Department of Endocrinology, NHC Key Laboratory of Endocrinology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences, Beijing 100730, China.
Department of Endocrinology, NHC Key Laboratory of Endocrinology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences, Beijing 100730, China; Musculoskeletal Research Laboratory and Bone Quality and Health Assessment Centre, Department of Orthopedics & Traumatology, The Chinese University of Hong Kong, Hong Kong.
Bone. 2020 Aug;137:115401. doi: 10.1016/j.bone.2020.115401. Epub 2020 May 1.
Hyperphosphatemic familial tumoral calcinosis/hyperostosis-hyperphosphatemia syndrome (HFTC/HHS) is a rare disorder caused by deficiency or resistance of fibroblast growth factor 23 (FGF23). Here we reported a Chinese family with HFTC/HHS, aiming at clarifying the clinical features, bone microarchitectures and molecular mechanisms of the disease.
Clinical manifestations, laboratory examinations and genetic analyses were collected from two HFTC patients. Bone microarchitectures were detected by HR-pQCT. In vitro expression and glycosylation of mutant and wild-type FGF23 proteins were analyzed by western blotting and wheat germ agglutinin affinity chromatography. Subcellular localizations of FGF23 proteins were detected by immunocytochemistry.
The two brothers carried previously unreported c.413T > G, p.Leu138Arg and c.491T > A, p.Ile164Asn compound heterozygous variants in the FGF23 gene, which was "likely pathogenic" according to American College of Medical Genetics (ACMG) Standards and Guidelines. Both patients had severe hyperphosphatemia and significantly elevated C-terminal FGF23. With HHS, patient 1 presented with lower extremity pain and widespread cardiovascular calcification. HR-pQCT of his distal radius and tibia revealed decreased volume BMD and cortical thickness, which were inconsistent with hyperostosis manifestations in X-ray. He received etidronate treatment, which improved his BMD and the ectopic calcification. His brother exhibited less bone involvement but had experienced recurrent painful calcified mass from a young age and undergone several resections. In vitro experiments showed that the mutant FGF23 proteins had defective O-glycosylation and impaired secretion. However, no difference in subcellular localization was found between the wild-type and mutant FGF23 proteins.
We have presented a Chinese HFTC/HHS family with novel FGF23 c.413T > G, p.Leu138Arg and c.491T > A, p.Ile164Asn variants. We clarified the bone microarchitectures of HFTC/HHS patients by HR-pQCT, and expanded the genotype-phenotype spectrum of the disease. In vivo studies suggested that O-glycosylation of FGF23 plays an important role in the pathogenesis of HFTC/HHS, providing further understanding of the disease mechanism.
高磷血症性家族性肿瘤性钙质沉着症/骨质硬化-高磷血症综合征（HFTC/HHS）是一种由成纤维细胞生长因子 23（FGF23）缺乏或抵抗引起的罕见疾病。本研究报道了一个中国 HFTC/HHS 家系，旨在阐明该病的临床特征、骨微观结构和分子机制。
收集 2 例 HFTC 患者的临床表现、实验室检查和基因分析资料。采用高分辨率外周定量计算机断层扫描（HR-pQCT）检测骨微观结构。通过 Western blot 和麦胚凝集素亲和层析分析，研究突变型和野生型 FGF23 蛋白的表达和糖基化。通过免疫细胞化学检测 FGF23 蛋白的亚细胞定位。
这两兄弟均携带 FGF23 基因中先前未报道的 c.413T > G，p.Leu138Arg 和 c.491T > A，p.Ile164Asn 复合杂合变异，根据美国医学遗传学学院（ACMG）标准和指南，该变异被归类为“可能致病性”。两名患者均有严重的高磷血症和显著升高的 C 端 FGF23。合并 HHS 的患者 1 表现为下肢疼痛和广泛的心血管钙化。其桡骨远端和胫骨的 HR-pQCT 显示体积 BMD 和皮质厚度降低，这与 X 线表现的骨质硬化不一致。他接受了依替膦酸治疗，改善了 BMD 和异位钙化。他的兄弟骨受累较少，但从小就经历过复发性疼痛性钙化肿块，并接受了多次切除术。体外实验表明，突变型 FGF23 蛋白存在缺陷的 O-糖基化和受损的分泌。然而，野生型和突变型 FGF23 蛋白的亚细胞定位没有差异。
我们报道了一个中国 HFTC/HHS 家系，存在新的 FGF23 c.413T > G，p.Leu138Arg 和 c.491T > A，p.Ile164Asn 变异。我们通过 HR-pQCT 阐明了 HFTC/HHS 患者的骨微观结构，并扩展了疾病的基因型-表型谱。体内研究表明，FGF23 的 O-糖基化在 HFTC/HHS 的发病机制中起重要作用，为进一步了解疾病机制提供了依据。
