<b>Cathepsin S (CTSS) in IgA Nephropathy: An Exploratory Study on Its Role as a Potential Diagnostic Biomarker and Therapeutic Target</b>

We conducted a comprehensive bidirectional and multivariable Mendelian randomization (MR) study using publicly available genetic data to explore the causal association between cathepsins and IgA nephropathy (IgAN), systematically. Additionally, immunohistochemical (IHC) staining and enzyme-linked immunosorbent assay (ELISA) were employed to evaluate cathepsin expression levels in renal tissues and serum of IgAN patients. We investigated the underlying mechanisms via gene set variation analysis (GSVA), gene set enrichment analysis (GSEA), and immune cell infiltration analysis. Molecular docking and virtual screening were also performed to identify potential drug candidates through drug repositioning.Univariate MR analyses demonstrated a significant link between increased cathepsin S (CTSS) levels and a heightened risk of IgAN. This was evidenced by an odds ratio (OR) of 1.041 (95% CI=1.009–1.073, <i>P</i>=0.012) as estimated using the inverse variance weighting (IVW) method. In multivariable MR analysis, even after adjusting for other cathepsins, elevated CTSS levels continued to show a strong correlation with an increased risk of IgAN (IVW <i>P</i>=0.020, OR=1.037, 95% CI=1.006–1.069). However, reverse MR analyses did not establish a causal relationship between IgAN and various cathepsins. IHC and ELISA findings revealed significant overexpression of CTSS in both renal tissues and serum of IgAN patients compared to controls, and this high expression was unique to IgAN compared with several other primary kidney diseases such as membranous nephropathy, minimal change disease and focal segmental glomerulosclerosis. Investigations into immune cell infiltration, GSEA, and GSVA highlighted the role of CTSS expression in the immune dysregulation observed in IgAN. Molecular docking and virtual screening pinpointed Camostat mesylate, c-Kit-IN-1, and Mocetinostat as the top drug candidates for targeting CTSS.本研究依托公开可用的遗传数据，开展了一项全面的双向多变量孟德尔随机化（Mendelian randomization, MR）研究，以系统性探究组织蛋白酶与IgA肾病（IgAN）之间的因果关联。此外，本研究采用免疫组化（IHC）染色与酶联免疫吸附试验（ELISA），检测IgAN患者肾组织与血清中的组织蛋白酶表达水平。本研究通过基因集变异分析（GSVA）、基因集富集分析（GSEA）以及免疫细胞浸润分析，对该疾病的潜在发病机制展开探究。同时，本研究还通过药物重定位策略开展分子对接与虚拟筛选，以识别潜在的靶向治疗候选药物。 单变量MR分析显示，组织蛋白酶S（CTSS）水平升高与IgAN发病风险增加存在显著关联。该关联经逆方差加权（IVW）法估算得到验证，其比值比（OR）为1.041（95%CI=1.009–1.073，*P*=0.012）。多变量MR分析结果显示，即便校正其他组织蛋白酶的影响后，CTSS水平升高仍与IgAN发病风险升高存在显著关联（IVW法*P*=0.020，OR=1.037，95%CI=1.006–1.069）。不过，反向MR分析并未证实IgAN与各类组织蛋白酶之间存在因果关联。 IHC与ELISA结果显示，与对照组相比，IgAN患者肾组织与血清中的CTSS均呈现显著高表达；且与膜性肾病、微小病变肾病、局灶节段性肾小球硬化等其他原发性肾脏疾病相比，该高表达仅见于IgAN患者。免疫细胞浸润分析、GSEA及GSVA结果均表明，CTSS表达在IgAN患者的免疫失调过程中发挥了关键作用。分子对接与虚拟筛选结果明确了Camostat mesylate、c-Kit-IN-1与Mocetinostat为靶向CTSS的最优候选药物。