Detection of oncometabolites 1-methylnicotinamide, nicotine imine and N-Methylnicotinium in nails of oral cancer patients and prediction of them as modulators of DNMT1

BackgroundThe prominent among various existing views on the role of nicotine and nicotine-metabolized products in Oral squamous cell carcinoma (OSSC) is metabolic adaptation that allows the use of methyl-donor S-adenosylmethionine (SAM) for non-epigenetic purpose including the methylation of nicotinamide and nicotine. In fact, channeling of SAM for generation of 1-methylnicotinamide (1-MNA) and methylated nicotine products is seen as a key event in cancer cells that allows favorable epigenetic states by forcing DNA hypomethylation. A better perception of such events can be appreciated by analyzing samples like nail, which represents a perfect biological material for studying long-term metabolic reflections of the body. MethodsPotential nicotine-metabolized products and 1-MNA in nails of OSCC patients were analyzed by using a novel approach of Vertical tube gel electrophoresis (VTGE)- assisted purification followed by their identification by LC-HRMS. Further, these identified nicotine metabolized products and 1-MNA were evaluated for their molecular interactions with known methyltransferases including cytosolic nicotinamide methyltransferase (NNMT), DNA methyltransferase (DNMT)1 and histone methyltransferases by molecular docking and molecular dynamics simulation (MDS) analyses. ResultsOur data suggests the presence of N-methylnicotinium ion and nicotine imine in the nail samples of OSCC patients. Further, 1-MNA is also detected in the nails as a major enzymatic product of a known detoxifying enzyme NNMT. Molecular docking of all nicotine and nicotine metabolized products with DNMT1revealed a specific binding affinity of nicotine imine only with a -6.2 Kcal/Mol docking energy. Importantly, binding of nicotine imine is within the CXCC regulatory domain of DNMT1 and it displays molecular interactions with the key amino acid residues, namely ARG690, PRO574, VAL658, PRO692 and ALA695. Furthermore, MDS data corroborated well with the specific binding affinity of nicotine imine to DNMT1 obtained by docking analysis. ConclusionIdentification of N-methylnicotinium ion, nicotine imine and 1-MNA in nail samples indicates their potential as predictive and detectable biomarkers for OSCC. Molecular docking and MDS data lead us to propose a role of nicotine imine in modulating the activity of DNMT1. These data further suggest a novel understanding on the role of nicotine metabolized products in modulating DNA methylation that may contribute to tumorigenicity in oral cancer patients.