J Bone Miner Res. 2014 Jun;29(6):1402-11. doi: 10.1002/jbmr.2173.
Osteogenesis imperfecta (OI) types V and VI are caused, respectively, by a unique dominant mutation in IFITM5, encoding BRIL, a transmembrane ifitm-like protein most strongly expressed in the skeletal system, and recessive null mutations in SERPINF1, encoding pigment epithelium-derived factor (PEDF). We identified a 25-year-old woman with severe OI whose dermal fibroblasts and cultured osteoblasts displayed minimal secretion of PEDF, but whose serum PEDF level was in the normal range. SERPINF1 sequences were normal despite bone histomorphometry consistent with type VI OI and elevated childhood serum alkaline phosphatase. We performed exome sequencing on the proband, both parents, and an unaffected sibling. IFITM5 emerged as the candidate gene from bioinformatics analysis, and was corroborated by membership in a murine bone co-expression network module containing all currently known OI genes. The de novo IFITM5 mutation was confirmed in one allele of the proband, resulting in a p.S40L substitution in the intracellular domain of BRIL but was absent in unaffected family members. IFITM5 expression was normal in proband fibroblasts and osteoblasts, and BRIL protein level was similar to control in differentiated proband osteoblasts on Western blot and in permeabilized mutant osteoblasts by microscopy. In contrast, SERPINF1 expression was decreased in proband osteoblasts; PEDF was barely detectable in conditioned media of proband cells. Expression and secretion of type I collagen was similarly decreased in proband osteoblasts; the expression pattern of several osteoblast markers largely overlapped reported values from cells with a primary PEDF defect. In contrast, osteoblasts from a typical case of type V OI, with an activating mutation at the 5'-terminus of BRIL, have increased SERPINF1 expression and PEDF secretion during osteoblast differentiation. Together, these data suggest that BRIL and PEDF have a relationship that connects the genes for types V and VI OI and their roles in bone mineralization.
成骨不全症（OI）类型 V 和 VI 分别由 IFITM5 中的独特显性突变引起，该基因编码 BRIL，一种跨膜 IFITM 样蛋白，在骨骼系统中表达最强，以及 SERPINF1 中的隐性纯合缺失突变，该基因编码色素上皮衍生因子（PEDF）。我们鉴定了一位 25 岁的女性患有严重的 OI，其真皮成纤维细胞和培养的成骨细胞显示出最小的 PEDF 分泌，但血清 PEDF 水平在正常范围内。尽管骨骼组织形态计量学与 VI 型 OI 一致且儿童期血清碱性磷酸酶升高，但 SERPINF1 序列正常。我们对先证者、父母和一位无病兄弟姐妹进行了外显子组测序。IFITM5 从生物信息学分析中脱颖而出，成为候选基因，并通过其成员关系证实了其存在于包含所有已知 OI 基因的鼠骨共表达网络模块中。在先证者的一个等位基因中发现了新发的 IFITM5 突变，导致 BRIL 细胞内结构域中的 p.S40L 取代，但在无病家庭成员中不存在。IFITM5 在先证者的成纤维细胞和成骨细胞中表达正常，BRIL 蛋白水平在 Western blot 分析分化的先证者成骨细胞和通过显微镜分析通透的突变体成骨细胞中与对照相似。相比之下，SERPINF1 在先证者成骨细胞中的表达降低；在先证者细胞的条件培养基中几乎检测不到 PEDF。先证者成骨细胞中 I 型胶原的表达和分泌也同样减少；几种成骨细胞标志物的表达模式与 PEDF 缺陷细胞的报道值基本重叠。相比之下，BRIL 5'-末端具有激活突变的典型 V 型 OI 患者的成骨细胞在成骨细胞分化过程中 SERPINF1 表达增加和 PEDF 分泌增加。总之，这些数据表明 BRIL 和 PEDF 之间存在关系，将 V 型和 VI 型 OI 的基因及其在骨矿化中的作用联系起来。
