PURPOSE

Mucopolysaccharidosis type I (MPS I), also called Hurler syndrome, is an autosomal recessive lysosomal storage disease caused by deficiency of α-L-iduronidase enzyme ¹. Consequently, glycosaminoglycans accumulate within the lysosomes causing cellular dysfunction. MPS I leads to progressive loss of cognitive function and substantial physical disease in children. Cognitive decline in children with MPS I correlate with focal white matter lesions (hyperintense on T2W MRI) often observed in the corpus callosum ². Recent study of a canine model of MPS I revealed abnormal DTI parameters in the corpus callosum, which was supported by reduced expression of myelin-related genes and abnormal composition of myelin ³. The purpose of this study was to demonstrate whether demyelination of the corpus callosum can be detected in vivo in a mouse model of MPS I.

METHODS

C57BL/6 knock-out mice deficient for IDUA were used as a well-established model of MPS I. MRI/MRS data from MPS I mice (male, N = 11) and from heterozygote littermates (male, N = 12) were collected at 12 months of age using 9.4 T Agilent/Varian MR scanner. Multi-slice FSE images in axial and sagittal orientation (slice thickness = 0.5 mm) were used for a precise positioning of the VOI (2.0 x 1.0 x 1.8 mm³) centered in the corpus callosum (Fig. 1). The B₀ field homogeneity was adjusted by FASTMAP shimming ⁴. In vivo ¹H MRS data were acquired using ultra-short TE STEAM (TE = 2 ms) localization sequence combined with VAPOR water suppression ⁵. Metabolites were quantified using LCModel with the spectrum of fast relaxing macromolecules included in the basis set. Spontaneously breathing animals were anesthetized with 1.0 – 1.5% isoflurane. Another group of MPS I and control mice (N = 5 for each group) was fed with cuprizone-rich diet for 6 weeks to induced demyelination. After switching back to normal chow for 6 weeks, animals were sacrificed at 16 weeks and the corpus callosum was dissected and analyzed by RT-PCR and ELISA.

RESULTS

The size of the mouse corpus callosum (thickness ranging from 0.15 – 0.35 mm) is too small for in vivo ¹H MRS, therefore, the VOI was centered on the midline encompassing its posterior part (Fig. 1). The spectral quality consistently accomplished in this study (Fig. 1) enabled reliable quantification of seventeen brain metabolites (Fig. 2). The comparison of neurochemical profiles revealed significantly decreased N-acetylaspartate (NAA, -0.5 µmol/g, p = 0.004) and phosphoethanolamine (PE, -0.6 µmol/g, p = 0.018) concentration in MPS I mice relative to controls (heterozygote littermates). In addition, a trend for increased level of ascorbate (Asc, +0.5 µmol/g, p = 0.086) has been observed. Preliminary tissue analysis data of dissected corpus callosum at 16 weeks indicate a consistent reduction in myelin related gene expression in MPS I mice compared to controls. However, intergroup differences in myelin basic protein and total cholesterol have not been observed.

DISCUSSION

Since phosphoethanolamine is the direct precursor for the synthesis of the phospholipid phosphatidylethanolamine, PE levels represent an in vivo marked of myelination ⁶. Therefore, a significantly decreased level of PE in the VOI encompasses the corpus callosum clearly indicates demyelination. These results are in agreement with preliminary issue analysis results showing reduced expression of myelin-related genes. Moreover, reduced PE levels are in excellent agreement with abnormal DTI parameters, reduced expression of myelin-related genes and abnormal myelin composition observed in a canine model of MPS I ³. A significant decrease in NAA well correspond with a loss in neuronal and axonal integrity characteristic for MPS I. In addition, a trend for increased level of Asc is in agreement with our previous finding in the hippocampus of MPS I mice ⁷, which indicate a protective response against the oxidative stress reported in lysosomal diseases ⁸. These consistent ¹H MRS results have been found despite challenges related to the partial volume effects.

CONCLUSIONS

In vivo ¹H MRS confirmed demyelination and a loss in neuronal/axonal integrity in the corpus callosum of MPS I mice. These results highlight underlying neurodegenerative processes that may substantially contribute to progressive loss of brain cognitive function in this disease.

Acknowledgements

Supported by: NIH grants P41 EB015894, P30 NS076408, R01 NS085381 and WM KECK Foundation

References

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