A Dominant Heterozygous Mutation in COG4 Causes Saul-Wilson Syndrome, a Primordial Dwarfism, and Disrupts Zebrafish Development via Wnt Signaling.

Zhi-Jie Xia,Xin-Xin I Zeng,Mitali Tambe,Hudson H Freeze,Bobby G Ng,P Duc S Dong

doi:10.3389/fcell.2021.720688

Abstract

Saul–Wilson syndrome (SWS) is a rare, skeletal dysplasia with progeroid appearance and primordial dwarfism. It is caused by a heterozygous, dominant variant (p.G516R) in COG4, a subunit of the conserved oligomeric Golgi (COG) complex involved in intracellular vesicular transport. Our previous work has shown the intracellular disturbances caused by this mutation; however, the pathological mechanism of SWS needs further investigation. We sought to understand the molecular mechanism of specific aspects of the SWS phenotype by analyzing SWS-derived fibroblasts and zebrafish embryos expressing this dominant variant. SWS fibroblasts accumulate glypicans, a group of heparan sulfate proteoglycans (HSPGs) critical for growth and bone development through multiple signaling pathways. Consistently, we find that glypicans are increased in zebrafish embryos expressing the COG4p.G516R variant. These animals show phenotypes consistent with convergent extension (CE) defects during gastrulation, shortened body length, and malformed jaw cartilage chondrocyte intercalation at larval stages. Since non-canonical Wnt signaling was shown in zebrafish to be related to the regulation of these processes by glypican 4, we assessed wnt levels and found a selective increase of wnt4 transcripts in the presence of COG4p.G516R. Moreover, overexpression of wnt4 mRNA phenocopies these developmental defects. LGK974, an inhibitor of Wnt signaling, corrects the shortened body length at low concentrations but amplifies it at slightly higher concentrations. WNT4 and the non-canonical Wnt signaling component phospho-JNK are also elevated in cultured SWS-derived fibroblasts. Similar results from SWS cell lines and zebrafish point to altered non-canonical Wnt signaling as one possible mechanism underlying SWS pathology.

Highlights

Saul–Wilson syndrome (SWS) is a rare skeletal dysplasia characterized by profound short stature and distinctive craniofacial features such as prominent forehead, prominent eyes, and micrognathia (Saul and Wilson, 1990; Ferreira et al, 2020)
We found that the gpc2 and gpc4 transcript levels are elevated in both COG4W T and COG4p.G516R embryos compared to the uninjected control, but there is no significant difference between COG4W T and COG4p.G516R (Figure 2G)
Increased glypicans were seen in both SWS-derived cells and COG4p.G516R-injected zebrafish embryos

Summary

INTRODUCTION

Saul–Wilson syndrome (SWS) is a rare skeletal dysplasia characterized by profound short stature and distinctive craniofacial features such as prominent forehead, prominent eyes, and micrognathia (Saul and Wilson, 1990; Ferreira et al, 2020). The COG4p.G516R variant disrupted protein trafficking by accelerating brefeldin-A (BFA)-induced retrograde transport and delaying anterograde transport, causing the collapse of the Golgi stacks. This interrupted bidirectional trafficking between the ER and the Golgi and altered decorin, a proteoglycan (Ferreira et al, 2018), indicate that modified proteoglycans may be involved in the pathogenesis of SWS. SWS individuals show very different features compared to COG4-CDG individuals, and SWS cells show accelerated BFA-induced retrograde trafficking in contrast to COG4-CDG cells Considering these facts, a zebrafish model for the SWS-specific variant is highly desired to investigate phenotypic features and the possible pathogenesis of this heterozygous mutation in COG4. These findings suggest that disrupted Wnt signaling is one possible mechanism underlying the pathogenesis of SWS

RESULTS

DISCUSSION

MATERIALS AND METHODS

ETHICS STATEMENT