866. Aortic Disease in Mucopolysaccharidodis I Mice Is Due to Increased Expression of Matrix Metalloproteinase 12 (MMP 12) Can Be Prevented with Neonatal Retroviral Vector-Mediated Gene Therapy
Authors: Xiucui Ma, Mindy Tittiger, Russ Knutson, Robert Mecham, Laura Schaller, Katherine Ponder
Journal: Molecular Therapy (2006)

Mucopolysaccharidosis I (Hurler Syndrome) is due to deficiency in α-L-iduronidase (IDUA) and results in the inability to degrade the glycosaminoglycans (GAGs) heparin and dermatan sulfate. GAGs accumulate in a variety of organs, which results in disease through poorly defined mechanisms. Cardiovascular disease is a frequent cause of death in patients with MPS I, which can be due to aortic dilatation with resulting aortic valve insufficiency, thickening of the heart values with resulting stenosis and/or insufficiency, and/or deposition of GAGs in the myocardium. MPS I in humans can be treated with enzyme replacement therapy, which involves intravenous injection of enzyme that is modified with mannose 6-phosphate (M6P), which can diffuse into organs and be taken up via the M6P receptor. Gene therapy could program cells to secrete M6P-modified IDUA, which could similarly diffuse into organs and correct disease. We previously noted that neonatal gene therapy with 109 transducing units/kg of a retroviral vector (RV) expressing canine IDUA resulted in stable expression from the liver, and correction of cardiovascular disease (Molecular Therapy 11:35–47, 2005). The goal of this study was to better understand the pathophysiology of aortic disease in MPS I, and to identify the mechanism by which gene therapy resulted in improvements. We had previously noted that aortas from untreated MPS I mice were markedly dilated and had broken elastic fibers at 8 months, suggesting that there was a defect in elastin assembly or accelerated elastin degradation. In this study, we demonstrated that aortas appeared normal at 6 weeks, and that histochemically-visible elastic fiber breakage progressed from mild at 3 months to severe at 6 months. Similarly, pressure/volume measurements of the ascending aorta were normal at 6 weeks, and became progressively dilated at all pressures by 6 months. Matrix metalloproteinases (MMPs) are proteins that can degrade elastin and collagen, which are important components of the extracellular matrix in the aorta. Analysis of RNA from the ascending aorta of untreated MPS I mice with real-time reverse transcriptase PCR demonstrated that MMP 12 was 20-fold normal at 6 months after birth, while osteopontin (a known inducer of 12) was 12-fold normal. In contrast, MMP 2 and MMP 9 levels were normal at 6 months or earlier. Elastin, collagen 1α1 and 6α1, and fibrillin mRNA levels were normal or modestly elevated in untreated MPS I mice at all times, suggesting that synthesis of the precursors of these matrix proteins was not impaired. MPS I mice that received neonatal gene therapy had normal pressure volume curves of the ascending aorta, and normal levels of the genes noted above. These data suggest that aortic disease in MPS I is due to activation of MMP 12, which can degrade elastin and collagen. Further experiments will attempt to understand the mechanism by which MMP 12 is elevated, and test if inhibitors of MMPs can reduce aortic disease in MPS I mice.