Background and objectives Many genome-wide association studies have uncovered over 30 common sequence variants influencing serum uric acid concentrations and/or gout in several genes, mostly in SLC2A9, SLC22A12 and ABCG2. In the present study, we describe the clinical, biochemical, molecular genetic and functional analysis of a phenotype caused by variants in the ABCG2 gene, a physiologically important urate transporter whose dysfunction plays a major role in pathogenesis of gout, in a patient with severe primary hyperuricemia and gout.
Materials and methods 47-year old male started to have gouty attacks at the age of 29. His family history was unremarkable, and no close relative was known to have gout or hyperuricemia. When first seen in our clinic, he already suffered from chronic debilitating polyarthritis with widespread bulky tophi located mainly on his hands, feet, and around elbows and knees, with intermittent discharge of a chalky substance. Sodium urate was microscopically verified from the synovial fluid aspirated from his swollen knee. Without antihyperuricemic therapy, his serum uric acid was 595 μmol/l.
Detailed metabolic investigations were performed and purine metabolic disorders associated with hyperuricemia were excluded. Hyperuricemia led us to suspect a defect in renal urate transport and thus a mutational analysis of the SLC22A12, SLC2A9 and ABCG2 gene was performed. To examine the effect of the found variant on the expression and function of the ABCG2 protein we performed the following in vitro experiments.
Results An unpublished one nucleotide heterozygous intron variant c.689+1G >A was found in ABCG2. Two abnormal splicing variants were identified: a) r.[532_689del] deletion of exon 6; b) r.[532_689del], r.[944_949del] deletion of exon 6 and deletion of first six bp of exon 9. Prediction for these variants showed extra stop codon resulting from the frame shift.
ABCG2 native protein was estimated to be about 70 kDa, whereas both variants were to be about 18 kDa. The protein expression levels of variants were lower than of native protein, suggesting the instability of the mutated protein. Immunocytochemical studies in HEK293A cells showed a strong signal for ABCG2 at the plasma membrane. Both variant proteins were detected only within the intracellular compartment. Finally, both variant proteins had no transport activity for urate.
Conclusions Our results strongly suggest a relationship between the novel dysfunctional mutation of the ABCG2 gene and a severe gout phenotype. The analysis of ABCG2 in cohort of 50 gouty patients is in process. In clinical practice, primary hyperuricemia/gout is an indication for investigation of purine metabolism and urate transporters.
This study was supported by the grants from the Czech Republic Ministry of Health: the project for conceptual development of Research organisation 023728 Institute of Rheumatology and by institutional support provided by the program at Charles University in Prague PRVOUK-P24/LF1/3.
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