Background/Purpose: ABCG2 is a high-capacity urate transporter gene. Common dysfunctional variants of ABCG2 that result in decreased urate excretion in humans are major causes of hyperuricemia and gout, especially in pediatric-onset patients [1]. In the present study, we identified and functionally characterized a novel ABCG2 variants in a family with three generations of early-onset hyperuricemia and gout.

Methods: A 12-year-old girl was examined for chronic asymptomatic hyperuricemia (397–405 μmol/L) with decreased fractional excretion of uric acid (2.2–3.5%).  From positive family history of early hyperuricemia and gout < 35 year (her mother, maternal uncle and maternal grandfather), we performed a metabolic investigation. The results suggested that their hyperuricemia were mainly due to a defect in the urate excretion system and did not result from an excess production of urate. Since this type of hyperuricemia is reportedly related to ABCG2 dysfunction, we analyzed ABCG2 coding regions of our patients using their genomic DNA. We carried out a series of biochemical analyses for the functional validation of the identified ABCG2 variants using ABCG2-expressing plasma membrane vesicles as we report previously [2]. This study was approved by the Ethics Committee of the Institute of Rheumatology in Prague (no. 6181/2015).

Results: We identified two novel heterozygous variants: c.393G >T (p.M131I) and c.706C >T (p.R236X) in ABCG2 gene in the proband. The segregation analysis showed existence of those variants in her mother, maternal brother and maternal grandfather.  Immunoblotting for N-glycosidase treated whole cell lysates of 293A cells transiently-expressing each ABCG2 variant demonstrated that p.M131I had minimal effect on the protein level and N-linked glycosylation status, while the p.R236X variant was detected as truncated forms with weaker band intensity compared with that of ABCG2 wild-type.  Confocal microscopy showed that similar to ABCG2 wild-type, p.M131I variant localized on the plasma membrane; the p.R236X variant exhibited little plasma membrane localization. Functional assay revealed that contrary to the wild-type, p.M131 had limited ATP-dependent urate transport activity; ABCG2-mediated urate transport activity of this variant was calculated as 14 ± 2% of wild-type control. Moreover, p.R236X variant was functionally null.  

Conclusion: We found a representative case of paediatric hyperuricemia with familial gout that harboured two dysfunctional variants of a physiologically important urate transporter ABCG2. Biochemical analyses revealed that ABCG2 p.M131I and p.R236X were functionally deficient and null, respectively. Our identification of novel dysfunctional ABCG2 variants confirmed a key role of ABCG2 transporter and its link with early-onset hyperuricemia and gout.

References

1. Stiburkova B, et al. Arthritis Res Ther. 2019 Mar 20;21(1):77.
2. Toyoda Y, et al. Cells. 2019 Apr 18;8(4). pii: E363.

Supported by the grants from the Czech Republic Ministry of Health RVO 00023728 (Institute of Rheumatology) and by the JSPS KAKENHI Grant Numbers 18KK0247 (to T.T.).

ACR Meeting Abstracts - https://acrabstracts.org/abstract/identification-of-two-novel-dysfunctional-variants-in-a-physiologically-important-urate-transporter-abcg2-in-paediatric-onset-familial-hyperuricemia-and-gout-patients-in-three-generations/