Dominant negative variants in KIF5B cause osteogenesis imperfecta via down regulation of mTOR signaling

Abstract

Background Kinesin motor proteins transport intracellular cargo, including mRNA, proteins, and organelles. Pathogenic variants in kinesin-related genes have been implicated in neurodevelopmental disorders and skeletal dysplasias. We identified de novo, heterozygous variants in KIF5B, encoding a kinesin-1 subunit, in four individuals with osteogenesis imperfecta. The variants cluster within the highly conserved kinesin motor domain and are predicted to interfere with nucleotide binding, although the mechanistic consequences on cell signaling and function are unknown. Methods To understand the in vivo genetic mechanism of KIF5B variants, we modeled the p.Thr87Ile variant that was found in two patients in the C. elegans ortholog, unc-116, at the corresponding position (Thr90Ile) by CRISPR/Cas9 editing and performed functional analysis. Next, we studied the cellular and molecular consequences of the recurrent p.Thr87Ile variant by microscopy, RNA and protein analysis in NIH3T3 cells, primary human fibroblasts and bone biopsy. Results C. elegans heterozygous for the unc-116 Thr90Ile variant displayed abnormal body length and motility phenotypes that were suppressed by additional copies of the wild type allele, consistent with a dominant negative mechanism. Time-lapse imaging of GFP-tagged mitochondria showed defective mitochondria transport in unc-116 Thr90Ile neurons providing strong evidence for disrupted kinesin motor function. Microscopy studies in human cells showed dilated endoplasmic reticulum, multiple intracellular vacuoles, and abnormal distribution of the Golgi complex, supporting an intracellular trafficking defect. RNA sequencing, proteomic analysis, and bone immunohistochemistry demonstrated down regulation of the mTOR signaling pathway that was partially rescued with leucine supplementation in patient cells. Conclusion We report dominant negative variants in the KIF5B kinesin motor domain in individuals with osteogenesis imperfecta. This study expands the spectrum of kinesin-related disorders and identifies dysregulated signaling targets for KIF5B in skeletal development.

Involvement of kinesins in skeletal dysplasia: a review

“…The impaired lysosomal function reported in the Kif5Blof zebrafish mutants correlated with decreased mTOR pathway activation that may have contributed to the decrease in the number of hypertrophic chondrocytes and suggested a role for KIF5B in regulating autophagy. Moreover, a heterozygous mutation in KIF5B was identified in four patients with osteogenesis imperfecta (OI) and analysis of primary patient fibroblasts showed an impaired intracellular transport of mitochondria and abnormal Golgi positioning and resulted in a downregulation of mTOR signaling supporting the implication of KIF5B in regulation of autophagy ( 98 ). In addition, Kinesin-1 and -3 proteins have been shown to drive lysosome movement along different microtubule tracks and particularly KIF5B selectively recruits lysosomes to the microtubules ( 97 ).…”