Morphologic analysis of BMP-9 gene therapy-induced osteogenesis

The present study was performed to determine the histological, ultrastructu ral, and radiographic changes that occur over time at intramuscular BMP-9 g ene therapy treatment sites. Several members of the bone morphogenetic prot ein (BMP) family have the potential to induce osteochondrogenesis when the protein is delivered to rodents, canines, rabbits, and nonhuman primates. P revious studies have also demonstrated that BMP gene therapy utilizing aden oviral vectors can also stimulate orthotopic and heterotopic bone formation in rodents and rabbits. Athymic nude and Sprague-Dawley rats were injected with Ad-BMP-9 or Ad-beta -Gal (3.75 x 10(9) particles) in their thigh musc ulature and light microscopic, electron microscopic, and computerized tomog raphy analysis was performed 3, 6, 9, 12, 15, 18, 21, and 100 days later. T o assess early mesenchymal cell proliferation, a bromodeoxyuridine (BrdU) i mmunohistochemical analysis was also performed 48, 60, and 72 hr postinject ion in athymic nude rats. All animals demonstrated extensive endochondral b one formation at the Ad-BMP-9 treatment sites within 3 weeks. The Sprague-D awley rats also exhibited a massive, acute inflammatory infiltrate during t he first week. Proliferating mesenchymal stem cells were clearly evident as early as 2 days after treatment, which differentiated into small or hypert rophied chondrocytes during the next week. During the third week, the carti laginous matrix mineralized and formed woven bone, which converted to lamel lar bone by 3 months. No evidence of bone formation was demonstrated at the Ad-beta -Gal injection sites in the athymic nude or Sprague-Dawley rats. I n addition, no cellular proliferation was seen at the Ad-beta -Gal treatmen t sites in the athymic nude animals as assessed by light microscopy and Brd U immunohistochemistry. The extensive bone formation induced by Ad-BMP-9 su ggests that BMP gene therapy may have potential utility in the treatment of degenerative, rheumatic, or traumatic bone pathology.