Glucose-6-Phosphate Dehydrogenase Deficiency and Neonatal Hyperbilirubinemia: Insights on Pathophysiology, Diagnosis, and Gene Variants in Disease Heterogeneity

Abstract

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a prevalent condition worldwide and is caused by loss-of-function mutations in the G6PD gene. Individuals with deficiency are more susceptible to oxidative stress which leads to the classical, acute hemolytic anemia (favism). However, G6PD deficiency in newborn infants presents with an increased risk of hyperbilirubinemia, that may rapidly escalate to result in bilirubin induced neurologic dysfunction (BIND). Often with no overt signs of hemolysis, G6PD deficiency in the neonatal period appears to be different in the pathophysiology from favism. This review discusses and compares the mechanistic pathways involved in these two clinical presentations of this enzyme disorder. In contrast to the membrane disruption of red blood cells and Heinz bodies formation in favism, G6PD deficiency causing jaundice is perhaps attributed to the disruption of oxidant-antioxidant balance, impaired recycling of peroxiredoxin 2, thus affecting bilirubin clearance. Screening for G6PD deficiency and close monitoring of affected infants are important aspects in neonatal care to prevent kernicterus, a permanent and devastating neurological damage. WHO recommends screening for G6PD activity of all infants in countries with high prevalence of this deficiency. The traditional fluorescent spot test as a screening tool, although low in cost, misses a significant proportion of cases with moderate deficiency or the partially deficient, heterozygote females. Some newer and emerging laboratory tests and diagnostic methods will be discussed while developments in genomics and proteomics contribute to increasing studies that spatially profile genetic mutations within the protein structure that could predict their functional and structural effects. In this review, several known variants of G6PD are highlighted based on the location of the mutation and amino acid replacement. These could provide insights on why some variants may cause a higher degree of phenotypic severity compared to others. Further studies are needed to elucidate the predisposition of some variants toward certain clinical manifestations, particularly neonatal hyperbilirubinemia, and how some variants increase in severity when co-inherited with other blood- or bilirubin-related genetic disorders.

Keywords: G6PD; Heinz bodies; bilirubin; favism; hemolytic anemia; molecular screening; neonatal jaundice; peroxiredoxin 2.

FIGURE 1

The three RBC antioxidant systems that are compromised in G6PD deficiency. The more familiar glutathione peroxidase/glutathione reductase (GPx/GR) and antioxidant mechanisms are shown in gray (middle) and the more recently recognized peroxiredoxin 2 system in light blue (top). Prdx2 is a thiol protein that is oxidized to an interchain disulfide then recycled predominantly by thioredoxin (Trx) and thioredoxin reductase [TrxR; (102)]. Prdx2, the third most abundant RBC protein, is present at a much higher concentration than glutathione peroxidase or catalase. It is highly reactive with peroxides and is favored to consume most of the intracellular H₂O₂ (17). NADPH is required as a reducing substrate for both GR and TrxR, and it protects catalase against inactivation. It is produced via the pentose phosphate pathway of which the first step is catalyzed by G6PD. By restricting the supply of NADPH, G6PD deficiency compromises the ability of all three antioxidant systems to detoxify hydrogen peroxide. O₂^–, superoxide; H₂O₂, hydrogen peroxide; GSH, reduced glutathione; GSSG, glutathione disulfide; SOD, superoxide dismutase. The “red” and “ox” subscripts refer, respectively, to the reduced and oxidized forms of the proteins.

FIGURE 2

A crystal violet supravital staining of a peripheral blood film, viewed at 40x magnification. Note the absence of Heinz bodies in this blood film from a G6PD-deficient neonate with hyperbilirubinemia.

FIGURE 3

Quantitative Polymerase Chain Reaction (qPCR) for G6PD variant analysis in a newborn infant with early onset jaundice. (A) This assay confirmed that the female infant with severe hyperbilirubinemia at day 3 of life but with G6PD enzyme activity reported within the normal reference range, was heterozygous for G6PD Mahidol (c.487G > A). DNA sequencing result is shown in (B).