Hypomorphic zebrafish models mimic the musculoskeletal phenotype of β4GalT7-deficient Ehlers-Danlos syndrome

β4GalT7 is a transmembrane Golgi enzyme, encoded by B4GALT7, that plays a pivotal role in the proteoglycan linker region formation during proteoglycan biosynthesis. Defects in this enzyme give rise to a rare autosomal recessive form of Ehlers-Danlos syndrome (EDS), currently known as 'spondylodysplastic EDS (spEDS-B4GALT7)'. This EDS subtype is mainly characterized by short stature, hypotonia and skeletal abnormalities, thereby illustrating its pleiotropic importance during human development. Insights into the pathogenic mechanisms underlying this disabling disease are very limited, in part due to the lack of a relevant in vivo model. As the majority of mutations identified in patients with spEDS-B4GALT7 are hypomorphic, we generated zebrafish models with partial loss of B4galt7 function, including different knockdown (morphant) and mosaic knockout (crispant) b4galt7 zebrafish models and studied the morphologic, functional and molecular aspects in embryonic and larval stages. Morphant and crispant zebrafish show highly similar morphological abnormalities in early development including a small, round head, bowed pectoral fins, short body-axis and mild developmental delay. Several craniofacial cartilage and bone structures are absent or strongly misshapen. In addition, the total amount of sulfated glycosaminoglycans is significantly diminished and particularly heparan and chondroitin sulfate proteoglycan levels are greatly reduced. We also show impaired cartilage patterning and loss of chondrocyte organization in a cartilage-specific Tg(Col2a1aBAC:mcherry) zebrafish reporter line. The occurrence of the same abnormalities in the different models confirms these are specifically caused by B4galt7 deficiency. A disturbed actin pattern, along with a lack of muscle tone, was only noted in morphants in which translation of b4galt7 was blocked. In conclusion, we generated the first viable animal models for spEDS-B4GALT7, and show that in early development the human spEDS-B4GALT7 phenotype is faithfully mimicked in these zebrafish models. Our findings underscore a key role for β4GalT7 in early development of cartilage, bone and muscle. These models will lead to a better understanding of spEDS-B4GALT7 and can be used in future efforts focusing on therapeutic applications.