Distinct Molecular Drivers Identified In Oral Cancers Among Non-Smokers And Non-Drinkers

Head and neck cancers, particularly those arising in the oral cavity, have long been associated with established risk factors such as tobacco use, alcohol consumption, and infection with oncogenic strains of Human papillomavirus (HPV).

These factors account for the majority of oral cavity squamous cell carcinomas (OCSCC), one of the most common forms of head and neck cancer.

However, clinicians are increasingly encountering cases among individuals with no identifiable exposure to these traditional risk factors, especially younger patients and women, raising serious concerns about previously unrecognised mechanisms underlying the disease.

To investigate this emerging trend, researchers carried out a comprehensive molecular analysis of oral cancers occurring in patients classified as having “No Identified Risk Factor” (NIRF) OCSCC. The study was led by Dr. Jiri Zavadil of the International Agency for Research on Cancer and Prof. François Virard. Their findings were published in the International Journal of Oral Science.

Explaining the rationale behind the study, Dr. Zavadil said the researchers sought to determine whether these cancers represented a biologically distinct subtype driven by mechanisms unrelated to conventional carcinogenic exposures. Since smoking and alcohol could not account for these cases, the team explored whether internal cellular processes or alternative environmental influences were involved.

The researchers analysed large-scale genomic datasets from The Cancer Genome Atlas. Following stringent quality assessment, 347 head and neck cancer samples were examined, including 253 oral cavity cancers and 94 laryngeal cancers that served as smoking-related controls. Using advanced multi-omics techniques, the team investigated DNA mutations, gene expression profiles, and epigenetic alterations.

A central component of the analysis involved identifying mutational signatures — characteristic patterns of DNA damage that reveal the biological processes responsible for tumour development. Through this approach, the researchers identified four distinct molecular clusters.

Two of these clusters were strongly associated with established carcinogenic exposures such as tobacco and alcohol use. These tumours displayed mutational signatures including SBS4 and SBS16, which are commonly linked to smoking-related DNA damage. The study also found that tobacco-associated mutations differed according to anatomical site, with laryngeal cancers carrying a substantially higher mutation burden than oral cancers, suggesting tissue-specific responses to carcinogens.

More significantly, two additional clusters were found to be enriched with NIRF cases. According to Prof. Virard, these tumours were dominated not by external carcinogens but by endogenous biological processes. One cluster was characterised primarily by the ageing-related SBS1 signature, while another showed strong activity of APOBEC-associated mutational signatures, namely SBS2 and SBS13. APOBEC enzymes are part of the body’s innate immune defence system but can also induce harmful mutations in DNA.

Importantly, the investigators found no mutational signatures associated with known environmental carcinogens in the NIRF tumour groups. Instead, these cancers displayed considerable biological heterogeneity, with one subgroup driven predominantly by age-related mutations and the other by APOBEC-mediated mutagenesis.

Further molecular analyses revealed additional distinguishing features of NIRF tumours. Researchers identified unique driver gene mutations involving immune regulation and cellular signalling pathways. Gene expression profiling also demonstrated activation of antimicrobial response pathways and keratinisation-related processes. Notably, bacterial components were detected within tumour tissues and subsequently confirmed through histopathological examination, suggesting that alterations in the oral microbiome may contribute to cancer development in certain patients.

The findings also carry potentially important clinical implications. The NIRF tumours demonstrated several features associated with immune evasion, including alterations in antigen presentation pathways that may influence responsiveness to immune checkpoint therapies. At the same time, the prominence of APOBEC-driven mutagenesis suggests that some of these cancers could be vulnerable to therapies targeting DNA damage response mechanisms, opening possible avenues for precision oncology.

Overall, the research highlights the role of endogenous mutational processes, underscores site-specific effects of traditional carcinogens, and points towards a possible contribution of microbial influences in tumour initiation and progression.

The researchers were of the opinion that oral cancers developing in the absence of recognised risk factors should be regarded as a separate disease entity with unique biological drivers.

“These findings may improve diagnostic classification, guide future investigations into disease causation, and support the development of more targeted and effective therapeutic strategies for affected patients,” said the study.