# G6PD (Glucose-6-Phosphate Dehydrogenase): NADPH Production, Favism, and Malaria Protection Variants ## Key Takeaways - G6PD is the rate-limiting enzyme of the pentose phosphate pathway, crucial for NADPH production in erythrocytes, which lack mitochondria and rely on this pathway for antioxidant defense via glutathione regeneration. - G6PD deficiency, the most common human enzymopathy, renders erythrocytes vulnerable to oxidative hemolysis, manifesting as favism, drug-induced hemolytic anemia, neonatal hyperbilirubinemia, or chronic non-spherocytic hemolytic anemia depending on the variant's severity (WHO Class I-V). - The X-linked inheritance of the *G6PD* gene explains the sex bias in deficiency, with hemizygous males and homozygous females fully deficient, while heterozygous females exhibit mosaicism due to X-chromosome inactivation. - G6PD deficiency variants are geographically concentrated in regions historically endemic for malaria, providing strong evidence for a heterozygote advantage against *Plasmodium falciparum* infection, a key driver of its high prevalence. - Beyond red blood cells, G6PD plays roles in cellular proliferation, cancer progression, and epigenetic regulation, with its activity often upregulated in cancers to support high NADPH demand for biosynthesis and oxidative stress resistance. - Pathogenic mutations, frequently missense, disrupt catalytic activity, enzyme stability, or dimer interface integrity, with specific hotspots identified at codons 68, 188, 385, 459, and 463, leading to diverse clinical phenotypes. --- ## Executive Summary & Key Metadata Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the oxidative pentose phosphate pathway (PPP), catalyzing the first and committed step: the conversion of glucose-6-phosphate (G6P) to 6-phosphoglucono-δ-lactone (6PGL) while concomitantly reducing nicotinamide adenine dinucleotide phosphate (NADP⁺) to NADPH. This reaction is the principal intracellular source of NADPH in mature erythrocytes, which lack mitochondria and therefore cannot generate NADPH via alternative oxidative pathways. NADPH serves as the obligate reducing equivalent for glutathione reductase, which regenerates reduced glutathione (GSH) from its oxidized form (GSSG). GSH is the primary antioxidant defense against hydrogen peroxide and other reactive oxygen species (ROS) in red blood cells. Consequently, G6PD deficiency—the most common human enzymopathy, affecting an estimated 400–500 million individuals worldwide—renders erythrocytes vulnerable to oxidative hemolysis. The clinical spectrum ranges from asymptomatic states to neonatal hyperbilirubinemia with kernicterus risk, acute hemolytic anemia triggered by fava beans (favism), infections, or specific drugs, and chronic non-spherocytic hemolytic anemia (CNSHA) in rare severely deficient variants. The G6PD gene is X-linked, which accounts for the pronounced sex bias in deficiency phenotypes: hemizygous males and homozygous females are fully deficient, whereas heterozygous females exhibit mosaic expression due to random X-chromosome inactivation (lyonization). The geographic distribution of G6PD deficiency alleles closely mirrors historical malaria endemicity, providing strong evidence for heterozygote protection against *Plasmodium falciparum* malaria. This selective advantage has driven the high prevalence of specific deficiency variants in African, Mediterranean, Middle Eastern, and Southeast Asian populations. Beyond its canonical role in erythrocyte redox homeostasis, G6PD is increasingly recognized as a central node in cellular metabolism, proliferation, and survival across multiple tissues. G6PD activity is upregulated in numerous cancers, where it supports the high NADPH demand for macromolecular synthesis, detoxification, and resistance to oxidative stress and chemotherapy. G6PD also influences epigenetic regulation through NADPH-dependent pathways, including DNA methylation and histone acetylation, thereby modulating gene expression programs in vascular smooth muscle cells and other cell types. This manual provides an exhaustive, publication-grade reference covering the genomic architecture, structural biology, molecular function, pathogenic mutations, host-pathogen interactions, [pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), and bioinformatic resources for G6PD. | **Metadata Field** | **Value** | |---|---| | HGNC Symbol | G6PD | | UniProt Accession | P11413 | | Representative PDB ID | 2BH9 | | Chromosomal Locus | Xq28 (GRCh38: chrX:154,531,390–154,547,569; minus strand) | | Primary Molecular Function | Glucose-6-phosphate dehydrogenase (EC 1.1.1.49); catalyzes NADP⁺ reduction and G6P oxidation in the pentose phosphate pathway | | Disease & Pathology Associations | G6PD deficiency (OMIM #300908); favism; drug-induced hemolytic anemia; neonatal hyperbilirubinemia; CNSHA; malaria protection; cancer progression; cardiovascular disease; COVID-19 severity | --- ## 1. Genomic Locus, Chromosomal Organization & Isoforms ### 1.1 Chromosomal Localization and Gene Structure The human *G6PD* gene is located on the long arm of the X chromosome at cytogenetic band Xq28, a gene-dense region rich in housekeeping genes and CpG islands. The gene spans approximately 18.5 kilobases (kb) of genomic DNA (GRCh38/hg38: chrX:154,531,390–154,547,569, minus strand orientation). The gene comprises 13 exons and 12 introns, with the coding sequence distributed across exons 2 through 13. Exon 1 is entirely non-coding and contains the 5' untranslated region (5' UTR). The coding sequence is 1,545 nucleotides in length, encoding a protein of 515 amino acids with a calculated molecular mass of approximately 59.3 kDa for the monomer. Comparative genomic analysis in the pufferfish *Fugu rubripes* revealed a conserved genomic organization, with the *G6PD* gene structure (exon–intron boundaries) largely preserved across vertebrates, underscoring its ancient evolutionary origin and essential housekeeping function. The *G6PD* gene is also conserved in *Drosophila melanogaster*, where adaptive evolution at this locus has been documented, and in the hyperthermophilic bacterium *Thermotoga maritima*, where the enzyme exhibits extreme thermostability, providing a model for structure–function studies. ### 1.2 Promoter Architecture and Regulatory Elements The *G6PD* promoter is a classic example of a housekeeping gene promoter: it lacks a canonical TATA box and instead contains a high-density CpG island spanning the 5' region, extending from approximately −600 bp to +100 bp relative to the transcription start site (TSS). This CpG island is constitutively unmethylated in somatic tissues, permitting ubiquitous expression. The promoter contains multiple Sp1 (specificity protein 1) binding sites, which are critical for basal transcription. Additionally, binding sites for other transcription factors, including AP-2, NF-κB, and CCAAT/enhancer-binding proteins, have been identified in the proximal promoter region. The promoter also contains a functional antioxidant response element (ARE)-like sequence, enabling transcriptional upregulation in response to oxidative stress via the Nrf2 (nuclear factor erythroid 2-related factor 2) pathway. This regulatory feature is particularly relevant in cancer cells, where constitutive Nrf2 activation drives G6PD overexpression to support NADPH production and redox balance. Transgenic mouse studies using a human *G6PD* minigene construct demonstrated that the proximal promoter (approximately 1.5 kb upstream of the TSS) is sufficient to confer high-level, regulated, and tissue-appropriate expression, although distal enhancer elements may contribute to fine-tuning in specific tissues. The promoter is also subject to negative regulation by the transcriptional repressor H3K9me3 (tri-methylation of histone H3 at lysine 9), which suppresses G6PD expression in human mesothelioma cells, thereby reducing PPP flux and ROS production. ### 1.3 Transcription Factor Binding and Epigenetic Regulation Chromatin immunoprecipitation followed by sequencing (ChIP-seq) data from ENCODE and other consortia reveal that the *G6PD* promoter is bound by a broad array of transcription factors, including MYC, MAX, and USF1/2, which are known to regulate metabolic gene expression. MYC, in particular, directly transactivates *G6PD* in cancer cells, linking oncogenic signaling to PPP activation. Epigenetic regulation of *G6PD* expression extends beyond promoter methylation. The H3K9me3 histone mark, deposited by the methyltransferase SUV39H1, represses *G6PD* transcription in mesothelioma cells. Pharmacological inhibition of H3K9 methylation (e.g., with chaetocin) derepresses *G6PD*, leading to increased PPP flux, elevated NADPH, and enhanced ROS production, which paradoxically suppresses tumor growth in this context. In vascular smooth muscle cells, G6PD activity itself regulates genome-wide DNA methylation and histone acetylation patterns, creating a feed-forward loop in which G6PD expression influences the epigenetic landscape that, in turn, controls gene expression programs involved in vascular disease. ### 1.4 Alternative Splicing and Isoforms The *G6PD* gene undergoes alternative splicing, although the functional significance of the resulting isoforms is not fully characterized. The predominant transcript (ENST00000369567.8) encodes the canonical 515-amino-acid protein. Minor splice variants that retain intronic sequences or utilize alternative 5' splice sites in the 5' UTR have been detected in various tissues, but these do not alter the open reading frame and are likely subject to nonsense-mediated decay or translational regulation. A notable finding from Thai female patients is the high frequency of intronic variants that affect splicing efficiency, leading to reduced G6PD enzyme activity without altering the coding sequence. These intronic variants may create cryptic splice sites or disrupt splicing enhancer/silencer elements, resulting in aberrant mRNA processing and reduced steady-state G6PD mRNA levels. This observation underscores the importance of non-coding variants in the molecular pathology of G6PD deficiency and has implications for genetic testing strategies that rely solely on exome sequencing. --- ## 2. 3D Protein Domain Architecture & Structural Biology ### 2.1 Overall Fold and Quaternary Structure Human G6PD is a homodimer or homotetramer in solution, with the dimer being the minimal catalytically active unit. Each monomer folds into two distinct structural domains: a large N-terminal NADP⁺-binding domain (residues approximately 1–200) and a C-terminal substrate-binding domain (residues approximately 201–515), connected by a flexible hinge region. The high-resolution crystal structure of human G6PD (PDB: 2BH9) at 2.9 Å resolution reveals the detailed architecture of the dimer interface and the cofactor/substrate binding sites. The N-terminal domain adopts a classic Rossmann fold, consisting of a central parallel β-sheet flanked by α-helices, which is characteristic of NAD(P)⁺-binding proteins. This domain binds the structural NADP⁺ molecule, which is required for dimer stability but is not catalytically utilized. The C-terminal domain contains the active site cleft, which accommodates the sugar phosphate substrate G6P and the catalytic NADP⁺ cofactor. ### 2.2 Catalytic Site and Reaction Mechanism The catalytic machinery of G6PD is centered on the conserved residues Asp154, His201, and Lys205. The reaction proceeds via a ternary complex mechanism: G6P binds first, followed by NADP⁺. The enzyme abstracts a hydride ion from the C1 position of G6P, transferring it to the C4 position of the nicotinamide ring of NADP⁺, yielding 6-phosphoglucono-δ-lactone and NADPH. The lactone product is subsequently hydrolyzed to 6-phosphogluconate by a separate enzyme (6-phosphogluconolactonase) in the PPP. The active site is highly specific for G6P; however, the enzyme can utilize alternative sugar phosphates, such as 2-deoxy-glucose-6-phosphate and galactose-6-phosphate, albeit with lower catalytic efficiency. The cofactor specificity is strict for NADP⁺; NAD⁺ cannot substitute, owing to the presence of a conserved lysine residue (Lys205) that forms a salt bridge with the 2'-phosphate of NADP⁺. ### 2.3 Structural NADP⁺ and Dimer Stability A second, non-catalytic NADP⁺ binding site is located at the dimer interface. This structural NADP⁺ is essential for maintaining the dimeric conformation and protecting the enzyme against proteolytic degradation. Mutations that disrupt this structural NADP⁺ binding site, such as those affecting residues in the interface region, result in reduced enzyme stability and increased susceptibility to denaturation, even if the catalytic site itself is intact. This phenomenon explains why many G6PD deficiency variants exhibit reduced enzyme activity in erythrocytes, where the protein must remain stable for the entire 120-day lifespan of the red blood cell in the absence of de novo protein synthesis. ### 2.4 Structural Basis of Pathogenic Mutations The vast majority of G6PD deficiency-causing mutations are missense mutations that result in single amino acid substitutions. These mutations exert their pathogenic effects through one or more of the following mechanisms: (i) disruption of the catalytic site, directly reducing kcat; (ii) perturbation of the structural NADP⁺ binding site, reducing enzyme stability; (iii) destabilization of the dimer interface, promoting monomerization and inactivation; or (iv) induction of protein misfolding, leading to accelerated degradation. For example, the common African variant G6PD A⁻ (Val68Met; rs1050828) is located in the N-terminal NADP⁺-binding domain, near the structural NADP⁺ site. This substitution reduces enzyme stability without significantly affecting catalytic activity, resulting in a mild-to-moderate deficiency phenotype with class III WHO classification. In contrast, the Mediterranean variant (Ser188Phe) is located in the active site cleft and severely impairs catalytic activity, resulting in a class II deficiency with a high risk of favism and drug-induced hemolysis. The p.(Gln195His) variant (G6PD Tainan) and the novel p.(Ser184Cys) variant (G6PD Toluca) identified through Mexican newborn screening are located in the C-terminal substrate-binding domain. Functional characterization demonstrated that both variants exhibit reduced catalytic activity and altered kinetic parameters, consistent with their classification as class II/III deficiency variants. Similarly, the G6PD Flores variant, caused by a novel point mutation in exon 12, results in a severe deficiency phenotype associated with chronic hemolysis. ### 2.5 Interactive 3D Visualization For a comprehensive exploration of the G6PD three-dimensional structure, including the dimer interface, catalytic site, and structural NADP⁺ binding pocket, the interactive visualizer tool is recommended: [Interactive 3D Protein Visualizer: Load G6PD (PDB: 2BH9)](/tools/protein-structure-viewer?source=direct&pdbId=2BH9) This tool allows users to rotate the molecule, highlight specific residues, and visualize the spatial relationships between pathogenic mutation sites and functional domains. --- ## 3. Cellular Signaling Pathways & Molecular Function ### 3.1 The Pentose Phosphate Pathway and NADPH Production G6PD catalyzes the first and rate-limiting step of the oxidative branch of the PPP, which is the primary source of NADPH in most cell types. The PPP operates in the cytosol and is divided into two phases: the oxidative phase (irreversible) and the non-oxidative phase (reversible). The oxidative phase, comprising G6PD and 6-phosphogluconate dehydrogenase (6PGD), generates two molecules of NADPH per molecule of G6P oxidized. The non-oxidative phase produces ribose-5-phosphate, a precursor for nucleotide biosynthesis, and regenerates glycolytic intermediates. In mature erythrocytes, which lack mitochondria and the tricarboxylic acid cycle, the PPP is the sole source of NADPH. The NADPH/NADP⁺ ratio in erythrocytes is maintained at a high level (typically >100:1), ensuring the continuous reduction of GSSG to GSH by glutathione reductase. GSH serves as a cofactor for glutathione peroxidase, which detoxifies hydrogen peroxide and lipid peroxides. When G6PD activity falls below approximately 20% of normal, the erythrocyte's capacity to regenerate GSH is overwhelmed, leading to oxidative damage to hemoglobin (forming methemoglobin and Heinz bodies), membrane lipids, and cytoskeletal proteins. This oxidative insult culminates in intravascular and extravascular hemolysis. ### 3.2 Regulation of G6PD Activity G6PD activity is regulated at multiple levels: transcriptional, post-transcriptional, and allosteric. **Transcriptional regulation:** As described in Section 1.2, the *G6PD* promoter is responsive to Nrf2, MYC, and other transcription factors. In response to oxidative stress, Nrf2 translocates to the nucleus and activates *G6PD* transcription, increasing NADPH production to restore redox balance. In cancer cells, oncogenic signaling pathways (e.g., PI3K/AKT, RAS/MAPK) converge on MYC and Nrf2 to upregulate G6PD, supporting the high NADPH demand of proliferating cells. **Post-transcriptional regulation:** G6PD mRNA is subject to regulation by microRNAs (miRNAs). For example, miR-199a-5p/3p directly targets the *G6PD* 3' UTR, and the long non-coding RNA SNHG1 functions as a molecular sponge for these miRNAs, thereby upregulating G6PD expression in hepatocellular carcinoma. Additionally, METTL14-mediated N6-methyladenosine (m6A) modification of *G6PD* mRNA enhances its stability and translation, promoting lung adenocarcinoma progression. **Allosteric regulation:** G6PD is allosterically inhibited by its product NADPH, which competes with NADP⁺ for binding to the catalytic site. This feedback inhibition ensures that NADPH production is matched to cellular demand. In cells with high NADPH consumption (e.g., during fatty acid synthesis or detoxification), the NADPH/NADP⁺ ratio decreases, relieving inhibition and increasing G6PD flux. ### 3.3 G6PD in Cellular Signaling and Gene Expression Beyond its metabolic role, G6PD influences cellular signaling pathways through NADPH-dependent mechanisms. NADPH is a cofactor for NADPH oxidases (NOX), which generate superoxide and other ROS as signaling molecules. In vascular smooth muscle cells, G6PD-derived NADPH fuels NOX enzymes, contributing to ROS-mediated signaling that promotes cell proliferation, migration, and vascular remodeling. G6PD inhibition reduces NOX activity, attenuates ROS production, and suppresses the expression of serum response factor (SRF)- and myocardin-driven smooth muscle cell-specific genes, thereby modulating the pathogenesis of vascular diseases. G6PD also regulates epigenetic modifications. NADPH is a cofactor for the methionine synthase and methylenetetrahydrofolate reductase enzymes, which generate S-adenosylmethionine (SAM), the universal methyl donor for DNA and histone methylation. G6PD deficiency or inhibition reduces NADPH availability, leading to altered SAM levels and changes in genome-wide DNA methylation patterns. In smooth muscle cells, G6PD activity is required for the maintenance of histone acetylation at specific gene loci, linking metabolic flux to transcriptional programs. ### 3.4 G6PD in Immune Function and Inflammation G6PD plays a critical role in immune cell function. In cytotoxic T lymphocytes (CTLs), G6PD expression is required for the production of granzyme B, a key effector molecule for tumor cell killing. Mechanistically, G6PD-derived NADPH supports the redox environment necessary for granzyme B transcription and protein stability. Inhibition of G6PD in CTLs impairs granzyme B expression and reduces their cytotoxic activity against tumor cells, suggesting that G6PD is a metabolic checkpoint for anti-tumor immunity. In macrophages, G6PD is upregulated during *Brucella* infection via the NF-κB signaling pathway. The resulting increase in PPP flux and NADPH production supports glycolysis and the pro-inflammatory response, enhancing macrophage bactericidal activity. Conversely, in microglia (the resident immune cells of the central nervous system), G6PD deficiency leads to impaired NADPH production, redox imbalance, and lysosomal dysfunction, which may contribute to neuroinflammation and autism spectrum disorders. ### 3.5 Protein-Protein Interaction Networks G6PD interacts with a diverse array of proteins, as cataloged in BioGRID and STRING databases. Key interacting partners include: - **6-Phosphogluconate dehydrogenase (6PGD):** The second enzyme of the oxidative PPP, which physically associates with G6PD in a metabolon complex, facilitating substrate channeling. - **Transketolase (TKT):** A non-oxidative PPP enzyme that interacts with G6PD to coordinate flux through the pathway. - **p53:** The tumor suppressor p53 binds to G6PD and inhibits its activity, providing a direct link between tumor suppression and metabolic regulation. Mutant p53 loses this inhibitory function, contributing to the elevated G6PD activity observed in many cancers. - **SQSTM1 (p62):** G6PD interacts with SQSTM1 and regulates its protein stability, thereby modulating autophagy and NF-κB signaling in glioma cells. - **DNA methyltransferases (DNMTs):** G6PD-derived NADPH influences DNMT activity, linking metabolic status to epigenetic regulation. ### 3.6 G6PD in Cancer Metabolism G6PD is consistently upregulated in a wide range of cancers, including lung, breast, colon, liver, ovarian, and pancreatic cancers. The elevated G6PD activity supports cancer cell proliferation through multiple mechanisms: 1. **NADPH production for biosynthesis:** NADPH is required for reductive biosynthesis of fatty acids, cholesterol, and deoxyribonucleotides, all of which are essential for rapid cell division. 2. **Antioxidant defense:** Cancer cells experience high levels of oxidative stress due to increased ROS production from aberrant metabolism. G6PD-derived NADPH maintains GSH levels, protecting cancer cells from oxidative damage and apoptosis. 3. **Metastasis:** G6PD-mediated NADPH production promotes the de novo biosynthesis of NADP⁺, which is required for the activity of NADP⁺-dependent enzymes involved in metastasis, including NOX and inositol-1,4,5-trisphosphate receptors. 4. **Chemoresistance:** G6PD overexpression is associated with resistance to cisplatin and other chemotherapeutic agents in ovarian cancer. Inhibition of G6PD sensitizes cancer cells to chemotherapy, suggesting that G6PD is a potential therapeutic target. Pan-cancer analyses have confirmed that G6PD expression is elevated in tumor tissues compared to normal tissues and that high G6PD expression correlates with poor prognosis in multiple cancer types. G6PD also drives glioma invasion by regulating SQSTM1 protein stability, and its expression is regulated by the circadian gene RORα in gastric cancer. --- ## 4. Pathogenic Hotspot Mutations & Clinical Differentials ### 4.1 Classification of G6PD Deficiency Variants The World Health Organization (WHO) classifies G6PD deficiency variants into five classes based on enzyme activity and clinical severity: - **Class I:** Severe deficiency (<10% activity) with chronic non-spherocytic hemolytic anemia (CNSHA). - **Class II:** Severe deficiency (<10% activity) without CNSHA; intermittent hemolysis triggered by oxidative stress. - **Class III:** Moderate deficiency (10–60% activity); hemolysis typically only with severe oxidative stress. - **Class IV:** Normal activity (60–150%); clinically silent. - **Class V:** Increased activity (>150%); clinically silent. A revised WHO classification system has been proposed, incorporating both biochemical and clinical data to better stratify variants. ### 4.2 Common Polymorphic Variants The most common G6PD deficiency variants worldwide include: | **Variant Name** | **Nucleotide Change** | **Amino Acid Change** | **WHO Class** | **Geographic Distribution** | **Clinical Features** | |---|---|---|---|---|---| | G6PD A⁻ | 202G>A, 376A>G | Val68Met, Asn126Asp | III | Sub-Saharan Africa, African diaspora | Mild-to-moderate deficiency; hemolysis with infection or drugs | | G6PD Mediterranean | 563C>T | Ser188Phe | II | Mediterranean basin, Middle East, India | Severe deficiency; favism, drug-induced hemolysis | | G6PD Mahidol | 487G>A | Gly163Ser | III | Southeast Asia | Moderate deficiency; primaquine-induced hemolysis | | G6PD Canton | 1376G>T | Arg459Leu | II | Southern China, Southeast Asia | Severe deficiency; neonatal jaundice, favism | | G6PD Kaiping | 1388G>A | Arg463His | II | Southern China | Severe deficiency; neonatal jaundice | | G6PD Viangchan | 871G>A | Val291Met | III | Southeast Asia | Moderate deficiency | | G6PD Union | 1360C>T | Arg454Cys | II | Philippines, Southeast Asia | Severe deficiency | | G6PD Chatham | 1003G>A | Ala335Thr | II | India, Middle East | Severe deficiency | The G6PD A⁻ variant is particularly notable for its high frequency in African populations, where it is associated with protection against severe *P. falciparum* malaria. The Val68Met substitution (rs1050828) reduces enzyme stability, leading to a class III deficiency phenotype. Red blood cells from individuals with this variant exhibit increased hemolytic propensity under oxidative stress and reduced post-transfusion recovery, highlighting the clinical significance of this polymorphism in transfusion medicine. ### 4.3 Pathogenic Hotspot Mutations The *G6PD* gene exhibits a remarkable degree of allelic heterogeneity, with over 200 distinct mutations reported to date. The majority are missense mutations, but nonsense, frameshift, and splice-site mutations have also been described. Several mutational hotspots have been identified: **Codon 385 (Cys385):** Three distinct variants at this codon—G6PD Tomah (C385R), G6PD Kangnam (C385G), and G6PD Madrid (C385Y)—have been characterized. All three are class I variants associated with CNSHA. Biochemical and in silico analyses revealed that these substitutions disrupt the local hydrophobic core, leading to protein misfolding and accelerated degradation. **Codon 188 (Ser188):** The G6PD Mediterranean variant (Ser188Phe) is the most common class II variant in Mediterranean populations. The substitution introduces a bulky aromatic residue into the active site, severely impairing catalytic activity. **Codon 68 (Val68):** The G6PD A⁻ variant (Val68Met) is the most common class III variant in African populations. The substitution destabilizes the structural NADP⁺ binding site, reducing enzyme stability. **Codon 459 (Arg459):** The G6PD Canton variant (Arg459Leu) is a common class II variant in Southern China. The substitution disrupts a salt bridge at the dimer interface, promoting monomerization and inactivation. **Codon 463 (Arg463):** The G6PD Kaiping variant (Arg463His) is another common class II variant in Southern China, with similar structural consequences to G6PD Canton. ### 4.4 Novel and Rare Variants Next-generation sequencing and newborn screening programs have identified numerous novel G6PD variants. In Mexico, newborn screening identified the novel p.(Ser184Cys) variant (G6PD Toluca) and the previously known p.(Gln195His) variant (G6PD Tainan). Functional characterization demonstrated that both variants exhibit reduced catalytic activity and altered kinetic parameters, consistent with class II/III deficiency. In Qatar, whole-genome sequencing of G6PD-deficient individuals identified seven novel variants, including missense, nonsense, and splice-site mutations. These variants were distributed across the gene, with no clear mutational hotspot, underscoring the allelic heterogeneity of G6PD deficiency in Middle Eastern populations. In Korea, a novel de novo mutation was identified in a boy with G6PD deficiency, highlighting the possibility of spontaneous mutations in the absence of a family history. Similarly, a novel deleterious variant was identified in three Chinese families with severe G6PD deficiency, emphasizing the importance of comprehensive genetic testing for accurate diagnosis and genetic counseling. ### 4.5 Clinical Differentials and Phenotype-Genotype Correlations The clinical phenotype of G6PD deficiency varies widely, ranging from asymptomatic to life-threatening hemolysis. The severity of the phenotype is influenced by the specific variant, the level of residual enzyme activity, and the nature of the oxidative stressor. **Neonatal hyperbilirubinemia (NHB):** G6PD deficiency is a major risk factor for NHB, which can progress to kernicterus if untreated. The risk of NHB is influenced by the interaction between G6PD genotype and polymorphisms in the UGT1A1 gene, which encodes bilirubin UDP-glucuronosyltransferase. A study of neonates demonstrated that the combination of G6PD deficiency and UGT1A1 promoter polymorphisms significantly increased the risk of NHB and the need for phototherapy. **Favism:** Ingestion of fava beans (*Vicia faba*) can trigger acute hemolytic anemia in individuals with class II or III G6PD deficiency. The hemolysis is thought to be mediated by oxidative compounds in the beans, such as vicine and convicine, which generate ROS upon metabolism. The severity of favism varies among individuals and even within the same individual over time, suggesting the involvement of additional genetic and environmental factors. **Drug-induced hemolysis:** Numerous drugs can trigger hemolysis in G6PD-deficient individuals, including antimalarials (primaquine, tafenoquine), sulfonamides, nitrofurantoin, dapsone, rasburicase, and high-dose aspirin. The Clinical Pharmacogenetics Implementation Consortium (CPIC) has published guidelines for the use of medications in the context of G6PD genotype, classifying drugs as "definitely" or "possibly" hemolytic. Rasburicase, used for tumor lysis syndrome prophylaxis, is a notable example of a drug that can cause severe hemolysis and methemoglobinemia in G6PD-deficient patients. **Chronic non-spherocytic hemolytic anemia (CNSHA):** Class I variants, which are typically sporadic or familial, cause chronic hemolysis with persistent anemia, reticulocytosis, and splenomegaly. These variants are often associated with severe enzyme deficiency (<5% activity) and are caused by mutations that severely disrupt [protein structure](/knowledge/bioinformatics/protein-structure-biophysical-levels-folding) or catalytic function. **Acquired G6PD deficiency:** Somatic mutations in the *G6PD* gene can occur in hematopoietic stem cells, leading to acquired G6PD deficiency in the context of clonal hematopoiesis. A case of acquired G6PD deficiency with severe hemolysis was reported in a patient with chronic myelomonocytic leukemia evolving into acute leukemia, where the G6PD defect was due to clonal expansion of precursor cells carrying a somatic mutation. ### 4.6 G6PD Deficiency and Comorbidities **Diabetes mellitus:** G6PD deficiency is associated with lower HbA1c levels for a given level of glycemia, which can lead to underdiagnosis and undertreatment of type 2 diabetes. This is particularly relevant in populations with a high prevalence of G6PD deficiency, such as individuals of African, Asian, and Mediterranean descent. **Cardiovascular disease:** G6PD deficiency has been linked to an increased risk of cardiovascular disease, possibly due to impaired NADPH production and increased oxidative stress in vascular tissues. Conversely, G6PD inhibition has been proposed as a therapeutic strategy for vascular diseases, as it reduces NOX-mediated ROS production and smooth muscle cell proliferation. **Obesity and metabolic syndrome:** Loss-of-function G6PD variants moderate high-fat diet-induced obesity, adipocyte hypertrophy, and fatty liver in male rats, suggesting that G6PD inhibition may have beneficial metabolic effects. **COVID-19:** G6PD deficiency has been proposed as a risk factor for COVID-19 severity, based on the observation that G6PD-deficient cells are more susceptible to coronavirus infection and that chloroquine, an early candidate drug for COVID-19, can trigger hemolysis in G6PD-deficient individuals. However, the clinical significance of G6PD deficiency in COVID-19 remains uncertain, and further studies are needed. **Autism spectrum disorders (ASD):** An association between G6PD deficiency and ASD has been proposed, possibly mediated by oxidative stress and impaired NADPH production in the developing brain. --- ## 5. Host-Pathogen & Viral Interactions ### 5.1 Malaria and G6PD Deficiency The geographic correlation between G6PD deficiency and malaria endemicity is one of the most compelling examples of natural selection in humans. The "malaria hypothesis" posits that G6PD deficiency confers a protective advantage against *Plasmodium* infection, thereby maintaining the high frequency of deficiency alleles in malaria-endemic regions. **Protective mechanisms:** The mechanisms underlying G6PD deficiency-mediated malaria protection are not fully understood but likely involve: 1. **Increased oxidative stress in parasitized erythrocytes:** *Plasmodium* parasites are highly susceptible to oxidative stress. G6PD-deficient erythrocytes have reduced NADPH and GSH levels, creating an environment that is hostile to parasite growth and replication. 2. **Impaired parasite development:** The parasite relies on the host erythrocyte's PPP for NADPH production. In G6PD-deficient erythrocytes, the parasite's ability to detoxify ROS is compromised, leading to reduced parasite viability. 3. **Enhanced phagocytosis:** G6PD-deficient erythrocytes, particularly those infected with *Plasmodium*, exhibit increased surface expression of "eat-me" signals (e.g., phosphatidylserine), promoting their clearance by macrophages. **Heterozygote advantage:** In females heterozygous for G6PD deficiency, X-chromosome inactivation results in a mosaic population of normal and deficient erythrocytes. It has been hypothesized that the presence of deficient erythrocytes impairs parasite growth while the normal erythrocytes maintain adequate oxygen-carrying capacity, providing a selective advantage. A fluorometric assay was developed to quantify the differential invasion rates of *P. falciparum* in heterozygous females, providing a proof-of-concept for this hypothesis. **Epidemiological evidence:** Studies in Ghana demonstrated that genotypic G6PD deficiency protects against *P. falciparum* infection. In Sri Lanka, specific G6PD variants were associated with reduced malaria risk. In Thailand, genetic analysis of G6PD deficiency among malaria patients provided insights into the safe use of 8-aminoquinolines (primaquine and tafenoquine) for radical cure of *P. vivax* malaria. **Clinical implications:** The protective effect of G6PD deficiency against malaria has important clinical implications. Primaquine and tafenoquine, which are used to eliminate *P. vivax* hypnozoites and *P. falciparum* gametocytes, can cause severe hemolysis in G6PD-deficient individuals. Therefore, G6PD testing is mandatory before administering these drugs. The development of point-of-care G6PD tests and genotyping assays is critical for the safe implementation of malaria elimination programs. ### 5.2 Viral Infections **Influenza virus:** Influenza virus infection down-modulates G6PD expression and activity in host cells, leading to decreased NADPH production, increased oxidative stress, and enhanced viral replication. This suggests that G6PD is a host factor that restricts influenza virus replication and that the virus has evolved mechanisms to suppress G6PD expression to create a favorable environment for its replication. **COVID-19 (SARS-CoV-2):** G6PD deficiency has been proposed as a risk factor for COVID-19 severity. Early studies suggested that G6PD-deficient cells are more susceptible to coronavirus infection and that G6PD deficiency may exacerbate the oxidative stress and inflammatory response associated with COVID-19. However, a large retrospective cohort study found no significant association between G6PD deficiency and COVID-19 incidence or severity. The use of chloroquine, an early candidate drug for COVID-19, was cautioned in G6PD-deficient individuals due to the risk of hemolysis. ### 5.3 Bacterial Infections **Brucella:** *Brucella* infection upregulates G6PD expression in macrophages via the NF-κB signaling pathway. The resulting increase in PPP flux and NADPH production supports glycolysis and the pro-inflammatory response, enhancing macrophage bactericidal activity. This finding highlights the role of G6PD in host defense against intracellular bacterial pathogens. --- ## 6. [Pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), Drug Targets & Small-Molecule Inhibitors ### 6.1 CPIC Guidelines for G6PD Genotype The Clinical Pharmacogenetics Implementation Consortium (CPIC) has published an expanded guideline for medication use in the context of G6PD genotype. The guideline classifies medications into three categories: - **Definitely hemolytic:** Drugs that consistently cause hemolysis in G6PD-deficient individuals, including rasburicase, dapsone, methylene blue, and nitrofurantoin. These drugs should be avoided in G6PD-deficient individuals unless the benefit outweighs the risk. - **Possibly hemolytic:** Drugs that may cause hemolysis in some G6PD-deficient individuals, including sulfonamides, primaquine, tafenoquine, and high-dose aspirin. These drugs should be used with caution and with monitoring for hemolysis. - **Low risk:** Drugs that are unlikely to cause hemolysis in G6PD-deficient individuals, including standard-dose acetaminophen and ibuprofen. The CPIC guideline emphasizes the importance of genotyping for G6
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