Novel Humanized X-Linked Alport Syndrome Missense Variants Induce Endoplasmic Reticulum (ER) Stress and Early Glomerular Dysfunction

Background: X-linked Alport Syndrome (XLAS) is caused by pathogenic COL4A5 variants and is characterized by progressive glomerular dysfunction and renal failure. While nonsense mutations are well characterized, the impact of disease-relevant missense variants on glomerular pathology and signaling remains poorly defined. We generated and characterized two humanized Col4a5 missense CRISPR/Cas9 knock-in mouse models—L1655R/Y and C1570S/Y— commonly reported in the Western United States to evaluate functional decline and molecular injury responses. Methods: Glomerular filtration rate (GFR) was longitudinally measured via transcutaneous FITC-sinistrin clearance from 3 months of age until 1 year of age. Transmission electron microscopy (TEM) was performed to examine ultrastructural changes in glomerular basement membrane (GBM) and podocytes. Whole kidney cortex was analyzed by qPCR to assess unfolded protein response (UPR), ER stress signaling, and TRPC5/Rac1 pathway activation. Results: Both Col4a5 missense models exhibited early GFR increases compared to WT controls, followed by a progressive decline in GFR. At 1 year, GFR was reduced to 594.2 ± 94.6 μL/min/100g in Col4a5C1570S/Y and 555.0 ± 78.0 μL/min/100g in Col4a5L1655R/Y versus 861.1 ± 152.4 μL/min/100g in WT and 752.7 ± 136.7 μL/min/100g in Col4a5+/Y littermates, indicating impaired renal function by 1 year. TEM revealed classic Alport features, including GBM thickening, lamellation, and podocyte foot process effacement in both models. qPCR revealed significant upregulation of UPR markers in both mutants, including HSPA5 (4.3- to 18.8-fold), DDIT3 (4.0- to 7.3-fold), and ATF4 (13.8- to 15.1-fold). TRPC5 and Rac1 expression were increased 3.7- to 13.7-fold and 3.0- to 3.1-fold, respectively, suggesting activation of stress-sensitive cytoskeletal signaling pathways. Conclusion: Humanized Col4a5 missense variants are sufficient to drive ER stress, TRPC5/Rac1 signaling, and glomerular injury in vivo. The L1655R variant produces a more severe phenotype than C1570S, highlighting allele-specific progression in XLAS. These models offer valuable tools for dissecting early disease mechanisms and support targeting ER and TRPC5 pathways as therapeutic strategies. Funding: NIDDK Support - NIDDK Support, NIDDK Support