Combining iPSC and gene editing with cell therapy to develop the next generation of MSC-based therapeutics to counteract bone fragility in OI.

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Bone loss and skeletal fragility impose a huge healthcare and economic burden, affecting millions of people in the world due to the ageing process, microgravity, prolonged immobilisation, or pathologies such as osteoporosis or the brittle bone disease osteogenesis imperfecta (OI). OI develops because a genetic defect causes the main component of the bone matrix (type I collagen) to be faulty, preventing the bone-forming cells (osteoblasts) from maturing, which causes the bones to become brittle. There is no cure for OI and patients need care all their lives. Thus, developing therapeutics to counteract bone fragility is of paramount importance. Using a pre-clinical model of OI, we have recently shown that transplantation of human mesenchymal stem cells (MSC) isolated from the pregnancy fluid surrounding healthy babies improved the quality of fragile bones, stimulated the maturation of resident osteoblasts and reduced the activity of resident bone resorbing cells (osteoclasts). However, there are two main hurdles to overcome before MSC can be routinely used in the clinic. First, MSC isolated from donor organs or pregnancy fluid progressively lose their regenerative potential whilst they are cultivated in the laboratory to reach sufficient numbers. Second, we need to understand how MSC work in vivo to improve transplantation protocols and develop tests to determine the repair potential of putative new donor cell samples. Luckily, a new source of MSC is rejuvenated cells, also called induced pluripotent stem cells (iPSC). Skin cells, which can be isolated from healthy or OI patients, can be rejuvenated in vitro and the genetic defect causing OI can be repaired through genome editing. Since iPSC do not change during culture, it is possible to obtain sufficient number of cells before converting them back into MSC. These cells are called induced MSC (iMSC). This raises the possibility to develop the next generation of cell therapy using iMSC derived from the patient's own cells. The aim of this project is to to test the potential of iMSC to counteract bone fragility and replace the use of primary MSC and also to understand how donor MSC ameliorate OI bones. This project will test whether transplantation of iMSC (derived from healthy iPSC or from OI-iPSC following correction of the genetic defect) into a pre-clinical model of OI improves bone quality and normalise osteoblast function. We will also establish whether the differentiation of donor MSC towards the osteoblast lineage directly contributes to bone formation and/or whether the tiny sacks (called exosomes) released by MSC function as activators to stimulate resident osteoblasts and/or inhibit osteoclasts. Finally, we will use ex vivo co-culture to assess how the OI genetic defect impedes the behaviour of OI human and mouse osteoblasts and determine whether the function of bone-forming and bone-resorbing cells can be modulated by human MSC through direct cell contact, or via exosomes or the soluble proteins secreted by human MSC. Our long-term goal is to develop personalized treatments for people suffering from bone loss and at risk of fractures using patients' own skin cells.