Bone dysplasia in Hutchinson-Gilford Progeria Syndrome is associated with dysregulated differentiation and function of bone cell populations.
Hutchinson-Gilford Progeria Syndrome (HGPS) is a premature aging disorder that affects tissues of mesenchymal origin. Most individuals with HGPS harbor a de novo c.1824C>T (p.G608G) mutation in the gene encoding lamin A (LMNA), which activates a cryptic splice donor site resulting in production of a toxic protein termed “progerin”. Clinical manifestations include growth deficiency, lipodystrophy, sclerotic dermis, cardiovascular defects and bone dysplasia. In this study we utilized the LmnaG609G knock-in (KI) mouse model of HGPS to further define mechanisms of bone loss associated with normal and premature aging disorders. Newborn skeletal staining of KI mice revealed altered rib cage shape and spinal curvature, and delayed calvarial mineralization with increased craniofacial and mandibular cartilage content. MicroCT analysis and mechanical testing of adult femurs indicated increased fragility associated with reduced cortical and trabecular bone mass, recapitulating the progressive bone deterioration that occurs in HGPS patients. We investigated underlying mechanisms of bone loss in KI mice at the cellular level in bone cell populations. Formation of wild-type and KI osteoclasts from bone marrow-derived precursors was inhibited in the presence of KI osteoblast-conditioned media in vitro, suggesting an extrinsic factor(s) responsible for the decreased number of TRAcP-positive osteoclasts observed on KI trabecular surfaces in vivo. Cultured KI osteoblasts exhibited abnormal differentiation characterized by reduced capacity to deposit and mineralize  extracellular matrix and increased lipid accumulation compared to wild-type, providing a mechanism for the reduced mineral apposition and bone formation rates observed in vivo. Furthermore, quantitative analyses of KI transcripts confirmed upregulation of adipogenic genes both in vitro and in vivo. These data demonstrate that osteoblast phenotypic plasticity, rather than dysregulated bone remodelling, contributes to abnormal bone formation in HGPS mice. Overall design: total RNA was extracted from femoral diaphyses of 8 week-old wild-type and homozygous LmnaG609G mice for gene expression comparison.