Role of microbiota in response to treatment in anal squamous cell carcinoma.

3518 Background: Anal squamous cell carcinoma (ASCC) is a rare tumor whose management and treatment have not changed since the 1970s, consisting of classical chemotherapy combined with radiotherapy. Microbiota has been recently included as a hallmark of cancer, playing a relevant role in oncogenesis and tumor progression. Therefore, the aim of this study is the characterization of tumor microbiota in ASCC. Methods: Paraffin samples from seventy-six ASCC patients were analyzed. First, DNA was extracted, followed by 16S rDNA sequencing to identify microbiota. Then, a bacterial reference proteome was built, including the bacteria genera identified by 16S, and proteins from human and microbiota were identified and quantified by mass-spectrometry proteomics. Results: Seventy-six ASCC patients were included in this study: 46 (60%) female; median age 61 years; 62 (81%) HPV-positive; 1 (1%) stage I, 30 (40%) stage II, 43 (57%) stage III, 1 (1%) stage IV, and 1 (1%) unknown stage. Of these, sixty-nine (stages II-III, treated with chemoradiotherapy) were considered for survival analyses. One thousand and fifty-seven bacteria genera were identified by 16S sequencing, with Cutibacterium , Bacteroides , and Fusobacterium being the most abundant. Using proteomics and after applying quality criteria, 1735 proteins were identified and quantified. Of them, 1727 were human proteins and eight were bacterial proteins. Two oncomicrobiota protein profiles were identified in ASCC, ASCC-OMP1 (36 [47%] pts) and ASCC-OMP2 (40 [53%] pts). ASCC-OMP1 presented a higher expression of proteins from Sutterella and Staphylococcus whereas ASCC-OMP2 showed higher expression of proteins from Dialister , Campylobacter A , and Dysosmobacter . In addition, the two microbiota profiles showed differences in human proteins related to immune response, adhesion, extracellular matrix, translation, and metabolism. Interestingly, significant differences in disease-free survival (DFS) (p=0.02, HR= 2.73, DFS at 5 years: ASCC-OMP1 78.43%, ASCC-OMP2 52.33%) were shown between ASCC-OMP1 and ASCC-OMP2. Overall survival (OS) is not significant between the two groups but a trend can be observed (p=0.06, HR=2.55, OS at 5 years: ASCC-OMP1 79.28%, ASCC-OMP2 62.92%). In a multivariate analysis including the classical clinical prognostic factors, the prognostic value in disease-free survival of the oncomicrobiota profiles remains. Conclusions: Two different oncomicrobiota protein profiles with implications in disease-free survival exist in anal squamous cell carcinoma.