Cellular and molecular analysis of the skeletal pathologies associated with mucopolysaccharidosis-VI (MPS-VI)

Periodic Reporting for period 1 - LYSOBONE (Cellular and molecular analysis of the skeletal pathologies associated with mucopolysaccharidosis-VI (MPS-VI))

Berichtszeitraum: 2018-03-01 bis 2020-02-29

Within the LYSOBONE project, the overall goal was to investigate and understand the effects of enzyme replacement therapy (ERT) for the lysosomal storage disorder mucopolysaccharidosis type VI (MPS-VI). As MPS-VI in patients is caused by inactivating mutations in the ARSB gene, resulting in a deficiency for the lysosomal enzyme ARSB, MPS-VI patients are currently being treated with a human recombinant form of ARSB (rhARSB, Naglazyme®) through ERT. It has however always been clear that ERT in these patients does not fully correct or prevent the development of the disorder, a problem for which the underlying mechanisms remained unknown and poorly investigated. Moreover, since the current treatment options for MPS-VI patients, especially conventional ERT, do not only have their limitations, but are also very expensive, the LYSOBONE project had a potentially high impact on life quality of the patients and their parents and the health care system.

To achieve the overall goal of the LYSOBONE project, several scientific and training objectives were defined. The overall scientific objective of the project was the full characterization of the skeletal and non-skeletal effects of Arsb deficiency and the impact of ERT with rhARSB in Arsb-deficient, a mouse model for MPS-VI. Another major objective was to unravel the cellular consequences of Arsb deficiency at a molecular level and to investigate mechanisms of rhARSB uptake and lysosomal delivery in different cell types. This project also had several training objectives, to improve my scientific and methodological knowledge, presentation skills, writing skills, language and administrative skills.

Overall, the data obtained within the LYSOBONE project, allow us to conclude that defects in many skeletal cell types are prevented and corrected by ERT in a valid mouse model for MPS-VI, whereas specifically chondrocyte defects are not targeted by ERT. This knowledge provides the basis to specifically improve chondrocyte delivery of the recombinant enzyme as well as patient monitoring. Future research towards novel therapeutic approaches could therefore not only provide improved treatment efficiency for MPS-VI patients, but possibly also a more affordable alternative than conventional ERT.

The first work package 1 (WP1) of the project focused on investigating the impact of ERT on the skeletal phenotype of Arsb-deficient mice. Here, I found that a great part of the MPS-VI-associated skeletal defects are fully prevented by ERT. In contrast, there was no positive effect of ERT on their impaired skeletal growth, suggesting that chondrocyte defects are not targeted by the treatment. The latter observation was further confirmed with electron microscopy of all skeletal cell types in Arsb-deficient mice and in a biopsy of a MPS-VI patient, demonstrating that chondrocyte defects caused by Arsb inactivation are not prevented or corrected by systemic delivery of rhARSB, thereby raising the question about the underlying causes.

WP2, including a detailed molecular analysis of primary cells from Arsb-deficient mice resulted in the identification of specific cellular defects caused by Arsb deficiency. Skeletal cells of Arsb-deficient mice were investigated through histomorphometry, ex vivo analysis of primary skeletal cell types and electron microscopy. Here it became clear that the high bone mass of Arsb-deficient mice is caused by a specific defect in the bone resorption activity of the osteoclasts and not by an increased activity or number of the bone-forming osteoblasts.

In a third scientific work package, WP3, I investigated the rhARSB uptake and lysosomal delivery mechanisms in primary cell types. The results of these analyses strongly suggest that reduced expression of Mrc1, encoding the mannose receptor, in chondrocytes underlies their decreased capacity to endocytose rhARSB and thus restricts efficacy of ERT in cartilage. As an excellent model of MPS-VI to study the efficacy of ERT, I additionally attempted to set up a valid biomarker approach to monitor disease progression and therapeutic effects in urine of Arsb-deficient mice receiving ERT and in MPS-VI patients. Here the data clearly implied that the quantification of 4-sulfated and 6-sulfated disaccharides in urine samples is a feasible and promising method to monitor disease progression and therapeutic effects in MPS-VI.

Data from the LYSOBONE project were exploited and disseminated in several ways throughout the course of the fellowship. More specifically, next to regular presentations at meetings with the research group or retreats with the Institute of Osteology and Biomechanics, I’ve presented these data to an international audience twice with an oral presentation and once with poster presentation. Results of this project were also published in two peer-reviewed journals, with me as a first and co-author.

Since the current treatment options for MPS-VI patients, especially conventional ERT (Naglazyme®), do not only have their limitations, but are also very expensive, my project has a potentially high impact on life quality of the patients and their parents and the health care system. Future research on the impact of plant-derived mannose-rhARSB on the phenotype of Arsb-deficient mice will inform us whether this could be an alternative therapeutic strategy, possibly one to target cells that remained uncorrected by conventional rhARSB. Interestingly, in comparison to the traditional form of rhARSB, the production of rhARSB from a plant-based platform could positively impact the price of ERT for MPS-VI patients and their families. This innovative output of the project could therefore create new market opportunities and reinforce the competitiveness and growth of companies producing and providing ERT to patients nowadays.

Furthermore, as there are several other forms of lysosomal storage disorders (including other mucopolysaccharidosis types), where the patients also display features of dysostosis multiplex, the findings obtained in Arsb-deficient mice could benefit future basic and clinical research on these disorders as well.

Finally, the severe affection of osteocytes and articular chondrocytes in Arsb-deficient mice closely links the LYSOBONE project results to two of the most prevalent skeletal disorders in the EU, i.e. osteoporosis and osteoarthritis. Based on these arguments, my research project could also have an innovative impact on the European Society.

Overview_Lysobone_Results

Periodic Reporting for period 1 - LYSOBONE (Cellular and molecular analysis of the skeletal pathologies associated with mucopolysaccharidosis-VI (MPS-VI))

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