Impaired bone remodeling and its correction by combination therapy in a mouse model of mucopolysaccharidosis-I

Mucopolysaccharidosis-I (MPS-I) is a lysosomal storage disease (LSD) caused by inactivating mutations of IDUA, encoding the glycosaminoglycan-degrading enzyme alpha-l-iduronidase. Although MPS-I is associated with skeletal abnormalities, the impact of IDUA deficiency on bone remodeling is poorly defined. Here we report that Idua-deficient mice progressively develop a high bone mass phenotype with pathological lysosomal storage in cells of the osteoblast lineage. Histomorphometric quantification identified shortening of bone-forming units and reduced osteoclast numbers per bone surface. This phenotype was not transferable into wild-type mice by bone marrow transplantation (BMT). In contrast, the high bone mass phenotype of Idua-deficient mice was prevented by BMT from wild-type donors. At the cellular level, BMT did not only normalize defects of Idua-deficient osteoblasts and osteocytes but additionally caused increased osteoclastogenesis. Based on clinical observations in an individual with MPS-I, previously subjected to BMT and enzyme replacement therapy (ERT), we treated Idua-deficient mice accordingly and found that combining both treatments normalized all histomorphometric parameters of bone remodeling. Our results demonstrate that BMT and ERT profoundly affect skeletal remodeling of Idua-deficient mice, thereby suggesting that individuals with MPS-I should be monitored for their bone remodeling status, before and after treatment, to avoid long-term skeletal complications.