Tobacco habituated and non-habituated subjects exhibit different mutational spectrums in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common non-skin cancer in the world. Tobacco chewing is implicated with most of the cases of HNSCC but this type of cancer is increasing in non-tobacco chewers as well. This study was instigated to provide comprehensive variant and gene-level data in HNSCC subjects of the Indian population and fill the gap in the literature on comparative assessment of gene mutations in cancer subjects with a habit of tobacco and those without any habit using targeted amplicon sequencing. We performed targeted Amplicon sequencing of 409 tumor suppressor genes and oncogenes, frequently mutated across many cancer types, including head and neck. DNA from primary tumor tissues and matched blood was analyzed for HNSCC patients with a habit of tobacco and those without any habit. PDE4DIP, SYNE1, and NOTCH1 emerged as the highly mutated genes in HNSCC. A total of 39 candidate causal variants in 22 unique cancer driver genes were identified in non-habitual (WoH) and habitual (WH) subjects. Comparison of genes from both the subjects, showed seven unique cancer driver genes (KIT, ATM, RNF213, GATA2, DST, RET, CYP2C19) in WoH, while WH showed five (IL7R, PKHD1, MLL3, PTPRD, MAPK8) and 10 genes (SETD2, ATR, CDKN2A, NCOA4, TP53, SYNE1, KAT6B, THBS1, PTPRT, and FGFR3) were common to both subjects. In addition to this NOTCH1, NOTCH2, and NOTCH4 gene were found to be mutated only in habitual subjects. These findings strongly support a causal role for tobacco, acting via PI3K and MAPK pathway inhibition and stimulation of various genes leading to oncogenic transformations in case of tobacco chewers. In case of non-tobacco chewers it appears that mutations in the pathway affecting the squamous epithelial lineage and DNA repair genes lead to HNSCC. Somatic mutation in CYP2C19 gene in the non-habitual subjects suggests that this gene may have a tobacco independent role in development and progression of HNSCC. In addition to sharing high mutation rate, NOTCH gene family was found to be mutated only in habitual sample. Further, presence of mutated genes not earlier reported to be involved in HNSCC, suggest that the Indian sub-continent may have different sets of genes, as compared to other parts of the world, involved in the development and progression of HNSCC.