Unraveling proteoglycan linkeropathies and associated diseases through zebrafish models

Proteoglycans are extracellular matrix components consisting of core proteins linked to glycosaminoglycan chains via a conserved tetrasaccharide linker region. Pathogenic variants in the enzymes responsible for linker region biosynthesis ( XYLT1 , XYLT2, B4GALT7 , B3GALT6 , and B3GAT3 ) give rise to a group of heritable connective tissue disorders, collectively termed "linkeropathies". These disorders manifest with a broad spectrum of clinical features, including skeletal dysplasia, joint hypermobility, cardiovascular anomalies, and skin fragility. Variants in enzymes for precursor synthesis or modification (UXS1, FAM20B, CHST3) cause overlapping phenotypes, termed linkeropathy-associated diseases. This review highlights the zebrafish ( Danio rerio ) as a powerful vertebrate model organism for elucidating the molecular and developmental mechanisms underlying linkeropathies and associated diseases. Zebrafish offer genetic tractability, rapid development, and conserved extracellular matrix architecture. Mutant zebrafish models have been established for nearly all known linkeropathy(-associated) genes, recapitulating key human disease traits such as craniofacial and skeletal malformations, disrupted glycosaminoglycan synthesis, and altered extracellular matrix organization. We discuss how zebrafish studies have advanced our understanding of proteoglycan biosynthesis and skeletal development. Comparison with mouse and human models underscores their translational relevance. Zebrafish models have uncovered previously unrecognized processes in skeletal development, including premature chondrocyte maturation, altered ossification timing, and the presence of a trisaccharide linker variant that suggests non-canonical linker biosynthetic pathways. Collectively, these insights position zebrafish as an indispensable platform for dissecting disease mechanisms and exploring therapeutic strategies in linkeropathies. We advocate for expanded phenotypic characterization and the generation of patient-specific knock-in models to accelerate mechanistic discoveries and identify potential therapeutic targets in linkeropathies.