Frishberg Yaacov, Ito Nobuaki, Rinat Choni, Yamazaki Yuji, Feinstein Sofia, Urakawa Itaru, Navon-Elkan Paulina, Becker-Cohen Rachel, Yamashita Takeyoshi, Araya Kaori, Igarashi Takashi, Fujita Toshiro, Fukumoto Seiji
Shaare Zedek Medical Center, Jerusalem, Israel.
J Bone Miner Res. 2007 Feb;22(2):235-42. doi: 10.1359/jbmr.061105.
Two hyperphosphatemic patients with mutations in GALNT3 showed low intact FGF23 levels with marked increase of processed C-terminal fragments. FGF23 protein has three O-linked glycans and FGF23 with incomplete glycosylation is susceptible to processing. Silencing GALNT3 resulted in enhanced processing of FGF23. Decreased function of FGF23 by enhanced processing is the cause of hyperphosphatemia in patients with GALNT3 mutation.
Hyperostosis-hyperphosphatemia syndrome (HHS) is an autosomal recessive entity manifesting as severe hyperphosphatemia associated with episodic bone pain and radiological findings of cortical hyperostosis and periosteal reaction. Persistent hyperphosphatemia is not counterbalanced by PTH or 1,25-dihydroxyvitamin D, posing a mirror image of hypophosphatemic states attributed to increased fibroblast growth factor (FGF)23 activity.
We describe two children with HHS who were found to be homozygous for a mutation in GALNT3 encoding a peptide involved in mucin-type O-glycosylation (ppGaNTase-T3). FGF23 levels were evaluated by two ELISAs and Western blotting. FGF23 protein was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Effect of silencing GALNT3 was evaluated using siRNA in cells transfected with expression vector for FGF23.
Both patients had low levels of the full-length FGF23 with markedly augmented amounts of the inactive fragments. Biologically active FGF23 has three O-linked glycans. FGF23 with only one or two O-linked glycans is processed into inactive fragments. Decreasing the expression of the GALNT3 gene by RNA interference resulted in enhanced processing of FGF23.
The primary defect in HHS is impairment of glycosylation of FGF23 resulting from mutations in GALNT3 and leading to augmented processing of FGF23. These changes in FGF23 abolish its phosphaturic effect and lead to severe persistent hyperphosphatemia. This study provides the pathogenetic mechanism of the first mucin-type O-glycosylation defect identified.
两名GALNT3发生突变的高磷血症患者表现出完整FGF23水平较低，而加工后的C末端片段显著增加。FGF23蛋白有三个O-连接聚糖，糖基化不完全的FGF23易被加工。沉默GALNT3导致FGF23的加工增强。FGF23因加工增强而功能降低是GALNT3突变患者高磷血症的原因。
骨肥厚-高磷血症综合征（HHS）是一种常染色体隐性疾病，表现为严重的高磷血症，伴有发作性骨痛以及皮质骨肥厚和骨膜反应的影像学表现。持续性高磷血症不能被甲状旁腺激素或1,25-二羟维生素D抵消，这与成纤维细胞生长因子（FGF）23活性增加导致的低磷血症状态形成镜像。
我们描述了两名患有HHS的儿童，他们被发现是编码参与粘蛋白型O-糖基化的肽（ppGaNTase-T3）的GALNT3发生突变的纯合子。通过两种酶联免疫吸附测定法（ELISA）和蛋白质印迹法评估FGF23水平。通过基质辅助激光解吸/电离飞行时间质谱法分析FGF23蛋白。使用小干扰RNA（siRNA）在转染了FGF23表达载体的细胞中评估沉默GALNT3的效果。
两名患者的全长FGF23水平均较低，无活性片段的量显著增加。具有生物学活性的FGF23有三个O-连接聚糖。只有一个或两个O-连接聚糖的FGF23被加工成无活性片段。通过RNA干扰降低GALNT3基因的表达导致FGF23的加工增强。
HHS的主要缺陷是由于GALNT3突变导致FGF23糖基化受损，进而导致FGF23的加工增强。FGF23的这些变化消除了其排磷作用，并导致严重的持续性高磷血症。本研究提供了所鉴定的首个粘蛋白型O-糖基化缺陷的发病机制。
