Trustee, Andrew Rankin, summarised the project’s findings:

In this short review, I have tried to explain what MMP-9 is, its role in the body and potential as a therapeutic target, drawing particularly on evidence from work in cancer and bone metastases. This hopefully provides supporting evidence and context for the single case study observations in the FOP patient R deficient in MMP-9 and subsequent experiments inhibiting MMP-9 in mouse models of FOP; and gives some idea of both the potential and complexities for developing a novel therapy targeting MMP-9 to prevent HO in FOP.

What is MMP-9 and what is its role in inflammation and disease?
Matrix metalloproteinases (MMPs) are zinc-dependent enzymes mainly devoted to the degradation of extracellular matrix proteins with a role in regulating pro-inflammatory mediators; they are involved in the inflammatory response and the remodeling of the extracellular matrix (ECM), the network consisting of extracellular components such as collagen, that provide structural and biochemical support to surrounding cells. MMP-9 specifically regulates inflammation in tissues and diseases and is mainly secreted by neutrophils and macrophages, which are a type of white blood cell that secrete both pro-inflammatory and antimicrobial mediators as part of the innate immune system. MMP-9 also plays a key role in tumor formation and as such has been considered a potentially attractive target for anticancer therapies. However, the complex mechanism of regulation of expression, synthesis and activation of MMP-9, which can lead to target specific contradictory roles for this enzyme, and on the other hand high similarity with other members of MMP family leading to concerns over off-target actions, has made development of effective and safe MMP-9 inhibitors as anticancer drugs challenging; similar challenges might be expected as a target for other diseases and conditions.

MMP-9: what is the evidence as a potential therapeutic target in bone diseases?
Growing experimental evidence suggests that these proteinases such as MMPs be implicated in bone remodeling processes associated with various physiological and pathological conditions, including bone metastasis formation in some cancers. A further association between MMP-9, Activin A and bone metabolism comes from work on prostate cancer with bone metastases where there appears to be complex feedback loops between MMP-9 and Activin A during the bone turnover process. Activin A appears implicated in the regulation of osteoblastic activity (to form new bones and add growth to existing bone tissue) and osteoclast differentiation (dissolve old and damaged bone tissue so it can be replaced with new, healthier cells created by osteoblasts), but also in the modulation of intracellular MMP-9 expression levels in osteoblasts, osteoclasts and in bone metastatic cells (i.e.. May not just be a one-way MMP-9 to Activin A activation pathway – potentially complex mechanism).

A clinical study of MMP-2, MMP-9 and Activin A Blood Levels in Patients with Breast Cancer (BC) or Prostate Cancer Metastatic to the Bone (PC) demonstrated that Activin A and MMP-2 were significantly increased in BC and PC patients as compared to sex-matched controls while MMP-9 levels were more elevated only in the PC patients. (ANTICANCER RESEARCH 27: 1519-1526 (2007)); also, activin A levels were significantly higher in the PC patients than those measured in the BC patients. Furthermore, a significant relationship was also highlighted between activin A concentration and the number of bone metastases and tumor grade in PC patients; a significant correlation was observed between PSA and activin A and MMP-9, and between Activin A and Gleason score and the number of skeletal metastases. This clinical study suggests that activin A and MMP-9 may have a preferential role in facilitating bone metastasis formation in prostate cancer, therefore activin A and MMP-9 may be regarded as possible novel therapeutic targets in the treatment for metastatic bone PC. Given the strength of evidence for MMP-9 involvement this bone disease, and its role in Activin A release, we should also consider mechanistically the potential of MMP-9 as a therapeutic target for the treatment of heterotopic ossification (HO) in FOP.

MMP-9: case study evidence and preclinical experiment studies identify as a potential therapeutic target in FOP
The key role of Activin A, an obligate and essential component in the pathway to Heterotopic Ossification in FOP is clear and this rightly remains a key therapeutic target for ongoing potential medicines in development. However, growing experimental evidence suggests that matrix metalloproteinases (MMPs) are implicated in bone remodeling processes associated with various physiological and pathological conditions, including bone metastasis formation in some cancers. Additionally, MMP-9 is known to induce endochondral ossification (one of the two essential pathways by which bone tissue is produced during fetal development); it is also expressed at elevated levels in ischemic and hypoxic skeletal muscle which is present early in HO lesions. A further association between MMP-9, Activin A and bone metabolism comes from work on prostate cancer with bone metastases where there appears to be complex feedback loops between MMP-9 and Activin A during the bone turnover process.

MMP-9 Deficiency Confers Resilience in Fibrodysplasia Ossificans Progressiva in a Man and Mice, Lounev et al., 2024
Giving further support to this evidence from cancer and inflammation for a potential role of MMP-9 in bone formation, and potential role driving heterotopic ossification (HO) in FOP, the case of Patient R is of particular interest. Patient R presents with the classical FOP mutation ACVR1R206H and congenital great toe malformation of FOP but little HO and near normal mobility for a man of 35 with this condition which one might have expected to have significantly progressed with multiple lesions by this age. The hypothesis that patient R lacked MMP-9 as a key mediator triggering inflammation in the HO process through the Activin A pathway was supported by both decreased MMP-9 secretion and activity: Patient-R had significantly lower levels of total MMP-9 when compared to all patients with classic FOP and when compared to patients with quiescent, classic FOP. The lack of MMP-9 in this patient was considered due to multiple identified mutations in genes driving MMP-9 production and activity, quite separate from the FOP mutation. The observation of a patient with classical FOP mutation but only minor lesions as a mature adult, and having different MMP-9 mutations that resulted in substantially reduced MMP-9 and minimal HO, stimulated further experimental research studies by the authors to explore the role of MMP-9 in HO and its potential as a therapeutic target. The link with Activin A was supported by demonstrating that human proinflammatory macrophages (a major source of MMP-9), having undergone gene editing to express a variant that generated significantly less MMP-9 (like Patient R), produced significantly less Activin A. This prompted the authors to conduct further studies in validated FOP mouse models where they inhibited MMP-9 activity, even partially, through genetic editing, pharmacological or biological tools. Inhibition of MMP-9 activity in these mouse models of FOP dramatically reduced or completely inhibited HO formation compared to control (placebo) treatments.

In summary, the publication by Lounev et al detailing the observations in patient-R with mutations reducing MMP-9 production and activity, and multiple FOP mouse model studies inhibiting MMP-9, support a role for MMP-9 as an inflammatory trigger for Activin A -mediated heterotopic ossification in FOP. The authors hypothesize that pharmacological reduction in tissue levels of MMP-9 in patients with FOP may offer some protection against trauma-induced HO through damping down the Activin A pathway to HO. More studies are required to fully understand the mechanistic relationship between MMP-9 and Activin A, and the exact role of MMP-9 in the trauma-induced formation of HO in FOP. Additionally, there is the potential for off-target activity with an MMP-9 therapeutic due to the high similarity with other MMPs that have different actions and roles in the body. Nevertheless, MMP-9 inhibition offers a potential novel additional therapeutic target for reducing HO formation in FOP patients beyond those currently in clinical development.

Andrew J Rankin, PhD DipPharmMed