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Acute intermittent porphyria: the in vitro expression of mutant hydroxymethylbilane synthase
P M Ong1, W G Lanyon, G Graham
1Duncan Guthrie Institute of Medical Genetics, Glasgow, Yorkhill, UK.
Abstract:
Acute intermittent porphyria (AIP) is an inborn error of haem biosynthesis caused by a variety of mutations in the gene coding for hydroxymethylbilane synthase (HMB-S). The entire coding sequence of this gene, from each of three South African AIP patients, was therefore screened for mutations using chemical cleavage mismatch (CCM) analysis and any changes detected characterized by DNA sequencing. Three single base changes were identified; a G77 to A in exon 3, a C346 to T in exon 8 and a G518 to A in exon 10. These missense mutations, previously reported to be present in other populations, are known to be responsible for the structurally deleterious amino acid replacements R26H, R116W and R173Q, respectively. The in vitro expression of the enzymes containing these mutations and the subsequent measurement of their specific activities revealed a reduction to approximately 4% of normal activity.
Insights
Three mutations in the hydroxymethylbilane synthase (HMB-S) gene cause acute intermittent porphyria (AIP). These genetic defects significantly reduce enzyme activity, impacting haem biosynthesis.
Area of Science:
- Biochemistry
- Genetics
- Molecular Biology
Background:
- Acute intermittent porphyria (AIP) is an inherited metabolic disorder affecting haem biosynthesis.
- Mutations in the hydroxymethylbilane synthase (HMB-S) gene are a known cause of AIP.
Purpose of the Study:
- To identify and characterize mutations in the HMB-S gene in South African AIP patients.
- To assess the functional impact of identified mutations on HMB-S enzyme activity.
Main Methods:
- Screening of the HMB-S coding sequence using chemical cleavage mismatch (CCM) analysis.
- DNA sequencing to characterize detected genetic variations.
- In vitro expression of mutant HMB-S enzymes and measurement of their specific activities.
Main Results:
- Three distinct single base changes were identified: G77A in exon 3, C346T in exon 8, and G518A in exon 10.
- These mutations correspond to previously reported missense changes (R26H, R116W, R173Q) known to be deleterious.
- In vitro analysis showed a significant reduction in HMB-S enzyme activity to approximately 4% of normal levels for all identified mutations.
Conclusions:
- The identified mutations in the HMB-S gene are causative for AIP in the studied South African cohort.
- These mutations lead to structurally detrimental amino acid substitutions and severely impaired enzyme function.
- The findings contribute to understanding the genetic basis and molecular pathology of AIP in diverse populations.