Primary hyperoxaluria type III gene HOGA1 (formerly DHDPSL) as a possible risk factor for idiopathic calcium oxalate urolithiasis

Abstract

Background and objectives: Primary hyperoxaluria types I and II (PHI and PHII) are rare monogenic causes of hyperoxaluria and calcium oxalate urolithiasis. Recently, we described type III, due to mutations in HOGA1 (formerly DHDPSL), hypothesized to cause a gain of mitochondrial 4-hydroxy-2-oxoglutarate aldolase activity, resulting in excess oxalate.

Design, setting, participants, & measurements: To further explore the pathophysiology of HOGA1, we screened additional non-PHI-PHII patients and performed reverse transcription PCR analysis. Postulating that HOGA1 may influence urine oxalate, we also screened 100 idiopathic calcium oxalate stone formers.

Results: Of 28 unrelated hyperoxaluric patients with marked hyperoxaluria not due to PHI, PHII, or any identifiable secondary cause, we identified 10 (36%) with two HOGA1 mutations (four novel, including a nonsense variant). Reverse transcription PCR of the stop codon and two common mutations showed stable expression. From the new and our previously described PHIII cohort, 25 patients were identified for study. Urine oxalate was lower and urine calcium and uric acid were higher when compared with PHI and PHII. After 7.2 years median follow-up, mean eGFR was 116 ml/min per 1.73 m(2). HOGA1 heterozygosity was found in two patients with mild hyperoxaluria and in three of 100 idiopathic calcium oxalate stone formers. No HOGA1 variants were detected in 166 controls.

Conclusions: These findings, in the context of autosomal recessive inheritance for PHIII, support a loss-of-function mechanism for HOGA1, with potential for a dominant-negative effect. Detection of HOGA1 variants in idiopathic calcium oxalate urolithiasis also suggests HOGA1 may be a predisposing factor for this condition.

Figure 1.

Reverse transcription (RT)–PCR results for the IVS700 + 5 G>T, c.944_946 del AGG (p.Glu315del), and c.763 C>T (p.R255X) HOGA1 mutations. (A) Gel electrophoresis. Analysis by RT-PCR of wild-type and mutant cDNA from patient 7 (heterozygous for the IVS700 + 5G>T variant) is shown. The presence of a larger product derived from the insertion of 51 bp (412 bp in lane 1 versus 361 bp in the normal control in lane 2) is apparent. (B) Sequencing results for the mutant cDNA (excised band) in patient 7. The G>T change in the + 5 position (3′ circle) weakens the wild-type donor site (5′ circle), allowing a downstream GT cryptic donor site at position + 52 to become active and leading to the in-frame insertion of 51 nucleotides (17 codons), including the actual wild-type donor site. (C) Sequencing results. Analysis by RT-PCR of mutation c.763 C>T (p.R255X) in patient 8 is shown. The mutant T nucleotide that generates a termination codon is as equally represented as the wild-type C nucleotide. This suggests that the truncating, mutant allele is stably expressed and is not subject to nonsense-mediated decay. (D) Sequencing results for the mutant cDNA generated from patient 3 (homozygous for the c.944_946 del AGG variant) demonstrating in-frame deletion of AGG (p.Glu315del). Mutant cDNAs were derived from Epstein Barr Virus-transformed lymphoblast cell lines in all three patients.

Figure 2.

Box plots of PH type versus urine oxalate (mmol/1.73 m² per 24 h [A]), urine calcium (mg/kg/24 h [B]), and urine uric acid (mg/1.73 m² per 24 h [C]). The box plots display the 25^th and 75^th percentiles (bottoms and tops of boxes), the median (inside lines), the means (+), and the minimum and maximum values (bottoms and tops of vertical lines).