## 4 Discussion and Conclusions  
Urate transport in the kidney is a complex process involving several transmembrane proteins that provide reabsorption (URAT1, GLUT9) and secretion (ABCG2). In genome-wide association studies, the _SLC2A9_ gene is a well-established locus that is significantly associated with hyperuricemia while the _ABCG2_ locus had the most significant association with gout susceptibility [15; 16; 17]. The dysfunctional variants in URAT1 and GLUT9 cause hereditary renal hypouricemia, and genetic analysis is needed to confirm the diagnosis and/or to identify the specific type of renal hypouricemia. The _SLC22A12_ gene is located on chromosome 11q13. Ten exons encode two transcript variants of the URAT1 transporter (332 and 553 amino acids), which are specifically expressed on the epithelial cells of the proximal tubules in the renal cortex [4]. At present, 52 variations in the _SLC22A12_ coding region (40 missense/nonsense, two splicing, three regulatory, three small deletions, two small insertions, one gross deletion, and one complex rearrangement) have been described (HGMD Professional 2018.4, [[http://www.hgmd.cf.ac.uk](http://www.hgmd.cf.ac.uk)]([http://www.hgmd.cf.ac.uk](http://www.hgmd.cf.ac.uk))). Thirty-six URAT1 variants are currently associated with the hypouricemia phenotype. Functional analysis confirmed in part of these variants impact on urate uptake ability and/or cytoplasmatic expression and localization [7; 11; 13; 14]. However, not all URAT1 allelic variants have effect on decreasing of protein expression and/or function (p.R228E, R477H) [7; 11]. The analysis of _SLC2A9_ coding regions in our proband revealed three common non-synonymous variants: heterozygous rs2276961 (p.G25R, Caucasian MAF = 0.53), rs16890979 (p.V282I, Caucasian MAF = 0.21), and homozygous rs2280205 (p.P350L, Caucasian MAF = 0.48). These variants have not been previously reported in association with hypouricemia. Variant p.V282I was previously described relative to the hyperuricemia and gout phenotype [18]. Moreover, in our previous study, which used association analysis together with functional and immunohistochemical characterization of these variants identified in the adult population, we did not find any influence of these allelic variants on expression, subcellular localization, or urate uptake of GLUT9 transporters [19]. Our analysis of _SLC22A12_ coding regions revealed two rare heterozygous non-synonymous variants: rs150255373 (p.R325W, Caucasian MAF = 0.001) and rs145200251 (p.R434C, Caucasian MAF unknown). Variant p.R434C was previously associated with renal hypouricemia 1 in a five-year-old Macedonian girl suffering from distal renal tubular acidosis and renal hypouricemia [14]. The patient had symptoms of dehydration, polyuria, and vomiting. The patient also had rickets and slow growth. There was evidence of hyperlicromic metabolic acidosis (pH 7.23, HCO\({}_{3}\) 13.6 mmol/L, BE = 12.6 mmol/L), hypokalemia (3.0 mmol/L), hypophosphatemia (0.84 mmol/L), hypouricemia (73 umol/L), and hyperuricosuria (EF-UA 24-31%). Bilateral nephracellosis and a solitary cyst in the left kidney were discovered during an ultrasound examination. The patient was given alkali therapy; metabolic compensation was achieved, serum electrolytes normalized, and low molecular proteinuria resolved. Only the hypouricemia parameter persisted during the two-year observational period. The mother of this patient was a heterozygote for the same missense variant (S-UA 136 umol/L, EF-UA 19%) and a history of renal colic and the passage of a single renal calculus. Functional studies of p.R434 Cs were previously performed using transiently transfected HEK293 cells [14]. Plasma membrane expression levels of the p.R434C variant were low, intracellular localization was not strongly observed, and urate uptake showed a significant reduction of urate transport function (P \(<\) 0.001). The structural model for URAT1 is mainly focused on the organization and alignment of residues within 12 transmembrane spanning domains.