Methylenetetrahydrofolate Reductase Polymorphism (rs1801133) and the Risk of Hypertension among African Populations: A Narrative Synthesis of Literature

In this review, we have gathered and analyzed the available genetic evidence on the association between the methylenetetrahydrofolate reductase gene (MTHFR), rs1801133 and the risk of Hypertension (HTN) in African populations, which was further compared to the global data evidence. This review was reported following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol and Human Genome Epidemiology Network (HuGENet) guidelines. Literature was retrieved through major search databases, including PubMed, Scopus, Web of Science, and African Journal Online. We identified 64 potential studies, of which 4 studies were from the African continent and 60 studies were reported globally. Among the studies conducted in Africa, only two (n = 2) reported a significant association between the MTHFR (rs1801133) and the risk of developing HTN. Only one (n = 1) study population was purely composed of black Africans, while others were of other ethnicities. Among studies conducted in other continents (n = 60), forty-seven (n = 47) studies reported a positive association between MTHFR (rs1801133) and the risk of developing HTN, whereas the remaining studies (n = 14) did not show a significant association. Available literature suggests an apparent association between rs1801133 and HTN in global regions; however, such information is still scarce in Africa, especially in the black African population.


Introduction
Hypertension (HTN) remains a major risk factor for the development of cardiovascular diseases (CVDs), which significantly contributes to high rates of mortality and morbidity worldwide. Globally, HTN affects over 1.4 billion individuals above the age of 18 years and the number is expected to increase to 1.56 billion by 2025 [1][2][3]. In Africa, HTN

Results
There are very limited studies reporting on MTHFR (rs1801133) association and HTN in African populations. Out of all identified relevant studies, only one study population was indigenous African (Cameroon) [34]; others were composed of Caucasian participants (Algeria, Morocco, Egypt) [15,16,35]. For this reason, a narrative synthesis of the findings was performed, instead of a meta-analysis.

Characteristics of Studies
Using our search strategy (Figure 1), we have identified 1230 related studies, of which four (n = 4) were from the African continent and (n = 60) were from non-African continents (globally). Based on our inclusion and exclusion criteria, there were 321 cases and 308 controls for the African population and 15,865 cases and 28,762 controls for other global populations globally, that were available for this analysis. The study characteristics are described in Table 1. In all the studies reported in Africa [15,16,34,35], HTN was defined as systolic/diastolic BP (SBP/DBP) ≥ 140/≥ 90 mm Hg. Among the included studies reported in the African region (n = 4), only two (n = 2) studies reported a significant association between the MTHFR (rs1801133) and the risk of developing HTN [16,34]. All African studies, where age was reported, included only patients aged above 40 years, except for Amin et al. [35], which included patients aged ≤ 45 years. Furthermore, most African studies included more females than males with exception of Amin et al. [35] which did not report on gender. In studies reported in other continents (n = 60), forty-seven separate studies showed a significant association between MTHFR (rs1801133) and the risk to develop HTN Table 2, whereas the remaining studies did not show any significant association (n = 14). described in Table 1. In all the studies reported in Africa [15,16,34,35], HTN was defined as systolic/diastolic BP (SBP/DBP) ≥ 140/≥ 90 mm Hg. Among the included studies reported in the African region (n = 4), only two (n = 2) studies reported a significant association between the MTHFR (rs1801133) and the risk of developing HTN [16,34]. All African studies, where age was reported, included only patients aged above 40 years, except for Amin et al. [35], which included patients aged ≤ 45 years. Furthermore, most African studies included more females than males with exception of Amin et al. [35] which did not report on gender. In studies reported in other continents (n = 60), forty-seven separate studies showed a significant association between MTHFR (rs1801133) and the risk to develop HTN Table 2, whereas the remaining studies did not show any significant association (n = 14).

Association of MTHFR (rs1801133) and HTN Reported in African Continent
In this section, we briefly summarize the evidence on MTHFR (rs1801133) associations based on the four available studies reporting on African populations ( Table 2).
The first study was conducted by Amin et al. [35], and it was aimed at evaluating the presence of MTHFR (rs1801133) polymorphism and its association with HTN and myocardial infarction among participants of Egyptian origin (n = 181, <45 and ≥45 years). The study showed that there was no association between MTHFR (rs1801133) and HTN. The study further demonstrated that individuals with HTN were smokers and presented with impaired lipid profiles such as significantly raised levels of total cholesterol (TC), triglycerides, lowdensity lipoprotein-cholesterol (LDL-c), and low high-density lipoprotein cholesterol (HDL-c), in comparison to the control group. The gender of the participants was not reported in this study. The authors clearly stated the guidelines (SBP/DBP ≥ 140/90 mm Hg) that were used to define HTN. However, the method used to adjust for patients who were already on treatment was not mentioned.
The second study by Nassereddine et al. [16] was carried out to evaluate the association between MTHFR (rs1801133) variant and HTN in a Moroccan population (n = 203, range 40-87 years). The authors demonstrated a significant association between rs1801133 and HTN. It was further demonstrated that the distribution of demographic and clinical characteristics of patients did not show a significant trend in relation to HTN. Thus, the study did not adjust for confounding factors. The study reported more females (n = 77) than Genes 2022, 13, 631 5 of 14 males (n = 24). Lastly, the study defined HTN as SBP/DBP ≥ 140/90 mm Hg. However, the authors did not provide any information about the treatment status of the cohort.  The third study by Amrani-Midoun et al. [15] reported a lack of association between MTHFR (rs1801133) and HTN in an Algerian population (n = 154, ≥42 years); however, the authors did acknowledge the impact of the small sample size used. Despite the small sample used, this study showed that there were significant differences between participants with HTN and controls with respect to age, SBP, DBP, and family history of HTN. The study was composed of more females (n = 84) than males (n = 70), and defined HTN as SBP/DBP ≥ 140/90 mm Hg. However, the method that was used for adjusting for the use Genes 2022, 13, 631 7 of 14 of antihypertensive medication was not mentioned. Also, the genotyping method used in this study (PCR-RFLP) could be a potential limitation.
The fourth study was conducted by Ghogomu et al. [34], and it reported an association between MTHFR (rs1801133) and HTN in the native Bantu ethnic group of the South-West region of Cameroon (n = 91, range 40-70 years). Of note, this was the only study that sampled participants from an indigenous African population. Lipid profile dispersion for all subjects reported that serum lipid levels were higher in hypertensive patients than in healthy controls. The study further demonstrated that the MTHFR (rs1801133) variant may influence individual susceptibility to HTN through a mechanism that involves an increase in the level of serum LDL-c. However, the sample size was very small and was likely accompanied by biasedness. Furthermore, the study did not report on the number of females/males that were sampled. HTN was defined as having elevated SBP ≥ 140 mm Hg and DBP of at least ≥90 mm Hg. Patients who were already placed on hypertensive medication were also categorized as hypertensive.

Discussion
The MTHFR gene has been among the most studied genes associated with the development and progression of HTN [26,36]. Indeed, numerous genetic studies have investigated the association between the genetic variant of MTHFR (rs1801133) and the risk of developing HTN [36][37][38]. However, these studies reported conflicting results. In our previous systematic review, the MTHFR gene (rs1801133) was reported as one of the most studied genes associated with HTN among African populations [95]. Thus, in the present review, we gathered and analyzed the available genetic evidence on the association between MTHFR (rs1801133) and HTN among Africans and further compared the evidence with global data.
We reviewed 60 published articles that examined the association between MTHFR (rs1801133) and HTN. Out of 60 published articles, 47 reported a positive association between HTN and the MTHFR variant. However, only 4 studies were conducted in the African continent, of which 2 reported a positive association between rs1801133 and HTN [16,34]. The inconsistencies observed between these studies may be due to: (a) the limited number of relevant African studies and their relatively small sample sizes, which makes comparisons with other studies challenging. Given the small sample size in these studies, many true associations with small effects will not be significant and many suggestive associations may be false. In addition, the use of various cohorts, to maximize sample size and increase statistical power, could interfere with the biased results as some associations may be due to heterogeneity [96]; (b) the low frequency of the MTHFR (rs1801133) T allele observed among the African populations [97], which may be influenced by folate deficiency due to malnutrition and impaired intestinal absorption of folic acid, which are common in Africa [98]. Lastly, a study by Amrani-Midoun et al. [15] also suggested that these differences may be due to the epigenetic mechanisms which are involved in the gene expression predisposed by environmental factors such as lifestyle and diet. All the aforementioned factors may lead to failure to replicate the association of MTHFR (rs1801133) with disease phenotypes.
Although all included African studies [15,16,34,35] defined HTN as SBP/DBP ≥ 140/90 mm Hg, there were great differences in these studies, partly because of the criteria used in selecting participants and methods applied in each study. A study by Ghogomu et al. [34] and Nassereddine et al. [16] reported an association between MTHFR (rs1801133) polymorphism and HTN. However, these studies did not adjust for confounding factors such as gender, age, and smoking status. This may introduce bias, thus making it difficult to compare the findings with other studies. Furthermore, the age inconsistencies among the four African studies [15,16,34,35] may impose challenges when comparisons are made with other studies. For instance, a study by Nassereddine et al. [16], included 101 outpatients with a mean age of 61.6 ± 9 (range 40-87 years) and 102 age and sex-matched unrelated healthy control subjects with a mean age of 59.24 ± 10.7 (range 40-87 years); whereas a study that was conducted by Amin et al. [35] sampled young adults aged <45 years and older adults aged ≥45 years. The use of antihypertensive medication was reported by African studies [15,16,34,35]; however, the methods used for adjustments were not mentioned. This may introduce bias when making comparisons across studies, as studies that make adjustments would not be comparable to studies that did not make adjustments. Ghogomu et al. [34] was the only study that was composed of participants from an indigenous African population [15,16,35], thus limiting comparisons across different racial groups, since the genetics of HTN vary across different populations and geographical regions [17,99].
Nonetheless, a recent systematic review and meta-analysis comprising of 57 studies with 14,378 patients and 25,795 control subjects examined the association between MTHFR (rs1801133) and HTN and revealed that the major reason for equivocal results might be the racial differences observed across the different studies [36]. In comparison with that study, the present review had the following advantages: First, there were 64 eligible studies with 16,186 hypertensive cases and 29,070 controls, which could provide more reliable conclusions. Second, since none of the previous systematic reviews and meta-analyses [26,27,[100][101][102] focused on the indigenous African populations, we assessed the comparison among studies reported on other cohorts with the ones reported on African populations. Therefore, future studies should pay more attention to the differences in the genetic background of indigenous African populations. For this reason, our review updates information from the previous systematic reviews [26,27,[100][101][102] with additional supplements and adjustments, which makes it a comprehensive study regarding the association between MTHFR (rs1801133) and HTN.

Strengths and Limitations of This Study
It must be pointed out that this is the first review that specifically assessed the effect of MTHFR (rs1801133) on HTN in Africa, as well as performing comparisons between African studies and available global data, which opens the door for future research. However, it should be noted that there were certain limitations to the present analysis, which inevitably prevented more in-depth analyses. First, the sample size of some of the selected studies was relatively small. Second, variations between population characteristics, phenotypic measures, and genotypic analyses could cause bias when comparing the current findings with previous reports. Third, literature was surveyed globally; unfortunately, in Africa, we were able to identify only four studies, which suggests that there is a lack of information regarding the black African ethnic groups in relation to genetic association studies. Furthermore, out of those identified studies (n = 4), only one study by Ghogomu et al. [34] was composed of a purely black African population and the remaining three studies were composed of other ethnicities [15,16,35]. Thus, our meta-analysis only included a few numbers of participants who were of African origin. As such, the analysis was unlikely to produce valid results (Figure 2), thus we conducted a narrative synthesis of the results. This indicates that there is an urgent need to carefully plan African-specific studies with large sample sizes in order to be able to draw conclusions on the association between MTHFR (rs1801133) and HTN. et al. [34] was composed of a purely black African population and the remaining three studies were composed of other ethnicities [15,16,35]. Thus, our meta-analysis only included a few numbers of participants who were of African origin. As such, the analysis was unlikely to produce valid results (Figure 2), thus we conducted a narrative synthesis of the results. This indicates that there is an urgent need to carefully plan African-specific studies with large sample sizes in order to be able to draw conclusions on the association between MTHFR (rs1801133) and HTN. Figure 2. Forest plot of the evaluation for the association between the MTHFR (rs1801133) and HTN in the dominant genetic model (Africa). We evaluated the risk of the TT or CT genotype on HTN compared with the CC genotypes. Then, pooled Odds ratios (OR) with 95% confidence intervals (CI) and z score were performed to estimate associations. All analyses were performed using R software (Version 3.3.3, using R package meta) [15,16,34,35,103].

Conclusions
Although the association between rs1801133 and HTN was predominantly reported in other global regions, the result from the current review opened avenues to further explore a possible association between rs1801133 and HTN among individuals of African origin. Furthermore, this study has demonstrated the need to generate African-specific genomic data. Such data could provide insights into human evolution and the role of genetic variants in disease phenotypes. These data could also increase our understanding of African population genomics and highlight its potential impact on biomedical research and genetic susceptibility to disease. Thus, future studies should sample a fair number of participants that completely represent the African population. Since African populations are well known to have high genetic diversity, because of their deep evolutionary history, and genetic differences, it is of utmost importance for future association studies to pay Figure 2. Forest plot of the evaluation for the association between the MTHFR (rs1801133) and HTN in the dominant genetic model (Africa). We evaluated the risk of the TT or CT genotype on HTN compared with the CC genotypes. Then, pooled Odds ratios (OR) with 95% confidence intervals (CI) and z score were performed to estimate associations. All analyses were performed using R software (Version 3.3.3, using R package meta) [15,16,34,35,103].

Conclusions
Although the association between rs1801133 and HTN was predominantly reported in other global regions, the result from the current review opened avenues to further explore a possible association between rs1801133 and HTN among individuals of African origin. Furthermore, this study has demonstrated the need to generate African-specific genomic data. Such data could provide insights into human evolution and the role of genetic variants in disease phenotypes. These data could also increase our understanding of African population genomics and highlight its potential impact on biomedical research and genetic susceptibility to disease. Thus, future studies should sample a fair number of participants that completely represent the African population. Since African populations are well known to have high genetic diversity, because of their deep evolutionary history, and genetic differences, it is of utmost importance for future association studies to pay more attention to African genetic studies and to understand the functional and biological relevance of associated rs1801133. Moreover, improved methods need to be developed to understand and compare heritability across populations and study participants from different parts of the African continent. It is also imperative for all studies to report more detail in the protocols used to enable better replication and minimize bias between studies.