Head and neck cancers arise from the mouth, tongue, and voice box with more than 90 percent diagnosed as squamous cell carcinoma. Known risk factors for head and neck cancer include a history of tobacco and alcohol exposure, and/or persistent human papillomavirus (HPV) infection. The only known cause of cervical cancer in women, HPV now afflicts men in epidemic numbers with more than 50 percent of malignancies presenting in non-smokers. It is not only the most common cause of throat cancer in men in their 50’s and 60’s, but HPV-associated throat cancer is also now more common than cervical cancer in women. In fact, more than 80 percent of sexually active individuals will be infected with HPV in their lifetime, according to the Centers for Disease Control.
Investigators at the Head and Neck Cancer Research Program at the Icahn School of Medicine at Mount Sinai are pondering, if HPV infection is so prevalent, then why do some people suppress the infection while others develop a persistent infection that leads to throat cancer? One answer may lie in analyzing the oral microbiome and its bacterial construct.
About the Human Microbiome and Immune Regulation
The human body is colonized by microbes, with each human host having a unique microbial colonization defining him/her. The microbes, bacteria, fungi, and viruses that live within our bodies are referred to the microbiome. The relationship between host and microbe can be one of symbiosis, living in harmony, or dysbiosis, living in disharmony. Certain microbes are known to neutralize carcinogens, while others are known to produce reactive oxygen species or other toxic metabolites proteins that trigger inflammation and promote disruption in immunity.
The microbiome plays a key role in immune regulation and barrier defenses of the human body and the oral cavity. Research has shown that imbalances in the microbiome, which may result from diet, antibiotic use, stress, and a variety of other factors, is associated with stomach cancer, colon cancer, depression, obesity, and new data may link the microbiome to autism.
Diseases such as gastrointestinal cancers have already been linked to inflammation and the microbiome. Hepatitis B and C and Helicobacter pylori are recognized in the development of three distinct cancers: hepatocellular carcinoma, gastric adenocarcinoma, and MALT gastric lymphoma. Bacterial metabolites colibactin and Bacteroides fragilis toxin (Bft) produced by B2 Escherichia coli and B. fragilis, respectively, have shown to play a causative role in certain gastrointestinal malignancies.
These examples highlight the role of microbiota in tumor development. Microbiome immune-modulating effects in the setting of malignancy are highlighted in a recent study that demonstrated increased response to immunotherapy in melanoma patients with higher alpha diversity of their gut microbiome in addition to a relative abundance of the Ruminococcaceae bacterial family compared to non-responders. Some microbial communities are protective and advantageous in the setting of malignancy, while others may be detrimental by compromising barrier immunity and triggering chronic inflammation.
The Link between the Oral Microbiome and Oral Cancer
The oral cavity has a unique microbiota made up of approximately 700+ known species and different areas within the oral cavity are colonized by distinct species. Several research investigations have demonstrated the association of poor oral hygiene and tooth loss with oral cavity squamous cell carcinoma (OCSCC), implicating the oral microbiome in the development of oral cancer. One study found an increased number of anaerobic and aerobic bacterial colonies at oral cavity tumor sites compared to healthy oral cavity mucosa, indicating a role for bacteria in the development of tumors.
However, other studies suggest that some bacteria have a protective nature against the development of cancer. One prospective study found that Corynebacterium was associated with a decreased risk of cancer, specifically laryngeal cancers.
It is unknown why some individuals are susceptible to persistent HPV infection and what the relationship is between the bacterial framework of the oral microbiome and persistent HPV infection. Factors associated with persistent oral HPV infection include elevated salivary IgG and decreased levels of matrix metalloproteinase 8 (MMP-8) in oral HPV+ infected patients, compared to non-infected controls. One study found differences in the microbiome of patients with HPV-associated throat cancer compared to patients with non- HPV-associated throat cancer.
Studying the Microbiome of Cancer Patients through Gene Sequencing and Functional Metagenomics
Advances in gene sequencing and metagenomics, the analysis of genetic material from a community of organisms, have enabled investigators to identify unculturable microbes and study the relationship between the microbiome and various diseases. Importantly, less than 2 percent of bacteria can be cultured in the laboratory; however, approximately 50 percent of the oral microflora are culturable. It is our aim at the Head and Neck Cancer Research Program to characterize the microbiome in cancer patients – including non-smokers and non-drinkers -- and identify ways to modify it to reduce the risk of cancer and cancer recurrence. This will also help us answer patients’ frequent question, “Why and how did I develop this cancer?”
Our researchers hypothesize that the oral microbiome modulates the development of carcinoma. We propose to characterize the microbiome using 16S rRNA gene sequencing and will use a shotgun metagenomics platform to identify specific species and genes expressed by those species that -generate effector proteins and metabolites that incite, potentiate, or protect again cancer types.
Functional metagenomics involves the extraction of microbial DNA and subsequent cloning of the DNA into cultured bacteria, enabling investigators to identify and isolate microbial metabolites and effector proteins, and study their effects on host-microbial interactions and role in malignant transformation. This will enable us to move beyond merely identifying associated microbes and their genes, and allow us to elucidate causative mechanisms underlying malignant transformation in oral and throat cancer.
In addition, we plan to collect oral cavity samples throughout the course of OCSCC treatment in a subset of patients to determine how the microbiome changes during treatment and whether specific microbiome signatures are associated with recurrence. We hope this study will lay the foundation for future studies in this rapidly advancing field.