Tumour microbiome and immune dysregulation in early-onset colorectal cancer.

3545 Background: Early-onset colorectal cancer (EOCRC) in patients under 50 years is increasing incidence in many countries worldwide, including New Zealand. The reason for this trend is yet unclear, but is associated with such risk factors as obesity, alcohol intake, lifestyle and diet. This suggests a multi-factorial exposome-related aetiology, with changes to the gut microbiome likely to play a role. Methods: In this study, we investigated differences in the tumour-resident microbiome and molecular characteristics between patient cohorts of EOCRC and late-onset CRC (LOCRC). Transcriptomic analysis was carried out on pre-treatment tumours from a cohort of 19 EOCRC patients and compared to a control group of 196 LOCRC aged over 65 years. Bioinformatics analysis of RNA sequencing data was used to analyse tumour microbial abundance and taxonomy, differential gene expression and gene-set enrichment between the two groups, as well as assign consensus molecular subtypes (CMS) . Results: We found an increase in expression of genes involved in the cell cycle in the EOCRC cohort, and of specific genes (e.g. HOXA11-AS, STMN2) involved in cell proliferation and metastasis. Converesly, enriched gene sets in the late-onset category were predominantly related to immune function. When grouping CMS subtypes as immune-rich (CMS1/CMS4) versus immune-depleted (CMS2/CMS3), there was a significant difference between the two groups with 94% of EOCRC tumours being immune-depleted, compared to 67% of late-onset tumours (p = < 0.05). Meanwhile, we found an increase in bacterial richness (observed alpha diversity) in the late-onset tumours compared to early-onset, while there were no differences in the Shannon alpha diversity measures (richness and evenness), and in beta diversity between the groups. We also found a depletion of the bacteria Helicobacter canadensis and Campylobacteria ureolyticus in the early-onset cohort, while there was an increase in Lachnospiraceae species. Conclusions: Our results add to the growing body of evidence that EOCRC is a distinct disease from LOCRC. EOCRC shows lower tumour-immune activation compared to LOCRC and very low rates of CMS1 and CMS4 subtypes, which is associated with a distinct tumour-resident microbiome. This may have implications for prognosis and targeted treatments.