The role of GGPS1 and CYP1A1 mutations in atypical femoral fracture

摘要 非典型股骨骨折(AFF)是一个主要的临床问题，对患者和 对于整个骨质疏松症人群来说，因为他们对患者的治疗意愿产生了巨大的影响 服用有效的骨质疏松症药物。此外，尽管AFF与长期的 在双磷酸盐(BPS)治疗中，许多患有AFF的患者(高达30%)从未服用过BPS。 这一观察表明存在罕见的基因突变，这些突变构成了这种情况和/或 通过增加某些患者对BPS的敏感度，使他们易于接受AFF。罕见突变的鉴定 预先给予AFF，特别是在使用BPS治疗之后，将允许预防许多此类情况 通过为这类患者选择适当的治疗方法。因此，理解遗传和 AFF的分子基础和骨干脆性在临床上具有极其重要的意义。到今天为止，几乎没有 已知AFF的发病机制，没有AFF的动物模型或骨干易感性的动物模型 骨折是可用的。值得注意的是，Diez-Perez博士团队最近的一项潜在突破可能会提供 有机会在填补这一知识缺口方面采取初步步骤。具体地说，3个姐妹的外显子组测序 接受BP治疗的AFF患者在3例中发现了突变，显示或预测会损害功能 甲氧丙戊酸途径的组成部分，正是BP靶向抑制骨吸收的途径。 此外，3名无血缘关系的患者中有1名和单独的队列显示出CYP1A1突变，而在 GGPS和法尼基焦磷酸合成酶(FPPS)也在2名患者中被发现 成骨不全(OI)V型进一步表明这一途径与骨脆性之间存在联系。事实是 这一途径是含氮双膦酸盐的靶标，这有力地表明这些 突变，通过模仿/放大BPS的作用，可能使患者倾向于AFF。在这里，我们将测试 甲氧丙戊酸途径中涉及的一个或多个关键酶的突变易患 对AFF，证据是BP治疗后骨干脆性增加 目的1)体外：确定Ggps1和/或Cyp1A1突变是否以及如何复制 AFF三姐妹对OC、OB、OCY分化和功能的影响及其意义 对含氮双膦酸盐的反应。 目的2)在体内：确定Ggps1和/或Cyp1A1杂合缺失的小鼠是否表现出 在OVX和NBPS治疗前和/或后骨干脆性。 尽管这一途径的突变可能只构成AFF患者的一个子集，但这一建议 可以证实特定的基因改变形成AFF的背景并提供第一个， 即使有潜在的不完善之处，动物模型也无法理解这些有害骨折的机制。

Abstract Atypical Femoral Fractures (AFFs) are a major clinical problem, both for the patients that suffers them and for the overall osteoporotic population because of their enormous impact on patients' willingness to be treated with effective osteoporosis medications. Also, although AFF has been associated with long-term bisphosphonates (BPs) treatment, a number of patients (up to 30%) who suffer AFF have never taken BPs. This observation suggests the presence of rare genetic mutations that form the basis of this condition and/or predispose certain patients to AFF by increasing their sensitivity to BPs. Identification of rare mutations that predispose to AFF, in particular after treatment with BPs, would permit the prevention of many of these fractures by selecting the appropriate treatment for such patients. Thus, understanding the genetic and molecular basis of AFF and diaphyseal fragility is of the utmost clinical importance. As of today, little is known about the pathogenesis of AFF and no animal model of AFF or of susceptibility to diaphyseal fractures is available. Notably, a recent potential breakthrough by Dr. Diez-Perez group may offer the opportunity to make initial steps in filling this knowledge gap. Specifically, exome sequencing in 3 sisters treated with BPs who suffered AFFs has identified mutations, shown or predicted to impair function, in 3 components of the mevalonate pathway, the very pathway targeted by BPs to inhibit bone resorption. Furthermore, 1 of 3 unrelated patients and a separate cohort showed mutations in CYP1A1, and mutations in GGPS and Farnesyl pyrophosphatase synthase (FPPS) have also been identified in 2 patients with Osteogenesis Imperfecta (OI) type V further suggesting a link between this pathway and bone fragility. The fact that this pathway is the target of nitrogen-containing bisphosphonates strongly suggests that these mutations, by mimicking/amplifying BPs action, may predispose patients to AFFs. Here, we will test the hypothesis that mutations in one or more of the key enzymes involved in the mevalonate pathway predispose to AFF, evidenced as increased diaphyseal fragility after BP treatment Aim 1) In vitro: Determine whether and how replication of the Ggps1 and/or Cyp1A1 mutations observed in the 3 sisters with AFF affect OC, OB and OCY differentiation and function and their responses to nitrogen-containing bisphosphonates. Aim 2) In vivo: Determine whether mice with heterozygous Ggps1 and/or Cyp1A1 deletion exhibit diaphyseal fragility before and/or after OVX and treatment with NBPs. Although mutations in this pathway probably constitute only a subset of the patients with AFF, this proposal may validate the concept that specific genetic alterations form the background of AFFs and provide the first, even though potentially imperfect, animal model to understand the mechanisms of these deleterious fractures.