rs76723693 — G6PD G6PD Nefza (c.968T>C)

G6PD Nefza: A Rare Enzyme Deficiency That Makes Ordinary Drugs and Foods Dangerous

Glucose-6-phosphate dehydrogenase (G6PD) is a housekeeping enzyme that every cell carries, but it matters most in red blood cells — which have no mitochondria and therefore depend 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. NADPH is the fuel that keeps the antioxidant defense of red blood cells running. When G6PD activity is significantly reduced, oxidative stress from drugs, infections, or certain foods overwhelms the red cell's 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.

The rs76723693 G allele creates the G6PD Nefza variant, a missense change (c.968T>C) that substitutes leucine with proline at position 323 of the enzyme. Proline's rigid ring structure disrupts local protein folding, reducing enzyme stability and activity to approximately 50% of normal. This variant was first characterized in Tunisian patients³³ first characterized in Tunisian patients
Benmansour et al. 2013, Blood Cells Mol Dis; 293 G6PD-deficient individuals across familial and asymptomatic cases and is classified by the WHO as a Class III G6PD deficiency variant — mild to moderate, with residual activity in the 10–60% range. G6PD deficiency is the most common human enzymopathy, affecting more than 400 million people worldwide⁴⁴ affecting more than 400 million people worldwide, distributed across Africa, the Mediterranean, Middle East, and South and East Asia in patterns that mirror historic malaria endemicity — the G6PD-deficient red cell is less hospitable to Plasmodium parasites, conferring selective advantage in malaria-endemic regions.

The Mechanism

G6PD catalyzes the first 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 glutathione⁵⁵ glutathione
The cell's primary intracellular antioxidant; G6PD deficiency depletes reduced glutathione, leaving hemoglobin and the cell membrane vulnerable to oxidative damage. When NADPH falls below a critical threshold — triggered by oxidative challenge from drugs, fava bean ingestion (which liberates vicine and convicine, potent oxidants), or febrile infection — unprotected hemoglobin forms Heinz bodies⁶⁶ Heinz bodies
Precipitates of denatured hemoglobin that attach to the red cell membrane and accelerate its removal by the spleen and the cell is destroyed. The Leu323Pro substitution in G6PD Nefza reduces enzyme stability and substrate affinity for both glucose-6-phosphate and NADP⁺, confirmed in functional studies⁷⁷ confirmed in functional studies showing approximately 50% residual activity relative to the wild-type enzyme.

Because the G6PD gene is on the X chromosome, the variant is X-linked: males carrying the G allele are hemizygous and express the full deficiency phenotype. Females with one G allele are carriers and may have partial deficiency depending on X-chromosome inactivation patterns⁸⁸ X-chromosome inactivation patterns
Random silencing of one X chromosome in each female 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

The G6PD Nefza variant (c.968T>C) alone produces approximately 50% residual enzyme activity — biochemical analysis by Ramírez-Nava et al. 2017⁹⁹ biochemical analysis by Ramírez-Nava et al. 2017
Biochemical Analysis of Two Single Mutants that Give Rise to a Polymorphic G6PD A-Double Mutant. Int J Mol Sci, 2017 showed that Leu323Pro is the functionally dominant destabilizing mutation. This variant frequently co-occurs in cis with a second mutation, c.376A>G (p.Asn126Asp), forming the G6PD Betica-Selma complex allele with more severe enzyme dysfunction and pathogenic hemolytic anemia (ClinVar variation 1065168). The c.968T>C single variant is classified in ClinVar as Pathogenic/Likely pathogenic with 17/21 submissions in agreement.

An important and underappreciated clinical consequence of G6PD deficiency is its effect on HbA1c measurements. Because G6PD-deficient red cells have a shorter lifespan, hemoglobin has less time to become glycated, producing systematically lower HbA1c readings despite normal or elevated blood glucose levels¹⁰¹⁰ systematically lower HbA1c readings despite normal or elevated blood glucose levels
Nature Medicine 2024 found that G6PDdef carriers showed significantly higher glucose but lower HbA1c during the pre-diabetes period, and accounted for 12% of diabetic retinopathy and 9% of neuropathy cases in African ancestry participants of the ACCORD trial. This diagnostic gap means G6PD-deficient individuals can meet diabetes diagnostic thresholds by blood glucose criteria while appearing pre-diabetic by HbA1c — a clinically dangerous misclassification that delays treatment and intensification of therapy.

Practical Actions

The primary goal for G6PD Nefza carriers is avoiding oxidative triggers. The most evidence-supported prohibitions are seven drugs with solid evidence for hemolysis induction¹¹¹¹ seven drugs with solid evidence for hemolysis induction
Youngster et al. 2010, Drug Safety, evidence-based review of MEDLINE, PubMed, Cochrane: primaquine, dapsone, rasburicase, nitrofurantoin, methylene blue, phenazopyridine, and toluidine blue. Fava beans (broad beans, Vicia faba) and their pollen must also be avoided — the pyrimidine glycosides vicine and convicine undergo oxidative metabolism that overwhelms G6PD-deficient red cells. Hemolytic episodes triggered by infection are common and typically self-limiting, but severe episodes require urgent medical evaluation.

For diabetes screening and management, carriers should request fasting plasma glucose or a 2-hour oral glucose tolerance test (OGTT) rather than relying on HbA1c alone. If HbA1c is used, clinicians should apply a genotype-adjusted threshold — current evidence supports treating an HbA1c reading as representing meaningfully higher true glycemic exposure than the number suggests.

Interactions

The rs76723693 G allele can occur in cis with rs2230037 (G6PD c.376A>G, p.Asn126Asp), forming the G6PD Betica-Selma double mutant (ClinVar 1065168). This compound allele causes more severe enzyme deficiency and a higher risk of nonspherocytic hemolytic anemia than either mutation alone. Individuals who test positive for rs76723693 should ideally have both variants assessed to determine whether the compound allele is present.

The HbA1c interference from G6PD deficiency interacts critically with common diabetes screening pathways that rely exclusively on HbA1c — an important interaction with variants in the glycation and hemoglobin pathway. Concurrent hemoglobinopathies (e.g., sickle cell trait, thalassemia) further complicate HbA1c reliability and may compound the interpretation challenge.