Fusobacterium in colonic flora and molecular features of colorectal carcinoma

Abstract

Fusobacterium species are part of the gut microbiome in humans. Recent studies have identified overrepresentation of Fusobacterium in colorectal cancer tissues, but it is not yet clear whether this is pathogenic or simply an epiphenomenon. In this study, we evaluated the relationship between Fusobacterium status and molecular features in colorectal cancers through quantitative real-time PCR in 149 colorectal cancer tissues, 89 adjacent normal appearing mucosae and 72 colonic mucosae from cancer-free individuals. Results were correlated with CpG island methylator phenotype (CIMP) status, microsatellite instability (MSI), and mutations in BRAF, KRAS, TP53, CHD7, and CHD8. Whole-exome capture sequencing data were also available in 11 cases. Fusobacterium was detectable in 111 of 149 (74%) colorectal cancer tissues and heavily enriched in 9% (14/149) of the cases. As expected, Fusobacterium was also detected in normal appearing mucosae from both cancer and cancer-free individuals, but the amount of bacteria was much lower compared with colorectal cancer tissues (a mean of 250-fold lower for Pan-fusobacterium). We found the Fusobacterium-high colorectal cancer group (FB-high) to be associated with CIMP positivity (P = 0.001), TP53 wild-type (P = 0.015), hMLH1 methylation positivity (P = 0.0028), MSI (P = 0.018), and CHD7/8 mutation positivity (P = 0.002). Among the 11 cases where whole-exome sequencing data were available, two that were FB-high cases also had the highest number of somatic mutations (a mean of 736 per case in FB-high vs. 225 per case in all others). Taken together, our findings show that Fusobacterium enrichment is associated with specific molecular subsets of colorectal cancers, offering support for a pathogenic role in colorectal cancer for this gut microbiome component.

Fig.1

Over-representation of F. nucleatum (left) and pan-fusobacterium (right) in CRC tissues relative to adjacent normal colonic mucosa in 89 paired cases. Statistical analysis was performed using the Wilcoxon signed-rank test.

Fig. 2

Distribution of Fusobacterium in CRC patients (n=149). The cases were ranked according to the amount of pan-fusobacterium (right=high amount, left=low amount). Note that all F. nucleatum high cases (n=8) were included in pan-fusobacterium high cases (n=14) and there is clear separation of FB high group (n=14, 9.4%) and FB low/negative group (n=135, 90.6%). Red, CIMP1, MLH1 methylated, BRAF, KRAS and TP53 mutated; blue, CIMP2; grey CIMP-negative, MLH1 unmethylated, BRAF, KRAS and TP53 wild type; white, not determined;

Fig. 3

Number of mutated genes determined by whole exome sequencing analysis in 11 CRCs. (2 FB-high, 6 FB-low/negative CIMP1 and 3 FB-low/negative CIMP2/CIMP-negative). Statistical analysis was performed using One-way ANOVA.