Submission Details

Mutations in G6PD have long been known as the cause of X linked G6PD deficiency. In most cases, G6PD deficiency, the most common enzymopathy, is better described as a genetic trait rather than a disease, with an estimated over 500 million affected individuals worldwide. These individuals, individuals without a chronic affected state, only present with acute hemolytic anemia upon exposure to an exogenous oxidative stressor, such as fava beans. In contrast, there exists a subset of very rare mutations that severely disrupt the enzyme function sufficient to cause chronic nonspherocytic hemolytic anemia, which likewise can cause acute hemolytic anemia after exposure to exogenous oxidative stressors. Patients with chronic nonspherocytic hemolytic anemia (CNSHA), also referred to as class 1 variants, may require blood transfusions and often present with severe neonatal jaundice.

Importantly, a decision to split chronic nonspherocytic anemia due to G6PD deficiency (class 1) from G6PD deficiency without chronic nonspherocytic hemolytic anemia was made by the general IEM GCEP and G6PD VCEP on 4/14/2023. This decision was based upon the specific phenotypic differences observed in patients with CNSHA from other G6PD patients. Additionally, the decision was made off observable differences in the underlying enzymatic function, whereby individuals with CNSHA typically have 1% to undetectable (but existing) enzymatic function that causes an insufficiency in NADPH causing hemolysis even in the presence of endogenous stressors. This curation will only classify the gene-disease relationship between G6PD and G6PD deficiency without CNSHA.

The underlying disease mechanism is loss of enzymatic function. In this curation, 8 variants were scored from 7 publications (PMIDs: 8081374, 22906047, 10782016, 17653668, 30279493, 16607506, 36145477), to reach a maximum score of genetic evidence, though over 100 variants have been associated with G6PD without CNSHA (PMID:32702756). All variants are missense variants. Complete loss of function is embryonic lethal. Evidence supporting this gene-disease relationship includes case-level data and experimental data. Experimentally, this gene-disease relationship is supported by the biochemical function of G6PD, which is a rate limiting step in the pentose phosphate pathway. When G6PD enzymatic activity is significantly diminished, insufficient NADPH is produced. NADPH is crucial to reducing oxidative stressors by mediating several cellular oxidative defense lines. In the absence of NADPH, red blood cells are particularly sensitive to oxidative stressors, resulting in phenotypes such acute hemolytic anemia upon exposure to an exogenous oxidative stressor such as fava beans (PMID:32702756). Further evidence comes from a knock out HeLa cell model, where transfection of G6PD WT rescues phenotypes observed in the KO cells such as, loss of G6PD enzymatic activity, increased sensitization to oxidative stressors including hydrogen peroxide, and decreased NADPH (PMID:36243112). Finally, the gene-disease relationship is supported by a G6PD deficient mouse model. Severely deficient mice are embryonic lethal, while mice with diminished G6PD deficiency recapitulated phenotypes such as significantly decreased NADPH levels, significantly decreased G6PD enzymatic activity, increased oxidative stress, and decreases in glutathione (PMIDs:19805580).

In summary G6PD is definitively associated with X linked G6PD deficiency without CNSHA. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time. This classification was approved by the ClinGen General Inborn Errors of Metabolism on the meeting date 6/23/2023 (SOP Version 9).