Cellular and Molecular Pathogenesis of Osteoarthritis

RESEARCH CONTEXT
Osteoarthritis is the leading cause of pain and disability. Its incidence has increased 50% in the last 30 years and it now affects more than 500 million people worldwide. 15% of the UK population suffer from osteoarthritis with an economic cost of £14.8 billion per annum. Despite the enormous personal, economic, and societal toll, osteoarthritis remains a generally neglected disease and a Lancet Commission has been established recently to address this problem directly. Osteoarthritis causes progressive cartilage wear and tear and failure of normal tissue repair that ultimately results in joint destruction. Understanding how osteoarthritis begins and progresses is limited because the joint is a complex structure consisting of different tissues and cell types, many of which are difficult to examine. Studies in humans are complicated by inclusion of patients of differing age, sex, ethnicity and aetiology. They also lack critical comparative information from normal joints as only tissue obtained at joint replacement for end-stage disease can be investigated. Thus, human studies cannot establish the causal relationships that connect changes in gene expression with mechanisms of disease. Accordingly, the only treatment for osteoarthritis is joint replacement and no drugs are available that can prevent disease onset or progression. A new and unbiased approach that uses a well-established and validated osteoarthritis disease model is now required to advance the field and address this critical unmet need.

AIMS and OBJECTIVES
We hypothesise that tissue specific patterns of gene expression (spatial transcriptomics) obtained from the mouse knee joint during the onset and progression of disease will identify the key target cells, genes, major pathways and signalling networks that define osteoarthritis.

AIM 1. Undertake detailed imaging and tissue specific spatial transcriptomics of knee joints from normal, aged and osteoarthritic mice.

AIM 2. Prioritise key genes and signalling pathways that underpin the onset and progression of osteoarthritis.

We recently identified severe early onset osteoarthritis in mice lacking one of the two copies of the Pitx1 gene. Thus, in Aim 3 we will establish the role of PITX1 in osteoarthritis and determine its mechanism of action in cartilage.

AIM 3. Define the role of Pitx1 in the onset and progression of osteoarthritis in vivo.

APPLICATIONS and BENEFITS
The studies in Aim 1 will: (i) define a cellular and molecular atlas of the knee joint and (ii) determine the sequential changes in the cellular and molecular programmes that occur during disease onset and progression. The studies in Aim 2 will: (i) integrate diverse and complementary mouse and human data sets and (ii) prioritise and validate the critical and tractable conserved genes and major signalling pathways involved in the onset and progression of osteoarthritis. The studies in Aim 3 will: (i) define the key target genes and downstream signalling pathways that are perturbed in articular cartilage following deletion of Pitx1 and (ii) provide an osteoarthritis disease model in which to determine the tractability and therapeutic potential of targeting PITX1 signalling.

Overall, these studies will provide a new and detailed understanding of the cellular and molecular basis of osteoarthritis and identify novel drug targets for the prevention and treatment of this common and debilitating, yet neglected disease.

We will use tissue specific spatial transcriptomics of knee joints during the onset and progression of osteoarthritis to identify the cellular and molecular mechanisms of disease pathogenesis.

Thus, we will undertake detailed phenotyping combining (i) back-scattered electron scanning-electron microscopy, (ii) iodine contrast-enhanced microCT, (iii) X-ray microradiography, and (iv) histological scoring together with 10x Genomics Visium spatial transcriptomic analysis in normal, aged and osteoarthritic knee joints. Subsequently, we will interrogate multi-omics and GWAS datasets to prioritise key genes that are expressed in disease relevant tissues. Finally, the credibility and tractability of candidate genes and downstream signalling pathways will be evaluated in a bespoke pipeline that integrates established informatics databases with our experimental datasets. Prioritised genes will be validated in vivo using our unique resource of disease models. In addition, we have recently identified a protective role for the homeobox transcription factor PITX1 in osteoarthritis pathogenesis and will investigate the cellular and molecular mechanisms responsible using inducible cell specific gene targeting in articular chondrocytes.

These studies will define a cellular and molecular atlas of the knee joint and determine the sequential changes in gene expression that occur during the onset and progression of disease. Integration of multi-omic datasets will prioritise and validate the critical and tractable genes and pathways involved in the pathogenesis of osteoarthritis. Investigation of the role of PITX1 will define key target genes and downstream signalling pathways that are perturbed in articular cartilage following deletion of Pitx1, providing a new disease model to determine the therapeutic potential of targeting PITX1 signalling.