Gut Microbiota and Colorectal Cancer: An Umbrella Review of Methodological Trends and Clinical Correlations

In this umbrella review, we analyze the effect of gut microbiota on the development and progression of colorectal cancer (CRC), a global health challenge. Following Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) 2020 guidelines, we searched multiple databases for the most relevant systematic reviews and meta-analyses from 2000 to 2023. We identified 20 articles that met our inclusion criteria. The findings include the identification of specific microbiota markers, such as Fusobacterium nucleatum, for potential early diagnosis and improvement of disease treatment. This thorough study not only establishes the connection between microbiota and CRC but also provides valuable knowledge for future research in developing microbiome-centered treatments and preventive methods.


Introduction And Background
Colorectal cancer (CRC) ranks as the second cause of cancer mortality in the United States and the third worldwide.Statistics indicate that in 2023, the estimated number of new cases will be 153,020, with 52,550 deaths [1].Alarmingly, by 2040, the global incidence of CRC will skyrocket to 3.2 million new cases, with 1.6 million deaths [2].The development of CRC is a multifaceted process involving genetics, lifestyle, age, and environmental factors.Interestingly, 85-90% of the CRC cases are related to environmental factors rather than genetics [3].The gut microbiota especially plays a critical role that spans from the initiation to the evolution of the disease [4,5].
Recent studies have highlighted the differences in the gut microbiome in healthy individuals compared to those with the disease, ranging from essential identification of the gut microbiome to utilizing and manipulating treatments such as probiotics, prebiotics, and fecal transplants.Even though these techniques show potential, assessing their risks and patient-specific aspects is crucial [6,7].Despite the advancements, we need a more profound understanding of which microbiota and by which mechanism leads to -to advance better-targeted therapies [8].
This umbrella review combines and evaluates previous studies to understand how different types of gut microbiota composition influence CRC development and advancement.We will investigate the potential of early diagnosis and individualized treatments using microbiota profiling.We aim to shed light on future directions to enhance public health strategies in CRC control by bridging the gaps between current research and clinical applications.

Review Methodology
We conducted this umbrella review strictly adhering to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) 2020 guidelines [9].

Search Strategy
To identify relevant systematic reviews, we searched the following electronic databases: Google Scholar, PubMed, Web of Science, and Science Direct.We focused on two key topics: microbiota and colorectal

Inclusion and Exclusion Criteria
Inclusion criteria included systematic reviews and meta-analyses that examine the relationship between gut microbiota alterations and CRC risk and progression in adult populations aged 18 years and older.We only considered studies that have been published in peer-reviewed journals and are in the English language.Exclusion criteria included narrative reviews, editorials, opinion pieces, case reports, studies in pediatric populations, or studies that did not distinguish CRC from other cancer types.We completed the comprehensive search after manually reviewing the reference lists of included articles published from January 1, 2000, to December 1, 2023.

Data Extraction
Our umbrella review used a Microsoft Excel worksheet (Microsoft Corporation, Redmond, Washington) for data extraction and analysis.Two reviewers (Alousious Kasagga and Chnoor Hawrami) independently screened titles and abstracts to determine their eligibility.We then conducted a full-text review to confirm their inclusion.The extracted data included the author(s), year of publication, journal name, funding source, references to the included studies, population characteristics, method of microbiota assessment, measured CRC outcomes, key findings, and conclusions.If there were any inconsistencies, they were resolved through consultation with a third reviewer (Erica Ricci) when deemed necessary.

Quality Assessment
We used the AMSTAR 2 (A Measurement Tool to Assess Systematic Reviews) tool to evaluate the quality of the included systematic reviews [10].

Evaluation of Study Overlap in Systematic Reviews
We used a comprehensive approach to measure the degree of study duplication among the systematic studies.This approach involved three primary metrics: overlap percentage (%), covered area (CA), and corrected covered area (CCA) [11].Overlap percentage (%) measures the proportion of primary studies cited in several systematic reviews.It is calculated by dividing the number of repeated primary studies by the total number of primary studies.It provides a clear indicator of the level of study overlap.The CA measures the scope of the research field covered by the included reviews.It is calculated by dividing the total number of citations by the product of the total number of primary studies and the number of included reviews.CCA refines the CA by considering the frequency of each primary publication across the included reviews.Assessing the degree of overlap using this metric offers a greater level of accuracy, classifying it as low (0-5%), moderate (6-10%), high (11-15%), and very high (>15%).

Study Characteristics
The included reviews covered a range of topics, exploring specific strains of gut microbiota, overall microbial diversity, and their impact on CRC risk and progression.

Quality Evaluation of the Included Reviews
We used the AMSTAR 2 assessment tool to evaluate the reviews' quality.Three reviews were of high quality, while 17 were of moderate quality.The most common observed limitation in the moderate reviews was not performing data searches and extraction in duplicate, not registering their protocols, and failing to report.
Table 3 shows how each study performed during the AMSTAR 2 evaluation.

Analysis of Study Overlap in Included Reviews
We reviewed a total of 662 citations, comprising 414 distinct primary studies, and identified 113 instances where primary studies were repeated in our 20 systematic reviews and meta-analyses.The most commonly cited study appeared in eight reviews.The calculation of the overlap parameters is as follows.
The overlap percentage of 27.29% indicates a significant redundancy, which means that more than a quarter of the primary publications are repetitive.The 7.99% CA shows a low redundancy, indicating that our included reviews are unique regarding their content, data, or findings, providing diverse and complete literature coverage.CCA is a more accurate measure for overlap analysis, with 3.15% indicating a low degree of overlap.
In the analysis, we also used a matrix heatmap to visualize the interrelationship among the included studies.Each cell indicates the number of primary studies shared between the two included studies.Figure 2 shows how each included study obtained its data from unique sources, emphasizing overall diversity among the included studies.

Synthesis of Findings
This section presents a comprehensive synthesis of results on the gut microbiota's role in CRC diagnosis and development.In 20 studies, researchers have found a significant correlation between the disease and a range of microbial taxa.[31].

Heterogeneity and Publication Bias
This umbrella review shows some heterogeneity among the reviewed papers because of the complex relationship between the CRC and gut microbiome, where methodological differences are seen, with some studies focusing on fecal samples while others focus on tissue samples.In addition, this article may have a review selection bias, favoring recent and peer-reviewed studies despite our efforts to include a variety of studies, and with positive publication bias becoming a problem in this discipline, we employed a detailed search in well-known databases to mitigate these.

Discussion
This umbrella review has thoroughly analyzed the existing literature on the role of the microbiota in colorectal cancer.This section addresses the potential implications, challenges, limitations, and future directions seen in the included studies.

Implications of the Results
As we comprehensively presented in the synthesis of results, there is a noteworthy correlation between dysbiosis in the gut and oral microbiota and CRC development and progression.Microbial markers, specifically F. nucleatum, can be low-cost, noninvasive tools for early detection, monitoring recurrence, and treatment response.Furthermore, integrating these microbial markers with traditional screening methods, like fecal occult blood tests, can increase the sensitivity and specificity of CRC diagnostics.
While the findings of studies on the correlation between specific microbiomes and CRC are promising, it is essential to acknowledge some challenges and limitations.Firstly, the cause-and-effect relationship between the disease and microbiomes is yet to be determined.Secondly, it is impractical to make direct comparisons due to different study methodologies like sample types (tissue vs. fecal), lab analysis techniques, and population demographics.Lastly, most studies did not account for variables that may influence the result, such as diet, age, genetics, and lifestyle, which can impact the gut microbiome.

Future Directions
There is a critical need for the standardization of microbiota methodologies, as it would allow comparable findings across studies.Future research should consider setting up large-scale multicenter longitudinal studies involving different populations and locations to verify the specific biomarkers needed to develop a universal diagnostic tool.Furthermore, studies should investigate how the microbiota influences CRC pathogenesis, leading to new approaches to targeted prevention and treatment therapies.

Bias, Flaws, and Low Study Overlap
According to a CA of 7.99% and a corrected CA of 3.15%, the studies we included have a low level of redundancy among the primary studies they covered, suggesting that a broad range of unique primary studies increases our findings' reliability while reducing the risk of citation bias.

Conclusions
This umbrella review extensively analyzes the literature on the role of the microbiota in colorectal cancer.It strongly links gut and oral microbiota changes with CRC development.Our review paper suggests that the microbial marker F. nucleatum can be used as a noninvasive, cost-effective tool to improve early detection and monitoring of CRC.However, the current challenges are the varying methodologies and cause-andeffect between gut microbiota and CRC, which must be better understood.So, future research should focus on conducting large-scale longitudinal studies with standardized methodologies and exploring the mechanisms of gut microbiota influence on CRC for targeted therapeutics.

FIGURE 1 :
FIGURE 1: PRISMA flow diagram.PRISMA: Preferred Reporting Item for Systematic Reviews and Meta-Analyses The diagram was drawn by the authors of this article.

TABLE 1 : Selected keywords and MeSH terms for microbiota and colorectal cancer
search query was created using the identified keywords and MeSH terms, using advanced search and Boolean operators (AND, OR).(Microbiota OR Microbiome OR Gut flora OR Intestinal bacteria OR Intestinal microbiome OR Gut bacteria OR Dysbiosis Microbiota [MeSH] OR Gastrointestinal Microbiome [MeSH] OR Dysbiosis [MeSH]) AND (Colorectal cancer OR Colorectal neoplasms OR Colorectal carcinoma OR Colon cancer OR Rectal cancer OR Colorectal tumor OR Colorectal Neoplasms [MeSH] OR Neoplastic Processes [MeSH] OR Colonic Neoplasms [MeSH] OR Rectal Neoplasms [MeSH]) A

Table 2
summarizes each selected study's characteristics, containing the author's name, journal name, publication year, number of primary studies included, funding source, study objective, and outcome.
markers and related metabolites that could serve as diagnostic indicators for CRC They identified nine distinct microbial markers in CRC patients compared to healthy controls.

TABLE 2 : Characteristics of included reviews
CRC: colorectal cancer

TABLE 3 : Quality evaluation using AMSTAR 2 checklist questions
Y: Yes; N: No; PY: Partial Yes; NA: Not applicable; AMSTAR: A Measurement Tool to Assess Systematic Reviews Hussan et al. and Peng et al. have demonstrated that Fusobacterium is a critical marker in CRC patients, as confirmed by several other studies showing its abundance in CRC tissue and fecal samples [21,25].There is a considerable amount of interest in oral microbiota, with studies like Negrut et al. suggesting the use of salivary Fusobacterium nucleatum DNA as a noninvasive diagnostic method; this is supported by Yu et al. and Eastmond et al., who showed variations of oral cavity microbiota in the CRC [18,24,30].Several studies, such as Gethings-Behncke et al., Ranjbar et al., Tabowei et al., and Valciukiene et al., emphasized the role of F. nucleatum in CRC diagnosis and prognosis and its potential as a treatment target [20,26-28].In contrast, some studies, like Fratila et al., discussed the possible role of Lactobacilli and Bifidobacteria in CRC control [19].Additionally, Zwezerijnen-Jiwa et al. suggested that combining traditional detection tests with microbiome markers can lead to more effective CRC detection