Our prospective study included 118 patients with urinary stone and 42 healthy individuals. Substantial evidence indicated that patients with urinary stone exhibited increased uCA-2 level was found in urine by ELISA. The key finding in this study is that an significant difference in uCA-2 excretion by ELISA analysis could discriminate patients with urinary stones from healthy controls. If our results are supported by many prospective validation studies, such a test might be clinically beneficial in the surveillance of high-risk patients, such as patients with metabolic syndrome1-4, inflammatory bowel disease18 and accepted bariatric surgery19 et al. High risk of stone recurrence patients may also reap the benefits of our results.
The small, hard mineral deposits that characterize urinary stone can cause severe pain, hematuria, obstruction or (and) hydronephrosis. The diagnosis of urinary stone depends on imaging examination and lacks effective biomarker, which means it is difficult to predict the risk recurrence of urinary stone. The AUA guideline has proposed that millions of patients newly diagnosed with urinary stone could be advised to a screening evaluation consisting to reduce the risk of stone recurrence 20. To achieve this goal, more advanced diagnostic approaches have to be developed and applied to earlier detection of urinary stone. The available clinical data today support the conclusion that leads to a decreased risk of stones recurrence of asymptomatic patients, because of an early dietary or drug intervention21,22.
To improve understanding on the significance of the excessive production of uCA-2 in patients with urinary stone, we evaluated data from our prospective database on patients with urinary stone and assigned uCA-2 level to patients with or without stone-related complications according to appropriate symptoms and signs. Our data showed that uCA-2 were significantly increased excretion in patients with urinary stone develop one or more complications. All 3 complications showed a high level of concordance and pain complication demonstrated superiority over the others with regard to predicting uCA-2 excretion.
To trace the phenomenon of higher uCA-2 level in patients with urinary stone develop one or more complications, our goal in this study was to describe the unclear effect of stone formation. The results of uCA-2 excretion and stone complications interactions had emphasized potential factors to pathophysiological mechanism, such as crystal-membrane interaction, microbial-stone interaction, or other possible interactions. The data show that the non-significantly higher uKIM-1 level excretion was found in hydronephrosis complications among the other two groups. Our results are seemingly at variance with a previous study evaluated that uKIM-1 as a biomarkers of acute renal injury was significantly higher in stone patients with hydronephrosis compared to without hydronephrosis23. The difference results may be explained by the variation of participants inclusion criteria.
The excretion of uCA-2 by the bacteria in patients with urinary stone is also a potentially important factor. Twenty bacterial genera have significant differences in relative abundance between urinary stone patients and healthy subjects. These findings may provide new and non-invasive potential biomarkers for the diagnosis of kidney stone24. Escherichia coli is one of most commonly detected pathogenic microorganisms attached to renal stone25, which also actively secrete PH-dependent CA-226. Both CA and urease in bacteria play a synergic role in promoting CaCO3 precipitation27. Microbial urease is a Ni-containing enzyme found in various microorganisms that hydrolyzes urea to NH4+ and CO2, whereas carbonic anhydrase transforms CO2 into HCO3−. In Bacillus megaterium, calcite precipitation is driven by the coupled activity of both urease and carbonic anhydrase enzymes. The precipitation of CaCO3, which predominantly consists of calcite crystals, by microbial carbonic anhydrase has been documented28. CA-2 may decrease the stone inhibitory protein through its potential effect on the g-glutamyl carboxylase reaction by providing CO2. CA-2 may also supply CO2 to g-glutamyl carboxylase that catalyzes glutamate residue carboxylation in some proteins, thereby enhancing their biological activity. Several proteins that require this post-translational carboxylation to become biologically active may be involved in the calcification processes in human tissues, including matrix Gla protein, growth arrest-specific protein 6, Gla-rich protein, and osteocalcin29. The relationship between CA-2 and other stone-promoting or -inhibiting factor will be the focus of our study in the future.
Although we have a larger number of samples, the overall sample size is still small. The study did not follow up healthy participants with a relatively increased uCA-2 excretion. Stone formation is a dynamic process that cannot easily be captured by a midstream urine collection. Finally, the relationship among uCA-2, Ca, oxalic acid and P excretion is difficult to evaluate in urine because the 24 h urine biochemical analysis was not measured.