The study of the microbiome has significantly contributed to our understanding of complex diseases including cancer, with a profound influence of the microbiota on clinical prognosis and the efficacy of cancer treatments. Oesophageal cancer is positioned amongst the most aggressive malignant diseases, resulting from a complex interaction between anthropometric, genetic, immune response, and environmental factors. Oesophageal squamous cell carcinoma (OSCC) is the most common type of oesophageal cancer and is a serious burden in Eastern Africa, in the area known as the African oesophageal cancer corridor (AOCC). OSCC is often diagnosed at a late stage, with patients already suffering from severe malnutrition and dehydration due to swallowing difficulties, leading to high mortality rates. So far, aetiological factors have been individually analysed with an inappropriate contextualisation. The upper digestive tract microbiome has been proposed to contribute to the onset and progression of OSCC but with limited understanding of the mechanisms behind this interaction. Data on African populations are limited, and the aetiology of AOCC is still poorly understood. This review discusses the current knowledge of the aetiology of OSCC in Africa, with special focus on the probable influence of the upper digestive tract microbiota.
Oesophageal cancer is extremely aggressive, ranking eighth in of most diagnosed cancers worldwide and being the sixth most mortal one . The 2 main histological types – oesophageal adenocarcinoma (OAC) and oesophageal squamous cell carcinoma (OSCC) – are easily distinguished in terms of epidemiology, pathogenesis, and tumour biology. OAC develops in the lower portion of the oesophagus and predominates in high-income settings, mainly affecting middle-aged and elderly people. OSCC occurs in all parts of the oesophagus and accounts for over 3 quarters of all oesophageal cancers in the world, yet is much less understood than the former type. In fact, in high-income settings, the main OAC risk behaviours include smoking, tobacco, and a diet poor in fruits and vegetables. In low-income regions, these factors hardly explain the high burden of the disease, especially among young people, and other factors such as micronutrient deficiencies, poor oral hygiene, dental fluorosis (a defect in the tooth enamel characterised by hypomineralisation), mycotoxins, as well as genetic factors, may explain the pattern observed . The existence of 2 main OSCC endemic regions in the world (Fig. 1) is striking: the so-called African oesophageal cancer corridor (AOCC) in Eastern and Southern Africa, and the Asian oesophageal cancer belt (AOCB), which extends from western/northern China through Mongolia, Kazakhstan, Turkmenistan, Uzbekistan, Tajikistan, the Middle-East, and eastern Turkey [3, 4]. The 2 regions present a 20-fold higher incidence than any other region, with variations in sex ratio that most likely reflect aetiological factors . OSCC is more common in men than in women, but the ratio varies greatly between countries, ranging from 4:1 in high-incidence areas such as China to 1:1 in other hotspot countries such as Mozambique . Yet there is an overall male excess in OSCC incidence across Africa, especially in individuals aged 30–39 years old . Factors driving the high prevalence in such marked geographical regions and the higher incidence in younger patients is nevertheless still poorly understood.
OSCC is usually preceded by a premalignant lesion called oesophageal squamous dysplasia (OSD) . This asymptomatic precursor lesion was found to be common even among asymptomatic residents of southwestern Kenya, especially in those aged over 50 years . Nevertheless, OSCC is usually detected at late stages of the disease in low-income settings, carrying a poor prognosis since the disease presents too late for a therapeutic intervention. Only 13% of patients in sub-Saharan Africa are potential surgical candidates, and many more do not even have access to healthcare providers . Despite the technological advancements in surgical therapies and chemoradiotherapy, the prognosis remains poor even in patients who have undergone complete resection. The 5-year survival rate in western countries does not exceed 15–25% frequency amongst African patients. This situation is aggravated as many African patients already suffer from severe malnutrition and dehydration due to swallowing difficulties at the time of diagnosis [7, 9]. The poor prognosis leads to high mortality rates, being the fourth and second cause of death in Mozambique and Malawi, respectively .
This review will sum up evidence for the multifactorial profile of OSCC, addressing not only relevant environmental and genetic risk factors for the disease development, but also mainly the oral, oesophageal, and gastric microbiomes. The correct evaluation of multiple possible factors may challenge the burden of oesophageal cancer in Africa, empowering medical staff, researchers, and politicians to understand the disease and the importance of implementing prevention strategies to improve overall quality of life in high-prevalence settings such as AOCC.
The bibliographic search consisted of a generic approach that provided analysis of recent and up-to-date literature on a wide range of topics related with oesophageal cancer in Africa (1990–2020). Terms such as “squamous cell oesophageal cancer,” “Africa oesophageal cancer corridor,” “epidemiologic risk factor,” “alcohol and tobacco,” “micronutrients deficiencies,” “thermal injury,” “mycotoxins,” “microbiome,” “oesophageal microbiota,” “oral microbiota,” and “gastric microbiota” were used in PubMed searches, with Boolean operators to broaden or narrow down the output results.
Epidemiological Risk Factors
The unusual existence of high OSCC incidence areas strongly suggests a population-dependent role of environmental risk factors. The list of potential carcinogens is long and ranges from thermal damage, cooking fire and inhaled smokes, to micronutrient deficiencies and dietary habits .
Tobacco and Alcohol
The evidence on the influence of tobacco on OSCC susceptibility is still uncertain (Table 1), especially in low-income countries . In China, a country lying within the AOCB, smoking does not seem to significantly affect the risk of disease , but in eastern Turkey it was indicated as a major risk factor . Besides susceptibility variation due to distinct smoking habits, different ethnicities with similar smoking behaviours show different OSCC risk. African Americans show incidence rates greater than twice as high as Hispanics, Asians, and Native Americans . The risk of OSCC increases with both exposure intensity and duration, with long-standing tobacco smokers presenting a higher risk .
The link between alcoholic beverage consumption and OSCC seems to be well established although data must be taken with caution due to improper multifactorial evaluation (Table 1). In Asia there is an OSCC increased risk of 1.6- to 5.3-fold and about 3-fold in Africa and South America associated with alcohol consumption. This increased susceptibility is even greater in low incidence areas, with about a 6-fold increase in Europe and 9-fold in America . In Africa, the traditional home-made alcoholic beverages often contain carcinogenic substances, such as a greater ethanol percentage or acetaldehyde, due to preparation methods which represent an additional risk factor. A case-control study in endoscopy-confirmed OSCC Kenyan patients indicated that alcohol consumption, particularly of busaa and chang’aa, may contribute to half of the OSCC cases in the region, with a linear increased risk with number of drinks . The identification of the carcinogenic compounds in drinks and their damaging effects is, nevertheless, a challenge due to the great heterogenicity of beverages and lack of systematic evidence [15, 16].
The combined consumption of tobacco and alcohol nearly doubles the risk of OSCC than either one alone . The underlying mechanism is not clear but most likely includes DNA damage, increased epithelial permeability to carcinogenic compounds, and/or altered upper digestive tract microbiota (Fig. 2) . Nevertheless, in Africa other dominant factors besides alcohol and tobacco consumption must explain the burden of the disease, particularly among young adults and women. Okello et al.  showed that only 13% of Uganda’s OSCC cases were linked to smoking and alcohol, supporting the main influence of other factors in AOCC carcinogenesis.
Micronutrient deficiencies have been implicated in OSCC predisposition by making the oesophageal epithelium more prone to inflammation (Fig. 2), which may lead to tumour development and progression . The association between unbalanced intake of minerals and oesophageal cancer risk is still debateable (Table 1), but evidence points to the protective effect of selenium and zinc [19-23]. Selenium deficiency seems to accelerate oxidative stress and DNA damage, and selenium supplementation trials in nearly 30,000 participants from Linxian, an area of north central China with some of the world’s highest rates of oesophageal cancer and a population with a chronically low intake of several nutrients, showed promising results , with beneficial effects on mortality still evident up to 10 years after ending supplementation . Zinc deficiency affects cell homeostasis and may induce the overexpression of proinflammatory mediators. Nevertheless, contrary to the selenium trials, the Linxian trials showed no significant results for multivitamin supplementation containing zinc . Other potential OSCC protective micronutrients are vitamins A, C, and E, riboflavin, carotenoids, or folate, though prospective data are missing. In contrast, diets with high levels of saturated fats and cholesterol seem to increase the risk of OSCC development .
Nutrient deficiency, mainly zinc, selenium, and iodine, in the AOCC is particularly severe, such as the region falling within the Rift Valley, where soils are greatly exposed to degradation causing micronutrient imbalance, perpetuated by a diet mostly or exclusively relying on local agriculture [3, 26]. Despite the striking co-location, there is an acute lack of studies to access this possible correlation. Schaafsma et al.  examined gender-specific oesophageal cancer incidence rates in relation to dietary nutrient supplies. The authors showed that AOCC countries appear to have lower estimated dietary supplies of selenium and zinc in similarity to studies performed outside Africa, supporting the importance of dietary nutrient deficiencies in OSCC. Several studies in Malawi, Tanzania, Kenya, and South Africa showed crop uptake of selenium varies greatly due to the geochemical variation of soils . In Malawi, the most affected country by OSCC, selenium intake was lower than the average adult requirements for over 80% of the population . Kenya and Mozambique were the other countries with the lowest mean dietary selenium and zinc supplies, whereas other nutrients such as magnesium or iron tended to have adequate supplies [3, 28]. In contrast to what was expected, a study that evaluated the association between higher serum selenium concentration and OSD found a positive association between the 2 parameters, implying that further studies are needed to clarify the role of selenium in OSCC carcinogenesis . Nevertheless, simple nutritional prevention measures might have a great impact in decreasing the burden of OSCC in Africa, such as implementation of fortified/enriched food or drinking water, supplements, fortified fertilizers or other agronomic strategies, and the promotion of nutritional diversity . A study was performed in a Chinese population exploring the efficacy of selenium-enriched rice in improving human selenium status to evaluate and potentially prevent the progression of oesophageal cancer and dysplasia . This simple approach might be tailored to AOCC crops and has great potential in the region, where simple but effective measures are urgent.
Thermal Damage and Cooking Habits
Consumption of hot food and beverages has been associated with an increased risk of OSCC (Table 1), especially in high incidence areas, by causing thermal injury and the production of inflammatory heat shock proteins, cytokines, and chemokines that promote carcinogenesis (Fig. 2). Thermal damage can also induce nitrosamines formation, which are carcinogenic compounds. The epithelium injury may also allow the permeation of carcinogens . Hot beverage consumption of hot tea, hot coffee, and hot maté shows increased OSCC risk, with the latter having been classified by the International Agency for Research on Cancer (IARC) as probably carcinogenic to humans . In African countries like Tanzania, Malawi, and Kenya, hot tea consumption, especially milk tea with a high fat content that retains heat, is quite frequent and the contribution of this custom to OSCC needs to be evaluated in the AOCC, together with other risk factors acting synergistically in this malignancy. Studies in the region described hot beverage consumption could reach over 71°C [32, 33]. Middleton et al.  conducted a case-control study in Kenya observing that “very hot” and “hot” beverage drinkers had a 3.7- and 1.4-fold OSCC risk, respectively, when compared to “warmer” drinkers. Yet, the potential carcinogenic mechanisms involved are unclear.
African populations are greatly exposed to polycyclic aromatic hydrocarbons (PAH) that are a large class of noxious and carcinogenic compounds formed by incomplete burning of several organic substances. In low-income settings, populations are more prone to use charcoal and wood for cooking, being more exposed to inhaled smoke with high amounts of PAH, predisposing to respiratory infections as well as respiratory tract cancers [2, 35]. Nevertheless, the heterogenicity of epidemiological evidence presents the need for more data to clarify thermal damage and PAH in OSCC pathogenesis.
The cereal-based diet adopted by most people in Africa expose them to maize contamination by fumonisins, human health carcinogenic mycotoxins produced by the fungus Fusarium spp. . Fumonisins are categorised as possibly carcinogenic to humans (Group 2B) by the International Agency for Research on Cancer . F. verticillioides, a fungus that is virtually present in almost all maize samples, produces the mycotoxin fumonisin B1 (FB1), a toxic compound linked to oesophageal cancer (Fig. 2) and neural tube defects in humans . The presence of the fungus does not necessarily implicate FB1 production since most strains do not produce the toxin. Nevertheless, its presence in low-income countries, especially Africa, represents a public health problem. FB1 contaminates a large fraction of the crops, including maize, cereals, groundnuts, and tree nuts, in high OSCC incidence areas . Besides crop contamination, mycotoxins are also found in traditional alcoholic beverages due to customary preparation methods . Mycotoxin exposure is more likely to occur where poor food handling and storage conditions are common, and poverty and malnutrition prevent the disposal of contaminated food. In Malawi it was found that rural populations were exposed to extremely high levels of aflatoxins and fumonisins , and in Mozambique high levels of aflatoxins were also found in groundnuts and maize, demonstrating that aflatoxin exposure is still a major problem . Few food safety regulations protect the exposed populations.
Fumonisins contamination is associated with an increased risk of hepatocellular and renal cell carcinoma, but its implication in OSCC is still unclear [2, 36, 38]. Kigen et al.  suggested that mycotoxins, particularly fumonisins, combined with home-made alcoholic beverages, dietary deficiencies, and viral infections, contribute to the development of oesophageal cancer in Kenyan communities along the Rift Valley. However, Shephard et al.  showed that fumonisin exposure in all age groups in 2 regions with high and low oesophageal cancer incidence was above the short-term maximum tolerable daily intake according to the FAO/WHO Expert Committee on Food Additives. In rodents, FB1 was shown to cause tumours of the liver and kidney [41-43]. The carcinogenic mechanism triggered remains unclear but most likely implicates oxidative damage, production of reactive oxygen species, and apoptosis [39, 44-46]. Mycotoxins also have a strong mutagenic effect causing permanent defects in the genome and disrupting sphingolipid metabolism . A recent study showed fumonisins inhibit sphingolipid metabolism, suggesting that disruption of the de novo pathway of sphingolipid biosynthesis may be a crucial event in diseases associated with the consumption of fumonisins .
A few epidemiological studies also support the association of fumonisins and OSCC. The substitution of sorghum (a plant that does not enable Fusarium fungi growth) for maize in South Africa and Italy led to an increase of oesophageal cancer incidence [48, 49]. Conversely, the prevalence of sorghum in Nigeria shows very low rates of OSCC prevalence . The only case-control study on fumonisin exposure and oesophageal cancer risk was conducted in China, where 98 OSCC cases were randomly selected from the Linxian trial cohort and compared to 185 controls. The study evaluated concentrations of serum sphingolipids as these molecules are biomarkers of fumonisin exposure. For each of the 3 sphingolipid measures no significant relationship was found, with odds ratios of 0.69 (95% CI 0.33 ± 1.4) for sphingosine, 0.63 (95% CI 0.29 ± 1.4) for sphinganine, and 0.98 (sphinganine 0.50 ± 2.0) for the sphinganine/sphingosine ratio, adjusted for covariates such as age, sex, smoking, and drinking . However, given the heterogenicity of OSCC risk factors across the world and the sharp delineation of high incidence areas, the extensive available results for AOCB may not reflect Africa’s analogous high incidence area.
The research community has made a great effort to perform molecular analysis on the most common malignancies, but OSCC is still relatively understudied. The Cancer Genome Atlas (TCGA) consortium, a leading cancer genomics program, has a single publication on oesophageal cancer . Besides, most published data refer to western countries, where OSCC has been in decline for decades, leaving this type of cancer poorly understood in terms of molecular studies. An inherited genetic component to OSCC ethology has been indicated in Chinese populations [53, 54], but family history involves a series of habits and inherited genes that most likely influence disease susceptibility. In China and Japan, OSCC molecular studies described frequent mutations in genes associated with other squamous cell carcinomas (Table 2) as well as other cancer-associated and histone modifier genes, such as TP53, RB1, CDKN2A, PIK3CA, NOTCH1, and NFE2L2 (Fig. 2) [55-58]. In sub-Saharan Africa, a few molecular studies have addressed OSCC, mostly targeting single-nucleotide polymorphisms (SNPs) and indels in specific genes and chromosome regions [59-67]. The findings point to population-specific differences, most likely reflecting differences across ethnic groups. Liu et al.  performed one of the few whole-exome DNA sequencing studies in Africa, characterising 59 untreated OSCCs and matched normal pairs from Malawian patients, reporting similar genetic alterations as shown in North American and Asian cohorts in TP53, CDKN2A, NFE2L2, CHEK2, NOTCH1, FAT1 and FBXW7 genes (Table 2). Another study observed a significant enrichment of inactivating or dominant-negative events in known tumour suppressor genes such as TP53, CDKN2A, NOTCH1/3, FAT1/2/3/4, and FBXW7, as well as known activating events of PIK3CA and NFE2L2(Table 2). The authors also reported an enrichment of mutations in genes encoding the chromatin-modifying enzymes KMT2D (MLL2), KMT2C (MLL3), and EP300 .
Dental fluorosis has a striking co-location with AOCC and there are specific mutations linked to fluorosis in different populations . The few epidemiological studies on the subject have shown that some ethnic groups are more likely to develop the disease, especially African American children . Genetic variants in some candidate genes have already been linked to fluorosis susceptibility, such as COL1A2, CTR, ESR, COMT, GSTP1, MMP-2, PRL, VDR, and MPO . Despite the high incidence of fluorosis in the AOCC, to date there have been no studies on the potential individual genetic background influence in African populations. One of the few molecular studies in OSCC studied genetic variants from 7 loci identified in Asian and European populations for association in South African populations. An SNP in CHEK2, a tumour-suppressor gene involved in cell cycle regulation and DNA repair, was significantly associated with OSCC indicating that genetic risk factors most likely change in African and non-African populations . Nevertheless, there is a great urge to fill in this gap given the high incidence of the disease in AOCC and the marked differences in population ancestry that may reflect in cancer epidemiological and clinical features. Ancestry-specific predisposing genetic variants is well established in other cancer types and needs to be better addressed in the context of AOCC. For instance, prostate and breast cancers show an increased genetic susceptibility risk in Africans in comparison to other populations, even though the actual magnitude of risk is not well characterised [72, 73].
Upper Digestive Tract Microbiome
Microbial organisms interact with each other and with the host immune system, having a great influence on the development of disease . Alterations in the microbiome’s composition may induce significant changes in host homeostasis, and lead to inflammatory diseases and increased cancer susceptibility . Homeostasis of the microbiome is essential for adequate host immune responses . The most efficient way to characterise the microbiome involves the analysis of microbial genomes through metagenomics. Prokaryotic 16S ribosomal RNA (rRNA) is usually used as a marker for the taxonomic classification and phylogenetic analysis of the microbiome, affording enough information for secure identification of strains . Sequencing is currently mostly performed by next-generation methods, which process thousands of low-sized reads (approx. 100 base pairs) that are then mapped against reference panels of bacterial genomes . The sequencing of thousands of reads allows us to identify and quantify many microbes that constitute the microbiome, even the ones that are less frequent.
The upper digestive tract (oral cavity and oesophagus) and gut microbiota assemble complex microbial compositions involved in diseases such as OSCC (Fig. 2) and OSD [78, 79]. Despite the numerous microorganisms harbouring in the human upper digestive system, little is known about the relationship of the oral and/or oesophageal microbiome with the host and OSCC susceptibility. Sampling of the oesophagus usually consists in endoscopy with brushing or biopsies with limited sample sizes due to the invasiveness of these procedures, impending, namely, informative microbiome studies. The available non-endoscopic procedures for oesophageal microbiome analysis, such as “Esophageal String Test” or “Cytosponge,” indirectly also sample the stomach or oral microbiome, biasing the results [80, 81].
Initial evidence suggested the oesophageal microbiome was the result of momentary exposure to the oral microbiota , but recent studies showed that, in addition to swallowed bacteria, the oesophagus is exposed to refluxed gastric microbes, being a sum up of both oral and gastric microbiota, and having its own identity . Several factors may alter the oesophageal microbiome, such as proton pump inhibitors by increasing the pH of gastric secretions, and also by directly targeting the bacterial proton pumps of certain bacteria such as Streptococcus pneumoniae and diet . Early studies in Chinese populations showed that Streptococcus is the dominant taxon in the healthy oesophageal mucosa, followed by the genera Prevotella and Veillonella [85, 86]. Later, a study of a Caucasian cohort that underwent upper gastrointestinal endoscopy showed the oesophageal microbiota clusters in 3 community types with unique dominant organisms defined by the relative abundances of Streptococcus and Prevotella, and microbial richness and evenness. However, the findings in healthy oesophageal microbiota are not completely consistent and some genera abundances differ between cohorts. These differences are most likely explained by the different geographical locations of the populations, reflecting diverse lifestyles, sex proportions in the cohorts, and age that together significantly affect the microbiome. Different sampling and sequencing methods are also important variables .
Evidence of different oesophageal microbiome composition in healthy individuals and in patients with cancerous oesophageal mucosa or cancer-predisposing conditions is indisputable. A cross-sectional study performed on a Chinese cohort provided the first evidence of increased Gram-negative bacteria associated with adenocarcinoma and the related cancer-predisposing Barrett’s oesophagus . Liu et al.  also showed that bacterial communities differed among normal oesophagus, reflux esophagitis, and Barrett’s oesophagus in Japanese patients. While the most prevalent genus in normal oesophagus and reflux esophagitis patients was Streptococcus, Veillonella was predominant in Barrett’s oesophagus. Also, while Fusobacterium was found in patients, it was absent in the normal oesophagus . Another cohort displayed dominance of Streptococcus and Prevotella and found a significant association of these genera with Barrett’s oesophagus and OAC risk factors . Later, Deshpande et al.  found that taxonomic and functionally distinct microbial compositions were associated with the OAC cascade, indicating that these functionally distinct community types should be further explored. The authors identified 3 main community types with distinct functional signatures: cluster 1, with average levels of Streptococcus and Prevotella but increased levels of Haemophilus (H. parainfluenzae) and Rothia (R. mucilaginosa), was found to be enriched for glycolysis as well as pathways related to short-chain fatty acid metabolism; cluster 2, dominated by Streptococcus (S. mitis/oralis/pneumoniae), was enriched for the pentose phosphate pathway and fructose and mannose metabolism; and cluster 3, dominated by Prevotella(P. melaninogenica and P. pallens) and to a lesser extent Veillonella, was enriched for lipopolysaccharide biosynthesis.
Far less is known about the association of oesophageal dysbiosis and OSCC as the great majority of studies refer to OAC that has a particularly higher incidence in high-income countries. These results may not be reflected in OSCC due to the epidemiologically and biologically distinct characteristics of the 2 types of tumour. The few studies addressing the association of OSCC and the oesophagus microbiota showed compelling evidence between bacterial composition and disease risk (Table 3). Yu et al.  showed that Chinese individuals with lower microbial richness are more prone to develop OSCC since there was an inverse association between microbial richness and the risk of OSD. However, this study examined the upper digestive tract cells collected by 2 different devices, which does not allow the distinct microbiota of the oral cavity, oesophagus, and stomach to be distinguished.
Gao et al.  analysed oesophageal tissues from OSCC Chinese patients, finding that the Gram-negative anaerobic bacterium Porphyromonas gingivalis infects the cancerous and adjacent oesophageal mucosa of the patients but not the healthy mucosa of controls. This opportunistic pathogen falls within the “red complex” of periodontal pathogens, which are the species most strongly associated with severe periodontitis – an inflammatory disease that causes oral dysbiosis. The presence of P. gingivalis in the unhealthy mucosa was positively correlated with cancer cell differentiation and metastasis, which may represent a good marker for poor clinical outcome. The presence of F. nucleatum (a bacterium that inhabits the oral cavity also causing periodontitis) in oesophageal cancer specimens of Japanese patients was also associated with a significantly shorter survival time since “cytokine-cytokine receptor interaction” pathways were significantly affected in F. nucleatum-positive oesophageal cancer tissues, suggesting that this bacterium contributes to a more aggressive tumour behaviour by activating chemokines such as CCL20 .
Given the proximity of the oral cavity and the oesophagus, the oral microbiome has been strongly implicated in OSCC susceptibility. Disruption of the oral microbiome is stimulated through direct metabolism of chemical carcinogens such as alcohol and tobacco, systemic inflammatory effects, or poor oral hygiene [94-99]. Few studies have addressed the association between the oral microbiota and OSCC, being almost limited to the Asian corridor (Table 3). Chen et al.  reported that 87 OSCC Chinese patients had lower microbiome diversity than 85 healthy individuals, including an increased relative abundance of Prevotella, Streptococcus, and Porphyromonas. Although these genera seem to be non-pathogenic to the host, certain bacteria are associated with increased risk of disease – F. nucleatum, Tannerella forsythia, and P. gingivalis – while others were linked to lower risk – Streptococcus and Neisseria [80, 92, 93]. The detection of adverse bacteria in pre-cancer lesions may become a key disease biomarker and prognostic indicator [92, 101]. A recent prospective study in an American cohort of 25 OSCC cases and 50 matched controls strengthened the association of P. gingivalis with OSCC, indicating an association of the bacteria with OSCC lymph node metastasis and decreased survival time . Peters et al.  found that ecological networks between bacteria may play an important role in oesophageal cancer risk since there are bacterial taxon associations unique to OAC and OSCC. The authors reported a promising new perspective on the correlation between oral microbiome and oesophageal cancer types, where specific ecological networks are associated with adenocarcinoma and OSCC, in line with the different origins of these cancers. Although the authors did not report significant associations between the oral microbiota diversity and disease risk, they strengthened the association of P. gingivalis with OSCC, showing an association of this bacterium with OSCC lymph node metastasis and decreased survival time. Despite the exciting evidence presented by Peters et al. , there is an overrepresentation of adenocarcinoma cases in relation to the number of OSCC which leaves this cancer type underexplored. Moreover, the study lacked the periodontal status of the patients to properly address the possible independent implication of periodontal pathogens in patient outcome.
Menya et al.  conducted one of the few studies on oral health and OSCC in western Kenya, where dental fluorosis is endemic due to early-life excessive fluoride intake. The authors concluded that 430 cases with poor oral hygiene and dental fluorosis had increased OSCC-associated risk in comparison with 440 controls. Moderate/severe fluorosis cases, which consisted of 44% of cases, had an increased risk as high as 9.4-fold. Moreover, the occurrence of oral leucoplakia and tooth loss/decay increased with fluorosis severity and, consequently, contributed to cancer risk. Geographic differences also showed the striking co-location of areas with a high OSCC incidence and those of high groundwater fluoride levels, highlighting an urgent need for focused research into primary prevention strategies. The encouraging results of the role of the oral microbiota in OSCC strengthens the need to further explore periodontal disease and/or periodontal pathogens in OSCC carcinogenesis. The identification of oral pathogenic bacteria and/or protective specimens could lead to preventive interventions for their eradication or colonisation in susceptible individuals. This has particular importance in OSCC hotspot regions, particularly in sub-Saharan Africa, where prevention strategies are crucial to decrease the high incidence of the disease.
The gastric microbiota has been associated with oesophageal cancer susceptibility . While in OAC the relationship with gastric microbiota may be more evident due to the biological proximity of this type of tumour to the stomach, the association with OSCC may not be so clear but nonetheless important. Observational studies disclosed an association of the gastric microbiota environment with OSCC (Table 3), implying that changes of gastric mucosa interfere with OSCC susceptibility [103, 104].
Gastric colonisation by Helicobacter pylori, a Gram-negative bacterium that grows in the mucus layer of the stomach, is the leading cause of gastric carcinoma. Contradictory, H. pylori is believed to reduce the risk of OAC, but the underlying mechanism is not understood . While some studies found a protective effect of the bacterium to the risk of the disease , others did not find a significant association  or even showed a higher risk of the disease . A recent meta-analysis study based on 345,886 patients addressed these inconsistent results, revealing that they are probably due to the high geographic heterogeneity, translated into multiple risk factors, such as smoking and alcohol consumption or dietary habits . It is worth noting that this systematic review with 35 studies only includes one African population from Uganda, with the vast majority being Caucasian individuals from westernised countries. The results presented by Gao et al.  strengthen the need for well-designed prospective cohort studies with representative samples of the great heterogeneity of OSCC.
Besides H. pylori, other organisms of the gastric microbiota have been associated with OSCC. By comparing the gastric corpus microbiota in OSCC and oesophageal dysplasia patients with healthy individuals, Nasrollahzadeh et al.  found significant differences between diseased subjects and controls, with patients showing a higher abundance of Clostridiales and Erysipelotrichales, both Firmicutesphyla.
Microbiome Potential Clinical Applications
Better understanding of the dynamics between OSCC and the microbiome would contribute to clarify cancer aetiology and potentially develop new approaches for prevention and treatment. The microbiome may act as a biomarker for diagnosis and/or clinical outcome and can greatly improve the development of targeted therapies and strategies by modulating the microbiota. Microbiota can be changed by antibiotics, probiotics, prebiotics, or microbiota transplants, representing an enormous potential for cancer prevention, particularly in high-risk populations . While antibiotics can remove or supress pathogenic organisms, probiotics may provide the introduction of lost beneficial microbial components and prebiotics can enhance the proliferation of microbes with beneficial functions. Moreover, treatment outcomes may be influenced by the microbiota due to microbial influence on the toxicity and efficacy of therapeutic intervention . The microbiome can also influence host response to immunotherapy. Host-microbial interactions may provide an important basis for a more inclusive view of pharmacology and nutrition .
Several studies strongly indicate a link between commensal bacteria and cancer therapeutic efficacy by modulating responses to cancer immunotherapy . The gut microbiota is the one most frequently implicated in cancer therapy response across a wide variety of treatments, such as chemotherapy, immune checkpoint blockade, and stem cell transplant, through several potential mechanisms that require deeper understanding. Iida et al.  conducted one of the first studies in antibiotics-treated and germ-free mice to show that tumour-infiltrating myeloid-derived cell response to immunotherapy and chemotherapy was mediated differentially by commensal bacteria, impairing the inflammatory effect and stressing the importance of manipulating the microbiome to improve treatment. Several other studies have subsequently demonstrated that differential gut microbiota environments were associated with patients who responded favourably to treatments with an enhanced systemic immunity response . Preclinical and clinical studies suggested that the gut microbiome affects antitumor immunity via numerous mechanisms such as the induction of cytokine production, enhancing the therapeutic response . The gut microbiota has also been shown to influence responses to a range of chemotherapies, with both beneficial and microbiome-dependent effects .
Increasing acknowledgment that the microbiome plays a critical role in carcinogenesis and therapeutic outcome offers a great opportunity for research, from basic to translational research in both clinical and epidemiological studies. Simple strategies such as probiotics supplementation may be easily implemented in low-income African countries to monitor and modulate disease development and outcome in order to realise the full potential of host-human dynamics.
Cancer Research, Prevention, and Health Care Resources in AOCC
Africa faces a rising burden of cancer, namely oesophageal cancer along the AOCC. Yet, cancer research is still scarce in the continent to meet this public health challenge and few people are equipped to conduct research in the area despite remarkable progress in building human research capacity over the last decade [7, 112, 113]. In Tanzania, a great part of oncology trainees would like to incorporate research into their careers, but inadequate training in research methodology and absence of longitudinal mentorship is a drawback .
Additionally, the lack of well-trained health professionals and differential infrastructures pose a great challenge for clinical and research oncology performed at low-income settings. In Mozambique, the knowledge in prevalent cancer domains is well characterised, as well as the prevalent cancers treated by surgery at Maputo Central Hospital (MCH), the main hospital in Mozambique, together with residents’ oncological knowledge. Results obtained, besides confirming oesophageal cancer as the most prevalent malignant tumour treated by general and thoracic surgery, with advanced-stage diagnoses requiring multimodal treatment, informed the need to improve residents’ oncology knowledge, supporting the necessity of a surgical oncology training tailored to suit local needs [7, 112]. The improvement of resources, such as tests and procedures to diagnose OSCC are also urgent. This problem is transversal to other African countries and personalised oncology education programs should be implemented in AOCC countries at the undergraduate level, with specific training for residents and continuing oncological education for general surgeons to improve the practice of surgical oncology .
The African Esophageal Cancer Consortium (AfrECC) coordinates aetiological and molecular studies of OSCC and could represent a great platform to implement multisite and multidisciplinary research programs . Teams of surgeons, gastroenterologists, and pathologists are crucial to develop promotion plans, identify staff needs, establish general clinical procedures, and select risk assessment models for each region . It is also important to design case studies to implement dietary diversification and the consumption of fortified/enriched food [3, 116], as well as call attention to soil degradation and the importance of fertilizer use and improved crop varieties to increase nutrient levels in harvests [26, 116]. The empowerment of people with oesophageal cancer information and decision-making skills will promote disease prevention strategies and ultimately improve the population’s quality of life. Awareness of cancer risk factors is urgently needed in Africa. For instance, a study in Tanzania showed that despite high awareness of the cancer risk of tobacco or alcohol, knowledge was quite low regarding infections or food contamination by fungi. Moreover, most participants were not aware of cancer warning symptoms and many myths still endure in the community .
OSCC is a burden in impoverished countries, with a special incidence in the AOCC. Oesophageal cancer symptoms are associated with impaired quality of life, especially if help is not sought in the earliest stages of the disease. This scenario worsens in Africa due to limited clinical organisations and weak public health education that delays not only treatment but also the implementation of prevention strategies and research. It is urgent to evaluate OSCC multifactoriality in Africa, addressing relevant environmental (e.g., alcohol and tobacco, diet, mycotoxins), biological (oral microbiome and metabolome), and genetic risk factors (Fig. 2). So far, these elements have been mostly individually analysed with an inappropriate contextualisation. Easy and inexpensive solutions may decrease the OSCC burden in Africa, and these solutions must be studied with the best up-to-date technologies and robust scientific approaches. An oesophageal cancer prevention approach will be crucial not only because of the OSCC burden, but also because prevention strategies are quite deficient throughout Africa in general. The urge to change population mentalities and capacitation is demanding and only makes sense in light of new initiatives and collaborations between academic, clinical, and scientific institutions. Nevertheless, prevention strategies are pointless if we do not understand the causes for such high incidence rates in the AOCC. Understanding of the molecular oesophageal cancer pattern and microbiome risk profile along with complementary classical epidemiological studies is essential to implement oesophageal cancer prevention strategies and programmes.
There are no acknowledgments to declare.
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Conflict of Interest Statement
The authors have no conflicts of interest to declare.
i3S is financed by FEDER – Fundo Europeu de Desenvolvimento Regional funds through the COMPETE 2020 – Competitiveness and Internationalization Operational Programme (POCI), Portugal 2020, and by Portuguese funds through FCT/Ministério da Ciência, Tecnologia e Inovação in the framework of the project “Institute for Research and Innovation in Health Sciences” (POCI-01-0145-FEDER-007274). J.B.P. has a Stimulus of Scientific Employment – individual position from Portuguese funds through FCT – Fundação para a Ciência e a Tecnologia/Ministério da Ciência, Tecnologia e Inovação (CEECIND/00134/2017).
L.L.S. designed the manuscript and coordinated all contributions. All authors drafted, read, and approved the final manuscript.
J.C. and J.B.P. contributed equally to this work.