Full Length Article Zoledronic acid improves bone histomorphometry in a murine model of Rett syndrome Jay R. Shapiro a , Adele L. Boskey b , Stephen B. Doty b , Lyudmila Lukashova b , Mary E. Blue a, ⁎ a Hugo W. Moser Research Institute at Kennedy Krieger, Inc., 707 North Broadway, Baltimore, MD 21205, USA b Hospital for Special Surgery, Mineralized Tissue Laboratory, 535 E 70th Street, New York, NY 10021, USA abstract article info Article history: Received 7 September 2016 Revised 6 February 2017 Accepted 15 March 2017 Available online 18 March 2017 Rett syndrome (RTT) is a neurodevelopmental disorder predominately affecting young females, caused by defi- ciency of the global transcriptional protein methyl CpG binding protein 2 (MeCP2). Osteoblasts express MeCP2 and girls with RTT experience early onset osteoporosis, decreased bone mass and an increased fracture risk. There is no defined treatment for osteoporosis associated with RTT. The present study evaluated the effects of zo- ledronic acid (ZA), a third generation nitrogen-containing bisphosphonate with primarily anti-osteoclastic activ- ity, in a mouse model of MeCP2 deficiency. Mice received weekly injections of 20 μg/kg ZA for six weeks. Due to the shortened lifespan of hemizygous male (Mecp2-null) mice, treatment began at 3 weeks of age for this group and corresponding wildtype (WT) male mice. Treatment for heterozygous (HET) and WT female mice began at 8 weeks of age. Micro-computed tomography (micro-CT) and dynamic analyses of bone turnover were per- formed. ZA treatment led to significant increases in bone volume fraction, number, connectivity density and ap- parent density of trabecular bone in all genotypes of mice. In contrast, cortical bone generally was unaffected by ZA injections. Parameters of bone turnover, including mineral apposition rate, labeled bone surface and bone for- mation rate decreased after treatment with ZA. Mecp2-null mice had reduced labeled bone surface and bone for- mation rate compared to WT male mice. The results indicate that ZA treatment significantly improved trabecular bone mass in a murine model of RTT with little effect on cortical bone. © 2017 Elsevier Inc. All rights reserved. Keywords: Rett syndrome Bone histomorphometry MeCP2 Micro-CT Zoledronic acid Bone turnover Mouse model 1. Introduction Rett Syndrome (RTT) is a neurodevelopmental disorder caused by mutations of methyl CpG binding protein 2 (MECP2) located on the X chromosome [1]. Primarily affecting young females, RTT is one of the most common genetic causes of severe intellectual disability in females [2,3]. Males with RTT due to X-chromosome mutations in the MECP2 gene are more severely affected and often die in infancy [4]. In RTT, brain development is impaired in the presence of MeCP2 deficiency as a consequence of defective neuron development and impaired synaptic function [5–9]. Of note, MeCP2 has both gene repressor and activator functions in neural tissues [10], which may alter gene expression as well as treatment responses in different tissues. In addition to neural tissues, MeCP2 also is expressed by osteoblasts [11]. Osteoporosis is a recognized complication of RTT. Approximately 50% of affected females with RTT have low bone mass, low bone mineral content (BMC), low bone mineral density (BMD) and an increased risk for fractures [12–16]. A clinical report in one patient with RTT who had severely osteoporotic bones, found a complete cessation of bone fractures and a significant regression of osteoporosis after treatment for 3 years with intravenous pamidronate, a second generation antiresorptive bisphosphonate [17]. Caffarelli et al. reported that treat- ment of an 18 year-old osteoporotic female with teriparatide for 8 months improved distal radius bone density and whole body BMD that was then maintained with no new fractures by treatment with the bisphosphonate neridronate [18]. Another clinical study of a 28 year-old patient with teriparatide also showed favorable results [19]. However teriparatide is not approved for use in young females in the USA. Recently, a consensus report on clinical guidelines for the Bone 99 (2017) 1–7 Abbreviations: ρ app , apparent density; BFR/BS, bone formation rate per unit of bone surface; BV/TV, bone volume fraction; Ct.Th, cortical thickness; Mecp2-null, hemizygous male missing Mecp2; HET, heterozygous female; MeCP2, human and mouse protein methyl CpG binding protein; MECP2, human gene for methyl CpG binding protein; MAR, mineral apposition rates; Mecp2, mouse gene for methyl CpG binding protein; MS/BS, labeled bone surface; pMOI, polar moment of inertia; RTT, Rett syndrome; SD, standard deviation; TMD, tissue mineral density; Conn.D, trabecular connectivity density; Tb.N, trabecular number; Tb.Sp, trabecular separation; Tb.Th, trabecular thickness; TRAP, tartrate resistance acid phosphatase; Col1α1, type 1 collagen alpha1 chain; VOI, volume of interest; WT, wild type; XCI, X-chromosome inactivation. ⁎ Corresponding author at: Room 400R, Kennedy Krieger Institute, 707 North Broadway, Baltimore, MD 21205, USA. E-mail addresses: BoskeyA@HSS.EDU (A.L. Boskey), DotyS@HSS.EDU (S.B. Doty), LukashovaL@HSS.EDU (L. Lukashova), blue@kennedykrieger.org (M.E. Blue). http://dx.doi.org/10.1016/j.bone.2017.03.040 8756-3282/© 2017 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Bone journal homepage: www.elsevier.com/locate/bone