rs5030868 — G6PD G6PD Mediterranean

G6PD Mediterranean: A Severe Enzyme Deficiency That Makes Ordinary Drugs Dangerous

Glucose-6-phosphate dehydrogenase (G6PD) is a housekeeping enzyme present in every cell, but it is most critically important in red blood cells — which have no mitochondria and therefore depend entirely on G6PD as their sole source of NADPH¹¹ NADPH
Nicotinamide adenine dinucleotide phosphate (reduced form) — a cellular reducing agent essential for regenerating glutathione and neutralizing oxidative stress. When G6PD activity is severely reduced, oxidative stress from certain drugs, foods, or infections overwhelms the red cell's antioxidant defenses, causing the cell membrane to rupture — a process called acute hemolytic anemia²² acute hemolytic anemia
Sudden, triggered destruction of red blood cells causing jaundice, dark urine, fatigue, and anemia that can require blood transfusion.

The rs5030868 A allele creates the G6PD Mediterranean variant — a missense change (c.563C>T on the coding strand) that substitutes serine with phenylalanine at position 188 of the mature protein (p.Ser188Phe, MANE Select transcript). This substitution disrupts a dimer interface contact residue, severely destabilizing the active enzyme structure and reducing G6PD activity to less than 10% of normal — classified by the WHO as Class II severe deficiency³³ Class II severe deficiency
Class II variants retain 1–10% of normal enzyme activity and cause acute hemolytic crises under oxidative challenge; Class I (most severe) causes chronic hemolysis even at rest. G6PD deficiency is the most common human enzymopathy, affecting more than 400 million people worldwide⁴⁴ affecting more than 400 million people worldwide
Cappellini & Fiorelli, Lancet 2008; geographic distribution mirrors historic malaria endemicity, and the Mediterranean variant is the predominant severe form across the Mediterranean basin, Middle East, and South Asia, where allele frequencies reach 1–10% in some sub-populations.

The Mechanism

G6PD catalyzes the first and rate-limiting step of the pentose phosphate pathway: converting glucose-6-phosphate to 6-phosphogluconate while reducing NADP⁺ to NADPH. In red blood cells, this NADPH is the only means of regenerating reduced glutathione, the cell's primary intracellular antioxidant. When NADPH falls below a critical threshold — triggered by oxidative challenge from drugs, fava bean ingestion (which liberates vicine and convicine, potent pyrimidine glycoside oxidants), or febrile infection — unprotected hemoglobin is oxidized, forms Heinz bodies⁵⁵ Heinz bodies
Precipitates of denatured hemoglobin that attach to the red cell membrane and trigger rapid removal by the spleen, and the red cell is destroyed.

The Ser188Phe substitution places a bulky aromatic phenylalanine residue at a dimer interface contact point, severely disrupting the quaternary structure on which G6PD catalytic activity depends. Mason et al. 2007⁶⁶ Mason et al. 2007
Blood Reviews; review of 160 G6PD mutations; severe variants predominantly destabilize dimer interface or structural NADP-binding residues identify this class of structural disruptions as responsible for the near-complete loss of enzyme activity in Class II variants. At <10% residual activity, the Mediterranean variant provides insufficient NADPH buffering capacity even under moderate oxidative challenge — lower than the Class III variants common in Africa (~10–60% residual), and substantially more dangerous in terms of hemolytic threshold.

Because the G6PD gene is on the X chromosome, the variant is X-linked: males carrying the A allele are hemizygous and express the full severe deficiency phenotype. Females with one A copy are carriers and may have partial-to-full deficiency depending on X-chromosome inactivation⁷⁷ X-chromosome inactivation
Random silencing of one X chromosome per cell; if the wild-type allele is preferentially silenced (skewed inactivation), a heterozygous female can have enzyme activity as low as a hemizygous male.

The Evidence

ClinVar VCV000100057⁸⁸ VCV000100057 classifies this variant as Pathogenic/Likely pathogenic based on 63 submissions (48 pathogenic with multiple submitters, no conflicts) — the highest evidence tier. Conditions include nonspherocytic hemolytic anemia due to G6PD deficiency, favism (fava-bean triggered hemolysis), malaria susceptibility, and neonatal hyperbilirubinemia.

A critical and underappreciated clinical consequence of G6PD deficiency is its interference with HbA1c measurements. Because G6PD-deficient red cells have shorter lifespans, hemoglobin accumulates less glycation, producing systematically lower HbA1c readings despite normal or elevated blood glucose. Breeyear et al. 2024 (Nature Medicine)⁹⁹ Breeyear et al. 2024 (Nature Medicine)
Adaptive selection at G6PD and disparities in diabetes complications; 42 institutions across the US found that G6PD deficiency masked pre-diabetic hyperglycemia in the years before clinical diabetes diagnosis, and estimated that 12% of diabetic retinopathy cases and 9% of neuropathy cases in African ancestry participants of the ACCORD trial were attributable to underdiagnosed/undertreated hyperglycemia caused by suppressed HbA1c readings. A case report by Danzig et al. 2011¹⁰¹⁰ A case report by Danzig et al. 2011
Adolescent with G6PD deficiency and type 1 diabetes; HbA1c consistently discordant with blood glucose measurements; hemolysis reduced red cell lifespan and glycated hemoglobin accumulation further illustrates this diagnostic pitfall in an individual patient.

Regarding drug safety, Youngster et al. 2010¹¹¹¹ Youngster et al. 2010
Drug Safety systematic review of MEDLINE, PubMed, Cochrane; evidence-based review of hemolysis-inducing drugs in G6PD-deficient patients identified seven drugs with solid evidence for prohibition: primaquine, dapsone, rasburicase, nitrofurantoin, methylene blue, phenazopyridine, and toluidine blue. The risk is particularly severe for Class II variants like G6PD Mediterranean because the minimal residual enzyme activity provides almost no buffer against oxidative drug stress.

Practical Actions

The primary goal for carriers of the G6PD Mediterranean variant is identifying and avoiding oxidative triggers before an acute crisis occurs. Seven drugs have solid evidence for hemolysis induction in G6PD-deficient patients and must be flagged with prescribing physicians before any new prescription. Fava beans (broad beans, Vicia faba) and their pollen must also be completely avoided. When an acute hemolytic episode occurs — sudden pallor, jaundice, dark or tea-colored urine — immediate medical evaluation is required; severe Class II crises can require blood transfusion.

For diabetes screening, carriers should request fasting plasma glucose (FPG) or a 2-hour oral glucose tolerance test (OGTT) as the primary screening method rather than relying on HbA1c alone, which will read artificially low. If HbA1c is the only test available, treat any value above 5.5% with heightened concern and correlate with self-monitored blood glucose values.

Interactions

rs5030868 (G6PD Mediterranean) can co-occur with rs2230037 (G6PD c.376A>G, p.Asn126Asp) on the same chromosome, forming a compound allele with further reduced enzyme activity. Individuals carrying rs5030868 should ideally have rs2230037 assessed to determine whether the compound allele is present, as this would shift management toward even greater vigilance and neonatal jaundice screening planning.

The HbA1c interference from G6PD deficiency interacts critically with diabetes screening protocols relying exclusively on HbA1c. Concurrent hemoglobinopathies (e.g., sickle cell trait, thalassemia) — which co-segregate with G6PD Mediterranean in overlapping populations — further complicate HbA1c reliability and compound the diagnostic challenge.