Altered lumbar spine structure, biochemistry, and biomechanical properties in a canine model of mucopolysaccharidosis type VII

Abstract

Mucopolysaccharidosis VII (MPS VII) is a lysosomal storage disorder characterized by a deficiency in beta-glucuronidase activity, leading to systemic accumulation of poorly degraded glycosaminoglycans (GAG). Along with other morbidities, MPS VII is associated with pediatric spinal deformity. The objective of this study was to examine potential associations between abnormal lumbar spine matrix structure and composition in MPS VII, and spine segment and tissue-level mechanical properties, using a naturally occurring canine model with a similar clinical phenotype to the human form of the disorder. Segments from juvenile MPS VII and unaffected dogs were allocated to: radiography, gross morphology, histology, biochemistry, and mechanical testing. MPS VII spines had radiolucent lesions in the vertebral body epiphyses. Histologically, this corresponded to a GAG-rich cartilaginous region in place of bone and elevated GAG staining was seen in the annulus fibrosus. Biochemically, MPS VII samples had elevated GAG in the outer annulus fibrosus and epiphyses, low calcium in the epiphyses, and high water content in all regions except the nucleus pulposus. MPS VII spine segments had higher range of motion and lower stiffness than controls. Endplate indentation stiffness and failure loads were significantly lower in MPS VII samples, while annulus fibrosus tensile mechanical properties were normal. Vertebral body lesions in MPS VII spines suggest a failure to convert cartilage to bone during development. Low stiffness in these regions likely contributes to mechanical weakness in motion segments and is a potential factor in the progression of spinal deformity.

Figure 1

A. Vertebral body schematic showing central, anterior, and lateral sites for end plate indentation testing. B. Intervertebral disc schematic showing the specimen excision site for AF tensile testing.

Figure 2

Structural comparisons of unaffected and MPS VII lumbar spine segments (ivd = intervertebral disc, vb = vertebral body, a = anterior, p = posterior, aaf = anterior AF, paf = posterior AF, NP = nucleus pulposus, e = epiphysis, gp = growth plate). A. Lateral radiograph of an unaffected spine segment. B. Lateral radiograph of MPS VII spine segment showing radiolucent lesions (*) in the vertebral bodies. C. Gross morphology of an unaffected spine segment (mid sagittal section). D. Gross morphology of an MPS VII spine segment (mid-sagittal section) showing lesions (*) localized to the peripheral epiphyses. E. Alcian blue-picorsirius red stained section from an unaffected spine segment. F. Alcian blue-picorsirius red stained section from an MPS VII spine segment showing high GAG staining (blue) in epiphyseal lesions (*) and throughout the AF.

Figure 3

High magnification images of alcian blue-picrosirius red strained sections illustrating structural and compositional differences in the AF and anterior epiphyses between MPS VII and unaffected samples. A. Anterior epiphysis of an unaffected sample containing trabecular bone. B. Similar region in an MPS VII sample showing cellular cartilaginous material with high GAG content (blue). C and D. Higher magnification views of insets indicated in A and B, respectively. E. Anterior AF of an unaffected sample. F. Similar region in an MPS VII sample showing elevated GAG staining within annulus lamellae. G and H. Higher magnification views of insets indicated in E and F, respectively.

Figure 4

Spine motion segment mechanical testing results. A. Compressive stiffness was significantly lower for MPS VII samples. B. Neutral zone stiffness was also significantly lower for MPS VII samples. C. Total range of motion was greater for MPS VII samples. E. Creep displacement was significantly greater for MPS VII samples. Mean ± SD, *p < 0.05.

Figure 5

Vertebral body endplate indentation testing results. A. Stiffness. B. Failure load. Both properties were significantly lower for MPS VII samples in all regions. C. Failure displacement was not significantly different in any region. Mean ± SD, *p < 0.05, **p < 0.005.

Figure 6

AF circumferential tensile mechanical testing results. A. Linear region elastic modulus. B. Toe region elastic modulus. C. Transition strain. None of these properties were significantly different between groups. Mean ± SD.

Figure 7

Comparison of biochemical properties between unaffected and MPS VII samples in the nucleus pulposus (NP), inner AF (IAF), outer AF (OAF), and anterior epiphysis (E). A. Water content was significantly higher for MPS VII samples in all regions except the nucleus pulposus. B. GAG content was significantly higher for MPS VII samples in the outer AF and epiphysis. C. Calcium content was significantly lower in the anterior epiphysis of MPS VII samples. Mean ± SD, *p < 0.05, **p < 0.005.