The Two-Sample MR study we conducted clearly showed the bidirectional causal relationship between esophageal cancer and gut microbes without the influence of confounding factors. The forward MR revealed that genera Eubacterium oxidoreducens, Lachnospira, Romboutsia and Turicibacter had a protective effect on esophageal cancer, while Coprobacter, Ruminococcus1, Senegalimassilia and Veillonella might be the risk factors for the formation of esophageal malignant neoplasms. Meanwhile, in the reversed MR, Esophageal cancer enriched the abundance of the genera Eggerthella, Coprococcus2 and Christensenellaceae R.7, and the abundance of the genera Intestinimonas was found to be decreased.
In recent years, there has been an increasing focus on the correlation between microorganisms and digestive tract diseases in academic research. For example, Helicobacter pylori has been the well-recognized key causative factor for gastric cancer [25]. Previous researches have clarified that virus infection such as human papillomavirus (HPV) and Epstein-Barr virus, as well as the changes of gut microbial composition contribute to the carcinogenesis of the esophagus [26]. In a recent study, the tumor development in LFD-fed GF mice, which received the fecal microbiota from HFD-fed L2-IL1B mice, indicated the crucial role of intestinal microbes in esophageal cancer development and verified its linkage with the IL8/CXCL1 chemokine family [27]. In an observational study, which recruiting 23 EC patient and 23 health individuals from Huai'an First People's Hospital (Huai'an, China), the abundance of Lachnospira was observed to be reduced in the EC patients, and Lachnospira, of which the AUC = 1, showed a great discrimination ability between EC patients and the healthy [17]. Reinforcing our analyses, it is plausible that Lachnospira plays a protective role in the pathogenesis of esophageal cancer. The genera Lachnospira, belonging to the phylum Firmicutes and the family Lachnospiraceae, generates acetic acid and butyric acid. It has been recognized that SCFAs not only maintain the stability of Gastrointestinal homeostasis but also function through immunoregulation [28]. Studies have shown that Lachnospira might connect with nonalcoholic fatty liver disease [29], Asthma [30], gastric cancer [31], colorectal cancer [32], depression and anxiety in patients with active ulcerative colitis [33]. However, most of them are association studies, lacking in exploration of mechanisms. Veillonella, one of the major oral microorganisms, promoted the cancer development according to our analyses. The genera contain Veillonella parvum and Veillonella alkalinogens. Oral microbial colonization of the intestine is concerned with many diseases, such as inflammatory bowel disease (IBD) [34] and colorectal cancer [35]. A latest study illustrated that the Veillonella ectopically colonize the intestine by taking advantage of inflammatory nitrates derived from inducible nitric oxide synthase (iNOS) activity in host cells [36]. As a member of Type II bacteria (Gram-negative and anaerobic/microaerophilic bacteria) of the Esophageal Flora, the genera Veillonella may serve as collaborative mechanism to facilitate gastric reflux through activating the iNOS pathway. It is considered that the alteration of Veillonella promotes the progression of gastroesophageal reflux to Barrett's esophagus, which eventually leads to adenocarcinoma [37]. The studies mentioned above all further validated our analysis. Further studies are urgently needed to determine the mechanism of intestinal bacteria in the pathogenesis of esophageal cancer.
In our reverse MR analyses, we revealed that the alteration of gut microbes after suffering from esophageal cancer. Eggerthella is among the top five most abundant genera of the phylum Actinobacteria in the human gut. A study found that the colonization of Eggerthella activates intestinal Th17 by removing from inhibition of the Th17 transcription factor Rorγt, exacerbating the severity of colitis [38]. Moreover, with proinflammatory properties, Eggerthella is involved in the synthesis of key neurotransmitters in depression, such as glutamate, butyrate, serotonin, and γ-aminobutyric acid (GABA) [39]. In another study, Eggerthella lenta was found to promotes the accumulation of uremic toxin in serum and exacerbate the progression of kidney disease in a CKD rat model [40]. However, there is little research on the association of Eggerthella and EC, let alone the mechanism. Our analyses showed the decreased abundance of Intestinimonas under the exposure of EC. Intestinimonas produces butyrate from both sugars and amino acids, protecting the host from harmful metabolites. The enrichment of the genus Intestinimonas in gastric intraepithelial neoplasia [41] and the reduced abundance of this genera in the obesity [42] has been observed. Besides, there's already evidence that Intestinimonas showed a good distinction ability between the patients with chronic renal diseases (CKD) and the healthy [43] and correlated with the total functional ability score and the IL-4 level in patients with Huntington's disease [44]. Currently, there is limited research on the intestinal microbiota identified in our reverse study. Further investigation is necessary to determine whether alterations in gut microbiota following esophageal cancer have a positive or negative impact, which could enhance the prognosis and adjuvant therapy of EC.
Our study has several strengths. Previous studies are more focused on esophageal cancer and esophageal microbiota. Moreover, changes in the microbiota in observational studies are merely clinical manifestations. We analyzed GWAS data by MR to confirm the possible interaction and causal relationship between EC and intestinal microbiota. Bidirectional analyses guarantee causality inferred from both directions. The selected bacteria lay the foundation for further exploration in the future. It also has contributed to the screening of biomarkers for esophageal cancer and the improvement of prognosis. In the procedure of the MR analyses, we strictly screened for strong-associated SNPs as instrumental variables (F statistics > 10). MR-PRESSO test and MR Egger regression was conducted to exclude SNPS with pleiotropy. Despite our exploration is meaningful, there are several limitations. The population we obtained from the UK Biobank was restricted to the European, in which the most common type of esophageal cancer is adenocarcinoma. We did not get the original information on the patients with EC, making it difficult for us to perform subgroup analysis. Besides, genus was the lowest taxonomic level in the gut microbe datasets, further exploration at the species level was restricted.
In conclusion, this two sample MR found that the bilateral causality between esophageal cancer and intestinal microbes. Several genera such as Lachnospira and Veillonella had a protective or carcinogenic effect on esophageal cancer. Moreover, the alteration of the genera Eggerthella and Intestinimonas demonstrated the influence of EC on the gut microbiome. Further RCT study and animal experiments are in an urgent need to figure out the mechanisms involved, thus making the candidate intestinal microbes useful for clinical screening and treatment.