Truncating mutations in BBS10 and BBS12 impair proteostasis and ciliary architecture in Bardet-Biedl Syndrome

Abstract

Bardet-Biedl Syndrome (BBS) is a rare autosomal recessive ciliopathy characterized by genetic heterogeneity. Despite significant progress in understanding the BBSome-coding genes associated with ciliopathies, the pathogenesis linked to mutations in chaperonin-coding genes (BBS6, BBS10, and BBS12) remains poorly defined. This study aims to confirm the genetic diagnosis of BBS and elucidate the pathological mechanisms in causative genes of BBS10 and BBS12. Clinical evaluations were performed on BBS patients, followed by targeted next-generation sequencing (NGS) to identify disease-causing variants. Pathogenicity was assessed using computational prediction tools. Mutant BBS10 and BBS12 constructs were transfected into HEK293T cells for protein stability (Western blot) and interaction analyses (co-immunoprecipitation). Ciliogenesis was evaluated in hTERT-RPE1 cell model via immunofluorescence. The results identified novel compound heterozygous mutants in BBS10 (c. 1391G > C, c.2056 G > A) and BBS12 (c.590-591del AT, c.2102 C > G) in probands from two families. These mutations correlated with the classical BBS features: obesity, polydactyly, and retinal dystrophy. Ophthalmic examinations revealed bone spicule-like deposits, macular outer nuclear layer thinning, and photoreceptor loss in the retina. Comparative analysis across species revealed that these mutations occurred at conserved residues. Structural predictions indicated truncation at the protein's C-terminus. Transfection studies in HEK293T and hTERT-RPE1 cells showed that although the mutant protein localized to primary cilia similar to their wild-type counterparts, their stability was compromised, leading to accelerated degradation through ubiquitin-proteasome pathway. Our findings showed that C-terminal deletions in chaperonin-like BBS proteins significantly impaired their function, particularly affecting protein-protein interactions with each other and with the core BBSome subcomplex protein BBS7. The identified novel compound heterozygous mutations in BBS10 and BBS12 significantly affected ciliary length and protein-protein interactions critical for BBSome assembly, contributing to the manifestation of BBS symptoms.