ReviewCan gene editing and silencing technologies play a role in the treatment of head and neck cancer?
Introduction
Head and neck cancer (HNC) is a major health problem and a leading cause of morbidity and mortality worldwide. It is regarded as the sixth most common cancer in the world. More than 90% of these malignancies are head and neck squamous cell carcinoma (HNSCC), which mostly comprise of oral squamous cell carcinoma (OSCC), oropharyngeal squamous cell carcinoma (OPSCC) and Laryngeal squamous cell carcinoma (LSCC) [1], [2]. Tobacco use in different forms (including smoking and smokeless tobacco), areca nut and alcohol consumption are the most common risk factors for the development of HNSCC [3], [4]. However, there is now irrefutable evidence of strong association of human papillomavirus (HPV) with HNSCC, especially for OPSCC in younger adult males [5], [6]. Current treatment of HNSCC still involves conventional surgery, chemotherapy, and radiotherapy, or a combination of these. Radical surgical excision followed by radiotherapy, with or without adjunct chemotherapy, is now commonplace. Due to the complex and delicate anatomical structure of the face and neck, the extent of surgery may be limited, as wide excision results in visible and functional facial deformities: radiotherapy and chemotherapy have low specificity for individual neoplasms and have toxicities affecting the whole body [7]. HNSCC show a wide arrangement of genetic aberrations, increasing with tumour progression and these contribute to a therapeutic failure with conventional therapies [8]. Although in Western countries the average 5-year survival rate for HNSCC patients is now ∼60%, one-third of patients develop local or/and regional metastases which reduce survival to less than a year [9], [10]. It is clear that there is a need for the development of new treatment approaches, which would better preserve the patient’s physical functions and their quality of life. Targeted molecular therapeutics are promising alternatives for treating cancer patients more efficiently and with limited side effects. One approach is blocking or silencing the expression of a specific gene (or genes) that is vital for continued growth of the neoplasm. Gene silencing or editing, via RNA interference (RNAi) and clustered, regularly interspaced, short palindromic repeats – associated with cellular apoptotic susceptibility protein (CRISPR/Cas) respectively, offer new possibilities for HNSCC treatment. The ability of RNAi to suppress the expression of target genes with high efficiency and specificity is well known. CRIPSR/Cas is a more recent discovery that allows the in situ manipulation of target genes but its clinical efficacy is yet to be proven [11]. The issue is which gene does one target and how will the therapy be delivered? Here we discuss gene editing and silencing mechanisms, potential candidate genes and review the various delivery technologies that could be used for HNSCC treatment. We highlight the potential path from bench to bedside for the improved treatment of HNSCC.
Section snippets
Gene editing and silencing
Both RNAi and CRISPR/Cas are programmable systems that allow one to specifically target a mRNA or gene, respectively. By providing each with sequence-specific RNA templates to their target, one is able to precisely change the output of that gene. RNAi achieves this via small non-coding RNAs such as short interfering RNA (siRNA), micro RNA (miRNA) or short hairpin RNAs (shRNAs) [12], [13], [14] and takes advantage of a normal cell regulatory system built into every eukaryotic cell.
CRISPR/Cas (gene editing) in cancer treatment
The recent advent of genome editing technologies based on programmable nuclease have improve the ability to delete or modify specific nucleic acid sequences in the genome of cells or in the animal models that aided us to explore the role of particular genes in the cancer progression and therapeutic responses. These nucleases include; the ZFNs, TALENs and most recently discovered the CRISPR associated nuclease Cas9 protein that enabled site-specific genome editing with the potential of using as
Currently available delivery technologies
The ultimate success of these therapies relies on the adequate delivery of the siRNA or CRISPR/Cas system into the cell. While siRNA delivery is either via synthetic RNA or a DNA plasmid, CRIPSR/Cas can be delivered in a variety of forms including guide RNA (gRNA)/DNA (expressing Cas protein), as gRNA/mRNA for Cas, or RNA/Cas protein. Nevertheless, the challenges for delivery are the same for both systems as delivery of RNA or DNA encounters a range of barriers that must be overcome. These
Target selection
The treatment-related toxicities, complications and recurrence rates, especially in advanced disease stages, with current conventional therapies have necessitated the focus on molecular targeted therapy. Currently, we are using the same treatment strategies to treat HPV-positive and HPV-negative HNSCC but they are clearly two different types of malignancy. Moreover, the key drivers of cancer formation and maintenance are also different in each type (and likely there are many more sub-types
Delivery in the HNSCC setting
Here we will consider delivery in the context of siRNA but as mentioned above, the challenges apply equally to CRISPR/Cas. In the HNSCC setting, the ability to deliver siRNA in the tumour microenvironment for effective gene silencing is a critical issue. Here we suggest several approaches that offer a potential way forward (Fig. 4). The preferred approach would be localised delivery directly to the tumour site as this avoids a number of issues found with systemic delivery, such as poor
Challenges and gaps
Both the RNAi and CRISPR/Cas system have the potential to play a major role in HNSCC treatment. RNAi-based therapies for HNSCC are still at the preclinical stage but show considerable promise. The CRISPR/Cas approach has the added advantages of being able to repress multiple target genes simultaneously and to permanently change genetic codes at genome level, but it is still too early to predict their clinical use. To date, only 10 early-phase clinical trials (phase I or phase I/II) of siRNA and
Conclusion
Even with the rapid progress of science and technology and advancement of treatment strategies, head and neck cancer remains a significant cause of morbidity and mortality worldwide. While RNAi therapies against cancer have not yet been tested in human trials, they have been shown to reduce the expression of targeted proteins with very limited side effects, both efficiently and specifically. This suggests their potential for treating cancers in humans efficiently. We believe that gene silencing
Conflict of interest
None declared.
Acknowledgements
MHS is funded by Griffith University, Australia. NAJM has received funding from the Cancer Council of Queensland and the National Health and Medical Research Council of Australia.
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