Department of Pathology, School of Medicine, University of Pittsburgh, Pittsburgh, PA, United States.
Department of Pathology, School of Medicine, University of Pittsburgh, Pittsburgh, PA, United States.
Mol Genet Metab. 2018 Nov;125(3):193-199. doi: 10.1016/j.ymgme.2018.08.010. Epub 2018 Aug 27.
Osteopenia is observed in some patients affected by phenylalanine hydroxylase (PAH) deficient phenylketonuria (PKU). Bone density studies, in diverse PKU patient cohorts, have demonstrated bone disease is neither fully penetrant nor uniform in bone density loss. Biochemical assessment has generated a muddled perspective regarding mechanisms of the PKU bone phenotype where the participation of hyperphenylalaninemia remains unresolved. Osteopenia is realized in the Pah mouse model of classical PKU; although, characterization is incomplete. We characterized the Pah bone phenotype and assessed the effect of hyperphenylalaninemia on bone differentiation. Employing Pah and control animals, cytology, static and dynamic histomorphometry, and biochemistry were applied to further characterize the bone phenotype. These investigations demonstrate Pah bone density is decreased 33% relative to C57BL/6; bone volume/total volume was similarly decreased; trabecular thickness was unchanged while increased trabecular spacing was observed. Dynamic histomorphometry demonstrated a 25% decrease in mineral apposition. Biochemically, control and PKU animals have similar plasma cortisol, adrenocorticotropic hormone, and 25-hydroxyvitamin D. PKU animals show moderately increased plasma parathyroid hormone while plasma calcium and phosphate are reduced. These data are consistent with a mineralization defect. The effect of hyperphenylalaninemia on bone maturation was assessed in vitro employing bone-derived mesenchymal stem cells (MSCs) and their differentiation into bone. Using standard culture conditions, PAH deficient MSCs differentiate into bone as assessed by in situ alkaline phosphatase activity and mineral staining. However, PAH deficient MSCs cultured in 1200 μM PHE (metric defining classical PKU) show significantly reduced mineralization. These data are the first biological evidence demonstrating a negative impact of hyperphenylalaninemia upon bone maturation. In PAH deficient MSCs, expression of Col1A1 and Rankl are suppressed by hyperphenylalaninemia consistent with reduced bone formation and bone turnover. Osteopenia is intrinsic to PKU pathology in untreated Pah animals and our data suggests PHE toxicity participates by inhibiting mineralization in the course of MSC bone differentiation.
骨质疏松症在一些患有苯丙氨酸羟化酶（PAH）缺乏型苯丙酮尿症（PKU）的患者中观察到。在不同的 PKU 患者队列中进行的骨密度研究表明，骨病既不完全穿透，也不均匀地丧失骨密度。生化评估对 PKU 骨表型的机制产生了混淆，其中高苯丙氨酸血症的参与仍未解决。骨质疏松症在经典 PKU 的 Pah 小鼠模型中表现出来；然而，其特征尚未完全确定。我们对 Pah 骨表型进行了特征描述，并评估了高苯丙氨酸血症对骨分化的影响。利用 Pah 和对照动物，细胞学、静态和动态组织形态计量学以及生物化学被用于进一步描述骨表型。这些研究表明，与 C57BL/6 相比，Pah 骨密度降低了 33%；骨体积/总体积也相应降低；小梁厚度不变，而观察到小梁间隔增加。动态组织形态计量学显示，矿化率降低了 25%。生化分析表明，对照和 PKU 动物的血浆皮质醇、促肾上腺皮质激素和 25-羟维生素 D 相似。PKU 动物表现出中等程度的甲状旁腺激素升高，而血浆钙和磷降低。这些数据与矿化缺陷一致。在体外利用骨源性间充质干细胞（MSCs）及其向骨的分化来评估高苯丙氨酸血症对骨成熟的影响。使用标准培养条件，PAH 缺陷型 MSCs 分化为骨，通过原位碱性磷酸酶活性和矿化染色来评估。然而，在 1200µM PHE（定义经典 PKU 的量度）中培养的 PAH 缺陷型 MSCs 矿化明显减少。这些数据是首次表明高苯丙氨酸血症对骨成熟有负面影响的生物学证据。在 PAH 缺陷型 MSCs 中，Col1A1 和 Rankl 的表达受高苯丙氨酸血症抑制，这与骨形成和骨转换减少一致。在未经治疗的 Pah 动物中，骨质疏松症是 PKU 病理的内在特征，我们的数据表明，PHE 毒性通过抑制 MSC 骨分化过程中的矿化参与其中。
