Chapter 18: Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency – Comprehensive Exam Notes

G6PD = first enzyme of Pentose Phosphate Pathway (PPP) (a.k.a. hexose-monophosphate shunt)
Main role is not glucose use (<10 %) but production of NADPH, vital for antioxidative defense
Oxygen radicals from Hb auto-oxidation → $O<em>2^{\bullet-}$ → $H</em>2O_2$
- Detoxified by catalase & glutathione-peroxidase (GSHPX)
- GSHPX requires GSH; GSH is regenerated by glutathione-reductase (needs NADPH)
G6PD-deficient RBCs lose NADPH → ↓GSH → extreme oxidative vulnerability
Complete G6PD KO is lethal in mice; null embryonic stem cells survive only in low O2

Ubiquitous, ancient housekeeping enzyme (found from bacteria to mammals)
Cytoplasmic; some peroxisomal (liver, kidney)
Active forms: dimer or tetramer of a 59-kDa subunit
Human AA sequence known; K205 critical for electron transfer; NADP structural molecule confirmed
Michaelis constants: $K<em>m^{NADP}$ ~10× < $K</em>m^{G6P}$
Regulation
- Instant: substrate/product levels (↑NADP & ↓NADPH activate)
- miR-1 down-regulates synthesis in nucleated cells

Gene symbol Gd at Xq28 → X-linked
Consequences: typical sex pattern; female mosaicism via X-inactivation
About 13 exons; giant intron 2 (~12 kb). GC-rich promoter, Sp1 sites essential
1966 WHO Classification
- Class I – severe (CNSHA) <10 % activity
- Class II/III – mild, asymptomatic baseline, risk AHA/NNJ
- Class IV – normal; Class V – ↑ activity (rare)
Mutation spectrum (186 known)
- 159 missense singles; 13 double; 2 triple; 10 in-frame deletions; 2 splicing
- Null alleles absent → embryonic lethality
- Polymorphic variants (≥1 %) selected by malaria (e.g. A−, Med, Mahidol)
- Severe Class I cluster in exons 10-11 (dimer interface)

Triggered by oxidant stress: fava beans (favism), infections, drugs (Table 18-3)
Presentation: dark "Coca-Cola" urine (hemoglobinuria), jaundice, anemia, splenomegaly, back/abdo pain
Drugs: primaquine, dapsone (7.5 mg/kg caused ↓Hb ~2 g/dL in 119 African kids)
Labs: normocytic anemia (Hb as low as 2.5 g/dL); ↑retics (≤30 %), bite cells, Heinz bodies (supravital), ↓haptoglobin, ↑bilirubin (unconj)
Pathogenesis chain: $\text{NADPH↓} \to \text{GSH↓} \to \text{\,Hb–SH oxidised}\to \text{Heinz bodies}\to \text{membrane damage/hemolysis}$
Older RBCs hemolyze first → post-crisis blood enriched in young higher-G6PD cells
Treatment guidelines
- Hb <7 g/dL → transfuse
- Hb 7–9 w/ hemoglobinuria → transfuse
- Monitor 48 h; renal failure rare in children

Peaks days 2-3; anemia mild/absent
Multifactorial: low UGT1A activity (Gilbert allele ↑risk), vit E low, ascorbate level etc.
Exacerbating factors: prematurity, breastfeeding, infection, naphthalene, favism in utero
Management per AAP: G6PD assay in jaundiced infant needing phototherapy; lower exchange threshold (15 mg/dL in first 48 h)

Caused by Class I mutations; chronic hemolysis + NNJ; variable severity, sometimes transfusion dependent
Pathology: membrane spectrin aggregates, shear fragility
Splenectomy may convert transfusion-dependent to independent
Gene therapy proof-of-principle in mice & rhesus

Quantitative spectrophotometric assay at 340 nm (IU/g Hb); leukocyte removal important
Screening (qualitative): Fluorescent spot; formazan spot; risk of false-normal in reticulocytosis/post-crisis
Heterozygotes: cytochemical metHb-reduction flow test; extreme lyonization may need DNA assay

Class I: severe instability/ catalytic defects → CNSHA (never polymorphic)
Class II/III polymorphic due to malaria advantage; clinical expression similar regardless of variant (Med, A−, Mahidol etc.)

Newborn screening (Sardinia, Malaysia, US proposals)
Education & avoidance list (favism prevention example in Sassari: admissions fell >80 %)
Drug safety vigilance (dapsone–chlorproguanil withdrawn)

Neutrophil dysfunction in class I variants → infection susceptibility; trauma patients G6PD- deficiency had ↑sepsis
Possible links to diabetes, cataract, pterygium, cardiovascular oxidative stress being studied
Coinheritance with SCA, thalassemia, enzyme defects may modulate phenotype (e.g., sickle + G6PD)

Epidemiologic & clinical data show G6PD deficiency (heterozygous females, some males) confers protection vs severe Plasmodium falciparum (and vivax)
Convergent evolution: multiple independent G6PD alleles risen to high frequency in endemic areas

NADPH from PPP essential for RBC antioxidative machinery
X-linked mosaicism explains variable female phenotypes, esp. extreme skewing
Hemolysis mechanism: oxidative stress → Heinz bodies → extravascular ± intravascular
Timing of NNJ vs β-mutation symptoms reflects globin switching
Screening pitfalls: post-hemolytic reticulocytosis may mask deficiency; always repeat later
Favism totally preventable with education + newborn screening; example of public health genetics
Absence of nonsense or frameshift mutations: housekeeping gene lethality concept
Classify mutations by enzyme activity & clinical syndrome (CNSHA vs AHA risk)
Pathogenic triggers & safe/unsafe drug list (memorize primaquine, dapsone definite)