Cell-Dependent Pathogenic Roles of Filamin B in Different Skeletal Malformations

Area of Science:

• Genetics
• Molecular Biology
• Skeletal Dysplasias

Background:

• Mutations in the filamin B (FLNB) gene are associated with a range of autosomal skeletal malformations, including Larsen syndrome (LRS) and boomerang dysplasia (BD).
• The precise molecular mechanisms driving the phenotypic variability observed in FLNB-related disorders remain incompletely understood.

Purpose of the Study:

• To investigate the pathogenic mechanisms of two novel FLNB variants identified in patients with autosomal dominant LRS and autosomal recessive vitamin D-dependent rickets type IA (VDDR-IA).
• To elucidate how these FLNB variants differentially regulate skeletal development and contribute to clinical heterogeneity.

Main Methods:

• Whole-exome sequencing was employed to identify novel FLNB variants.
• In vitro studies using HEK293, Saos-2, and ATDC5 cell lines were conducted to assess the functional impact of the identified variants.
• Analysis of filopodia formation, protein localization, and key signaling pathways (AKT, SHIP2, Smad3, Runx2) involved in endochondral osteogenesis.

Main Results:

• Two novel missense FLNB variants, c.4846A>G (p.T1616A) and c.7022T>G (p.I2341R), were identified.
• Both variants resulted in a lack of filopodia and perinuclear accumulation of filamin B in HEK293 cells.
• The variants differentially modulated endochondral osteogenesis by affecting AKT and Smad3 pathways and Runx2 expression in a cell-type-specific manner.

Conclusions:

• The identified FLNB variants contribute to skeletal malformations through distinct molecular mechanisms.
• FLNB variants can directly cause skeletal disorders and exacerbate symptoms in the context of other skeletal diseases.
• Differential effects of FLNB variants on skeletal development underlie the clinical heterogeneity of FLNB-related disorders.