Biallelic pathogenic variants in <i>FLNB</i> are associated with paediatric steroid-resistant nephrotic syndrome via podocyte cytoskeletal dysfunction.

Abstract

Steroid-resistant nephrotic syndrome (SRNS) is a severe paediatric kidney disease and a leading cause of end-stage kidney disease in children, with a high genetic contribution. While over 80 monogenic causes of SRNS have been identified, a significant proportion of affected patients still lack a clear genetic diagnosis, indicating that additional causative genes remain to be discovered. Through whole-exome sequencing of a paediatric SRNS cohort, we identified three probands carrying biallelic <i>FLNB</i> pathogenic variants. Sanger sequencing was performed for familial cosegregation verification and ACMG classification. Expression of Filamin B, Nephrin and Synaptopodin in renal tissues was assessed by immunohistochemistry/immunofluorescence. Wild-type and patient-derived variant <i>FLNB</i> plasmids were constructed and transfected into HEK293T cells and immortalised human podocytes (HPCs). The effects of these variants on protein expression, localisation and cytoskeletal organisation were assessed by western blotting and immunofluorescence. <i>FLNB</i> expression in HPCs was silenced using shRNA to evaluate the impact on podocyte marker proteins, cytoskeletal integrity and migratory capacity. A zebrafish <i>flnb</i> knockdown model was employed to validate its effects on renal development. All three probands presented with isolated SRNS without skeletal developmental abnormalities, and renal tissues showed significantly reduced Filamin B protein expression. In vitro, p.L117P and p.M1803L variants led to markedly reduced protein expression, while p.R470L and p.K2586R induced perinuclear aggregation of Filamin B accompanied by F-actin rearrangement. <i>FLNB</i> silencing led to downregulation of Nephrin and Synaptopodin, cytoskeletal disorganisation and impaired cell migration. Zebrafish <i>flnb</i> knockdown exhibited pericardial oedema, defective nephron development and abnormal podocyte foot processes. We report for the first time that biallelic <i>FLNB</i> pathogenic variants are associated with paediatric SRNS by disrupting Filamin B expression, cytoskeletal integrity and podocyte function, providing evidence that <i>FLNB</i> is a novel monogenic cause of SRNS.