GAG synthesis and skeletal dysplasia: role of SLC10A7 – SKELGAG

.Skeletal dysplasias with multiple dislocations (SDM) are severe disorders characterized by dislocations of large joints, scoliosis and pre and postnatal growth retardation. More than 10 recessive disorders have been described so far and the majority of them have been linked to pathogenic variants in genes encoding glycosyltransferases (“linkeropathies”), sulfotransferases, epimerases or sulfate transporters, all requested for the biosynthesis of the heparan sulfate (HS) and chondroitin sulfate (CS) glycosaminoglycan (GAG) chains attached to HS and/or CS proteoglycan (HSPG and CSPG) core protein. These findings support the recognition of a new group of inborn errors leading to defects in GAG biosynthesis. This process is tightly regulated in the Golgi, especially through ion homeostasis.
In our cohort of patients with SDM, pathogenic variants have been also identified in genes encoding proteins with no known functions directly related to proteoglycan synthesis, such as calcium activated nucleotidase-1 (CANT-1), that were associated with defective proteoglycans synthesis.
More recently, we identified homozygous mutations in SLC10A7, which encodes a member of the SLC family of uncharacterized transporters. Furthermore, we developed a Slc10a7-deficient mouse model that mimics the human bone phenotype. Preliminary results, interestingly, demonstrated a strong specific decrease in HS level in both patient fibroblasts and Slc10a7-/- mouse cartilages and a congenital disorder of glycosylation type II (CDG II) profile, i.e. resulting in the presence of truncated and abnormal N-glycan structures, in SLC10A7 deficient patients. The link of SLC10A7 with divalent ion homeostasis is not clear but suggested by yeast studies showing that SLC10A7 orthologs could act as negative regulator of cytosolic calcium homeostasis.
This proposal is built on the hypothesis that mutations identified in genes encoding transporters or ion binding proteins, such as SLC10A7 or CANT-1 lead to impaired Golgi ion homeostasis then resulting in Golgi glycosylation and specific GAG synthesis defects. As such, it is ambitious to hypothesize that the restoration of Golgi ion homeostasis will lead to a normalization of the GAG biosynthesis process, and will open the field of novel therapies for SDM patients. In order to avoid a scattering of our efforts, we will strategically focus our work on SLC10A7, for which both patient fibroblasts and Slc10a7-/- mouse model are already available. The ambition is to develop a framework that will then serve as a model to study the functions of the other identified transporters.
We will combine the synergistic complementary expertise of three teams) in skeletal dysplasia and ossification process 2) in Golgi glycosylation and Golgi homeostasis and 3) in GAG biosynthesis.
This team complementarity will be essential in pursuing our three main objectives:
WP1: Elucidation of the roles of SLC10A7 in Golgi glycosylation and ion homeostasis (Team 2)
The contribution of SLC10A7 in Ca2+/ Mn2+ homeostasis will be fully studied.
WP 2: Analysis of SLC10A7 deficiency specific consequences on endochondral and membranous ossification using Slc10a7-/- mouse model (Teams 1 and 3). Ion supplementation efficiency will be tested to reverse the GAG synthesis and glycosylation defects in Slc10a7-/- mice
WP 3: Golgi glycosylation, toward new therapeutics (Teams 1, 2 and 3).
It will include i) a structural analysis of glycoconjugates and a characterization of GAG synthesis defects in patient and mouse samples, ii) whole exome analysis on samples from SDM patients with still unknown molecular bases. The established framework will be applied to any relevant new gene.