Analysis of intestinal mucosal microbiome changes before and after chemoradiation in locally advanced rectal cancer patients

Purpose: Dysbiosis of gut microbiota has been reported to participate in the pathogenesis of colorectal cancer, but changes in microbiota due to radiotherapy have not been studied. In this study, we tried to elucidate the changes in the microbiome in rectal cancer after chemoradiotherapy using RNA sequencing analysis. Materials and methods: We included 11 pairs of human rectal cancer tissues before and after irradiation between August 2016 and December 2017 and performed RNA sequencing analysis. Mapped reads to human reference genomes were used for pair-wise transcriptome comparisons, and unmapped (non-human) reads were then mapped to bacterial marker genes using PathSeq. Results: At microbiome level, interindividual variability of mucosal microbiota was greater than the change in microbial composition during radiotherapy. This indicates that rapid homeostatic recovery of the mucosal microbial composition takes place short after radiotherapy. At single microbe level, Prevotella and Fusobacterium, which were identified as important causative microbes of the initiation and progression of rectal cancer were decreased by radiotherapy. Moreover, changes in Prevotella were associated with changes in the human transcriptome of rectal cancer. We also found that there was a gene cluster that increased and decreased in association with changes in microbial composition by chemoradiation. Conclusion: This study revealed changes in tumor-associated microbial community by irradiation in rectal cancer. These findings can be used to develop a new treatment strategy of neoadjuvant therapy for locally advanced rectal cancer by overcoming radio-resistance or facilitating radio-sensitivity.