Mechanism of growth failure in mucopolysaccharidosis VII mice

Abstract

Short stature due to progressive growth failure in mucopolysaccharidosis (MPS) does not respond to current treatment. The mechanism behind impaired bone growth in MPS is poorly understood but is crucial to the development of improved strategies to alter bone growth. Bone length was measured in 5 mouse models of MPS to determine the best model for further analysis. The severe MPS VII mouse model (Gus mps/mps strain) [mps/mps superscript] displayed the greatest reduction in bone length and was chosen as the model for studying bone shortening in MPS. Bone formation was delayed in both the primary and secondary ossification centres in MPS VII mice. The growth plate was thickened with enlarged chondrocytes in the resting (RZ) and hypertrophic zones (HZ) but there was a reduced number of chondrocytes in the proliferative zone (PZ) and HZ. Chondrocytes progress through the cell cycle to proliferate and withdraw from the cell cycle to differentiate. Immunohistochemical analysis of cell cycle regulators in the MPS VII growth plate revealed that fewer chondrocytes progressed to mitotic division for proliferation. Fewer HZ chondrocytes progressed to cell cycle withdrawal for terminal differentiation in MPS VII growth plate. Thus, MPS VII chondrocytes while committed to the cell cycle were unable to progress normally through the different stages. Circulating GH, T3 and IHH levels in MPS VII mice were not significantly different from normal. However, IGF1 production in MPS VII mice was reduced both in the circulation and in primary hepatocytes culture, suggesting an impaired GH/IGF1 signaling pathway in MPS VII mice, which may limit the proliferation of chondrocytes in the growth plate. This dysfunction of GH/IGF1 signaling was not caused by a deficiency of the hepatic growth hormone receptor but was associated with a reduction in tyrosine phosphorylation of STAT5 in MPS VII liver. Responsiveness of MPS VII chondrocytes to GH was decreased and local growth plate expression of GHR was reduced, indicating that GHR deficiency may cause the reduced proliferation in MPS VII growth plate. Proliferation and differentiation of chondrocytes are also regulated by locally expressed factors such as IHH, PTHrP and IGF1. Reduced IHH protein level was observed in MPS VII growth plate and in chondrocyte culture, suggesting a potential relationship between IHH and the reduced number of chondrocytes in the PZ of MPS VII growth plate. Persistent expression of Pthrpr and Sox9 and elevated IGF1 secretion by chondrocytes indicated relationships between altered PTHrP and IGF1 signalling pathways and the delayed hypertrophic transition of chondrocytes in the MPS VII growth plate. This thesis highlights that the bone phenotype of MPS is established before birth and suggests that the decreased numbers of chondrocytes in the PZ and HZ of MPS VII growth plate are the results of disruptions in the pace of cell cycle progression. Decreased GHR level would also contribute to reduction of chondrocyte proliferation. Although a decrease in circulating IGF1 level and a decrease in IHH expression in the growth plate was observed, a direct relationship between these observations and short stature cannot be established.