4 Mutations in the heparan sulfate backbone elongating enzymes EXT1 and EXT2 have no major effect on endothelial glycocalyx and the glomerular filtration barrier 75 patients and controls. We found no difference in PBR and vascular density between patients with an EXT1 or EXT2 mutation and controls, indicating that these mutations do not significantly alter the endothelial glycocalyx. Also, we found no specific glomerular phenotype in patients with osteochondromas. These results contribute to the widely discussed topic of the role of HS in the GFB.(25, 55) Interestingly, one patient in this group showed a complicated medical history with an unexplained renal insufficiency that resulted in renal replacement therapy. Our EM analysis of this patient showed glomerular deposition of large collagen fibrils in the mesangium and GBM, similar to those found by Roberts et al. (56). Previously, circulating HS derived from MO patients was found to be structurally normal, but quantitatively diminished compared to healthy controls (63). Whether MO patients have structurally aberrant HS in the GFB and glycocalyx is currently unknown. Our results could imply that the mono-allelic germline mutation in EXT1 or EXT2 in MO does not lead to these structural changes. Potentially, one functional allele is sufficient to prevent a phenotype from developing. Because most MO patients showed a normal glomerular morphology, we hypothesize that a second trigger is required to develop glomerular pathology specific to the EXT1 or EXT2 gene defect. In MO pathogenesis, germline heterozygous mutations in EXT1 or EXT2 are not sufficient to cause osteochondroma formation. Loss of heterozygosity is needed for osteochondroma to develop.(69, 70) Analogous to this tenet, our data supports the notion that a germline heterozygous mutation in the EXT1 or EXT2 gene alone is not enough to cause a pathologic phenotype in the glomerulus or endothelial glycocalyx. Cases such as those described by Roberts et al. and here, are likely to have a distinct, local loss of heterozygosity and as such, loss of HS, leading to a specific phenotype. Potentially, factors other than HS could also play a role in these rare cases. Our group has previously shown that a bi-allelic germline mutation of EXT in zebrafish also does not lead to a renal phenotype, although it does result in a specific cartilaginous and dental phenotype.(31, 33, 55, 71) Overall, based on the results from the current study and others, MO patients do not seem to have an increased risk of developing clinically significant microvascular complications. However, rare pathology exists in this disease and the current study might be underpowered to fully assess that risk. A limitation of this study is that both glycocalyx measurements and urine samples were not collected multiple times. Glycocalyx measurements are of a dynamic nature with a small variability, and multiple measurements, as we have performed, increase reproducibility.(72)
RkJQdWJsaXNoZXIy MTk4NDMw