Knock-in human FGFR3 achondroplasia mutation as a mouse model for human skeletal dysplasia

Achondroplasia (ACH), the most common genetic dwarfism in human, is caused by a gain-of function mutation in fibroblast growth factor receptor 3 (FGFR3). Currently, there is no effective treatment for ACH. The development of an appropriate human-relevant model is important for testing potential therapeutic interventions before human clinical trials. Here, we have generated an ACH mouse model in which the endogenous mouse Fgfr3 gene was replaced with human FGFR3G380R (FGFR3ACH) cDNA, the most common mutation in human ACH. Heterozygous (FGFR3ACH/+) and homozygous (FGFR3ACH/ACH) mice expressing human FGFR3G380R recapitulate the phenotypes observed in ACH patients, including growth retardation, disproportionate shortening of the limbs, round head, mid-face hypoplasia at birth, and kyphosis progression during postnatal development. We also observed premature fusion of the cranial sutures and low bone density in newborn FGFR3G380R mice. The severity of the disease phenotypes corresponds to the copy number of activated FGFR3G380R, and the phenotypes become more pronounced during postnatal skeletal development. This mouse model offers a tool for assessing potential therapeutic approaches for skeletal dysplasias related to over-activation of human FGFR3, and for further studies of the underlying molecular mechanisms.

form of human FGFR3 14 , parathyroid hormone 15 , and statins 16 have been shown to improve bone growth in genetically manipulated ACH or TD I mouse models in vivo or ex vivo. The effects of statins have been examined using the in vitro human-relevant model of chondrocytes differentiated from induced pluripotent stem cells from either TD I or ACH patients. The C-type natriuretic peptide analogue has reached clinical trials 17 . It is important to develop a human-relevant in vivo model to provide a robust system for testing potential therapeutic interventions before human clinical trials.
In this report, we developed a human-relevant ACH mouse model by replacing mouse Fgfr3 with human FGFR3 cDNA containing the FGFR3 G380R ACH mutation. The clinical phenotypes and histology of bone abnormalities were characterised in the mutant mice. This FGFR3 ACH mouse model closely recapitulates human ACH. As such, it offers a valuable tool for assessing potential therapeutic approaches designed to target the over-activation of human FGFR3.

Results
Generation of FGFR3 ACH and FGFR3 WT mice. To generate FGFR3 ACH mice, we used a gene-targeting approach to replace the mouse Fgfr3 with human FGFR3 cDNA carrying the ACH mutation (FGFR3 ACH ) under the full control of the endogenous mouse Fgfr3 promoter, intron 1, and 5′ and 3′ untranslated regions (Fig. 1A). Human WT FGFR3 (FGFR3 WT ) cDNA was introduced into Fgfr3 through the same approach to generate control mice for comparison. Southern blotting (Fig. 1B) and polymerase chain reaction (PCR) of genomic DNA detected the FGFR3 ACH cDNA within Fgfr3 in embryonic stem cells (Fig. 1C). PCR of genomic DNA detected the human FGFR3 ACH cDNA and mouse genomic Fgfr3 DNA from heterozygous (FGFR3 ACH/+ ), homozygous (FGFR3 ACH/ACH ) and WT mice (Fig. 1D). The expression of the human FGFR3 ACH gene and endogenous mouse Fgfr3 gene in ACH and WT mice was determined in the left hind-limb of neonatal mice by RT-PCR using gene specific primers (Fig. 1E).
Skeletal abnormalities in newborn FGFR3 ACH mice. The features of human ACH patients can be readily identified clinically and radiologically at birth. At birth, there were no obvious differences in appearance between FGFR3 ACH/+ or FGFR3 ACH/ACH mice, collectively termed FGFR3 ACH , and their WT littermates (Supplemental Fig. 1A). We therefore analysed the bone structure of newborn mice. The newborn FGFR3 ACH mice showed proximal limb shortening with relatively normally sized trunks ( Fig. 2A). Femur length was reduced by 15% in FGFR3 ACH/+ mice and 42% in FGFR3 ACH/ACH mice compared with WT mice (Fig. 2C). A closer view of the skull structure revealed the skull was rounded and the calvarial bones were distorted in FGFR3 ACH mice, due to a positional shift and compression of the frontal and parietal bones (Fig. 2B). The jugum limitans, i.e., the cranial suture that separates the frontal and nasal bones, was absent in FGFR3 ACH mice (Fig. 2B). The metopic sutures, which line the midline between the two nasal bones, were unilaterally fused or partially absent in FGFR3 ACH mice (Fig. 2B). Thus, newborn FGFR3 ACH mice exhibited premature suture closure and abnormal skull shapes. Furthermore, a shorter intervertebral distance between cervical vertebrae (Supplemental Fig. 1B) and a narrower rib cage (Supplemental Fig. 1D) were observed in FGFR3 ACH newborns. These phenotypes are similar in many respects to the skeletal deformities in human ACH newborns 18 , and the bone abnormalities are more evident in FGFR3 ACH/ACH mice than in FGFR3 ACH/+ mice.  (Fig. 3C) phenotypes could be readily observed in FGFR3 ACH mice at 10 days to 1 month of age. All FGFR3 ACH/ACH mice developed kyphosis phenotypes at around 2 weeks of age, and about 90% of FGFR3 ACH/+ mice developed kyphosis phenotypes before 1 month of age. In addition, protrusion of the lower incisors was observed in FGFR3 ACH mice (Fig. 3D) because of changes in the skull affecting the alignment of the incisors. FGFR3 ACH/ACH mice had a significantly lower survival rate at birth relative to expectations and a higher mortality rate before 4 weeks of age compared with FGFR3 ACH/+ and WT mice (Fig. 3E), and the majority of FGFR3 ACH mice died at around 1 year of age. Mean body weights and body lengths were decreased in FGFR3 ACH/+ and FGFR3 ACH/ACH mice (Fig. 3F). FGFR3 ACH/+ mice exhibited intermediate body weights and lengths between those of the WT and FGFR3 ACH/ACH mice, indicating a dose-dependent effect of activated FGFR3 G380R . In contrast, the control FGFR3 WT/+ or FGFR3 WT/WT mice expressing non-mutated human FGFR3 showed identical external phenotypes to those of WT (Supplemental Fig. 2A). The growth rates of WT, FGFR3 WT/+ , and FGFR3 WT/WT mice were the same (Supplemental Fig. 2B).

Pronounced skeletal abnormalities in
Two-dimensional micro-computed tomography (micro-CT) was used to examine the skeletal abnormalities in FGFR3 ACH mice. The skeletal bone revealed dwarfism, rounded skulls, and severe curvature of the cervical and upper thoracic vertebrae in FGFR3 ACH mice ( Fig. 4A-C). FGFR ACH/ACH mice exhibited more severe phenotypes compared with those of FGFR3 ACH/+ mice ( Fig. 4A-C). Furthermore, these phenotypes became more pronounced in older mice (based on comparison among the phenotypes of 1-, 4-, and 12-month-old mice in Fig. 4A-C. Close observation of the skulls and vertebrae of FGFR3 ACH mice revealed shortened snouts and dome-shaped skulls ( Fig. 4D-F), and almost completely folded upper thoracic vertebrae in FGFR ACH/ACH and older FGFR ACH/+ mice ( Fig. 4G-I). The severities of these phenotypes were more consistent among FGFR3 ACH/ACH mice, as compared with FGFR3 ACH/+ mice, as shown by the smaller variation in the body lengths of FGFR3 ACH/ACH mice compared with that of FGFR3 ACH/+ mice (Fig. 3F). This is relevant because the variation in the severities of the short snout, rounded-head, and kyphosis phenotypes is represented in the body length.
Patients with ACH present with rhizomelic (short-limbed) dwarfism. This phenotype was reproduced in the FGFR3 ACH/+ mice, which showed a 22% shortening of femur length along with a 7.1% shortening of body length at 1 month of age, compared with the corresponding measurements in WT mice ( Table 1). The results suggested that the limbs were disproportionately shortened relative to body length in FGFR3 ACH/+ mice. Furthermore, the Scientific RepoRts | 7:43220 | DOI: 10.1038/srep43220 femurs were short, curved, and thick with widened diaphyses and flared metaphyses in FGFR3 ACH mice (Fig. 4J), which are very similar to phenotypes observed in ACH patients.
Altered chondrocyte proliferation and differentiation in FGFR3 ACH mice. Femur length is significantly reduced in FGFR3 ACH mice ( Fig. 2C and Table 1). To examine defects in the long bones of FGFR3 ACH mice more closely, we performed a histological analysis of the distal femur from WT and FGFR3 ACH mice at different developmental stages. The epiphyseal structure was similar between the WT FGFR3 ACH mice at birth (Fig. 5A). The secondary ossification centre was readily formed in WT mice at 1 week of age, whereas its formation was markedly delayed in FGFR3 ACH mice (Fig. 5A,B), suggesting a delay in chondrocyte terminal differentiation. In endochondral ossification, chondrocytes sequentially transit through resting, proliferating, and hypertrophic The neomycin resistance cassette in the identified stem cells was removed by Flp/FRT excision and analysed by PCR amplification and EcoRI digestion. A 528 bp PCR product was present in the stem cells without the neomycin resistance cassette, and the 328 bp and 254 bp fragments produced by EcoRI digestion of the PCR product could be detected. (D) PCR amplification analysis of genomic DNA isolated from WT and FGFR3 ACH mice. A 1067 bp PCR product was amplified from the mouse Fgfr3 locus. A 506 bp PCR product was amplified from the human FGFR3 G380R targeted allele. (E) The mRNA expression of targeted human FGFR3 G380R and endogenous mouse Fgfr3 in the heterozygous FGFR3 G380R , homozygous FGFR3 G380R , and WT mice was determined by RT-PCR using sequence-specific primers. ACH/+, the heterozygous FGFR3 ACH/+ mice; ACH/ACH, the homozygous FGFR3 ACH/ACH mice; +/+, wild type littermates.
stages. The FGFR3 ACH mice showed good development of each stage. However, the growth plates were significantly shorter in FGFR3 ACH mice with a shorter proliferative zone at 2, 4, and 8 weeks of age (Fig. 5B,C). This was caused by a reduction in the number of proliferative chondrocytes, indicating that chondrocyte proliferation was compromised in FGFR3 ACH mice. Despite the shorter proliferative zone, the arrangement of chondrocyte columns in the growth plate remained normal in FGFR3 ACH mice before 2 weeks of age. The disturbed arrangement of chondrocyte columns in FGFR3 ACH mice can be appreciated at 4 and 8 weeks of age, and their arrangement was disrupted by an increased amount of space between the columns (Fig. 5B). We further showed that FGFR3 ACH mice had higher FGFR3 phosphorylation in chondrocytes of growth plates (Fig. 5D) and the primary chondrocytes had lower proliferation rates compared with those from WT mice (Fig. 5E), suggesting that FGFR3 activation inhibited chondrocyte proliferation in FGFR3 ACH mice.
Altered bone formation in FGFR3 ACH mice. Low bone density has been reported in adult ACH patients 19 , which may have clinical relevance and lead to subsequent bone damage. The development of the long bones is coordinated between chondrogenesis and osteogenesis. Reduced growth of the longitudinal trabecular bone was observed in the distal femoral metaphysis of FGFR3 ACH mice at several stages of postnatal development (Fig. 5A, stained in blue). Furthermore, the expression of osteocalcin, which is associated with the early stages of matrix ossification, was increased in the chondrocytes of the hypertrophic zone of the distal femur of FGFR3 ACH mice at 2 weeks of age (Fig. 5F). A reduced hypertrophic zone was observed in FGFR3 ACH mice at 8 weeks of age (Fig. 5B,C). These results indicate that the bone-forming process was disturbed in FGFR3 ACH mice. To determine the structure of trabecular bone, we performed a micro-CT analysis. Three-dimensional images of the distal femoral metaphysis showed a lower bone volume with thinner and fewer trabecular bones and larger intertrabecular spaces in newborn and 1-year-old FGFR3 ACH mice compared to WT mice (Fig. 5G). A histomorphometric analysis of bone formation showed that the trabecular bone volume (BV/TV), trabecular thickness (Tb.Th), and trabecular number (Tb.N) were decreased, along with an increased trabecular separation (Tb.Sp) and structure model index (SMI) in the distal femoral metaphysis of FGFR3 ACH mice compared with WT mice at birth and at 1 year of age (Table 2). Furthermore, we observed fewer osteoblasts and osteoclasts in the femurs of FGFR3 ACH mice at 1 year of age (Fig. 5H), suggesting that the bone turnover rate might be altered in FGFR3 ACH mice.

Discussion
In this study, we describe an ACH mouse model (FGFR3 ACH ), which expresses human FGFR3 G380R , the most common mutation in human ACH patients. Our mouse model recapitulates the main human ACH phenotypes and offers a valuable tool for studying pathological conditions and testing potential therapeutic approaches designed to target the over-activation of human FGFR3 and its downstream signaling pathway in vivo. In addition to the postnatal phenotypes observed in previous ACH mouse models 8,10,11 , we showed that FGFR3 ACH mice shared most of the key clinical phenotypes observed in human ACH patients at birth, including rhizomelic dwarfism, rounded skull, and midface hypoplasia. Furthermore, we described craniosynostosis and low bone density in newborn FGFR3 ACH mice. These phenotypes became more pronounced during postnatal skeletal development and were correlated with the dose of FGFR3 ACH . Kyphosis, which is common in human ACH during postnatal The body length, measured from nose to tail base as indicated in (C). Body weights and body lengths of mice were measured monthly from birth until 12 months of age. Each curve shows the average Body weights and body lengths of animals from several litters. Data points represent means ± SD; n values are shown in parentheses after group names. development, develops postnatally in FGFR3 ACH mice. Although previously generated ACH mouse models share some ACH phenotypes, some phenotypes have not been fully observed or described in these ACH mouse models. Here, we described the full range of ACH abnormalities in this mouse model. A comparison of skeletal phenotypes between human achondroplasia and ACH mouse models is summarised in Table 3.
Previously, two ACH mouse models were generated by either targeting 8 or transgenically expressing 10 murine Fgfr3 G374R (an ortholog of the human mutation FGFR3 G380R , Fgfr ACH ). Another ACH mouse model transgenically expresses human FGFR G380R under the control of the mouse Fgfr3 promoter region 11 . All these ACH mouse models exhibit some human ACH phenotypes during postnatal development. However, rhizomelic (short-limbed) dwarfism was not obvious, and dwarfism phenotypes were not described at birth in both Fgfr ACH models. In addition, the transgenic model expressing Fgfr3 ACH under the control of the collagen II promoter presented some specific defects that are not usually observed in human ACH patients due to misexpression of Fgfr3 ACH 10 . The transgenic human FGFR3 G380R mice presented the rhizomelic dwarf phenotype at birth 11 . However, thoracic kyphosis was not described for this model 11 . Moreover, homozygous FGFR3 G380R transgenic mice expressed more severe phenotypes than the two Fgfr3 ACH mouse models or our FGFR3 ACH model, and died shortly after birth. The severe phenotypes in homozygous FGFR3 G380R transgenic mice might be because the mice express both endogenous Fgfr3 and human FGFR3 G380R . Most homozygous ACH patients are stillborn or die during the neonatal period. Although our FGFR3 ACH/ACH mice exhibited a higher mortality rate than the WT mice at birth and before 4 weeks of age, 60% of FGFR3 ACH/ACH mice survived after 1 month in our study. The homozygous knock-in or transgenic Fgfr3 ACH mice survived after birth, but their neonatal survival rate was not reported. The human and mouse FGFR3 genes share 92.7% identity and 94.9% similarity. The various phenotypes and different levels of severity observed in different ACH mouse models might be caused by the different promoter chosen and various copy numbers of activated FGFR3 in the transgenic models, the different knock-in approaches used in the KI models, and the different genetic backgrounds of the mice assayed. Having access to various ACH mouse models provides the opportunity for choosing particular ACH phenotypes of interest for further studies. Nevertheless,   Table 1. Body length and femur length of 1-month-old mice. WT: wild-type littermates; ACH/+: FGFR3 ACH/+ mice. Each value is expressed as the mean ± SD (n values shown in column headers). ***p < 0.001.  The biological basis of dwarfism phenotypes in ACH patients involves a specific defect in endochondral ossification and longitudinal bone growth 1 . The molecular consequences of over-activation of FGFR3 on chondrocyte proliferation and differentiation in the growth plate have been well established 1 . Here, we demonstrated that over-activated human FGFR3 G380R has inhibitory effects on chondrocyte proliferation and maturation in an in vivo mouse model. FGFR3 mutations might also affect membranous ossification. Gain-of-function mutations of FGFR3 (P250R and A391E) have been shown to cause human Muenke syndrome and Crouzon syndrome with acanthosis nigricans 20,21 . Affected patients undergo premature fusion of the cranial sutures. Furthermore, TD patients also frequently exhibit severe craniosynostosis phenotypes 22 . ACH patients present craniofacial phenotypes suggesting the possibility of craniosynostosis defects. Recent report showed that ACH is associated with craniosynostosis and suggested that craniosynostosis may be under-reported 23 . An increased understanding of premature fusion of the cranial sutures and craniofacial phenotypes in ACH at early stages will facilitate improved treatment of these defects. For the first time, we have demonstrated here that the cranial sutures undergo premature fusion in newborn ACH mice, which offers the opportunity to better understand the development of cranial anomalies in ACH, and further study membranous ossification.
During the postnatal and adult stages, bone undergoes continuous remodelling through the coordinated processes of bone formation and bone resorption 24 . Low bone density has been reported in adult ACH 19 , which may have clinical relevance and lead to subsequent bone damage, such as increased fragility and risk of fracture. Enhanced osteoblast differentiation has been observed in long bone growth plates of Fgfr3 G369C mice at the age of 15 days 9 , suggesting advanced ossification at an early stage. Recent studies in Fgfr3 G369C mice revealed enhanced osteogenic differentiation in cultured bone marrow stromal cells, and this was associated with decreased bone mass at 2 months of age 25 . Here, we demonstrated that the expression of osteocalcin was increased in the chondrocytes of the hypertrophic zone of the distal femur in FGFR3 ACH mice at 2 weeks of age, suggesting enhanced osteoblast differentiation in FGFR3 ACH mice. Recently, a study revealed that bone density was reduced in the majority of ACH and HCH patients in the age range of 10-33 years, which is indicative of osteopenia 26 . Here, we provide direct evidence showing a low bone density in newborn and adult FGFR3 ACH mice. Furthermore, we observed fewer osteoclasts and osteoblasts in the femur of FGFR3 ACH mice at 1 year of age. Both endochondral ossification and bone remodelling regulate bone mass in adults, suggesting that altered bone remodelling might also contribute to the lower bone mass of adult FGFR3 ACH mice. Potential changes in bone structure should be further evaluated in neonatal ACH patients to determine whether an adequate diet and exercise may help to prevent any such osteopenia at early developmental stages. Our mouse model offers a good opportunity for testing interventions for early onset osteopenia in ACH.
The mouse is a convenient animal model for the study of human genes and diseases, which has led to the development of treatments for many serious diseases and conditions. However, mice are not always reliable as preclinical models for human disease. Many drugs have shown promising results in preclinical trials in mice but later failed in human clinical trials. As such, there is a need to develop reliable preclinical mouse models of human diseases for clinical research and drug development. Mice expressing a mutated version of a human gene known to be associated with a specific human disease and faithfully mimicking the disease phenotypes can be useful for studying disease pathology, conducting preclinical research, and testing compound efficacy in vivo. We have generated an ACH mouse model that faithfully and comprehensively recapitulates the disease phenotypes, enabling the evaluation of potential treatments targeting the over-activation of human FGFR3 and its downstream signaling.

Methods
(Full experimental details are provided in the Supplemental Data).  Table 3. Similarity of skeletal features found in human achondroplasia and observed in achondroplasia mouse models. Tg mFgfr3 ACH , transgenic mice expressing mouse Fgfr3 G374R using the type II collagen promoter and enhancer sequences 10 . KI mFGFR3 ACH , gene targeting mouse Fgfr3 G374R 8 . Tg hFGFR3 ACH , transgenic mice expressing human FGFR3 G380R using the mouse Fgfr3 promoter 11 . KI hFGFR3 ACH , gene targeting human FGFR3 G380R as described in this report. ND, not described; NS, not significant. a The time point when the specific phenotype was first observed in each ACH mouse model.