Fibrillary Glomerulonephritis Associated with Eosinophilic Esophagitis

Introduction: Fibrillary glomerulonephritis (FGN) is a rare glomerular disease, whose pathophysiology is relatively unclear, and is associated with neoplasms, infectious and autoimmune diseases. Discovering the DNAJ homolog subfamily B member 9 (DNAJB9) has contributed to a better understanding of the pathophysiology and diagnosis of FGN. We report a case of FGN associated with eosinophilic esophagitis (EE), with a challenging diagnosis. Case Description: A 41-year-old woman, with progressive dysphagia, was diagnosed with EE in 2020. Despite treatment with corticosteroids, her condition worsened, requiring a gastrostomy. She developed renal dysfunction associated with nephrotic syndrome. Tests: creatinine 2.3 mg/dL; serum albumin 2.4 g/dL; proteinuria 7.6g/24h; microscopic hematuria; ANA 1/320; C3 76 mg/dL (normal: 88-165 mg/dL); Other tests were negative, including anti-DNA, anti-Sm, ANCA, serology for hepatitis B and C, HIV and syphilis. Lupus nephritis was suspected and a kidney biopsy was performed. Light microscopy (LM): 3/7 global glomerulosclerosis, 4/7 segmental glomerulosclerosis, discrete mesangial expansion with preserved cellularity and mild interstitial fibrosis and tubular atrophy (IFTA). Positive immunofluorescence (IF) for IgG, C3, kappa and lambda in mesangium and capillary loops. Electron microscopy (EM) was not performed. Treatment was initiated with methylprednisolone and cyclophosphamide. In December 2023, continuing with nephrotic syndrome and renal dysfunction, a new biopsy was performed: LM 35/45 global glomerulosclerosis, discrete, diffuse mesangial matrix expansion and cellularity, severe IFTA; positive IF for the same previous pattern; ME: diffuse thickening of the basement membrane and frequent structured deposits arranged diffusely in the subepithelial, subendothelial, intramembranous and mesangial regions. Deposits averaged 15.6 nm. Immunohistochemistry was performed and was strongly positive for DNAJB9 throughout the glomerular compartment Discussion: Around 10%-30% of FGN are associated with autoimmune diseases. To our knowledge, this is the first report of FGN associated with EE. We emphasize the importance of EM for the correct diagnosis of glomerulopathies, including FGN. Currently, DNAJB9 is considered an excellent biomarker for FGN and probably a new gold standard for diagnosis