成骨不全症(OI)是一种遗传性骨骼发育不良,其特征是骨密度低,骨脆性和复发性骨折。其异质性遗传基础的表征允许鉴定骨骼发育中的新型参与者。2016年,我们描述了由半合子MBTPS2错义变异导致中度至重度表型的OI的第一个X连锁隐性形式。MBTPS2编码位点2蛋白酶(S2P),激活参与骨(OASIS)和软骨发育(BBF2H7)的转录因子,ER应激反应(ATF6)和脂质代谢(SREBP)经由过程调控膜内卵白水解。在内质网应激或甾醇缺乏的时候,上述转录因子被位点1蛋白酶(S1P)和S2P顺序切割。它们的N端片段穿梭到细胞核以激活基因转录。有趣的是,MBTPS2的其他位置的错义突变引起皮肤谱疾病,尽管存在一些致病变体,但无毛和畏光(IFAP)和尖锐湿疣角化病(KFSD)与OI没有临床重叠。为了了解S2P中的单个氨基酸取代如何导致非重叠表型,我们旨在比较MBTPS2-OI和MBTPS2-IFAP/KFSD的分子特征,最终目标是解开MBTPS2-OI背后的病理机制。来自健康对照(n=4)的原代皮肤成纤维细胞的基于RNA测序的转录组谱,MBTPS2-OI(n=3),和MBTPS2-IFAP/KFSD(n=2)患者进行鉴定与对照组相比在MBTPS2-OI和MBTPS2-IFAP/KFSD个体中差异表达的基因。我们观察到SREBP依赖性基因在OI中比在IFAP/KFSD中更下调。这与体外MBTPS2-OI成纤维细胞中脂肪酸相对丰度的改变有关。而未观察到固醇谱的一致变化。在MBTPS2-OI中,很少有OASIS依赖性基因被抑制,而BBF2H7和ATF6依赖性基因在OI和IFAP/KFSD患者和对照成纤维细胞之间是相当的。重要的是,我们鉴定了与软骨生理学有关的基因,这些基因在MBTPS2-OI中差异表达,但在MBTPS2-IFAP/KFSD成纤维细胞中不差异表达.总之,我们的数据提供了致病MBTPS2突变如何通过改变脂肪酸代谢或软骨发育导致骨骼畸形的线索,这可能会影响骨骼发育。矿化和软骨内骨化。
Osteogenesis imperfecta (OI) is an inherited skeletal dysplasia characterized by low bone density, bone fragility and recurrent fractures. The characterization of its heterogeneous genetic basis has allowed the identification of novel players in bone development. In 2016, we described the first X-linked recessive form of OI caused by hemizygous
MBTPS2 missense variants resulting in moderate to severe phenotypes.
MBTPS2 encodes site-2 protease (S2P), which activates transcription factors involved in bone (OASIS) and cartilage development (BBF2H7), ER stress response (ATF6) and lipid metabolism (SREBP) via regulated intramembrane proteolysis. In times of ER stress or sterol deficiency, the aforementioned transcription factors are sequentially cleaved by site-1 protease (S1P) and S2P. Their N-terminal fragments shuttle to the nucleus to activate gene transcription. Intriguingly, missense mutations at other positions of MBTPS2 cause the dermatological spectrum condition Ichthyosis Follicularis, Atrichia and Photophobia (IFAP) and Keratosis Follicularis Spinulosa Decalvans (KFSD) without clinical overlap with OI despite the proximity of some of the pathogenic variants. To understand how single amino acid substitutions in S2P can lead to non-overlapping phenotypes, we aimed to compare the molecular features of MBTPS2-OI and
MBTPS2-IFAP/KFSD, with the ultimate goal to unravel the pathomechanisms underlying
MBTPS2-OI. RNA-sequencing-based transcriptome profiling of primary skin fibroblasts from healthy controls (n = 4), MBTPS2-OI (n = 3), and MBTPS2-IFAP/KFSD (n = 2) patients was performed to identify genes that are differentially expressed in
MBTPS2-OI and
MBTPS2-IFAP/KFSD individuals compared to controls. We observed that SREBP-dependent genes are more downregulated in OI than in IFAP/KFSD. This is coupled to alterations in the relative abundance of fatty acids in MBTPS2-OI fibroblasts in vitro, while no consistent alterations in the sterol profile were observed. Few OASIS-dependent genes are suppressed in MBTPS2-OI, while BBF2H7- and ATF6-dependent genes are comparable between OI and IFAP/KFSD patients and control fibroblasts. Importantly, we identified genes involved in cartilage physiology that are differentially expressed in MBTPS2-OI but not in
MBTPS2-IFAP/KFSD fibroblasts. In conclusion, our data provide clues to how pathogenic MBTPS2 mutations cause skeletal deformities via altered fatty acid metabolism or cartilage development that may affect bone development, mineralization and endochondral ossification.