HMBS gene mutations and hydroxymethylbilane synthase activity in acute intermittent porphyria: A systematic review

BACKGROUND: Acute intermittent porphyria (AIP) is caused by a partial deficiency of hydroxymethylbilane synthase and affects heme biosynthesis. Mutations in the HMBS gene result in HMBS deficiency. AIP is a rare disease, and there been insufficient studies on it. This report describes the molecular epidemiology of HMBS gene defects and hydroxymethylbilane synthase activity levels in classical AIP. METHODS: Databases of PubMed, CNKI, and Wang Fang Database were searched for eligible studies to investigate HMBS gene mutations in peripheral blood samples and HMBS activity in erythrocytes of patients with classical AIP. Relevant studies published up to July 15, 2023, from several databases were independently searched and selected by 2 reviewers. Accuracy data and relevant information were extracted from each eligible study by 2 independent researchers and analyzed using statistical software. RESULTS: After pooling the accuracy data from 232 patients of the 15 eligible studies, 90.5% (210/232) of AIP patients had decreased erythrocyte hydroxymethylbilane synthase activity (<70%), and 96 different mutations were identified in 232 patients, including 33 missense (34.4%), 27 splice (28.1%), 19 deletion (19.8%), 8 nonsense (8.3%), 9 insertion (9.4%) mutations. Residual enzyme activities (%) for different groups of type were expressed using mean and 95% confidence interval (95% CI): missense (51.2, 48.5–53.9), splice (57.5, 52.0–59.1), deletion (54.9, 50.7–59.1), nonsense (52.2, 44.4–60.0), insertion (53.2, 47.4–59.0), group analysis P = .17. Subgroups of missense mutations, domain 1 (50.2, 46.0–54.4), domain 2 (52.8, 49.1–56.4), and domain 3 (49.2, 38.3–60.0), Subgroup analysis, P = .62. CONCLUSION: Different mutation types and mutation positions are not associated with the level of hydroxymethylbilane synthase activity. Erythrocyte hydroxymethylbilane synthase activity is often reduced to half of normal in patients with AIP, and the enzyme activity assay has a high diagnostic value in AIP. AIP is highly molecularly heterogeneous, with missense mutations being the most common, followed by splice mutations. R173W and G111R are high-frequency mutations and have been found in multiple families from different countries.