Backgrounds:Colorectal cancer(CRC)is the most common and deadliest gastrointestinal tumor in the world,but it is not a single entity;colon cancer and rectal cancer each have their own characteristics in terms of genetics,anatomy,treatment options,and metastatic patterns.Most colorectal cancer tumors are thought to be caused by precancerous lesions called adenomas.Gut flora are strongly associated with each other phenotypically and genetically,and they are thought to be directly involved in colorectal carcinogenesis,but the epidemiological evidence that gut microbes influence CRC risk comes almost exclusively from observational studies.It is unclear whether microbial alterations are a cause or a consequence of CRC development,and it is not clear how microbial interactions affect the identification of causality.An agnostic statistical framework using two-sample multivariate Mendelian randomization based on Bayesian model averaging(MR-BMA)analysis can disentangle the causal role of correlated microbial risk factors from shared genetic predictors.Furthermore,there is growing evidence to support the notion that dietary factors are key determinants of gut microbial composition and function and influence the development of CRC.However,much of the evidence on dietary factors altering gut microbial composition comes from cross-sectional or short-term intervention studies.Cross-sectional and short-term intervention studies are unable to determine how changes in dietary habits modulate gut flora and thus affect the risk of CRC.Objective:To comprehensively explore the causal relationship between gut microbiota and CRC at different cancer subsites(colon,rectum,and colorectum)and stages(benign tumors and malignant tumors)and to search for causal pathways by which diet alters gut bacteria and thereby affects CRC risk.Methods:Univariate Mendelian randomization(MR)analysis,MR-BMA-based potential biomarkers ranking analysis,and microbial risk factor identification approach were used to comprehensively explore the microbial risk factors for CRC at different cancer subsites and stages using genome-wide association studies(GWAS)of gut microbiota from the Dutch population and CRC at different cancer subsites and stages from FinnGen databases.Among them,the MR-BMA approach can address the high correlation between microorganisms and identify microbial combinations that may have a causal relationship with outcomes.A series of sensitivity analyses were performed to ensure the robustness of the results.We collected human gut microbiota 16S rRNA datasets from 6 European CRC cohorts from the National Center for Biotechnology Information(NCBI)to construct a multi-population cohort.The above multipopulation cohort was also analyzed using Sparcc-based Network Module Structure Shift(NetMoss),which can be used to assess changes in microbial network modules while eliminating batch effects,to identify microbial markers of colorectal adenoma or CRC,and to validate the causal association between the above microbial-disease outcomes.Finally,mediation analysis was used to reveal how 14 dietary habits associated with the development of CRC mediated gut microbiota influence CRC risk,i.e.,to explore the diet-gut microbiotacolorectal cancer pathway.Results:By combining the estimated univariate MR results(P<0.05 or Pfdr<0.2),the results of potential biomarkers ranking(MIP>0.1),the results of microbial risk factor identification(MIP were ranked in the top 10)based on MR-BMA,and the results of NetMoss(NetMoss score>1),9 bacteria were identified by at least three methods as being associated with CRC occurrence and development with a causal relationship.Among them,Streptococcus thermophilus and Erysipelotrichaceae reduced the risk of CRC;Eubacterium ventriosum and Streptococcus were protective microbiota of malignant tumors of the colon(CC);Erysipelotrichaceae,Coprococcus sp.strain ART55-1 and Eubacterium siraeum protected against malignant tumors of the rectal(CR);Bacteroides ovatus and Bifidobacterium adolescentis increased the risk of benign tumors of the colorectum(BCR).Bacteroides ovatus was also a risk factor for benign tumors of the colon(BC)and benign tumors of the rectal(BR);Sutterellaceae unclassified was a protective factor for BR.Sensitivity analyses also confirmed the robustness of these results.Finally,the mediation analysis identified 10 causal pathways in which diet-regulated bacteria affected the risk of CRC,including:alcohol consumption increased the risk of CC by reducing the abundance of Eubacterium ventriosum(mediated proportion:43.044%);Cereal type:biscuit cereals(e.g.,Vitamar)reduced CRC by increasing the abundance of Erysipelotrichaceae(mediated proportion:13.973%);and excessive ferritin intake led to a decrease in the abundance of Streptococcus thermophilus,which in turn led to an increased risk of CRC(mediated proportion:34.22%).Bifidobacterium adolescentis was a mediator in regulating BCR risk from milk type:never/rarely(mediated proportion:435.399%);Bacteroides ovatus was an important gut mediator,simultaneously mediated the influence of multiple dietary factors on the benign tumors.Including intake of vitamin and mineral supplements:multivitamin +/-minerals could have an impact on both BCR(mediated proportion:7.026%)and BC(mediated proportion:8.817%)through Bacteroides ovatus;minerals and other dietary supplements:calcium could simultaneously increase the risk of BCR(mediated proportion:11.854%),BC(mediated proportion:7.23%),and BR(mediated proportion:1 1.85%)by regulating Bacteroides ovatus;Bacteroides ovatus also mediated the effect of bread intake on BCR(mediated proportion:12.639%).Conclusion:The findings of this study highlighted 6 protective microbes that may reduce the risk of malignancy and 2 dangerous microbial traits that may increase the risk of benign tumors.These findings will help to understand the different carcinogenic mechanisms and tumor transformation risks of the colon and rectum of the gut microbiota,which in turn will help with the prevention,early screening,and development of future CRC risk reduction strategies.Dietary modification may be used as a form of cancer prevention in healthy individuals.This study elucidates the causal pathways by which some dietary habits affect CRC risk by modulating gut bacteria.These findings could further guide probiotic and dietary interventions to reduce the prevalence of CRC or develop new treatment strategies. |