A homozygous hypomorphic BNIP1 variant causes an increase in autophagosomes and reduced autophagic flux and results in a spondylo-epiphyseal dysplasia

BNIP1 (BCL2 interacting protein 1) is a soluble N-ethylmaleimide-sensitive factor-attachment protein receptor involved in ER membrane fusion. We identified the homozygous BNIP1 intronic variant c.84+3A>T in the apparently unrelated patients 1 and 2 with disproportionate short stature. Radiographs showed abnormalities affecting both the axial and appendicular skeleton and spondylo-epiphyseal dysplasia. We detected ~80% aberrantly spliced BNIP1 pre-mRNAs, reduced BNIP1 mRNA level to ~80%, and BNIP1 protein level reduction by ~50% in patient 1 compared to control fibroblasts. The BNIP1 ortholog in drosophila, Sec. 20, regulates autophagy and lysosomal degradation. We assessed lysosome positioning and identified a decrease in lysosomes in the perinuclear region and an increase in the cell periphery in patient 1 cells. Immunofluorescence microscopy and immunoblotting demonstrated an increase in LC3B-positive structures and LC3B-II levels, respectively, in patient 1 fibroblasts under steady-state condition. Treatment of serum-starved fibroblasts with or without bafilomycin A1 identified significantly decreased autophagic flux in patient 1 cells. Our data suggest a block at the terminal stage of autolysosome formation and/or clearance in patient fibroblasts. BNIP1 together with RAB33B and VPS16, disease genes for Smith-McCort dysplasia 2 and a multisystem disorder with short stature, respectively, highlight the importance of autophagy in skeletal development. This article is protected by copyright. All rights reserved.

MTD-like motif of a BH3-only protein, BNIP1, induces necrosis accompanied by an intracellular calcium spike

The mitochondrial targeting domain (MTD) of Noxa has necrosis-inducing activity when conjugated with cell-penetrating peptide (CPP). In this study, we report another MTD-like motif, B1MLM, found in BNIP1, a pro-apoptotic BH3-only protein found in the endoplasmic reticulum membrane. The B1MLM peptide, conjugated with CPP, induced necrosis in a way similar to that of R8:MTD. R8:B1MLM caused an intracellular calcium spike, mitochondrial reactive oxygen species generation, and mitochondrial fragmentation. The cytosolic calcium spike was likely due to the opening of the mitochondrial permeability transition pore.