Supplementary Material for: Serum Calciprotein Monomers and Chronic Kidney Disease Progression

<b><i>Introduction:</i></b> Elevated levels of fibroblast growth factor-23 (FGF23) in chronic kidney disease (CKD) are associated with progression of CKD. FGF23 inhibits proximal tubular phosphate reabsorption, raising phosphate concentrations in the tubular fluid of functioning nephrons, predisposing to spontaneous precipitation of calcium phosphate crystals and resultant tubular injury. Calciprotein monomers (CPM) form spontaneously in biological fluids when clusters of calcium phosphate ions are bound by the liver-derived glycoprotein fetuin-A. Serum CPM are elevated in CKD and are postulated to trigger FGF23 secretion. CPM are also readily filtered at the glomerulus into the tubular fluid, suggesting that higher CPM levels could be associated with progression of CKD via FGF23-mediated increased phosphate load but also through direct effects in the proximal tubule. <b><i>Methods:</i></b> ACADEMIC was a prospective observational study of 200 stable outpatients with CKD stages 3 and 4. Participants were followed until commencement of dialysis or death. In this study, we examined a sub-cohort of 189 participants who had baseline serum available for measurement of CPM. Cox proportionate hazard regression models were used to examine the association between CPM and a composite kidney disease outcome (commencement of dialysis or reduction in estimated glomerular filtration rate [eGFR] &gt;30%). Linear regression models were used to examine the association between CPM and annualized eGFR slope. <b><i>Results:</i></b> Relative to the lowest tertile, the highest tertile of CPM was associated with increased risk of the composite kidney disease outcome in univariate models and after sequential adjustment for conventional risk factors for progression of CKD (adjusted hazard ratio 4.22; 95% confidence interval [CI] 1.91, 9.33, <i>p</i> &lt; 0.001). Natural log-transformed CPM was also inversely associated with eGFR slope in univariate and multivariate adjusted models (adjusted β-coefficient −1.66, 95% CI: −3.10, −0.22, <i>p</i> = 0.024). In exploratory mediation analysis, the association between serum CPM and eGFR slope was partially mediated by iFGF23; however, the majority of the association was direct and independent of the iFGF23 pathway. <b><i>Conclusion:</i></b> Elevated levels of CPM are associated with the progression of CKD. This association was partially mediated via FGF23, consistent with recent evidence that FGF23 predisposes to spontaneous precipitation of calcium phosphate crystals leading to tubular injury. However, serum CPM also appeared to have a direct association with eGFR slope, raising the possibility that CPM may also be associated with progression of CKD through additional pathways.

<b><i>引言：</i></b> 慢性肾脏病（chronic kidney disease, CKD）患者体内成纤维细胞生长因子23（fibroblast growth factor-23, FGF23）水平升高与CKD进展相关。FGF23可抑制近端肾小管磷酸盐重吸收，升高功能性肾单位小管液中的磷酸盐浓度，易诱发磷酸钙晶体自发析出并引发肾小管损伤。钙蛋白单体（calciprotein monomers, CPM）是当磷酸钙离子簇被肝脏来源的糖蛋白胎球蛋白-A（fetuin-A）结合时，在生物体液中自发形成的复合物。CKD患者血清CPM水平升高，且被推测可触发FGF23分泌。CPM亦可经肾小球滤过进入小管液，这提示较高的CPM水平可能通过FGF23介导的磷酸盐负荷增加，以及对近端肾小管的直接作用，与CKD进展相关。
<b><i>研究方法：</i></b> ACADEMIC是一项针对200名CKD 3~4期稳定门诊患者的前瞻性观察性研究，受试者被随访至开始透析或死亡。本研究纳入其中189名具备基线血清样本以用于CPM检测的亚队列进行分析。采用Cox比例风险回归模型检验CPM与复合肾脏结局（开始透析或估算肾小球滤过率[estimated glomerular filtration rate, eGFR]下降＞30%）之间的关联；采用线性回归模型检验CPM与年化eGFR斜率之间的关联。
<b><i>研究结果：</i></b> 与最低三分位组相比，CPM最高三分位组在单变量模型及依次校正CKD进展传统危险因素后，复合肾脏结局的风险均显著升高（校正后风险比4.22；95%置信区间[confidence interval, CI] 1.91~9.33，<i>p</i>＜0.001）。经自然对数转换的CPM在单变量及多变量校正模型中也与eGFR斜率呈负相关（校正后β系数为-1.66，95%CI：-3.10~-0.22，<i>p</i>=0.024）。探索性中介分析显示，血清CPM与eGFR斜率之间的关联部分由全段FGF23（intact FGF23, iFGF23）介导，但大部分关联是直接且独立于iFGF23通路的。
<b><i>结论：</i></b> 血清CPM水平升高与CKD进展相关。该关联部分由FGF23介导，与近期研究证据一致——FGF23易导致磷酸钙晶体自发析出并引发肾小管损伤。但血清CPM似乎也与eGFR斜率存在直接关联，这提示CPM可能通过其他通路参与CKD进展。