Tobacco smoking and alcohol consumption are the strongest risk-factors for head and neck carcinogenesis around the world and is the major public health problem in Asian countries. Worldwide, more than 1.1 billion people smoke tobacco. But the incidence of tobacco smoking is high and is ever-increasing in low or middle-income countries like India 8. Despite the fact that use of tobacco and alcohol annually kills more than seven million people, these products are legally sold by manufacturers in all over the world. As per the recent Global Adult Tobacco Survey India Report (2016-2017), more than 199 million smokeless tobacco users are living in India 9. These lifestyle factors play a significant role in the aetiology of HNCs 10. Globally, ~90% of HNCs are associated with avoidable well-known lifestyle factors such as tobacco chewing/smoking and alcohol drinking 111213141516171819. Regardless of the fact that majority of research studies on HNCs and other diseases are primarily focused on the use of alcohol and tobacco as well-established classical factors, their rates of consumption in many countries specially in low and middle income countries remain high, requiring a better understanding of the mechanisms in HNCs that are exposed to these risk-factors 20. In addition to the carcinogenic effects of tobacco, alcohol, and their metabolites, studies have demonstrated that tobacco products along-with other life style factors (virus/bacterial infections and poor oral hygiene) can directly damage DNA, inducing DNA repair activity. Defective DNA repair can influence and modulate different genetic and epigenetic signalling pathways which effect transcriptional activation, DNA methylation, histone modifications, as well as the altered expression of noncoding RNAs (miRNAs, PIWI-RNAs and LncRNAs etc.), which play critical roles in the genesis of different cancers including HNCs 14151618.

Majority of the human genome is transcribed (ENCODE Consortium) and the 20,000 protein-coding genes represent only ⁓2% of the total genome, whereas about 98% is transcribed into RNA that does not code for protein. These non-protein coding RNA molecules comprise of functional RNA molecules 2122 which are categorized into housekeeping RNAs and regulatory RNAs 23. The housekeeping RNAs are constitutively transcribed and includes ribosomal (rRNA), transfer (tRNA), small nuclear (snRNA), and small nucleolar RNAs (snoRNAs). The regulatory group of ncRNAs on the other hand are divided into two groups, one that are shorter than 200 nucleotides in length (small ncRNAs) and the ones that greater than 200 nucleotides (long ncRNAs). ncRNA molecules are represented by microRNAs (miRNAs), small interfering RNAs (siRNAs), and PIWI-interacting RNAs (piRNAs), circular RNAs (circRNAs), tRNA-derived small RNAs (tsRNAs) and long ncRNAs (lncRNAs) (Figure 1).

Long non-coding RNAs are a highly diverse group of regulatory ncRNAs with respect to characteristics, localization and modes of action 24. LncRNAs are ubiquitously transcribed in the human genome and are emerging as essential players regulating genomic imprinting, shaping chromosome conformation and allosterically regulating enzymatic activity. On the basis of their functions and regulation, lncRNAs are categorised into 3 types, (i) non-functional lncRNAs that are likely to be the result of transcriptional noise, (ii) lncRNAs for which the act of transcription alone is sufficient for their function but the transcript itself is not necessary and (iii) functional lncRNAs that can act in cis and/or in trans25. lncRNAs with a length of >200 nts, are localized in the nucleus. In the nucleus, they modulate transcription by sequestering transcription factors (TFs) or chromatin-modifying enzyme complexes and thus, regulating biological function. The altered expression of lncRNAs is documented as a new hallmark feature in variety of cancers including HNCs 26. Furthermore, disrupted expression of lncRNAs in HNC cells is associated with different stages of cancer and cancer progression (Figure 2).

LncRNAs, may act as oncogenic and tumor-suppressive thereby modulating clinical outcome of HNC patients. There are few reports that have found a link between tobacco smoking/alcohol consumption and altered expression of lncRNAs. These lncRNAs regulate molecular processes such cellular differentiation, apoptosis, angiogenesis, proliferation and inflammatory pathways which in turn lead to carcinogenesis 27. The studies on the association between lncRNAs and addiction habits in head and neck cancer are still preliminary and sparse 2829.

In a study, Yu and co-workers (2016) demonstrated altered role of lncRNAs in alcohol induced HNC. The authors found the altered expression of lnc-PSD4-1 and lnc-NETO1-1 in alcohol and acetaldehyde induced HNCs 30. In a recent study, expression profiling analysis of lncRNAs showed that 9 out of 11 analysed, were significantly dysregulated in tobacco chewer/smoker HNC patients. Further, altered regulation of linc-RoR ceRNA was observed in undifferentiated tumors which altered treatment response 31. The altered expression level of HOTAIR (HOX transcript antisense RNA) is associated with many types of cancers including oncogenesis, metastasis and poor prognosis in HNC. HOTAIR is emerged as a potential biomarker for HNC diagnosis and prognosis 7323334. It is reported that polymorphisms could affect the expression of HOTAIR. The rs874945 polymorphism is located in the intron of HOTAIR gene. Recently, rs4759314 polymorphism present in the first intron of HOTAIR, contributes to an increased risk of HNC and serves as a potential biomarker 35. Loss of expression of LINC01133was found in oesophageal cancer tissues and cell lines indicating that it may have an anti-tumor effect in the early grades of oesophageal cancers. Interestingly, LINC01133 expression is markedly lower in patients who were regular drinker and serves as a possible poor prognostic marker and drug target for oesophageal cancers patients 36. Further, alcohol intake has been associated with reduced expression of AC012456.4 in oral cancer patients 37. rs11160608 polymorphism in MEG3 was significantly associated with alcohol-induced oral cancer patients and thus, might play an essential role in oral cancer pathogenesis 38.

LncRNA expression is altered in various types of cancer. A number of lncRNAs play major roles in HNC metastasis based on their subcellular localization 39. Most of these lncRNAs are located in nucleus and function in chromatin modifications and transcriptional regulation, examples include HOTAIR, MALAT1. On the other hand, lncRNAs such as FOXCUT, CCAT1, LIN00312 and NKILA (Nuclear Factor-kB interacting lncRNA) are localized in the cytoplasm and function as regulator/modulators of mRNA stability, translational controls and protein stability 39.

The above mentioned studies provide evidence for lncRNA as potential therapeutic targets. A number of techniques are available, that target lncRNAs. These have been reviewed by Arun et al., 2018 and include RNAi, antisense oligonucleotides (ASO), CRISPR/Cas, RNA blocking oligonucleotides and small-molecule modulators. RNAi as a therapeutic technique is challenged due to the complexity of in-vivo experiments as well as lack of proper delivery methods. Advancements in ASO chemistry has led to the development of Locked Nucleic Acids (LNA) and S-constrained ethyl (cEt) modifications, which have considerably improved potency and pharmacokinetics profiles 40. Subcutaneously delivered ASO targeting Malat1 in luminal B breast cancer mouse model led to reduction (80%) in metastasis 41. MALAT1 ASO is now being used in many types of metastatic cancers as a potential therapy 42. The engineered CRISPR/Cas13 technique can be used to knockdown the lncRNAs 43. This technique is engineered for mammalian RNA binding and knockdown 43. Small molecules inhibitors may be utilized to target secondary or tertiary structures for lncRNAs, although this technique is still in its infancy 44.