1725P Metastasis-resident bacteria in advanced hormone receptor-positive breast cancer are related to primary tumor microbiota and show distinct composition

Abstract

Tumor-resident bacteria are an emerging component of the tumor microenvironment. Recent studies have shown their presence over multiple cancer types. Lately, mechanistic evidence in a murine breast cancer model indicates that these bacteria promote the metastatic process. However, the presence and conformation of the microbiome of human metastatic tumors have not been determined. Here, we characterized tumor-resident bacteria in a cohort of metastatic hormone receptor-positive breast cancer (MHRBC) patients with matched primary tumors. We performed bacterial 16S rDNA sequencing targeting hypervariable regions V2-4 and V6-9 on FFPE tissues from 40 patients with MHRBC and their matched primary tumors. Sequence data were processed using high resolution sample inference with DADA2. Controls included normal breast tissue, paraffin from all blocks and a simulated bacterial community, comprising known intra and extracellular bacteria. Taxonomy was assigned using the SILVA database v138. Feature selection was used to determine amplicon sequences in primary tumors related to their metastasis. A machine learning classifier was generated to predict the metastatic site from selected amplicons. α-diversity was similar among sample types, while β-diversity showed segregation between metastatic and primary tumors. Alphaproteobacteria, Gammaproteobacteria and Bacilli had increased relative abundance across metastatic and primary tumors. Differential abundance of Proteobacteria and Firmicutes species was identified in metastatic tumors. A machine learning classifier using the 5 top ranked amplicons in primary tumors was capable of 100% precision and high recall for prediction of bone and liver metastatic site, but not lung metastases. We identified and categorized the tumor-resident bacteria of MHRBC. To our knowledge, this is the first study looking at the composition of metastatic breast cancer microbiome. While insights have been gained on primary tumor microbiota, the role of metastasis-resident bacteria including local immunity and treatment resistance warrants further study.