13C/31P MRS Metabolic Biomarkers of Disease Progression and Response to AAV Delivery of hGAA in a Mouse Model of Pompe Disease

Abstract

The development of therapeutic clinical trials for glycogen storage disorders, including Pompe disease, has called for non-invasive and objective biomarkers. Glycogen accumulation can be measured in vivo with 13C MRS. However, clinical implementation remains challenging due to low signal-to-noise. On the other hand, the buildup of glycolytic intermediates may be detected with 31P MRS. We sought to identify new biomarkers of disease progression in muscle using 13C/31P MRS and 1H HR-MAS in a mouse model of Pompe disease (Gaa-/-). We evaluated the sensitivity of these MR biomarkers in vivo after treatment using an adeno-associated virus vector 2/9 encoding hGAA driven by the desmin promotor. 31P MRS showed significantly elevated phosphomonoesters (PMEs) in Gaa-/- compared to control at 2 (0.06 ± 0.02 versus 0.03 ± 0.01; p = 0.003), 6, 12, and 18 months of age. Correlative 1H HR-MAS measures in intact gastrocnemius muscles revealed high glucose-6-phosphate (G-6-P). After intramuscular AAV injections, glycogen, PME, and G-6-P were decreased within normal range. The changes in PME levels likely partly resulted from changes in G-6-P, one of the overlapping phosphomonoesters in the 31P MR spectra in vivo. Because 31P MRS is inherently more sensitive than 13C MRS, PME levels have greater potential as a clinical biomarker and should be considered as a complementary approach for future studies in Pompe patients.

Keywords: AAV; MR spectroscopy; MRI; Pompe disease; carbon; gene therapy; glucose-6-phosphate; glycogen; glycogen storage disorder; phosphorus.

Figures

Graphical abstract

Figure 1

In Vivo 31P MRS Representative spectra from the lower leg muscle of (A) a control animal and (B) a Gaa−/− animal. The different peaks were assigned to (1) PME, (2) Pi, (3) PCr, (4) adenosine triphosphate γ (ATPγ), (5) ATPα, and (6) ATPβ. (C) PME levels determined by peak signal integration and expressed as PME/ATPtotal were significantly higher compared to age-matched controls by a factor of 2 or more in all age groups (2, 6, 12, 18 months old). Results are presented as mean ± standard deviation. *p < 0.05; **p < 0.01.

Figure 2

In Vivo 13C MRS Representative in vivo 13C MR spectra acquired from the lower leg muscles of a Gaa−/− mouse (A) and a control mouse (B) in vivo. The inserts show a zoom on the 90 to 110 ppm region, with the C1 glycogen resonance at 100.5 ppm. No signal could be detected in control animals due to very low glycogen concentrations. The other peaks are largely dominated by signal from the lipids overlapping with other resonances such as other glycogen carbons, creatine, taurine, and other sugars.

Figure 3

Glycogen Measurements Measures of glycogen levels (A) with 13C MRS in vivo, (B) with 1H HR-MAS of intact muscles, and (C) with quantitative biochemical assays of muscle extracts in μg/mL. All three types of measurements showed significantly higher glycogen levels in Gaa−/− compared to controls and significantly lower glycogen level after rAAV2/9-DES-hGAA treatment (Gaa−/− + AAV) as compared to untreated Gaa−/−. Data are presented as individual data points and box plots indicating the lower and upper quartiles and the median. The bars indicate the minimum and maximum values. *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 4

Effect of rAAV2/9-DES-hGAA Treatment on GAA Activity and Glycogen Concentration Glycogen concentration in μg/mL determined by biochemical assay on muscle homogenates for control, Gaa−/−, and Gaa−/− + AAV (A) and as a function of GAA activity (B). The dashed lines represent the average control value and the upper and lower 95% confidence interval. GAA activity of 10 nmol/hr/mg was sufficient to induce glycogen clearance. GAA activity above that value did not induce lower glycogen concentrations than found in control animals. *p = 0.03, ***p < 0.0001.

Figure 5

Relationship between Glycogen Levels and GAA Activity Overlay of the results obtained from the three different glycogen measurements as a function of the corresponding GAA concentration. The solid black line represents the result of a fit of the quantitative measurements by biochemical assay to a monoexponential decay, and the dashed lines represent the 95% confidence interval of the fit. Values are normalized to their maximum for display purposes.

Figure 6

Effect of rAAV2/9-DES-hGAA Treatment on 31P MRS PME Resonances In Vivo and 1H HR-MAS Glucose-6-Phosphate in Isolated Muscles Both in vivo PME levels and glucose-6-phosphate levels from intact muscles were significantly elevated in Gaa−/− mice compared to control. rAAV2/9-DES-hGAA induced a significant decrease in both PME (A) and glucose-6-phosphate (B). Data are presented as individual data points and box plots indicating the lower and upper quartiles and the median. The bars indicate the minimum and maximum values. *p < 0.05; **p < 0.005.

Figure 7

Description of the rAAV2/9-DES-hGAA Injection Sites Representative coronal (A) and axial (B) images from a 3D T1 weighted MR image of the lower legs of a Gaa−/− animal are shown. The fascia appear in black on the images. The muscles are delineated in white, and the white arrows indicate the three rAAV2/9-DES-hGAA injection sites (TA, tibialis anterior; GM, medial gastrocnemius; GL, lateral gastrocnemius). 3D T1 weighted images were obtained using a gradient echo sequence on with 4.7T Agilent scanner (Agilent, Inc., Palo Alto, CA) and a custom build 3 cm-diameter 1H volume coil. TR/TE = 50/7 ms, field of view = 1.5 × 1.8 × 1.8 cm3, matrix size = 256 × 192 × 96.