The role of the DNA repair proteins in bone health and disease

BACKGROUND: The DNA repair systems of a cell function to protect the genetic code from deleterious mutations that may result in the development of chronic aging-associated diseases including osteoporosis, Alzheimer’s and cancer(1). When we age these repair systems become down regulated through the inhibition of the DNA repair proteins by specific inhibitors or from transcriptional suppression(2). This results in reduced DNA repair capacity, enhanced rates of mutation load and the onset of diseases such as osteoporosis(3). Osteoporosis is a disease characterised by an imbalance between bone formation and resorption, which leads to reduced bone density and increased risk of fractures(4). Approximately 66 % of Australians over the age of 50 have osteoporosis or osteopenia causing significant mortality. More than 400 fractures occur every day due to osteoporosis resulting in ~A$3 billion in direct health costs annually.

Our lab has identified a key protein that functions to inhibit the removal of oxidised DNA lesions within the genome. This protein binds to another DNA repair protein, a polymerase, inhibiting its enzymatic activity and thus reducing the cells ability to repair damaged DNA. Interestingly, a single point mutation in the key protein, that enhances its binding and suppression of polymerase activity, causes a severe premature ageing syndrome leading to severe osteoporosis(5). However, the exact molecular links between these two proteins and the development of osteoporosis are yet to be established.

Furthermore, our lab has since developed and patented a drug that functions to prevent the binding of the DNA repair protein to the polymerase. In preliminary work on immortalised cell lines, the drug led to improved repair of DNA lesions and increased mitochondrial health.

In this project, we hypothesise that by blocking the interaction between the polymerase and its protein inhibitor the new drug will reactivate the DNA repair systems in skeletal stem cells and improve their bone-forming capacity.

AIM 1 (year 1): Development and testing of our new drug on bone marrow-derived mesenchymal stem cells (MSC) obtained from young and old donors (n≥4). Cells with and without drug treatment will be analysed for respiration, mitochondrial health, oxidative stress, DNA repair capacity and proteomic and transcriptomic expression data of osteogenic and senescent markers.

AIM 2 (Years 1-2): Development of a three-dimensional (3D) bone model with young and old MSC. 3D printing will be used to generate a scaffold that will then be combined with a hydrogel material resembling tissue matrix, in which cells can form mineralized tissue. The density of the bone-like matrix will be measured by micro computed tomography (microCT) scans and the cellular composition by confocal microcopy. Drug treatment of the 3D cultures will then be initiated to understand how our drug impacts this model system and if the bone density can be enhanced.

AIM 3 (Years 2-3): Establishment of a preclinical mouse model of postmenopausal osteoporosis (i.e., animals undergoing ovaryectomy surgeries) and testing the drug effect in this model. Mice at different ages 6, 12 and 18 months will be treated with the drug and their bone density measured by CT scan. Immunohistochemistry (IHC) looking for markers of osteoblasts, osteoclasts and immune infiltrates will be performed after sacrifice as will bone stress tests.

IMPORTANCE: The project takes an important step towards the clinical development of our drug for the treatment of osteoporosis. When successful, data from this project will pave the way to toxicology and phase I clinical trials for the use of our drug in the treatment of osteoporosis.