DNAJB9 in Fibrillary Glomerulonephritis: Diagnostic Biomarker, Putative Autoantigen, or Disease-Associated Scaffold? A Case-Based Narrative Review.

Fibrillary glomerulonephritis (FGN) is an uncommon glomerular deposition disease characterized by randomly oriented, nonbranching fibrils that are usually Congo-red-negative and larger than amyloid fibrils on electron microscopy. The discovery of DNAJ homolog subfamily B member 9 (DNAJB9) has transformed FGN from a primarily ultrastructural diagnosis into a molecularly recognizable disease. However, the pathogenic significance of DNAJB9 remains unresolved: it may represent a highly specific biomarker, a putative autoantigen, a chaperone-associated scaffold, or a marker of disturbed protein quality control. We report two patients with positive glomerular DNAJB9 staining and markedly divergent clinical phenotypes. The first patient, a 58-year-old man, presented with severe acute kidney injury, nephritic urinary sediment, severe hypertension, crescentic immune-complex glomerulonephritis, and dialysis-requiring kidney failure. Electron microscopy and IgG subclass staining were unavailable; therefore, the findings were interpreted as probable rather than definitive DNAJB9-positive FGN. Diagnostic and causal interpretation was further complicated by concurrent