A Novel FN1 Nucleotide Variant c.3051G>C (p.Trp1017Cys) in a Pediatric Patient with Fibronectin Glomerulopathy: Case Report and Literature Review

Background/Objectives: Fibronectin glomerulopathy (FNG) is a rare autosomal dominant inherited kidney disease. Approximately 40% of genetically confirmed FNG cases are associated with likely pathogenic variants in FN1. Patients with FNG have similar clinical features as those with chronic nephritis. Due to nonspecific clinical manifestations mimicking common childhood glomerular diseases, FNG poses significant diagnostic challenges in children, frequently resulting in delayed diagnosis. Case Description: A 9-year-old Chinese girl presented with manifestations suggestive of acute poststreptococcal glomerulonephritis (APSGN), including edema, hypertension, hypocomplementemia, nephrotic-range proteinuria (3.34 g/24 h), and microscopic hematuria (45–55 cells/HP). Despite resolution of edema and normalized complement C3 after initial therapy, proteinuria and hematuria persisted. Renal biopsy revealed prominent mesangial deposits extending to glomerular capillary walls, with strong fibronectin (FN) immunoreactivity and fibrillary electrondense deposits on electron microscopy. Genetic testing identified a heterozygous FN1 missense variant c.3051G>C (p.Trp1017Cys) in the proband and her asymptomatic father, classified as likely pathogenic per ACMG guidelines (supporting evidence: PS1, PM2, PP3, PP4). mRNA and cDNA sequencing confirmed the transcription of the mutant allele in the family members. Notably, these transcriptional analyses cannot provide direct evidence for the functional pathogenicity of the variant. The patient received combined angiotensin-converting enzyme inhibitor (ACEI) and angiotensin receptor blocker (ARB) therapy, and renal function remained stable during 3 years of follow-up. Conclusions: The FN1 c.3051G>C represents a novel nucleotide variant, while the corresponding amino acid alteration p.Trp1017Cys has been reported in the previous literature. This case expands the variant spectrum of FN1 and emphasizes the critical value of renal biopsy and genetic testing for diagnosing FNG in pediatric patients with persistent renal manifestations after suspected APSGN. Family screening is essential for identifying asymptomatic carriers. Our findings also highlight the phenotypic heterogeneity of FNG.