AGXT Gene Mutations and Their Influence on Clinical Heterogeneity of Type 1 Primary Hyperoxaluria

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder that is caused by a deficiency of alanine: glyoxylate aminotransferase (AGT), which is encoded by a single copy gene (AGXT). Molecular diagnosis was used in conjunction with clinical, biochemical, and enzymological data to evaluate genotype-phenotype correlation. Twenty-three unrelated, Italian PH1 patients were studied, 20 of which were grouped according to severe form of PH1 (group A), adult form (group B), and mild to moderate decrease in renal function (group C). All 23 patients were analyzed by using the single-strand conformation polymorphism technique followed by the sequencing of the 11 AGXT exons. Relevant chemistries, including plasma, urine and dialyzate oxalate and glycolate assays, liver AGT activity, and pyridoxine responsiveness, were performed. Both mutant alleles were found in 21 out of 23 patients, and 13 different mutations were recognized in exons 1, 2, 4, and 10. Normalized AGT activity was lower in the severe form than in the adult form (P < 0.05). Double heterozygous patients presented a lower age at the onset of the disease (P = 0.025), and they were more frequent in group A (75%) than in the group B (14%; P = 0.0406). The T444C mutation was more frequent in the severe form (P < 0.05), and the opposite was observed for G630A (P < 0.05). G630A mutation homozygotes had a higher AGT residual activity (P = 0.00001). This study confirms the allelic heterogeneity of the AGXT, which could to some extent be responsible for the phenotypic heterogeneity in PH1.