miRNAs as therapeutics and biomarkers for pancreatic cancer

Despite recent advances in our understanding of this disease, pancreatic cancer is one of the world’s most lethal malignancies. Although the disease accounts for only about 3% of all cancers, it is regarded as the 7^th leading cause of cancer related deaths worldwide (Siegel, R et al., 2019). Worryingly, the overall 5-year survival rate for pancreatic cancer is less than 6%; where most patients die within 3 to 8 months after diagnosis and can be regarded as the ultimate death penalty. Chemoresistance and radioresistance are leading to tumour recurrence and metastatic lesions with enhanced aggressiveness, and a consequent poor prognosis, is a major reason for treatment failure and shortening of patient survival (Swayden, M et al., 2018, Seshacharyulu, P et al., 2017). Hence, there is an urgent need for a useful therapeutic target for enhancing chemosensitivity and radiosensitivity in pancreatic cancer.

MicroRNAs (miRNA) have attracted significant attention because of their potential to modulate multiple biological processes including cell development, cell differentiation, and cell survival (Hammond, S. 2015). MiRNAs belong to the non-protein-coding RNA family which are short, single-stranded RNA approximately 18-24 nucleotides in length that negatively regulates gene expression at the post transcriptional level (Figure 1).They are transcribed by RNA polymerase II and are responsible for the degradation of messenger RNA (mRNA) via complete complementarity or may result in the inhibition of mRNA translation into protein via incomplete complementarity. MiRNAs are considered as ideal therapeutic targets because many miRNAs can target a specific mRNA, and many mRNAs can be targeted by a specific miRNA. Furthermore, as the majority of miRNAs are associated in all pathways and cellular processes within the cell, it is not surprising that miRNA dysfunction is viewed as a fundamental feature of cancer and is considered instrumental in the acquisition of the hallmarks of cancer; such as invasion, angiogenesis and evasion of apoptosis (Hwang, H. and Mendell, J, 2006). Accumulating evidence has also revealed that miRNAs play a pivotal role in sensitising cancer cells to chemotherapy and radiation, thus improving and prolonging quality of life (Mognato, M. and Celotti, L, 2015).

Figure 1. Biogenesis of miRNA. MiRNA genes are transcribed in the nucleus by RNA polymerase II producing a long, single stranded RNA (pri-miRNA); which folds into a hairpin structure and is processed by Drosha and DGCR. The 5’ and 3’ ends are asymmetrically cleaved to produce the pre-miRNA. The pre-miRNA is exported into the cytoplasm via exportin-5 and Ran-GTP on the nuclear membrane where it is further processed in the cytoplasm by Dicer and TRBP. The loop of the hairpin structure is cleaved to produce the miRNA duplex. The passenger strand is degraded whilst the mature strand complexed with Argo proteins. The Ago proteins constitute the major functional element of the RNA induced silencing complex (RISC). The exposed bases of the mature miRNA bind to mRNA via perfect or imperfect complementarity, Subsequently, translation of the mRNA is suppressed. 

Interestingly, it is proposed that cancer-associated miRNAs either have an oncogenic or tumour suppressive property, this relates to tissue type and location of the cancer (Zhang, B, et al., 2007). Moreover, miRNAs may act as biomarkers and can be appropriately inhibited or replaced, depending on the properties they present with (Figure 2).

Figure 2. MicoRNAs as Biomarkers; Inhibition/Replacement Therapy. MiRNAs have either oncogenic or tumour suppressive properties. And if these miRNAs become dysfunctional, an appropriate treatment can be applied to either suppress the overexpressed oncomiRs or to overexpress the suppressed TsmiRs, restoring cellular normality and functionality.

The overexpression of miRNAs that target tumour suppressor genes in cancer led to the development of the term oncomiR. Numerous therapeutic strategies have been designed including sequence-based inhibitors, developed to efficiently target and suppress the overexpressed miRNA (Oliveto, S et al., 2017). These inhibitors can potentiate the cleavage or sequestering of the target miRNA into processing bodies, thus the threat of miRNA inhibiting mRNA translation is eliminated. Generally, miRNA inhibitors can be classified accordingly into antagomiR or decoys known as miRNA sponges. MiRNA inhibitors which directly target mature miRNA via perfect complementarity are essentially antisense oligonucleotides. Currently, one of the leading antimiR is antimiR-122, which was designed to target thus suppress miR-122 in the liver (Jopling, C. 2010). MiR-122 is the most profusely expressed miRNA within the liver and is necessary for hepatitis C virus (HCV) replication; which is known to increase the risk of liver hepatitis and hepatocellular carcinoma. Astonishingly, it was demonstrated that HCV infection could be reduced by up to 300-fold when administrating antimiR-122 (Jopling, C. 2010), displaying the power behind modulation of a single miRNA.

MiRNAs themselves can also be tumour suppressors in addition to miRNAs that target tumour suppressors; and the downregulation of these particular miRNAs may result in cancer progression (Oliveto, S et al., 2017). Essentially, the reintroduction or overexpression of tumour suppressing miRNAs have been enabled by miRNA plasmids or lentiviral based expression vectors, where miRNA mimics are also exploited. Intriguingly, one particular group designed and developed TargomiRs (Winata, P et al., 2017) which is a nanocell based delivery system to transport a miR-15 & miR-107 sequence which targets the anti- coated tumour in non-small cell lung cancer (NSCLC) and malignant pleural mesothelioma (MPM) patients. Amazingly, this research entered Phase I trial in MPM back in 2014, conveniently named MesomiR-1, and has so far isolated five out of six patients benefiting through disease control, with acceptable tolerance for the drug.

MiR-31 is encoded on a genomic fragile site, 9p21.3, which is reportedly lost in the majority of pancreatic tumours, and its prognostic value is currently equivocal. By using a combination of patient tumour sample cohorts and novel cell models, it has been recently demonstrated in a variety of cancers that miRNA-31 loss influences cellular sensitivity to chemotherapy (Moody, H et al., 2016) and radiotherapy (Lynam-Lennon, N et al., 2012). Furthermore, it has also been displayed that replacement of lost microRNA-31 with a synthetic mimic, or suppression of endogenously expressed microRNA-31 with an anti-sense inhibitor, can significantly modulate sensitivity to chemotherapy (5-FU and cisplatin/oxaliplatin) and radiation (Moody, H et al., 2016, Lynam-Lennon, N et al., 2012). Finally, the mechanism(s) by which miRNA-31 regulates sensitivity is, in part, through the modulation of expression of multiple DNA repair genes and altered DNA damage response-repair efficiency, and through the regulation of intracellular chemotherapy drug trafficking between the inner cell cytoplasmic compartment and the nuclear environment housing the target DNA.

As new roles for miRNAs in cancer continue to be discovered, their future effect on diagnosis, prognosis, and treatment of patients remains to be seen. MiRNA profiling holds potential for differentiating between normal and tumour cells and between different tumour subtypes. The potential to use miRNAs as biomarkers for disease is reinforced by their stability in human serum and plasma, making miRNAs ideal therapeutic targets. Instead of using invasive procedures to extract tissue from patients’ tumours, miRNAs can be measured directly from the patients’ blood products which may significantly reduce patient stress and laboratory costs. MiRNAs plays a significant role in promoting cellular sensitivity to chemotherapy and radiation, therefore, may be used to help improve standard of care for patients with pancreatic cancer and have a positive impact on prolongation in overall survival.

References

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Lynam-Lennon, N., Reynolds, J., Marignol, L., Sheils, O., Pidgeon, G. and Maher, S. (2012). MicroRNA-31 modulates tumour sensitivity to radiation in oesophageal adenocarcinoma. Journal of Molecular Medicine, 90(12), pp.1449-1458.

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Winata, P., Williams, M., McGowan, E., Nassif, N., van Zandwijk, N. and Reid, G. (2017). The analysis of novel microRNA mimic sequences in cancer cells reveals lack of specificity in stem-loop RT-qPCR-based microRNA detection. BMC Research Notes, 10(1).

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