]. Recently, non-gel-based separation and detection proteomics techniques, particularly hyphenated technology, which are typically represented by an online combination of a separation technique and one or more spectroscopic detection techniques has fast gained popularity
[100]. Amongst the hyphenated technologies, liquid chromatography tandem mass spectrometry (LC-MS/MS) has emerged as the most preferred methodology
[101] for proteomics-based (cancer) research
[102][103][104].
Further, quantitative proteomics has recently developed in its ability to generate reasonably accurate quantitation of the expression of proteins, especially in cancer research
[105]. The coupling of mass spectrometry that uses ionisation techniques, paired with mass analysers with quantitative labelling strategies was reported to improve the detection and quantification of proteins
[106]. Moreover, MS-based targeted proteomics methods such as multiple reaction monitoring (MRM) and parallel reaction monitoring (PRM) are now rising as promising tools for the validation of identified proteins in biomedical applications
[107].
Proteomics technologies were likewise widely applied in the investigation of PDAC in pursuit of the identification of potential PDAC-associated proteins intended for diagnosis and staging, assessment of tumour resectability, prognosis and predicting responses to treatment in patients
[108]. For this, various clinical specimens including pancreatic juice
[109], tumour tissues
[110] and cyst fluid
[111], plasma or serum
[112] and urine
[113] have been previously extensively used.
In addition to this, PanC cell lines are a useful source and/or model for biomarker investigations. This is because cell lines can be easily obtained and allow the analysis of secreted proteins by means of culturing and harvesting the cells
[92]. Different PanC cell lines (e.g., Capan-2, PANC-1) exhibit distinct phenotypes and genotypes that represent the different subclasses of PDAC (e.g., classical, quasi-mesenchymal, respectively) which when their proteins are profiled, may provide insights on the differential expressions of specific proteins associated with tumour growth
[114], metastasis
[115] as well as response to therapy
[116]. For example, by comparing a primary cell line, BxPC-3, with a metastatic cell line, AsPC-1, researchers were able to detect the differentially expressed proteins involved in the metastasis of PDAC that may serve as potential biomarker(s), once validated
[117].
Aside from this, the stromal compartment, which constitutes 80–85% of the tumour
[118], consisting of the extracellular matrix
[119] infiltrated by cancer-associated fibroblasts (CAF)
[120], inflammatory cells
[121], and immune cells (e.g., lymphocytes and macrophages)
[122] is another choice of sample for biomarker investigations. Researchers were able to identify differentially regulated proteins in the stroma of PDAC by co-culturing the PanC cell lines together with the components of stroma such as CAF
[123]. For example, the differentially regulated proteins expressed in the stromal compartment such as hemopexin were previously reported to regulate the progression of tumours, thus revealing association with lymph-node metastasis resulting in poor prognosis in patients
[124]. Similarly in another study, Tao et al. reported that PDAC has an extensive stroma and abundant extracellular matrix that lacks vascularisation, resulting in hypoxia within the pancreatic tumour environment, thus revealing candidate biomarkers that may be useful in monitoring the treatment response in PDAC patients
[125].
On the other hand, exosomes are extracellular vesicles produced by cells that are involved in cell communication
[126]. The isolation and identification of cancer-specific proteins derived from exosomes may be developed into potential candidate biomarkers since the altered protein profiles of exosomes correlate with the pathogenesis of cancer
[127]. For example, Melo et al. reported glypican-1 (GPC1), a cell surface proteoglycan that is specifically enriched in exosomes derived from cancer cells in PDAC patients by using MS techniques
[128]. According to their research, GPC1 may serve as a potential biomarker for early stage PDAC. This shows that the analysis of exosomes can also be a useful target for potential clinical utility.
Glycosylation, a type of post-translational modification of proteins, is a hallmark of cancer
[129]. As such, targeting the aberrations occurring during glycosylation either in terms of the levels of expressions of proteins or the glycans (glycome; e.g., truncated
O-glycans, increased branching and fucosylation of
N-glycans, upregulation of specific proteoglycans and galectins, and increased
O-GlcNAcylation
[130]) may be beneficial for the identification of diagnostic and/or therapeutic targets
[131] as well as for aiding in treatment decisions for PDAC
[132]. Along this line, mucin, a highly glycosylated protein, could also be a potential biomarker
[133]. The levels of expression of mucin were previously reported to be altered in the course of development and progression of PDAC
[134]. According to Wang et al., PDAC tissues expressed high levels of MUC1, MUC4, MUC5AC, MUC5B, MUC6, MUC13 and MUC16, all of which are involved in promoting the aggressive phenotype of this disease
[134]. These findings further highlight the roles of aberrant protein glycosylation in the progression of PDAC.
The most critical step in a proteomics workflow is the optimal preparation and/or processing of samples, which in turn, ascertain the degree of sensitivity for detection of proteins in downstream application
[135]. The complexity of the samples due to high biological variations and/or post-translational modifications remains a major limitation in proteomics study. Furthermore, samples such as serum and/or plasma contain significant amounts of proteins of higher abundance (e.g., albumin, immunoglobulins) that are involved in various yet important biological and physiological functionality, hence may not be useful as potential markers
[136]. In turn, due to their predominant presence, these proteins are infamously known for masking away other proteins, particularly low molecular weight proteins that are present in lower abundance, but may have biomarker potential (e.g., small secreted proteins or peptide hormones)
[137]. The complexity of serum or plasma is usually reduced via depletion of high-abundance proteins
[138] or using sub-proteome-specific enrichment method
[139]. While depletion workflows help to improve the detectability of low-abundance proteins, enrichment methods are focused on increasing the sensitivity of targeted sub-proteomes for detection
[140]. On this note, Hashim et al. reviewed the use of lectins to enrich glycosylated proteins which are differentially expressed in cancer, as they might not be detected using conventional methods due to their low abundance
[139]. Apart from this, immunoprecipitation is another method commonly used to enrich target proteins by using antibodies that bind to specific antigens resulting in immune complexes that are captured on solid phase support such as chromatography resin and magnetic beads
[141].