Abstract 1784: Absence of intratumor microbes induces methylation of tumor suppressors and cell cycle-related genes in papillary thyroid carcinoma
Authors: Gnanasekar Aditi , Castaneda Grant , Iyangar Anjali , Magesh Shruti , Chakladar Jaideep , Li Wei T. , Wong Lindsay M. , Ongkeko Weg M. 
Published: July 2021

Background: Papillary thyroid carcinoma (PTC) is characterized by varied prognosis between gender and cancer subtype, but the cause for gender and subtype-based dissimilarities is unclear. Women are more frequently diagnosed with PTC, while men suffer more advanced-staged PTC. Additionally, tall cell and poorly differentiated variants are more aggressive than classical and follicular variants of PTC. While the intratumor microbiome has become increasingly implicated in cancer development, the microbial landscape of PTC is essentially uninvestigated. Past studies have shown how microbes within tumor tissue can contribute to or inhibit tumor growth by regulating gene expression and immune and cancer pathways. For example, a study published by Bahmani S. et. al. found that the absence of folate-producing Bifidobacterium and Lactobacillus was correlated with hypomethylation at regions of the p53 gene in colorectal cancer. We hypothesized that intratumor microbiome composition distinctly alters the immune landscape by preventing or inducing methylation of critical genes and may therefore predict cancer progression between PTC subtypes and between patient genders.

Methods: Raw whole-transcriptome RNA-sequencing data for PTC and adjacent normal tissue was downloaded from The Cancer Genome Atlas, and microbial reads were extracted using Pathoscope 2.0 software. The Kruskal-Wallis test was performed to identify differentially abundant microbes between tumor and normal samples. Level 3 normalized DNA methylation 450k sequencing was downloaded from the GDC portal. We converted B-values to M-values and then performed the probe-wise differential methylation analysis on the matrix of M-values in limma to obtain t-statistics and p-values for each CpG site. The Kruskal-Wallis test was used to correlate microbe abundance to extent of methylation. R

Results: Overall, PTC tumor tissue significantly lacked microbes that are populated in adjacent normal tissue. Tissue from different genders and subtypes was characterized by dysbiosis of specific microbe species, and microbes distinctly abundant in tall cell and male patient cohorts were correlated with greater methylation of tumor suppressors. The most significantly methylated sites occurred on chromosomes 1 and 17 and specifically at cell cycle related genes. Lower microbe abundance (Anabaena sp. K119, Trueperella pyogenes, Frankia sp.) in tumor tissue was predominantly correlated with greater extent of methylation at known tumor suppressor genes including NEURL1B, POLE, and SRCIN1.

Conclusions: We identified microbes that are uniquely abundant in specific PTC types and are predominantly negatively correlated with methylation of tumor suppressors and cell cycle-related genes. Our results may provide a basis for developing specialized prebiotic and probiotic treatments for varied PTC tumors.

Citation Format: Aditi Gnanasekar, Grant Castaneda, Anjali Iyangar, Shruti Magesh, Jaideep Chakladar, Wei T. Li, Lindsay M. Wong, Weg M. Ongkeko. Absence of intratumor microbes induces methylation of tumor suppressors and cell cycle-related genes in papillary thyroid carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1784.