Hypochondroplasia gain-of-function mutation in FGFR3 causes defective bone mineralization in mice.

Hypochondroplasia (HCH) is a mild dwarfism caused by missense mutations in fibroblast growth factor receptor 3 (FGFR3), with the majority of cases resulting from a heterozygous p.Asn540Lys gain-of-function mutation. Here, we report the generation and characterization of the first mouse model (Fgfr3Asn534Lys/+) of HCH to our knowledge. Fgfr3Asn534Lys/+ mice exhibited progressive dwarfism and impairment of the synchondroses of the cranial base, resulting in defective formation of the foramen magnum. The appendicular and axial skeletons were both severely affected and we demonstrated an important role of FGFR3 in regulation of cortical and trabecular bone structure. Trabecular bone mineral density (BMD) of long bones and vertebral bodies was decreased, but cortical BMD increased with age in both tibiae and femurs. These results demonstrate that bones in Fgfr3Asn534Lys/+ mice, due to FGFR3 activation, exhibit some characteristics of osteoporosis. The present findings emphasize the detrimental effect of gain-of-function mutations in the Fgfr3 gene on long bone modeling during both developmental and aging processes, with potential implications for the management of elderly patients with hypochondroplasia and osteoporosis.

A Mouse Model with a Frameshift Mutation in the Nuclear Factor I/X (NFIX) Gene Has Phenotypic Features of Marshall-Smith Syndrome.

The nuclear factor I/X (NFIX) gene encodes a ubiquitously expressed transcription factor whose mutations lead to two allelic disorders characterized by developmental, skeletal, and neural abnormalities, namely, Malan syndrome (MAL) and Marshall-Smith syndrome (MSS). NFIX mutations associated with MAL mainly cluster in exon 2 and are cleared by nonsense-mediated decay (NMD) leading to NFIX haploinsufficiency, whereas NFIX mutations associated with MSS are clustered in exons 6-10 and escape NMD and result in the production of dominant-negative mutant NFIX proteins. Thus, different NFIX mutations have distinct consequences on NFIX expression. To elucidate the in vivo effects of MSS-associated NFIX exon 7 mutations, we used CRISPR-Cas9 to generate mouse models with exon 7 deletions that comprised: a frameshift deletion of two nucleotides (Nfix Del2); in-frame deletion of 24 nucleotides (Nfix Del24); and deletion of 140 nucleotides (Nfix Del140). Nfix +/Del2, Nfix +/Del24, Nfix +/Del140, Nfix Del24/Del24, and Nfix Del140/Del140 mice were viable, normal, and fertile, with no skeletal abnormalities, but Nfix Del2/Del2 mice had significantly reduced viability (p < 0.002) and died at 2-3 weeks of age. Nfix Del2 was not cleared by NMD, and NfixDel2/Del2 mice, when compared to Nfix +/+ and Nfix +/Del2 mice, had: growth retardation; short stature with kyphosis; reduced skull length; marked porosity of the vertebrae with decreased vertebral and femoral bone mineral content; and reduced caudal vertebrae height and femur length. Plasma biochemistry analysis revealed Nfix Del2/Del2 mice to have increased total alkaline phosphatase activity but decreased C-terminal telopeptide and procollagen-type-1-N-terminal propeptide concentrations compared to Nfix +/+ and Nfix +/Del2 mice. Nfix Del2/Del2 mice were also found to have enlarged cerebral cortices and ventricular areas but smaller dentate gyrus compared to Nfix +/+ mice. Thus, Nfix Del2/Del2 mice provide a model for studying the in vivo effects of NFIX mutants that escape NMD and result in developmental abnormalities of the skeletal and neural tissues that are associated with MSS. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Thyroid Hormone Transporter MCT10 Is a Novel Regulator of Trabecular Bone Mass and Bone Turnover in Male Mice.

Thyroid hormones (TH) are essential for skeletal development and adult bone homeostasis. Their bioavailability is determined by specific transporter proteins at the cell surface. The TH-specific transporter monocarboxylate transporter 8 (MCT8) was recently reported as a regulator of bone mass in mice. Given that high systemic triiodothyronine (T3) levels in Mct8 knockout (KO) mice are still able to cause trabecular bone loss, alternative TH transporters must substitute for MCT8 function in bone. In this study, we analyzed the skeletal phenotypes of male Oatp1c1 KO and Mct10 KO mice, which are euthyroid, and male Mct8/Oatp1c1 and Mct8/Mct10 double KO mice, which have elevated circulating T3 levels, to unravel the role of TH transport in bone. MicroCT analysis showed no significant trabecular bone changes in Oatp1c1 KO mice at 4 weeks and 16 weeks of age compared with wild-type littermate controls, whereas 16-week-old Mct8/Oatp1c1 double KO animals displayed trabecular bone loss. At 12 weeks, Mct10 KO mice, but not Mct8/Mct10 double KO mice, had decreased trabecular femoral bone volume with reduced osteoblast numbers. By contrast, lack of Mct10 in 24-week-old mice led to trabecular bone gain at the femur with increased osteoblast numbers and decreased osteoclast numbers whereas Mct8/Mct10 double KO did not alter bone mass. Neither Mct10 nor Mct8/Mct10 deletion affected vertebral bone structures at both ages. In vitro, osteoblast differentiation and activity were impaired by Mct10 and Mct8/Mct10-deficiency. These data demonstrate that MCT10, but not OATP1C1, is a site- and age-dependent regulator of bone mass and turnover in male mice.

Bone Phenotyping Approaches in Human, Mice and Zebrafish - Expert Overview of the EU Cost Action GEMSTONE ("GEnomics of MusculoSkeletal traits TranslatiOnal NEtwork").

A synoptic overview of scientific methods applied in bone and associated research fields across species has yet to be published. Experts from the EU Cost Action GEMSTONE ("GEnomics of MusculoSkeletal Traits translational Network") Working Group 2 present an overview of the routine techniques as well as clinical and research approaches employed to characterize bone phenotypes in humans and selected animal models (mice and zebrafish) of health and disease. The goal is consolidation of knowledge and a map for future research. This expert paper provides a comprehensive overview of state-of-the-art technologies to investigate bone properties in humans and animals - including their strengths and weaknesses. New research methodologies are outlined and future strategies are discussed to combine phenotypic with rapidly developing -omics data in order to advance musculoskeletal research and move towards "personalised medicine".

A Roadmap to Gene Discoveries and Novel Therapies in Monogenic Low and High Bone Mass Disorders.

Genetic disorders of the skeleton encompass a diverse group of bone diseases differing in clinical characteristics, severity, incidence and molecular etiology. Of particular interest are the monogenic rare bone mass disorders, with the underlying genetic defect contributing to either low or high bone mass phenotype. Extensive, deep phenotyping coupled with high-throughput, cost-effective genotyping is crucial in the characterization and diagnosis of affected individuals. Massive parallel sequencing efforts have been instrumental in the discovery of novel causal genes that merit functional validation using in vitro and ex vivo cell-based techniques, and in vivo models, mainly mice and zebrafish. These translational models also serve as an excellent platform for therapeutic discovery, bridging the gap between basic science research and the clinic. Altogether, genetic studies of monogenic rare bone mass disorders have broadened our knowledge on molecular signaling pathways coordinating bone development and metabolism, disease inheritance patterns, development of new and improved bone biomarkers, and identification of novel drug targets. In this comprehensive review we describe approaches to further enhance the innovative processes taking discoveries from clinic to bench, and then back to clinic in rare bone mass disorders. We highlight the importance of cross laboratory collaboration to perform functional validation in multiple model systems after identification of a novel disease gene. We describe the monogenic forms of rare low and high rare bone mass disorders known to date, provide a roadmap to unravel the genetic determinants of monogenic rare bone mass disorders using proper phenotyping and genotyping methods, and describe different genetic validation approaches paving the way for future treatments.

Analgesic use and the risk of renal cell carcinoma - Findings from the Consortium for the Investigation of Renal Malignancies (CONFIRM) study.

PURPOSE: The incidence of renal cell carcinoma (RCC) is rising. Use of analgesics such as non-steroidal anti-inflammatory drugs (NSAIDs) and paracetamol may affect renal function. The aim of this study was to assess associations between analgesic use and risk of RCC. METHODS: A population-based case-control family design was used. Cases were recruited via two Australian state cancer registries. Controls were siblings or partners of cases. Analgesic use was captured by self-completed questionnaire. Logistic regression was used to estimate odds ratios (OR) and 95% confidence intervals (CI) for RCC risk associated with regular analgesic use (at least 5 times per month for 6 months or more) and duration and frequency of use. RESULTS: The analysis included 1064 cases and 724 controls. Regular use of paracetamol was associated with an increased risk of RCC (OR 1.41, 95%CI 1.13-1.77). Regular use of NSAIDs was associated with increased risk of RCC for women (OR 1.71, 95% CI 1.23-2.39) but not men (OR 0.83, 95% CI 0.58-1.18; p-interaction=0.003). There was no evidence of a dose-response for duration of use of paracetamol (linear trend p = 0.77) and weak evidence for non- aspirin NSAID use by women (linear trend p = 0.054). CONCLUSION: This study found that regular use of paracetamol was associated with increased risk of RCC. NSAID use was associated with increased risk only for women.

Gaps in the tuberculosis preventive therapy care cascade in children in contact with TB.

BACKGROUND: Young children (<5 years) and children living with HIV in contact with an adult with tuberculosis (TB) should receive TB preventive therapy (TPT), but uptake is low. AIMS: To determine gaps in the uptake of and adherence to TPT in child TB contacts under routine primary care clinic conditions. METHODS: A cohort of child TB contacts (age <5 years or living with HIV <15 years) was followed at a primary care clinic in Johannesburg, South Africa. RESULTS: Of 170 child contacts with 119 adult TB cases, only 45% (77/170) visited the clinic for TPT eligibility screening, two of whom had already initiated TPT at another clinic. Of the 75 other children, 18/75 (24%) commenced TB treatment and 56/75 (75%) started TPT. Health-care workers followed the guidelines, with 96% (64/67) of children screened for symptoms of TB and 97% (36/37) of those symptomatic assessed for TB, but microbiological testing was low (9/36, 25%) and none had microbiologically confirmed tuberculosis. Only half (24/46, 52%) of the children initiating TPT completed the 6-month course. Neither sociodemographic determinants (age, sex) nor clinical factors (HIV status, TB source, time to TPT initiation) was associated with non-adherence to TPT. CONCLUSION: Most child contacts of an adult TB case do not visit the clinic, and half of those initiating TPT did not adhere to the full 6-month course. These programme failures result in missed opportunities for early diagnosis of active TB and prevention of progression to disease in young and vulnerable children.

Osteocyte transcriptome mapping identifies a molecular landscape controlling skeletal homeostasis and susceptibility to skeletal disease.

Osteocytes are master regulators of the skeleton. We mapped the transcriptome of osteocytes from different skeletal sites, across age and sexes in mice to reveal genes and molecular programs that control this complex cellular-network. We define an osteocyte transcriptome signature of 1239 genes that distinguishes osteocytes from other cells. 77% have no previously known role in the skeleton and are enriched for genes regulating neuronal network formation, suggesting this programme is important in osteocyte communication. We evaluated 19 skeletal parameters in 733 knockout mouse lines and reveal 26 osteocyte transcriptome signature genes that control bone structure and function. We showed osteocyte transcriptome signature genes are enriched for human orthologs that cause monogenic skeletal disorders (P = 2.4 × 10-22) and are associated with the polygenic diseases osteoporosis (P = 1.8 × 10-13) and osteoarthritis (P = 1.6 × 10-7). Thus, we reveal the molecular landscape that regulates osteocyte network formation and function and establish the importance of osteocytes in human skeletal disease.

A molecular quantitative trait locus map for osteoarthritis.

Osteoarthritis causes pain and functional disability for over 500 million people worldwide. To develop disease-stratifying tools and modifying therapies, we need a better understanding of the molecular basis of the disease in relevant tissue and cell types. Here, we study primary cartilage and synovium from 115 patients with osteoarthritis to construct a deep molecular signature map of the disease. By integrating genetics with transcriptomics and proteomics, we discover molecular trait loci in each tissue type and omics level, identify likely effector genes for osteoarthritis-associated genetic signals and highlight high-value targets for drug development and repurposing. These findings provide insights into disease aetiopathology, and offer translational opportunities in response to the global clinical challenge of osteoarthritis.

Osteoclasts recycle via osteomorphs during RANKL-stimulated bone resorption.

Osteoclasts are large multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage-derived precursors that are thought to undergo apoptosis once resorption is complete. Here, by intravital imaging, we reveal that RANKL-stimulated osteoclasts have an alternative cell fate in which they fission into daughter cells called osteomorphs. Inhibiting RANKL blocked this cellular recycling and resulted in osteomorph accumulation. Single-cell RNA sequencing showed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and express a number of non-canonical osteoclast genes that are associated with structural and functional bone phenotypes when deleted in mice. Furthermore, genetic variation in human orthologs of osteomorph genes causes monogenic skeletal disorders and associates with bone mineral density, a polygenetic skeletal trait. Thus, osteoclasts recycle via osteomorphs, a cell type involved in the regulation of bone resorption that may be targeted for the treatment of skeletal diseases.

Accelerating functional gene discovery in osteoarthritis.

Osteoarthritis causes debilitating pain and disability, resulting in a considerable socioeconomic burden, yet no drugs are available that prevent disease onset or progression. Here, we develop, validate and use rapid-throughput imaging techniques to identify abnormal joint phenotypes in randomly selected mutant mice generated by the International Knockout Mouse Consortium. We identify 14 genes with functional involvement in osteoarthritis pathogenesis, including the homeobox gene Pitx1, and functionally characterize 6 candidate human osteoarthritis genes in mouse models. We demonstrate sensitivity of the methods by identifying age-related degenerative joint damage in wild-type mice. Finally, we phenotype previously generated mutant mice with an osteoarthritis-associated polymorphism in the Dio2 gene by CRISPR/Cas9 genome editing and demonstrate a protective role in disease onset with public health implications. We hope this expanding resource of mutant mice will accelerate functional gene discovery in osteoarthritis and offer drug discovery opportunities for this common, incapacitating chronic disease.

Role of thyroid hormones in craniofacial development.

The development of the craniofacial skeleton relies on complex temporospatial organization of diverse cell types by key signalling molecules. Even minor disruptions to these processes can result in deleterious consequences for the structure and function of the skull. Thyroid hormone deficiency causes delayed craniofacial and tooth development, dysplastic facial features and delayed development of the ossicles in the middle ear. Thyroid hormone excess, by contrast, accelerates development of the skull and, in severe cases, might lead to craniosynostosis with neurological sequelae and facial hypoplasia. The pathogenesis of these important abnormalities remains poorly understood and underinvestigated. The orchestration of craniofacial development and regulation of suture and synchondrosis growth is dependent on several critical signalling pathways. The underlying mechanisms by which these key pathways regulate craniofacial growth and maturation are largely unclear, but studies of single-gene disorders resulting in craniofacial malformations have identified a number of critical signalling molecules and receptors. The craniofacial consequences resulting from gain-of-function and loss-of-function mutations affecting insulin-like growth factor 1, fibroblast growth factor receptor and WNT signalling are similar to the effects of altered thyroid status and mutations affecting thyroid hormone action, suggesting that these critical pathways interact in the regulation of craniofacial development.

IGSF1 Deficiency Results in Human and Murine Somatotrope Neurosecretory Hyperfunction.

CONTEXT: The X-linked immunoglobulin superfamily, member 1 (IGSF1), gene is highly expressed in the hypothalamus and in pituitary cells of the POU1F1 lineage. Human loss-of-function mutations in IGSF1 cause central hypothyroidism, hypoprolactinemia, and macroorchidism. Additionally, most affected adults exhibit higher than average IGF-1 levels and anecdotal reports describe acromegaloid features in older subjects. However, somatotrope function has not yet been formally evaluated in this condition. OBJECTIVE: We aimed to evaluate the role of IGSF1 in human and murine somatotrope function. PATIENTS, DESIGN, AND SETTING: We evaluated 21 adult males harboring hemizygous IGSF1 loss-of-function mutations for features of GH excess, in an academic clinical setting. MAIN OUTCOME MEASURES: We compared biochemical and tissue markers of GH excess in patients and controls, including 24-hour GH profile studies in 7 patients. Parallel studies were undertaken in male Igsf1-deficient mice and wild-type littermates. RESULTS: IGSF1-deficient adult male patients demonstrated acromegaloid facial features with increased head circumference as well as increased finger soft-tissue thickness. Median serum IGF-1 concentrations were elevated, and 24-hour GH profile studies confirmed 2- to 3-fold increased median basal, pulsatile, and total GH secretion. Male Igsf1-deficient mice also demonstrated features of GH excess with increased lean mass, organ size, and skeletal dimensions and elevated mean circulating IGF-1 and pituitary GH levels. CONCLUSIONS: We demonstrate somatotrope neurosecretory hyperfunction in IGSF1-deficient humans and mice. These observations define a hitherto uncharacterized role for IGSF1 in somatotropes and indicate that patients with IGSF1 mutations should be evaluated for long-term consequences of increased GH exposure.

Treatment outcomes among HIV-positive orphaned and non-orphaned children on antiretroviral therapy in Johannesburg, South Africa.

BACKGROUND: Limited research investigating treatment outcomes for HIV-positive orphans compared with non-orphans has shown mixed results, with several studies indicating that HIV-positive orphans are at greater risk of delayed access to HIV care and poor antiretroviral therapy (ART) adherence, while other data suggest that ART outcomes of orphans can be similar to those of non-orphans. Understanding the impact of orphan status on short-term ART outcomes could improve targeted intervention strategies, and subsequent long-term treatment and developmental outcomes, for HIV-positive infants, children and adolescents. OBJECTIVES: To evaluate the relationship between orphan status and ART outcomes among HIV-positive infants, children and adolescents initiating ART at two large public sector HIV clinics in Johannesburg, South Africa. METHODS: This was a retrospective cohort study of HIV-positive children aged <18 years initiating standard first-line ART between June 2004 and May 2013. Using propensity scores, orphans and non-orphans were matched for age, sex, World Health Organization stage and ART regimen. The effect of orphanhood on attrition from care (all-cause mortality and loss to follow-up) was evaluated using Cox proportional hazards regression analysis, and its effect on having a detectable viral load (≥400 copies/mL) at 12 months on ART using binomial regression analysis with modified Poisson distribution. RESULTS: A total of 251 (29.4%) orphans (maternal, paternal or both) and 603 (70.6%) non-orphans were included at ART initiation. Following multiple imputation for missing data and propensity score matching, 222 orphans and 222 non-orphans were included. Orphans had a median age of 8.0 years (interquartile range (IQR) 4.9 - 10.7) and non-orphans 7.4 years (IQR 4.2 - 10.2). A total of 12 (5.4%) orphans and 33 (14.9%) non-orphans experienced attrition from care during the first 12 months on ART (adjusted hazard ratio 0.32, 95% confidence interval (CI) 0.17 - 0.63). Among those alive and in care, with a viral load at 12 months on ART, 18.0% of orphans (33/183) and 14.8% of non-orphans (24/162) had a detectable viral load (adjusted risk ratio 1.15, 95% CI 1.04 - 1.28). CONCLUSIONS: Orphans were less likely than non-orphans to experience attrition, but among those in care at 12 months, orphans were more likely to have detectable viral loads. Lower attrition among orphans may be due to their being in institutional or foster care, ensuring that they make their visits; however, their higher rates of non-suppression may result from lack of psychosocial support or stigma resulting in struggles to adhere. Additional research investigating age-specific outcomes will be important to elucidate these effects further.

PYY is a negative regulator of bone mass and strength.

OBJECTIVE: Bone loss in anorexia nervosa and following bariatric surgery is associated with an elevated circulating concentration of the gastrointestinal, anorexigenic hormone, peptide YY (PYY). Selective deletion of the PYY receptor Y1R in osteoblasts or Y2R in the hypothalamus results in high bone mass, but deletion of PYY in mice has resulted in conflicting skeletal phenotypes leading to uncertainty regarding its role in the regulation of bone mass. As PYY analogs are under development for treatment of obesity, we aimed to clarify the relationship between PYY and bone mass. METHODS: The skeletal phenotype of Pyy knockout (KO) mice was investigated during growth (postnatal day P14) and adulthood (P70 and P186) using X-ray microradiography, micro-CT, back-scattered electron scanning electron microscopy (BSE-SEM), histomorphometry and biomechanical testing. RESULTS: Bones from juvenile and Pyy KO mice were longer (P < 0.001), with decreased bone mineral content (P < 0.001). Whereas, bones from adult Pyy KO mice had increased bone mineral content (P < 0.05) with increased mineralisation of both cortical (P < 0.001) and trabecular (P < 0.001) compartments. Long bones from adult Pyy KO mice were stronger (maximum load P < 0.001), with increased stiffness (P < 0.01) and toughness (P < 0.05) compared to wild-type (WT) control mice despite increased cortical vascularity and porosity (P < 0.001). The increased bone mass and strength in Pyy KO mice resulted from increases in trabecular (P < 0.01) and cortical bone formation (P < 0.05). CONCLUSIONS: These findings demonstrate that PYY acts as a negative regulator of osteoblastic bone formation, implicating increased PYY levels in the pathogenesis of bone loss during anorexia or following bariatric surgery.

Transferrin receptor 2 controls bone mass and pathological bone formation via BMP and Wnt signaling.

Transferrin receptor 2 (Tfr2) is mainly expressed in the liver and controls iron homeostasis. Here, we identify Tfr2 as a regulator of bone homeostasis that inhibits bone formation. Mice lacking Tfr2 display increased bone mass and mineralization independent of iron homeostasis and hepatic Tfr2. Bone marrow transplantation experiments and studies of cell-specific Tfr2 knockout mice demonstrate that Tfr2 impairs BMP-p38MAPK signaling and decreases expression of the Wnt inhibitor sclerostin specifically in osteoblasts. Reactivation of MAPK or overexpression of sclerostin rescues skeletal abnormalities in Tfr2 knockout mice. We further show that the extracellular domain of Tfr2 binds BMPs and inhibits BMP-2-induced heterotopic ossification by acting as a decoy receptor. These data indicate that Tfr2 limits bone formation by modulating BMP signaling, possibly through direct interaction with BMP either as a receptor or as a co-receptor in a complex with other BMP receptors. Finally, the Tfr2 extracellular domain may be effective in the treatment of conditions associated with pathological bone formation.

Genetic and Pharmacological Targeting of Transcriptional Repression in Resistance to Thyroid Hormone Alpha.

Background: Thyroid hormones act in bone and cartilage via thyroid hormone receptor alpha (TRα). In the absence of triiodothyronine (T3), TRα interacts with co-repressors, including nuclear receptor co-repressor-1 (NCoR1), which recruit histone deacetylases (HDACs) and mediate transcriptional repression. Dominant-negative mutations of TRα cause resistance to thyroid hormone alpha (RTHα; OMIM 614450), characterized by excessive repression of T3 target genes leading to delayed skeletal development, growth retardation, and bone dysplasia. Treatment with thyroxine has been of limited benefit, even in mildly affected individuals, and there is a need for new therapeutic strategies. It was hypothesized that (i) the skeletal manifestations of RTHα are mediated by the persistent TRα/NCoR1/HDAC repressor complex containing mutant TRα, and (ii) treatment with the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) would ameliorate these manifestations. Methods: The skeletal phenotypes of (i) Thra1PV/+ mice, a well characterized model of RTHα; (ii) Ncor1ΔID/ΔID mice, which express an NCoR1 mutant that fails to interact with TRα; and (iii) Thra1PV/+Ncor1ΔID/ΔID double-mutant adult mice were determined. Wild-type, Thra1PV/+, Ncor1ΔID/ΔID, and Thra1PV/+Ncor1ΔID/ΔID double-mutant mice were also treated with SAHA to determine whether HDAC inhibition results in amelioration of skeletal abnormalities. Results:Thra1PV/+ mice had a severe skeletal dysplasia, characterized by short stature, abnormal bone morphology, and increased bone mineral content. Despite normal bone length, Ncor1ΔID/ΔID mice displayed increased cortical bone mass, mineralization, and strength. Thra1PV/+Ncor1ΔID/ΔID double-mutant mice displayed only a small improvement of skeletal abnormalities compared to Thra1PV/+ mice. Treatment with SAHA to inhibit histone deacetylation had no beneficial or detrimental effects on bone structure, mineralization, or strength in wild-type or mutant mice. Conclusions: These studies indicate treatment with SAHA is unlikely to improve the skeletal manifestations of RTHα. Nevertheless, the findings (i) confirm that TRα1 has a critical role in the regulation of skeletal development and adult bone mass, (ii) suggest a physiological role for alternative co-repressors that interact with TR in skeletal cells, and (iii) demonstrate a novel role for NCoR1 in the regulation of adult bone mass and strength.