Head and Neck Cancers Are Not Alike When Tarred with the Same Brush: An Epigenetic Perspective from the Cancerization Field to Prognosis

Abstract

Head and neck squamous cell carcinomas (HNSCC) are among the ten most frequent types of cancer worldwide and, despite all efforts, are still diagnosed at late stages and show poor overall survival. Furthermore, HNSCC patients often experience relapses and the development of second primary tumors, as a consequence of the field cancerization process. Therefore, a better comprehension of the molecular mechanisms involved in HNSCC development and progression may enable diagnosis anticipation and provide valuable tools for prediction of prognosis and response to therapy. However, the different biological behavior of these tumors depending on the affected anatomical site and risk factor exposure, as well as the high genetic heterogeneity observed in HNSCC are major obstacles in this pursue. In this context, epigenetic alterations have been shown to be common in HNSCC, to discriminate the tumor anatomical subsites, to be responsive to risk factor exposure, and show promising results in biomarker development. Based on this, this review brings together the current knowledge on alterations of DNA methylation and microRNA expression in HNSCC natural history, focusing on how they contribute to each step of the process and on their applicability as biomarkers of exposure, HNSCC development, progression, and response to therapy.

Keywords: DNA methylation; HPV; alcohol; biomarker; field cancerization; head and neck cancer; microRNA; tobacco.

Figure 1

Overview of gene regulation by DNA methylation and miRNA activity. Genomic DNA, found in the nucleus of eukaryotic cells, contains millions of CpG sites (lollipops) that can be either methylated (dark blue) or demethylated (light blue). These CpG sites are located in gene promoters and bodies, miRNAs, and intergenic regions, as well as in the vicinity of transposable elements (TE). When a promoter region is demethylated (Gene A), gene transcription may take place, leading to mRNA synthesis. However, when the promoter is hypermethylated (Gene B), transcription is inhibited. TEs transcription goes through the same regulatory mechanism. The transcription of some of these elements leads to the production of the transposition machinery, which enables their retrotransposition to a new genomic region, in a “copy–paste” fashion. DNA methylation similarly regulates the expression of miRNAs. Once transcribed, the primary miRNA (Pri-miRNA) is processed still in the nucleus by the RNAse Drosha, forming the precursor miRNA (Pre-miRNA). The Pre-miRNA is exported to the cytoplasm, where it is further processed by the RNAse Dicer, forming the miRNA duplex. After the miRNA duplex unwinds and the degradation of the passenger strand takes place, the lead strand (mature miRNA) assembles to the RNA-induced silencing complex (RISC) and, by nucleotide complementarity, identifies their target mRNAs. A single miRNA may have several mRNA targets and will lead to their degradation (perfect annealing) or translation inhibition (partial annealing).

Figure 2

Risk factors associated with HNSCC development affect DNA methylation and miRNA expression, showing different mechanisms by which tobacco components, alcohol, and HPV infection may lead to aberrant DNA methylation profiles and aberrant miRNA expression. NNK, 4-(methylnitro-samino)-1-(3-pyridyl)-1-butanone; BaP, benzo(a)pyrene; TCA Cycle, tricarboxylic acid cycle.

Figure 3

The natural history of HNSCC development through an epigenetic perspective. The exposure of the head and neck healthy epithelium to risk factors, such as tobacco and alcohol, may lead to the accumulation of DNA methylation alterations and miRNA aberrant expression even before the first morphological changes are observed. This characterizes a molecular cancerization field that may favor the development of pre-malignant lesions. In head and neck, the presence of leukoplakias and erythroplakias, frequently associated with dysplasia, increases the risk of tumor development, and these lesions are characterized by specific epigenetic alterations. The epigenetic aberrations observed in both the cancerization field and in the pre-malignant lesions, have the potential to be used as early markers of HNSCC carcinogenesis in screening programs. Finally, tumor onset and progression may be followed by further aberrant DNA methylation and miRNA expression. HPV-positive HNSCC epigenetic natural history is largely unknown.

Figure 4

Venn diagram with the number of microRNAs differentially expressed (upregulated and downregulated) between tumors and nontumor tissues in several studies with specific subsets of HNSCC (larynx [241,242,243,244], nasopharynx [245], hypopharynx [246,247], and oral cavity [248,249,250,251]) or aggregating several sites in the analysis (Misc./HNSCC) [252,253,254]. The list of differentially expressed microRNAs can be found in Table S1.