In this twin-sample Mendelian randomization analysis study, we identified four microbiota taxonomic groups associated with the risk of DLBCL. These groups include Ruminococcus torques.id.14377, Ruminococcaceae UCG014.id.11371, Ruminococcaceae UCG002.id.11360, all belonging to the Ruminococcus genus, and Eubacterium oxidoreducens.id.11339 from the Eubacterium genus. The investigation utilized Mendelian randomization to examine the potential causal association between the gut microbiota and DLBCL. While genetic and environmental factors can influence disease phenotypes in populations, our Mendelian randomization analysis directly used GWAS (Genome-Wide Association Studies) data based on genetic correlations to assess the potential causal relationship between the microbiota and DLBCL. Furthermore, we conducted strict quality control on the included SNPs to eliminate the influence of confounding factors and reverse causality.
The human gut microbiota plays a crucial role in physiological regulation, and almost all human cells interact with it[39]. The gut microbiota can mediate the occurrence and development of tumors, increasing cancer susceptibility. The interaction between genomic abnormalities and microbial signals may be bidirectional, with host TET (Ten-Eleven Translocation) deficiencies making individuals more susceptible to the effects of microbial signals driving myelodysplastic syndromes[40]. Similar research also suggests that the loss of a symbiotic microbiota is crucial in promoting the accumulation of white blood cell abnormalities[41]. Helicobacter hepaticus could induce the aggregation of macrophages and neutrophils in the colon, resulting in the upregulation of inducible nitric oxide synthase (iNOS) and increased production of nitric oxide, thus promoting atypical hyperplasia and carcinogenesis in the colon[42]. Similarly, a study using a Rag2-deficient model of mice infected with colonic Helicobacter hepaticus significantly promoted breast cancer through a TNF-α-dependent mechanism[43]. Metabolites and other molecules produced by the gut microbiota could also affect the immune system, regulating the balance between pro-inflammatory and anti-inflammatory mechanisms[44]. For example, short-chain fatty acids (SCFAs), which are metabolites of bacteria, can promote the production of thymic regulatory T (Treg) cells, regulating the function of colonic Treg cells[44, 45]. SCFAs also act as histone deacetylase (HDAC) inhibitors, which could prevent Epstein-Barr virus reactivation. Given the prevalence of the Epstein-Barr virus in lymphoma patients, further exploration of the interaction between SCFAs and the virus is warranted[46].
In our study, we identified two species belonging to the Ruminococcus genus as beneficial, with a substantial decrease in DLBCL risk (OR = 0.44) associated with an increased abundance of Ruminococcus torques.id.14377. Similarly, Ruminococcaceae UCG002.id.11360, classified as a probiotic, also contributed to approximately a 40% lower risk of lymphoma development. Previous studies have shown a deficiency of Ruminococcus in the gut microbiota of lymphoma patients[25].
Recent MR analysis has shown that enrichment of Ruminococcaceae and Bacteroidetes was associated with a reduced risk of liver cancer[19]. Another multi-tumor MR analysis showed that Ruminococcus genus UCG013 protectd against breast cancer[20]. The Ruminococcus genus was also closely related to the efficacy of immunotherapy. A study on hematologic CAR-T therapy showed that Ruminococcus, Prevotella, and Eubacterium were significantly associated with confirming the effectiveness of this immunotherapy[47]. Another study also suggested that a higher abundance of the Ruminococcus genus was associated with a higher complete response rate on day 100 after CAR-T therapy[48]. Moreover, patients with advanced liver cancer and melanoma who respond to PD-1 inhibitors typically had a higher abundance of the Ruminococcus genus[49, 50]. However, some studies had contrasting findings. A study by Jin et al.[51] suggested that Ruminococcus unclassified was associated with PD-1 therapy ineffectiveness, and a study by Peters et al.[52] indicated that Ruminococcaceae was significantly related to poor lung cancer prognosis.
On the other hand, Ruminococcaceae UCG014.id.11371, a member of the Ruminococcus genus, substantially increased the risk of DLBCL (OR = 1.69). This observation suggests that different species or strains may exhibit different characteristics or traits even within the same genus. Such differences may be due to environmental conditions, growth conditions, genetic variations, or other factors. Further research into these specificities and diversities within genera is necessary.
Additionally, Eubacterium oxidoreducens.id.11339 increased the risk of DLBCL (OR = 1.80). Lu et al.'s research indicated that Eubacterium, particularly Eubacterium rectale, was significantly deficient in gastrointestinal lymphoma and could produce high concentrations of butyrate[16]. Butyrate could induce apoptosis in lymphoma cells and inhibit the growth of lymphoma cells[53]. Furthermore, in Lu et al.'s in vitro experiments, Eubacterium rectale treatment reduced TNF levels and the incidence of lymphoma in mice[16]. Therefore, we also focused on this genus in our MR analysis. Unfortunately, Eubacterium rectale did not show a causal link with DLBCL risk in our MR analysis. However, Eubacterium oxidoreducens.id.11339, another Eubacterium genus member, exhibited characteristics promoting DLBCL risk. Reports on this bacterium are relatively scarce, but a 2022 ASCO abstract suggests that patients who respond effectively to immune checkpoint inhibitors (ICIs) show enrichment in Eubacterium oxidoreducens [54]. Thus, there is a need for in-depth investigation and research on this genus.
Considering that the phyla Firmicutes and Bacteroidetes have exhibited significant differences in multiple DLBCL-related observational studies[12–14, 27, 28], we also focused on these two phyla. In these studies, the Firmicutes phylum was highly enriched in DLBCL, and within the Firmicutes phylum, the family Enterobacteriaceae was significantly associated with the refractory outcomes of DLBCL[14]. On the other hand, the Bacteroidetes phylum exhibited reduced abundance in DLBCL patients[27]. Multiple studies have shown that an increase in Bacteroidetes and enhanced resistance to colitis[55], as well as the effectiveness of immunotherapy[47, 54, 56, 57], was associated with this phylum. Although the numerical trends of these two phyla were visible in our results (Fig. 2), these differences did not exhibit statistical significance in the IVW analysis.
As mentioned, specific microbial groups did not yield statistically significant results in the reverse MR analysis. Given the robust focus of MR on causal temporal relationships, DLBCL may not be amenable to specific microbial changes in the current analysis settings. This phenomenon may be due to interference from confounding factors or limitations in the study design. The lack of meaningful results at this stage may suggest that definitive causal conclusions cannot be drawn, and further research is needed. DLBCL, as a highly heterogeneous immunosystem tumor, exhibits different immune dysfunctions in different individuals, leading to considerable variability in microbial dysbiosis. Furthermore, DLBCL patients often use glucocorticoids or rituximab, which increases the instability of the immune microenvironment. These factors may have contributed to our inability to obtain meaningful results and present challenges to researchers in lymphoma microbiome research.
To more accurately recapitulate the relationship between gut microbiota and DLBCL in the real world, we relied on existing observational studies of gut microbiota rather than analyzing microbiota included in all GWAS databases. This approach helps to delve deeper into the heterogeneity of current microbiota research. By analyzing existing observational studies of gut microbiota, we gain a more comprehensive understanding of the relationship between gut microbiota and DLBCL without being influenced by potential microbiota heterogeneity in GWAS databases. With existing research to help pinpoint relevant microbiota, this strategy allows us to focus on known microbiota data and more accurately explore their potential roles in DLBCL development. Additionally, we can more precisely assess the interactions between microbiota and their impact on DLBCL susceptibility. This analytical approach provides us with finer and more reliable results, further advancing our understanding of the correlation between gut microbiota and DLBCL.
However, this study still has limitations. Firstly, the smallest analyzed taxonomic unit was at the genus level, and more precise information, such as species or strains, was unavailable, which may affect the depth of our analysis results. Secondly, the population included in GWAS summaries is predominantly of European ancestry, potentially limiting the generalizability of study results to other racial and ethnic groups. Thirdly, we used a SNP threshold of 1.0 × 10^−5 to obtain sufficient exposure variables for analysis, which is higher than the usual threshold of 5 × 10^–8.