Skeletal diseases occur in many forms that altogether affect 40% of adult Americans, more than any other disease. This prevalence keeps increasing, along with aging and obesity, which are major risk factors for degenerative skeletal diseases. Skeletal diseases can drastically impact the quality of life of affected individuals. They also constitute a socio-economic burden and present with major clinical challenges. Skeletal diseases can be inherited or acquired. Congenital skeleton malformations present themselves in hundreds of different types. The most severe are generally part of complex, early-lethal syndromes. The least severe can be risk factors for early adult-onset degenerative diseases, such as osteoporosis and osteoarthritis. Osteoporosis is characterized by gradual bone mass loss and is associated with a high risk for fractures. Osteoarthritis is characterized by irreversible loss of cartilage in articular joints and it can cause severe joint pain, deformation and incapacitation. Better understanding of mechanisms underlying skeleton development, adult maintenance, and diseases has led in recent years to new and improved treatments for osteoporosis and a few other conditions, but today most skeletal diseases remain untreatable or incompletely treatable. Research must thus be pursued to find novel strategies to prevent and treat effectively all types of skeletal diseases. Our laboratory team participates in this research effort in multiple ways.
Several of our research projects are designed to advance understanding of the genetic mechanisms whereby specific cell types fulfill the unique functions of building our skeleton during embryonic and postnatal development and maintaining it through adulthood. These cells include skeletal stem cells, chondrocytes (cartilage-making cells) and osteoblasts (bone-making cells). We are studying proteins called transcription factors that control skeletal cell identity and activity. New knowledge gained from these studies is prerequisite to uncovering the causes of skeletal diseases and to propose new treatment strategies. Other projects focus on studying how mutations found in the genes for skeletogenic transcription factors cause skeletal and other defects in human patients and finding ways to overcome the negative effects of these mutations in patients. They also include the development of protocols to derive healthy skeletal cell types from patient-derived pluripotent stem cells and thereby to generate or regenerate healthy skeletal tissues in vivo.
Recent accomplishments are described in details in the cited publications. Most focused on transcription factors belonging to the SOX family. This family comprises many members that have master roles in controlling cell identity and activity in many processes, from cell stemness to sex determination, heart formation, neurogenesis, and hematopoiesis. Mutations in the genes encoding these factors or mutations affecting the expression or activity of these factors cause severe human diseases. Our laboratory most specifically studies the roles of SOX proteins in skeletal cell types. SOX9 and its co-factors SOX5 and SOX6 form a trio that is required for chondrocyte differentiation and thereby for cartilage formation and maintenance. In contrast, SOX4, SOX11 and SOX12, referred to as SOXC proteins, for a non-chondrogenic trio. They have key roles in specifying skeletal progenitor/stem cells and in ensuring that these cells commit to non-chondrogenic lineages in the skeleton. Chia-Feng Liu, research associate, recently analyzed the genomic actions of this chondrogenic SOX trio.
Pallavi Bhattaram, Project Staff, recently demonstrated that the SOXC transcription factors are highly expressed in skeletogenic cells in the embryo (Bhattaram et al., J. Cell. Biol., 2014). The three proteins act largely in redundancy to ensure cell survival. They also stabilize beta-catenin through physically interacting with the protein. This activity results in amplification of canonical WNT signaling in presumptive joint cells and perichondrium cells and thus in securing the non-chondrogenic fate of these cells. SOXC proteins critically participate thereby in delineating the boundaries and the articulation of the multiple cartilage primordia that constitute the embryonic vertebrate skeleton. Kenji Kato, postdoctoral fellow, studied the contribution of SOXC proteins in further steps of skeletogenesis (Kato et al., J. Bone Miner. Res., 2015). He showed that SOXC proteins are required to form cartilage growth plates, that is, the specialized type of cartilage that drives skeletal elongation in fetal and postnatal vertebrates and that also initiates the progressive replacement of cartilage by bone. Kenji demonstrated that SOXC proteins fulfill this function primarily through actions in perichondrium cells, but also through actions in growth plate chondrocytes. They control the expression of the genes for WNT5A and other factors involved in non-canonical WNT signaling. This permits initiation of growth plate development in cartilage primordia and also permits proper organization and function of established growth plates. New SOXC projects in the laboratory are addressing the roles of SOXCs in other skeletal cell types in development, in adult skeleton maintenance, and in human diseases. We are characterizing both the modes of actions and the modes of regulation of the SOXC genes and proteins.
Chia-Feng Liu, postdoctoral fellow, recently analyzed the whole spectrum of genomic targets of SOX9 and SOX5/SOX6 in growth plate chondrocytes (Liu and Lefebvre, Nucleic Acids Res., 2015). She uncovered that the proteins act primarily in concert with each other, bind to thousands of enhancers and thereby activate a large panel of cartilage-specific genes. A main, novel finding is that most genes that are critical in chondrogenesis are under the control of super-enhancers, that is, clusters of enhancers. These super-enhancers span large distances upstream of the genes and sometimes within gene introns. Most of them are bound by the chondrogenic SOX trio. In the meantime, Baojin Yao, postdoctoral fellow, focused on identifying mechanisms controlling the expression of the SOX9 gene in chondrocytes (Yao et al., Nucleic Acids Res., 2015). Interested in identifying direct mechanisms, he searched for enhancers driving SOX9 transcription. He identified several of them within a 300-kb genomic region located upstream of SOX9. This region is subject to chromosomal translocations causing Campomelic Dysplasia, a severe skeleton malformation syndrome in humans. Interestingly, these enhancers overlap in activity during sequential stages of chondrogenesis and appear to be controlled by distinct mechanisms. These mechanisms include positive feedback loops of activation by the SOX9 protein. Thus, SOX9 expression in chondrocytes involves various transcriptional modules regulated by SOX9 itself and by other factors. New SOX chondrogenic trio projects in the laboratory are addressing the roles of the proteins in chondrogenic precursor cells and in adult articular chondrocytes. We are characterizing both the modes of actions and the modes of regulation of the SOX9 and SOX5/SOX6 genes and proteins.
Other projects in the laboratory focus on the mechanisms whereby mutations in SOX transcription factors cause developmental brain disorders. In one of these projects, we seek to understand how inactivation of one allele of the SOX5 gene causes LAMSHF syndrome. This syndrome causes global developmental delay, with intellectual disability and autism spectrum disorder features. We are developing in vivo models for the disease in the mouse, and in vitro models starting from human pluripotent stem cell lines. We are studying the molecular pathways affected by SOX5 partial inactivation in specialized cells in the brain, called corticofugal projection neurons. We are also looking for ways to rescue the defects of these cells, hoping thereby to suggest strategies for safe and effective treatment of children with this disease.
Further information on research projects can be obtained by directly contacting Dr. Véronique Lefebvre by email or by phone.
- Dy P, Wang W, Bhattaram P, Wang Q, Wang L, Ballock R.T., Lefebvre V. Sox9 directs hypertrophic maturation and blocks osteoblast differentiation or growth plate chondrocytes. Dev Cell 22, 597–609 (2012).
- Bhattaram P, Penzo-Méndez A, Kato K, Bandyopadhyay K., Gadi A, Taketo MM, Lefebvre V. SOXC proteins amplify canonical WNT signaling to secure non-chondrocytic cell fates in skeletogenesis. J Cell Biol 207, 657-671 (2014).
- Kato K, Bhattaram P, Penzo-Méndez A, Gadi A, Lefebvre V. SOXC Transcription Factors Induce Cartilage Growth Plate Formation in Mouse Embryos by Promoting Noncanonical WNT Signaling. J Bone Miner Res 30, 1560-1571 (2015).
- Yao B, Wang Q, Liu CF, Bhattaram P, Li W, Mead TJ, Crish JF, Lefebvre V. The SOX9 upstream region prone to chromosomal aberrations causing campomelic dysplasia contains multiple cartilage enhancers. Nucleic Acids Res 43, 5394-5408 (2015).
- Liu CF, Lefebvre V. The transcription factors SOX9 and SOX5/SOX6 cooperate genome‐wide through super‐enhancers to drive chondrogenesis. Nucleic Acids Res 43, 8183-8203 (2015).