This is the first systemic review to comprehensively summarize all genetic variants associated with MSUD in the MENA region. In this systematic review, we captured 105 genetic variants from 16 eligible studies 1, 3, 4, 11, 14, 22, 25, 27, 31–35, 43, 48, 49, comprising 291 patients presented with at least one variant associated with MSUD. We identified 105 variants in four genes (Table 2, Table S3). Among the four genes that have been reported, BCKDHA (38%) and BCKDHB (38%) were the most frequently affected genes (Fig. 2) among patients with MSUD in the MENA region, followed by DBT (23%), and PPM1K (1%). These findings are in concordance with other studies in Chinese and Indian populations in which the most commonly affected genes were reported to be either BCKDHA or BCKDHB28, 40, 41, 53, and that the frequency of variants in BCKDHA and BCKDHB is nearly equal 25, 40, 41. Nevertheless, other populations, such as Malaysian population, have reported different distribution patterns, in which almost half of the variants were identified in BCKDHB, and the other half was distributed closely between BCKDHA and DBT10, suggesting that the distribution of variants varies according to ethnicity. In this study, the variant BCKDHA: rs34442879; c.452C > T was the most frequently reported variant among all variants captured (Table 1), reported four times in two different countries in association with MSUD (Table S3). BCKDHA: rs34442879; c.452C > T is a missense variant that has been reported in other non-Arab ethnicities in association with MSUD and has been reported in ClinVar as benign/likely benign for MSUD (Table S3).
Variants unique to the MENA region
We identified 105 variants associated with MSUD among patients from MENA countries. Of these variants, 75 (71%) were unique to the MENA region, with the majority of them being reported in Iran and Turkey, followed by Saudi Arabia and Egypt. Most of the captured unique variants were located in BCKDHA and BCKDHB (Table 1), of which few variants were compound heterozygous (n = 15), while the majority were identified as homozygous variants (n = 90) (Table 1).
The high rate of consanguinity among MENA populations could explain the higher frequency of homozygous genotypes observed in our study 12. Consanguinity is a cultural practice where individuals marry their close relatives, which can increase the frequency of homozygous alleles in their offspring. This has been shown to be a common practice in the MENA region, leading to an increased frequency of genetic disorders.
Among the homozygous variants, four unique missense variants (c.896A > C, c.731G > A, c.773G > A, c.373C > G) were reported in severe/classic MSUD cases. The missense variant c.896A > C was identified as a novel mutation in the original study among the Saudi population and was predicted to be disease-causing and probably damaging. This variant resulted in an amino acid substitution from Aspartic acid to Alanine at position 299 (p.Asp299Ala) 32 (Table 1). In the same Saudi study, nineteen other variants were reported as novel variants associated with MSUD.
The remaining variants that were found to be unique to the MENA region were located in BCKDHB and DBT (Table 1). A total of 26 variants located in BCKDHB were found to be unique to populations in the MENA region (Table 1). Among which, one variant (c.688G > T), a homozygous nonsense variant that results in a premature stop codon, was classified as pathogenic in association with MSUD, according to ACMG. A total of 21 variants located in DBT were found to be unique to populations in the MENA region (Table 1). The variant c.85_86ins AACGA is an insertion (c.85_86ins AACG) that leads to a frame-shift mutation and premature stop codon (position of a stop codon in WT/Mut CDS 1449/147) and the resulting mRNA will be degraded by nonsense-mediated-decay (NMD). (Table 1). Furthermore, one start-loss mutation was located in the PPM1K gene in Turkey. It led to a mild-intermediate MSUD diagnosis 43. There is only 1 other reported variant located in PPM1K and it was first recorded in Spain in 2013 42.
The identification of unique variants in MENA populations highlights the importance of studying genetic diversity across different populations and emphasizes the need for population-specific genetic screening programs to improve diagnosis and management of MSUD. The development of population-specific genetic screening programs could lead to early diagnosis and treatment of MSUD, ultimately reducing the morbidity and mortality associated with this disorder. In addition, the identification of unique variants in MENA populations could have broader implications for precision medicine, highlighting the importance of considering genetic diversity in the development of targeted therapies for genetic disorders.
Variants shared with other ethnic groups
In the current systematic review, 30 variants located in BCKDHA, BCKDHB, DBT, and PPM1K were found were found to exhibit genotype-phenotype correlations that are shared between populations in the MENA region and other ethnicities (Table S3).
The majority of variants in this particular category were found in the BCKDHB gene (n = 14), which is the most prevalent gene associated with MSUD-related variants in populations from the MENA region (Table S3). These variants have been documented in Egypt, Iran, KSA, and Turkey (Table S3). Among them is the missense variant rs190867671; c.1149T > A, which has been identified in MSUD patients from Turkey (Table S3). This variant results in the nucleotide change 1149T > A, leading to the introduction of a stop codon at Y383. It has been linked to severe classic type MSUD in Turkey 48 and has also been previously reported in association with MSUD in China 54. BCKDHB mutations have been implicated in MSUD and have been extensively studied worldwide, with documented cases in various ethnic groups such as Spanish, Korean, Japanese, and German populations 25, 30, 37, 45 (Table S3).
BCKDHA was the second most prevalent gene in this category, with 12 identified variants linked to MSUD in multiple countries within the MENA region, including Egypt, Iran, Turkey, Jordan, and KSA, as well as in non-Arab countries such as India, Spain, and Mexico (Table 2, Table S3). Among the BCKDHA variants associated with MSUD, several were reported in the MENA region, while others were linked to cases in India, Spain, and Mexico (Table S3). Out of the 12 shared BCKDHA variants, 11 were homozygous.
In relation to the DBT gene, a total of 3 variants showed genotype-phenotype correlations that were shared among populations in the MENA region and other ethnicities (Table S3). Previously reported DBT variants were identified, such as c.827 T > G (p.Phe 276 cys), along with a novel mutation found in Indian populations, c.190 G > A (P.Val641 le). Additionally, the following variants were also detected: c.61delC, c.74delAT, c.137A > G, c.939-2A > G, c.1195T > C, and c.1281 + 3A > G.
Furthermore, some shared variants were observed in patients from different countries within the MENA region, including c.890G > A, c.452C > T, and c.859C > T in BCKDHA, and c.633 + 1G > A, c.508G > T, c.853C > T, c.995C > T, and c.410C > T in BCKDHB, as well as c.1291C > T in DBT. The distribution of these variants among the MENA population is depicted in Fig. 3.
Distribution of Genetic variants in the MENA region and their respective phenotypes
The identification of MSUD-associated variants in Arab populations play an important role in disease identification, disease management, and genetic counselling. The mutational spectrum of MSUD has been assessed in many countries, and the distribution of BCKDHA, BCKDHB, DBT, and PPM1K for patients with MSUD was found to vary widely between different populations 10. Over 500 disease-causing variants have been described in Human Gene Mutation Database (HGMD) (http://www.hgmd.cf.ac.uk/ac/index.php) in BCKDHA, BCKDHB DBT, PPM1K genes. The number of affected people with MSUD within the MENA region was reported to be higher than the reported prevalence around the world 1. For example, in Saudi Arabia, the prevalence rate was estimated to be 1 per 21490 birth 32. This is mainly attributed to consanguinity; the average rates of consanguinity ranges between 40–50% in the Arab world, and these numbers may be up to the level of 60% in some societies, as in the United Arab Emirates and Saudi Arabia 13, hence the rate of autosomal disease is higher in proportion than western countries.
To date, the genotype-phenotype correlations for patients with MSUD and severity of the disease remain poorly understood. Many articles have linked gene variants to the severity of MSUD. Still, no clear connection has been made to whether all variants within a specific gene location can cause a similar phenotype among other populations across the world, for example, in an Indian study designed to explore the genotype – phenotype correlation among MSUD patients, it was noticed the majority of MSUD cases had a variant in the BCKDHB gene. They were all Classic MSUD cases 28, furthermore a few studies have reported classic and intermediate MSUD-associated variants in BCKDHA 17, 52. In addition, variants in the DBT gene have been linked to three types of MSUD, including classic, intermediate and thiamine response MSUD phenotypes 18, 24, 51. However, even with the same gene variant, the reported phenotype might vary from one population to another.
In the present study, one patient in Iraq was identified to have a variant in BCKDHA with classic MSUD phenotype. In Iran, 33 MSUD patients were identified, with the majority of variants in the BCKDHB gene (n = 14) followed by BCKDHA (n = 11) with classic MSUD, DBT (n = 6) and the remaining patients with unreported variants. In KSA, 52 patients were identified, with the majority of cases having variants in BCKDHB (n = 9), followed by BCKDHA (n = 8) and DBT (n = 6), all of whom had classic MSUD.
In Lebanon out of five patients identified with MSUD, two patients had variants in the DBT gene and two patients with intermediate MSUD had variants in BCKDHA and BCKDHB. In turkey, 97 patients were identified, with the majority of MSUD patients with reported variants being in BCKDHA (n = 13), followed by BCKDHB (n = 10), DBT (n = 5), and PPM1K (n = 1), all of whom had classic MSUD. In Tunisia out of three patients with classic type of MSUD, two were in the BCKDHB gene and one was in DBT gene. In Egypt, out of 33 patients, the majority had variants in BCKDHB (n = 8), followed by BCKDHA (n = 5), and then the DBT gene (n = 4). All patients had the classic type of MSUD.
Studying the most frequent variants in one population may help elucidate the molecular etiology of MSUD, and thus commence treatment of the disease as early as possible. Moreover, studying the spectrum of genetic mutation may help identify any founder effect that exist in a specific population and the phenotype-genotype similarities within populations and ethnic groups. In addition, the identification of pathogenic mutation causing MSUD will be highly beneficial for molecular diagnosis from patients within the region in general, with respect to prevention of this disease in the form of carrier testing, prenatal testing, pre-martial screening, and pre-implantation genetic diagnosis.
The strengths of this study lie in the fact that it is the first systematic study to comprehensively summarize all reported MSUD-associated genetic variants in the MENA world. We conducted a comprehensive search using stringent predetermined inclusion and exclusion criteria and thorough analyses of each article included. Despite precluding a meta-analysis, key findings were reported narratively. Nevertheless, our study has some limitations. First, we encountered several discrepancies in MSUD classification, where some articles used uncommon terms to describe the phenotype of their patients, with no clear or unified guidelines of how those MSUD phenotypes have been classified. In addition, due to the limited number of patients with known variants, no clear picture could be drawn with regards to the genotype and phenotype correlations.
Another significant hurdle was the heterogeneity among the available studies. We observed considerable diversity in study designs, operational quality, sample sizes, and measured variables. This wide variation in methodologies made it challenging to synthesize evidence and draw conclusive results. Additionally, the degree to which these studies controlled for confounding factors varied significantly, further complicating the interpretation of results.
Despite an extensive literature search on MSUD-associated genetic variants in the Arab world, we discovered relatively few genetic association studies focusing on MSUD risk in the MENA region. The scarcity of research in this specific population limits our understanding of the genetic factors contributing to MSUD in this region. More comprehensive and region-specific studies are required to address this gap in knowledge.
To ensure clarity and consistency in future research, it is essential for the scientific community to establish standardized classification criteria and methodologies for studying MSUD and its genetic associations. Moreover, efforts should be made to increase collaboration and data sharing among researchers to facilitate a more comprehensive analysis of MSUD across diverse populations. By addressing these challenges, we can advance our understanding of this rare metabolic disorder and its implications for affected individuals in different regions of the world.