Molecular genetics of glucose-6-phosphate dehydrogenase deficiency in Mexico

An Overall View of the Functional and Structural Characterization of Glucose-6-Phosphate Dehydrogenase Variants in the Mexican Population

Glucose-6-phosphate dehydrogenase (G6PD) deficiency, affecting an estimated 500 million people worldwide, is a genetic disorder that causes human enzymopathies. Biochemical and genetic studies have identified several variants that produce different ranges of phenotypes; thus, depending on its severity, this enzymopathy is classified from the mildest (Class IV) to the most severe (Class I). Therefore, understanding the correlation between the mutation sites of G6PD and the resulting phenotype greatly enhances the current knowledge of enzymopathies' phenotypic and genotypic heterogeneity, which will assist both clinical diagnoses and personalized treatments for patients with G6PD deficiency. In this review, we analyzed and compared the structural and functional data from 21 characterized G6PD variants found in the Mexican population that we previously characterized. In order to contribute to the knowledge regarding the function and structure of the variants associated with G6PD deficiency, this review aimed to determine the molecular basis of G6PD and identify how these mutations could impact the structure, stability, and function of the enzyme and its relation with the clinical manifestations of this disease.

Hematological and molecular analysis of patients with G6PD deficiency revealed coexistent hereditary spherocytosis and alpha thalassemia

BACKGROUND

Glucose-6-phosphate dehydrogenase (G6PD) deficiency, hereditary spherocytosis (HS), and alpha thalassemia (α-thal) are frequent erythrocyte pathologies with different geographic distributions worldwide. Our aim is to report hematological and molecular findings of G6PD deficient Mexican patients in coinheritance with suggestive hereditary spherocytosis (sHS) and α-thal.

METHODS

We studied 78 G6PD deficiency patients. Hematological parameters, acidified glycerol lysis test, erythrocyte morphology, electrophoresis, and hemoglobin quantification were obtained. G6PD and HBA2/HBA1 variants were identified using ARMS-PCR, Gap-PCR, or Sanger sequencing.

RESULTS

Nine G6PD variants were identified; A^-202A/376G , A^-376G/968C , and A^+376G as the most frequent. G6PD Santiago de Cuba^1339A and Kamiube^1387T were detected in Mexicans for first time. Hematological analysis revealed additional erythrocyte pathologies in 52 patients, 32 with positive osmotic fragility test and spherocytes in blood smear (suggestive hereditary spherocytosis, sHS), 12 with microcytosis and 8 with all three defects who had the most severe phenotype, with significantly lower hematological parameters (Hb, PCV, MCV, and MCH). α-thal variants (α^Hph α, α^-59C>T α and -α^3.7 ) were observed in 65% of patients with microcytosis.

CONCLUSION

Additional erythrocyte defects were observed in 69.3% of G6PD deficiency patients. We stress the importance of searching for the presence of additional erythrocyte hereditary diseases in patients with G6PD deficiency.

Functional and structural analysis of double and triple mutants reveals the contribution of protein instability to clinical manifestations of G6PD variants

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common polymorphism and enzymopathy in humans, affecting approximately 400 million people worldwide. Over 200 point mutations have been identified in g6pd and the molecular mechanisms underlying the severity of G6PD variants differ. We report the detailed functional and structural characterization of 11 recombinant human G6PD variants: G6PD Asahi, G6PD A, G6PD Guadalajara, G6PD Acrokorinthos, G6PD Ananindeua, G6PD A-(202), G6PD Sierra Leone, G6PD A-(680), G6PD A-(968), G6PD Mount Sinai and G6PD No name. G6PD Guadalajara, G6PD Mount Sinai and G6PD No name are inactive variants and, correlating with the observed clinical manifestations, exhibit complete loss of enzyme activity. Protein structural instability, causing a reduction in catalytic efficiency, contributes to the clinical phenotypes of all variants. In double and triple mutants sharing the G6PD A mutation, we observed cooperative interaction between two and three mutations to cause protein dysfunction. The G6PD A (Asn126Asp) mutation exhibits no effect on protein activity and stability, indicating that the additional mutations in these G6PD variants significantly contribute to enzyme deficiency. We provide insight into the molecular basis of G6PD deficiency, which can explain the severity of clinical manifestations observed in individuals with G6PD deficiency.

Glucose-6-phosphate dehydrogenase deficiency in northern Mexico and description of a novel mutation

Glucose-6-phosphate dehydrogenase deficiency (G6PD) is the most common enzyme pathology in humans; it is X-linked inherited and causes neonatal hyperbilirubinaemia, chronic nonspherocytic haemolytic anaemia and drug-induced acute haemolytic anaemia. G6PD deficiency has scarcely been studied in the northern region of Mexico, which is important because of the genetic heterogeneity described in Mexican population. Therefore, samples from the northern Mexico were biochemically screened for G6PD deficiency, and PCR-RFLPs, and DNA sequencing used to identify mutations in positive samples. The frequency of G6PD deficiency in the population was 0.95% (n = 1993); the mutations in 86% of these samples were G6PD A(-202A/376G), G6PDA(-376G/968C) and G6PD Santamaria(376G/542T). Contrary to previous reports, we demonstrated that G6PD deficiency distribution is relatively homogenous throughout the country (P = 0.48336), and the unique exception with high frequency of G6PD deficiency does not involve a coastal population (Chihuahua: 2.4%). Analysis of eight polymorphic sites showed only 10 haplotypes. In one individual we identified a new G6PD mutation named Mexico DF(193A>G) (rs199474830), which probably results in a damaging functional effect, according to PolyPhen analysis. Proteomic impact of the mutation is also described.

G6PD deficiency in Latin America: systematic review on prevalence and variants

Plasmodium vivax radical cure requires the use of primaquine (PQ), a drug that induces haemolysis in glucose-6-phosphate dehydrogenase deficient (G6PDd) individuals, which further hampers malaria control efforts. The aim of this work was to study the G6PDd prevalence and variants in Latin America (LA) and the Caribbean region. A systematic search of the published literature was undertaken in August 2013. Bibliographies of manuscripts were also searched and additional references were identified. Low prevalence rates of G6PDd were documented in Argentina, Bolivia, Mexico, Peru and Uruguay, but studies from Curaçao, Ecuador, Jamaica, Saint Lucia, Suriname and Trinidad, as well as some surveys carried out in areas of Brazil, Colombia and Cuba, have shown a high prevalence (> 10%) of G6PDd. The G6PD A-202A mutation was the variant most broadly distributed across LA and was identified in 81.1% of the deficient individuals surveyed. G6PDd is a frequent phenomenon in LA, although certain Amerindian populations may not be affected, suggesting that PQ could be safely used in these specific populations. Population-wide use of PQ as part of malaria elimination strategies in LA cannot be supported unless a rapid, accurate and field-deployable G6PDd diagnostic test is made available.

Molecular characterization of glucose-6-phosphate dehydrogenase deficiency in the Kurdish population of Western Iran

A total of 1,000 school boys ages 14-18 years, were randomly selected from six schools of Kermanshah and screened for G6PD deficiency. Fifty-three out of 1,000 were found to be severely G6PD deficient giving a frequency of 5.3% for G6PD deficiency in males in Kermanshah. We performed DNA analysis on 68 G6PD deficient children, 52 school boys and 16 children with the history of favism and hemolytic anemia using a combination of PCR-RFLP technique, single strand conformation polymorphism (SSCP) analysis and DNA sequencing. Three polymorphic G6PD mutations were identified: G6PD Mediterranean, Chatham and Cosenza. The commonest allele was found to be the G6PD Mediterranean (91.2%) followed by the G6PD Chatham (7.3%) and the G6PD Cosenza (1.5%). Haplotype analysis revealed that 56 out of 62 (90%) G6PD Mediterranean was linked to haplotype BclI(+) (1311T). In contrast, all G6PD Chatham and Cosenza had haplotype BclI(-) (1311C). The polymorphism IVS11-93 (T-->C) was present in 88.5% of the subjects studied. Only 4/55 (7.3%) of the Mediterranean alleles were associated with the T form and were always related to the nt 1311C. Our findings indicate that the allele frequency of G6PD Mediterranean mutation in Kermanshah is higher than those from two Fars ethnic groups living in Northern and Southern Iran. Nevertheless, they are in strict accordance with a previous report of the prevalence of the G6PD Mediterranean in Kurdish and Middle East population. Also, of strong association of the G6PD Mediterranean mutation and the presence of the polymorphism nt 1311C-->T in the Kermanshah population demonstrate that the presence of this mutation may be the result of migrations that have taken place through the history of Iran.

Glucose-6-phosphate dehydrogenase (G-6-PD) mutations in Mexico: four new G-6-PD variants

Screening for mutations at the G-6-PD gene by PCR-SSCP combined with restriction enzyme analysis and DNA sequencing was performed in nine G-6-PD deficient individuals with negative results for the presence of the most frequent G-6-PD mutations previously observed in Mexican population. The variants G-6-PD Valladolid(406T), G-6-PD Durham(713G), and G-6-PD Viangchan(871A) and four new G-6-PD mutant alleles were identified. The new mutations are located at cDNA nt 376 A --> T (126 Asn --> Tyr), nt 770 G --> T (257 Arg --> Leu), nt 1094 G --> A (365 Arg --> His), and nt 1285 A --> G (429 Lys --> Glu) and they were named G-6-PD San Luis Potosi, G-6-PD Zacatecas, G-6-PD Veracruz, and G-6-PD Yucatán, respectively. To date, a total of 18 different G-6-PD variants have been observed in Mexico and several of them are common in Africa, South Europe, and Southeast Asia.

Molecular heterogeneity of G6PD deficiency in an Amazonian population and description of four new variants

To characterize the molecular variation in the glucose-6-phosphate dehydrogenase gene (G6PD), 196 asymptomatic and unrelated male G6PD-deficient blood donors from Belém, an Amazonian metropolis (Brazil), were analyzed. This deficiency was detected by horizontal agarose gel electrophoresis and quantitative spectrophotometric assay for enzyme activity. The mutations were searched by PCR/RFLP, SSCP, and direct DNA sequencing. The most frequent G6PD variant was the widespread and common G6PD A- (202G --> A, 376A --> G) observed in 161 subjects (82.1%). Besides this, we found another form of G6PD A- (968T --> C, 376A --> G) in 14 (7.1%) individuals, G6PD Seattle (844G --> C) in 4.6%, G6PD Santamaria (542A --> T, 376A --> G) in 2.5%, and G6PD Tokyo (1246G --> A) in one blood donor. Four novel variants were also identified: G6PD Belém (409C --> T; Pro137His), G6PD Ananindeua (376A --> G, 871G --> A; Asn126Asp, Val291Met), G6PD Crispim with four point mutations (375G --> T, 379G --> T, 383T --> C, and 384C --> T) leading to three amino acid substitutions (Met125Ile, Ala127Ser, and Leu128Pro), and G6PD Amazonia (185C --> A; Pro62His). The reported frequencies do not reflect the real values for blood donors from Belém, since an excess of individuals with "non A-" phenotype was included in this study to enhance the probability to find rare variants. Haplotype analyses were carried out for the less common G6PD variants identified in our study using PCR/RFLP for five polymorphic sites (FokI, PvuII, PstI, BclI, NlaIII). G6PD Crispim and G6PD Amazonia variants presented the most common haplotype found in G6PD B (- - + - -). G6PD Belém presented two haplotypes (- - + + +, - + + + +) and G6PD Ananindeua was found with the + - + - + haplotype. The reported heterogeneity probably is due to the great miscegenation, characteristic of the population of the Amazonian region, besides the apparently common occurrence of recurrent mutations in the G6PD gene.

Molecular heterogeneity of glucose-6-phosphate dehydrogenase deficiency in Mexico: overall results of a 7-year project

Several years ago, a project aiming to determine (i) the molecular basis of G-6-PD deficiency, (ii) the distribution of four different mutant alleles previously detected, and (iii) the whole of polymorphic alleles that account for the overall prevalence of G-6-PD deficiency in Mexico was implemented. Nearly 5000 individuals-from the general population and patients with hemolytic anemia-belonging to at least 14 States were screened for G-6-PD deficiency. Seventy-six G-6-PD-deficient subjects were detected and the prevalence of G-6-PD deficiency in 4777 individuals from the general population was 0.71%. Screening for both mutations associated with enzyme deficiency and silent polymorphisms at the G-6-PD gene was performed in the enzyme-deficient individuals by PCR-SSCP combined with restriction enzyme analysis; the silent polymorphisms as well as the nondeficient variant G-6-PD A(376G) were also investigated in 366 G-6-PD normal individuals from the general population. In 88% of the enzyme-deficient individuals it was possible to define the mutation responsible and the type G-6-PD A- variants were the more common in both individuals from the general population and patients with hemolytic anemia. G-6-PD deficiency is heterogeneous at the DNA level in Mexico and up to date 10 different variants-8 in the present project and 2 previously-have been observed: G-6-PD A(-202A/376G), G-6-PD A(-376G/968C), G-6-PD Santamaria(376G/542T), G-6-PD Vanua Lava(383C), G-6-PD Tsukui(del561-563), G-6-PD "Mexico City"(680A), G-6-PD Seattle(844C), G-6PD Guadalajara(1159T),G-6-PD Nashville(1178A), and G-6-PD Union(1360T). The G-6-PD A(-) variants have a relatively homogeneous distribution and along with G-6-PD Santamaria(376G/542T), they account for 82% of the overall prevalence of G-6-PD deficiency in Mexico; all other seven variants represent 9% of the mutant alleles examined, and in the rest of the chromosomes the mutation responsible for the enzyme deficiency remains to be defined. Several of the variants observed in Mexico are common in Africa, South Europe, and Southeast Asia. The prevalence for the variant G-6-PD A(376G) was 1.64%. From 256 possible haplotypes only 14 were observed and haplotype analysis suggests that some of the G-6-PD variants probably were imported to Mexico by population flow from South Europe, Africa, and Southeast Asia. This work (i) identified the G-6-PD variants prevalent in Mexico, (ii) defines their geographical distribution, (iii) contributes to the knowledge of the genetic structure of the Mexican population, and (iv) will facilitate the molecular analysis of the G-6-PD gene in enzyme-deficient Mexican individuals.

Glucose-6-phosphate dehydrogenase deficiency in Portugal: biochemical and mutational profiles, heterogeneity, and haplotype association

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common human enzymopathy. This deficiency in erythrocytes has a prevalence of 0.51 +/- 0.109 in the Caucasoid male population of Portugal. The frequency for deficiency-conferring genes is 0.39% in the Portuguese population. In the herein study populations males from areas of Portugal presenting with the highest prevalence of G6PD deficiency (Castelo Branco, Setúbal, Faro, and Lisbon) as well as similar subjects located in the border Center/North area of the country (Viseu) have been analyzed for biochemical parameters and screened for mutations and haplotype-associated mutations commensurate with G6PD deficiency. Six intragenic restriction fragment length polymorphisms (RFLPs) were studied: exon 5, nt 376 A -->G, FokI; intron 5, nt 611 C--> G, PvuII; intron 8, nt 163 C--> T, BspHI; exon 10, nt 116 G --> A, PstI; exon 11, nt 1311 C--> T, BclI; and intron 11, nt 93 T -->C, NlaIII. New haplotypes were constructed with the inclusion of intron 11, nt 93 T--> C, NlaIII, and only 5 of 64 possible haplotypes were found to show a marked linkage disequilibrium for several RFLPs and also for mutations and specific haplotypes. The control population (n = 168 males) presented the G6PD B variant and corresponded to haplotypes I (- - + + - -), Ia (- - + + - +), and VIIa (- - + + + +), in 91.8, 2.3, and 5.9%, respectively. The PCR and sequencing analysis of extracted DNAs from the deficient G6PD group showed 48.6% (16/33) of individuals with the G6PD A- mutation, corresponding to haplotype VIa (+ + - + - +); 9% (3/33) with the Betica mutation and 18% (6/33) with the Santa Maria mutation, both of them associated with haplotype IVa (+ - - + \- +); 6.1% (2/33) with the Mediterranean mutation associated with haplotype VIIa; 12.3% (4/33) with the Seattle mutation, 3.0% (1/33) with Gaohe mutation; and a new mutation, 3.0% (1/33), which we designated by G6PD Flores, all of them associated with haplotype I.

On the allelic spectrum of human disease

Human disease genes show enormous variation in their allelic spectra; that is, in the number and population frequency of the disease-predisposing alleles at the loci. For some genes, there are a few predominant disease alleles. For others, there is a wide range of disease alleles, each relatively rare. The allelic spectrum is important: disease genes with only a few deleterious alleles can be more readily identified and are more amenable to clinical testing. Here, we weave together strands from the human mutation and population genetics literature to provide a framework for understanding and predicting the allelic spectra of disease genes. The theory does a reasonable job for diseases where the genetic etiology is well understood. It also has bearing on the Common Disease/Common Variants (CD/CV) hypothesis, predicting that at loci where the total frequency of disease alleles is not too small, disease loci will have relatively simple spectra.

Glucose-6-phosphate dehydrogenase mutations and haplotypes in Mexican Mestizos

In a screening for glucose-6-phosphate dehydrogenase (G-6-PD) deficiency in 1985 unrelated male subjects from the general population (Groups A and B) belonging to four states of the Pacific coast, 21 G-6-PD-deficient subjects were detected. Screening for mutations at the G-6-PD gene by PCR-restriction enzyme in these 21 G-6-PD-deficient subjects as well as in 14 G-6-PD-deficient patients with hemolytic anemia belonging to several states of Mexico showed two common G-6-PD variants: G-6-PD A-(202A/376G) (19 cases) and G-6-PD A-(376G/968C) (9 cases). In 7 individuals the mutations responsible for the enzyme deficiency remain to be determined. Furthermore, four silent polymorphic sites at the G-6-PD gene (PvuII, PstI, 1311, and NlaIII) were investigated in the 28 individuals with G-6-PD A- variants and in 137 G-6-PD normal subjects. As expected, only 10 different haplotypes were observed. To date, in our project aiming to determine the molecular basis of G-6-PD deficiency in Mexico, 60 unrelated G-6-PD-deficient Mexican males-25 in previous studies and 35 in the present work-have been studied. More than 75% of these individuals are from states of the Pacific coast (Sinaloa, Nayarit, Jalisco, Michoacán, Guerrero, Oaxaca, and Chiapas). The results show that although G-6-PD deficiency is heterogeneous at the DNA level in Mexico, only three polymorphic variants have been observed: G-6-PD A-(202A/376G) (36 cases), G-6-PD A-(376G/968C) (13 cases), and G-6-PD Seattle(844C) (2 cases). G-6-PD A- variants are relatively distributed homogeneously and both variants explain 82% of the overall prevalence of G-6-PD deficiency. The variant G-6-PD A-(202A/376G) represents 73% of the G-6-PD A- alleles. Our data also show that the variant G-6-PD A-(376G/968C)-which has been observed in Mexico in the context of two different haplotypes-is more common than previously supposed. The three polymorphic variants that we observed in Mexico are on the same haplotypes as found in subjects from Africa, the Canary Islands, and Spain.