The Shore Lab, as part of the Center for Research in FOP and Related Disorders and in close collaboration with Frederick Kaplan, MD, investigates the genetic regulation of cell development and differentiation through studies of extra-skeletal (heterotopic) bone formation in two rare genetic diseases, fibrodysplasia ossificans progressiva (FOP) and progressive osseous heteroplasia (POH). While several features distinguish POH from FOP, both of these disorders induce ectopic bone formation during early childhood and are progressive throughout life, forming bone in soft tissues such as skeletal muscle.
Although FOP and POH are very rare genetic conditions that directly impact relatively few numbers of people, they provide unique insight into fundamental cellular and molecular mechanisms. By identifying the underlying genetic mutations that cause these diseases, we uncovered key regulatory proteins and cell signaling pathways that determine where and when bone forms.
To investigate the cellular and molecular mechanisms of cartilage and bone formation and to identify treatment strategies and conduct pre-clinical drug testing, we develop, characterize, and apply in vitro cell assays and in vivo models. On-going work includes investigating the stem cells that are mis-directed to form bone along with their interactions with other cells and tissues that influence and permit extra-skeletal bone formation, including the tissue microenvironment, biomechanical signaling, and the immune system.
Our work, which began when few knew of FOP, POH, and heterotopic ossification or appreciated the value of understanding rare diseases, has stimulated an active and expanding field of basic research and translational application.
Our research is conducted through a highly collaborative and multi-faceted program. While our long-standing focus is on heterotopic ossification, we also interested in the effects of the underlying genetic mutations in FOP and POH on the function of other tissues and cells and on the regulation of signaling mechanisms. Some of our ongoing project areas (with recent publications indicated) are described in more detail below.
Genetics and Signaling Mechanisms
- We made the surprising discovery that nearly all FOP patients carry the identical mutation (causing a single amino acid substitution) in the ACVR1 gene, indicating that very specific alterations in ACVR1 lead to this disease. ACVR1 is a type I receptor that mediates and regulates BMP pathway signaling. Ongoing studies are identifying the molecular mechanisms that are altered by the mutation, bringing new insight into how these signaling proteins normally regulate the necessary and precise signaling control. Recent work (Allen et al. 2020, eLife; collaboration with the Mullins Lab) and ongoing work is revealing that ACVR1 mutations that occur in FOP activate BMP pathway signaling by by-passing the normal receptor regulatory mechanisms that control how the pathway is turned on and off.
Heterotopic ossification - FOP
- Heterotopic ossification in FOP forms in soft connective tissues, such as skeletal muscle, through endochondral ossification, inducing cartilage formation prior to transition to bone; this is the same process through which most of our skeletal bones form during embryonic development. We determined that FOP mutations directly alter mesenchymal progenitor (stem) cells to enhance differentiation to osteoblast (bone) and chondrocyte (cartilage) fates and that ACVR1 is critical during early cell fate commitment stages of chondrogenesis. This aberrant cell differentiation is directed by increased BMP pathway signaling. However, cells also receive differentiation cues through their physical environment. Cells with the FOP mutation lose their ability to correctly interpret their microenvironment, responding with increased activation of biomechanical signaling, a characteristic ofosteoblastic cell fates (Haupt et al. 2019, MBoC; Stanley et al. 2019, JBMR; collaboration with the Mauck and Mourkioti Labs).
In addition to mis-directing cell fates of mesenchymal progenitor cells, our work has demonstrated that the FOP mutation alters key processes in cells and tissues that precede ectopic cartilage and bone formation that direct and support the formation of heterotopic bone, including immune cells (Convente et al. 2018, JBMR) and the physical properties (stiffness) of the connective tissues where HO will form (Haupt et al. 2019, MBoC).
Heterotopic ossification - POH
- Heterotopic ossification in POH is caused by inactivating mutations in the GNAS gene. We demonstrated that GNAS inactivation acts as a regulatory cell fate ‘switch’ that leads to increased osteogenesis and decreased adipogenesis. Ongoing single cell RNAseq analyses are being used to characterize the progenitor cells in detail (collaboration with the Seale Lab). Using a recently developed mouse model for POH, we determined that prior to initiation of heterotopic ossification in dermal adipose tissue, as occurs in POH patients, the adipose cells and tissue are dramatically altered with reduced lipid and increased collagen (Brewer et al. 2021 Front. Genet.), implicating key roles of the tissue microenvironment in supporting the formation of heterotopic ossification.
Pre-clinical drug testing
At present, there are no established treatments for the prevention of heterotopic ossification in FOP or POH, although ongoing clinical trial are promising (Fred Kaplan, MD and Mona Al Mukaddam, MD). In additional to having a wide range of research applications, our genetically-engineered mouse models of FOP and POH are used in pre-clinical studies to test candidate drugs for treating FOP. Novel treatment strategies are informed by lab research investigation, and both re-purposed drugs and novel compounds are tested.
Beyond heterotopic ossification
- Both ACVR1 and GNAS regulate signaling pathways that are important during development of skeletal bones. We have determined that inactivating GNAS mutations negatively impact bone maintenance and quality (Ramaswamy et al. 2017 Sci. Reports). We have also recently shown that FOP ACVR1 mutation alters bone and joint formation during embryonic skeletal development (Towler et al 2020 Dev. Biol.).