This is the first study to elaborate the detailed feactures of diversity and composition of gut microbiome in WD and explored the possible microbiome-related functions, some results of which were different from the study performed by Geng [11]. Though the two studies both showed an elimination, decreased density and loss of bacterial diversity of the microbial ecosystem in the WD patients, however, some opponent characterizations of the gut microbiome in WD were discovered in this study.
The gut microbiome of the WD group showed a significantly lower abundance of Firmicutes compared to the healthy controls at the phylum level. However, the abundance of Firmicutes in WD patients reported by Geng was significantly higher than that of healthy controls (26.18% vs. 19.83%, respectively, P < 0.05). Firmicutes covers the majority of butyrate-producing bacteria in human bacterial communities, and is known for its function in transforming undigested proteins and carbohydrates into acetic acid, and producing energy for the organism of host[12]. Meanwhile, the crucial short-chain fatty acids (SCFAs) butyrate participates in activating multiple physiological signal pathways, such as anti-inflammatory activities, and the differentiation and proliferation of regulatory T cells[13, 14]. Under this circumstance, the lower abundance of Firmicutes in present study may be more reasonable. We speculated that the alteration of composition in Firmicutes phylum may cause a low concentration of intestinal SCFAs, leading to skewed physiological functions in WD patients. Subsequent KEGG and COG pathway analysis further confirmed reduced transport and metabolism functional groups in the gut microbiome of the WD group, which intensify the possible effect of decreased Firmicutes in WD. WD maybe alleviate by increasing intestinal permeability, interfering with the intestinal microbiota by transplanting fecal bacteria rich in SCFAs, and supplementing with butyric acid. As all known, Penicillamine is a metabolic by-product of microbiota, which is one of the standard therapies introduced as a treatment for WD by Walshe in 1956 [15]. Penicillamine can combine with copper deposited in the tissue to form a soluble complex, and then excreted from the urine. Recently published study have found that oral penicillin can induce gut microbiota dysbiosis in mice, which may associate with host lipid metabolism dysfunction and low-grade inflammation[16]. The human gut microbiome has been confirmed to play a causal role in the development of pathologies in animal models of metabolic disease such as obesity, Alzheimer’s disease and Type 1 diabetes (T1D)[17–19]. While WD is an autosomal-recessive disorder characterized by abnormal copper metabolism leading to copper excretion disorder and deposition in target organs. The human gut microbiome participates in nutrient metabolism, inhibition of pathogenic growth, angiogenesis stimulation, and the maturation and maintenance of the immunological system to ensure a balance or homeostasis in the host[20, 21, 5]. Therefore, basing on the multiple roles of the gut microbiome played in metabolic diseases and the satisfactory efficacy of Penicillamine on WD, the alteration of composition and diversity of gut microbiome in WD maybe cause by metabolic reactions.
Meanwhile, the WD group showed significantly decreased phylum levels of Acidobacteria and Verrucomicrobia compared to the control group. The Acidobacteria is known for the capability of surviving in extreme, even toxic environments[22]. Verrucomicrobia is mucin-degrading bacteria residing in the intestinal mucosa that contribute to intestinal health and glucose homeostasis, and plays as an interface between the human gut microbiome and host tissues[23]. Therefore, the decrease of these two probiotics probably contributes to the disorder of physiological functions as Firmicutes in WD patients. The gut microbiome in the WD group also showed a significantly higher abundances of Proteobacteria and Fusobacteria than healthy individuals, consistent with the results reported by Geng[11]. The opportunistic pathogen of Proteobacteria created a major structural imbalance of gut microbiota in WD patients, while Fusobacteria have been widely recognized for the potential inducer of T regulatory cells or carcinogens promoting autophagy activation[24]. The WD group also showed higher abundance of Cyanobacteria, which has ability to perform nitrogen and carbon fixation and are involved in complex metabolic pathways with different mechanisms. Moreover, Cyanobacteria have an exceptionally high iron demand because of their involvement in the functions of a variety of crucial enzymes[12]. We speculated that microbiota-derived products may be act as triggers for promoting a proinflammatory and metabolically dysfunctional environment in WD patients. Microbiota-produced butyrate has been reported to provide energy for colonocytes and then prevent autophagy in the intestine of host[13]. Additionally, physiological homeostasis may be disrupted by gut microbiota, resulting in host metabolism disruption, and immune, neurological or cognitive system dysregulation and others[25, 26]. Therefore, the decreased abundance of ABC transporters system signaling pathways further confirmed potential alteration of energy metabolism related to gut microbiota in WD. To our best knowledge, it is the first report to analyze the correlation between transporters system signaling pathways and the microbiome of WD.
We also noticed that WD patients exhibited a lower ratio of Firmicutes to Bacteroidetes than healthy individuals. The dysbiosis of gastrointestinal tract metabolism was reported to be associated with low Firmicutes/Bacteroidetes ratio, causing a low concentration of circulating SCFAs, influencing elements of immune and inflammation system. These observations suggest that the gut microbiome has a potential impact on conserved functions, which may further induce the pathogenesis of WD. Human lymphocyte antigen (HLA) gene alleles have been confirmed to have a significant effect on the composition of the gut microbiota in late infancy[22]. We thus speculated that some metabolic product of gut bacteria may be recognized by high risk genotypes in WD leading to alteration of the taxa in the human gut.
The features of gut microbiome in WD group at the family and genus levels were more complex and varied significantly from the control group, presenting a different pathogen microbiome and well characterized structures. The Bacteroides showed a greater abundance in the WD group than in healthy individuals. Bacteroides are correlated with diets high in animal protein and saturated fats. Moreover, the Bacteroides genus was reported to enhance the efficacy of the anti-CTLA4 immune checkpoint in mice and is speculated to contact and stimulate T cells and DCs directly by means of pathogen-associated molecular patterns in the host[27]. The Enterobacteriaceae were markedly more abundant in the WD than the control group. The significant opportunistic pathogen Enterobacteriaceae is present in the human gut without causing symptoms or diseases under normal circumstance. However, host immunity and environmental factors, such as redox state and availability of oxygen, may result in significant variation of Enterobacteriaceae. The richness of Enterobacteriaceae in WD further suggested the correlation between pathogen microbiome and WD.
Moreover, the WD also presented unique richness of Gemellaceae, Pseudomonadaceae and Spirochaetaceae at family level in WD group, which were hardly detected in healthy controls. Under this condition, the three bacteria may be developed as biomarker. There is no gold standard for the diagnosis of WD, which is often delayed due to the non-specific clinical features and the need for a combination of clinical and laboratory tests for diagnosis. The development of targeting identified bacterial biomarkers of gut microbiome may be an alternative diagnostic method to WD in future.
The gut microbiome of controls and WD patients has a comparable abundance of Prevotella genus, which is more universal in populations with high carbohydrate and sugar consumption, as observed in agrarian and vegetarian societies[28]. Furthermore, the WD group exhibited significantly lower abundance of Blautia, Ruminococcus and Coprococcus genus in comparison with control group, which are considered as important components of commensal microbiota and play important roles in several homeostatic functions such as immunity, neurohormones and metabolism. Blautia plays a role in digestion of complex carbohydrates and also shows decreased abundance in diabetes, irritable bowel syndrome, Crohn’s disease and and nonalcoholic fatty liver diseases[29]. Coprococcus is a genus of anaerobic cocci which is part of the human feacal microbiota that functions in producing butyrate[30]. The decreased of these probiotics indicated the defections of multiple physiological function presented in WD may cause by the dysbiosis of gut microbiome. Additionally, Megamonas, Megasphaera and Clostridiales are genera of Firmicutes bacteria that showed increased abundance in the WD group. Clostridiales includes bacterial species that produce short-chain fatty acid, which are important for the equilibrium between regulatory T cells and helper T type17 (Th17) cells[25]. These results further indicated the structure of bacterial communities is more various than previous study and some potential gut microbiome may be identified as bacterial biomarkers.
Additionally, the analysis of KEGG and COG pathway indicated that these bacteria affect the host by shedding different microbial bioactive molecules, including transcription factors, metabolism of fructose, mannose, butanoate, glyoxylate and dicarboxylate, different transporters system such as ion-coupled transporters, ABC transporters and TRAP-type C4-dicarboxylate transport system. Increasing evidences demonstrated that gut microbiota related transporters system appeared to be key microbial bioactive signaling pathways. These transport systems regulate the energy system by promoting utilization of glucose, ribose/galactoside. We speculated that the dysfunction of gut microbiota in WD may influent the metabolic of copper via transporters system signaling pathways. In the next step, specific metabolites of the microbiome may be searched for as therapeutic chemicals like penicillamine in WD.