Impaired proteoglycan glycosylation, elevated TGF-β signaling, and abnormal osteoblast differentiation as the basis for bone fragility in a mouse model for gerodermia osteodysplastica

Gerodermia osteodysplastica (GO) is characterized by skin laxity and early-onset osteoporosis. GORAB, the responsible disease gene, encodes a small Golgi protein of poorly characterized function. To circumvent neonatal lethality of the GorabNull full knockout, Gorab was conditionally inactivated in mesenchymal progenitor cells (Prx1-cre), pre-osteoblasts (Runx2-cre), and late osteoblasts/osteocytes (Dmp1-cre), respectively. While in all three lines a reduction in trabecular bone density was evident, only GorabPrx1 and GorabRunx2 mutants showed dramatically thinned, porous cortical bone and spontaneous fractures. Collagen fibrils in the skin of GorabNull mutants and in bone of GorabPrx1 mutants were disorganized, which was also seen in a bone biopsy from a GO patient. Measurement of glycosaminoglycan contents revealed a reduction of dermatan sulfate levels in skin and cartilage from GorabNull mutants. In bone from GorabPrx1 mutants total glycosaminoglycan levels and the relative percentage of dermatan sulfate were both strongly diminished. Accordingly, the proteoglycans biglycan and decorin showed reduced glycanation. Also in cultured GORAB-deficient fibroblasts reduced decorin glycanation was evident. The Golgi compartment of these cells showed an accumulation of decorin, but reduced signals for dermatan sulfate. Moreover, we found elevated activation of TGF-β in GorabPrx1 bone tissue leading to enhanced downstream signalling, which was reproduced in GORAB-deficient fibroblasts. Our data suggest that the loss of Gorab primarily perturbs pre-osteoblasts. GO may be regarded as a congenital disorder of glycosylation affecting proteoglycan synthesis due to delayed transport and impaired posttranslational modification in the Golgi compartment.

骨发育不良性皮肤松弛症（Gerodermia osteodysplastica, GO）以皮肤松弛及早发性骨质疏松症为特征。其致病基因GORAB编码一种功能尚未明确的小型高尔基体蛋白（Golgi protein）。为规避GorabNull全基因敲除（full knockout）模型的新生儿致死性，研究团队分别在间充质祖细胞（mesenchymal progenitor cells, Prx1-cre）、前成骨细胞（pre-osteoblasts, Runx2-cre）及晚期成骨细胞/骨细胞（late osteoblasts/osteocytes, Dmp1-cre）中对Gorab进行条件性敲除。在三种基因修饰小鼠模型中均观察到骨小梁密度（trabecular bone density）降低，但仅GorabPrx1与GorabRunx2突变小鼠表现出皮质骨（cortical bone）显著变薄、多孔及自发性骨折。GorabNull突变小鼠皮肤及GorabPrx1突变小鼠骨组织中的胶原纤维（collagen fibrils）均呈现排列紊乱，该表型也在1例GO患者的骨活检（bone biopsy）样本中得到验证。对糖胺聚糖（glycosaminoglycan）含量的检测显示，GorabNull突变小鼠皮肤与软骨组织中的硫酸皮肤素（dermatan sulfate）水平降低。而GorabPrx1突变小鼠骨组织的总糖胺聚糖水平及硫酸皮肤素的相对占比均显著下降。相应地，蛋白聚糖双糖链蛋白聚糖（biglycan）与饰胶蛋白聚糖（decorin）的糖基化修饰（glycanation）水平降低。在GORAB缺陷型成纤维细胞（fibroblasts）中同样观察到饰胶蛋白聚糖糖基化水平降低。该类细胞的高尔基体区室（Golgi compartment）可见饰胶蛋白聚糖蓄积，但硫酸皮肤素相关信号减弱。此外，研究团队在GorabPrx1突变小鼠骨组织中发现转化生长因子-β（TGF-β）的激活水平升高，进而导致下游信号通路增强，该现象在GORAB缺陷型成纤维细胞中也得到重现。本研究数据显示，Gorab缺失主要影响前成骨细胞。GO或可被视为一类先天性糖基化紊乱（congenital disorder of glycosylation），其由高尔基体区室中蛋白聚糖合成的运输延迟及翻译后修饰（posttranslational modification）受损所导致。