COPB2haploinsufficiency causes a coatopathy with osteoporosis and developmental delay

Coatomer complexes function in the sorting and trafficking of proteins between subcellular organelles. Pathogenic variants in coatomer subunits or associated factors have been reported in multi-systemic disorders, i.e., coatopathies, that can affect the skeletal and central nervous systems. We have identified loss-of-function variants inCOPB2, a component of the coatomer complex I (COPI), in individuals presenting with osteoporosis, fractures and developmental delay of variable severity. Because the role of COPB2 in bone has not been characterized, we studied the effect ofCOPB2deficiency on skeletal development in mice and zebrafish.Copb2+/−mice showed low bone mass and decreased bone strength. In zebrafish, larvae carrying acopb2heterozygous frameshift variant showed delayed mineralization.copb2-null embryos showed endoplasmic reticulum (ER) and Golgi disorganization, and embryonic lethality.COPB2siRNA-treated fibroblasts showed delayed collagen trafficking with retention of type I collagen in the ER and Golgi, and altered distribution of Golgi markers. Our data suggest thatCOPB2haploinsufficiency leads to disruption of intracellular collagen trafficking and osteoporosis, which may improve with ascorbic acid supplementation. This work highlights the role of COPI complex as a critical regulator of bone mass and identifies a new form of coatopathy due toCOPB2deficiency.