Mice bearing germ line mutations of p53 develop sarcomas at a significant rate. Since they are susceptible to a variety of other malignancies, they are not ideally suited to the study of sarcomas. To test the possibility that targeted mutation of tumor suppressor genes in early mesenchymal cells would induce formation of sarcomas, the Prx1-cre transgenic mouse was crossed to mice-bearing floxed alleles of p53 and Rb. Mice with homozygous deletion of p53 (Prx1-cre p53(lox/lox)) developed sarcomas in the extremities at a mean time of 50 weeks. Osteosarcomas (OS) were the most common type of sarcoma (61%) followed by poorly differentiated soft tissue sarcomas (PDSTS) (32%). Homozygous deletion of p53 produced sarcomas significantly more rapidly than heterozygous deletion, which resulted in sarcoma formation after a mean of 96 weeks. Mice with homozygous Rb mutation (Prx1-cre Rb(lox/lox)) developed normally and had no ostensible defects in the limbs. In contrast to p53, targeted deletion of Rb did not produce sarcomas in the limbs. However, simultaneous deletion of Rb and p53 accelerated the time to sarcoma formation, and a greater percentage of PDSTS were found. Deletion of p53 in committed osteoblasts by the Col1a1-cre transgenic mouse bearing an osteoblast-specific enhancer resulted in a high percentage of OS. These findings suggest that deletion of p53 in mesenchymal cells that give rise to osteoblasts is a powerful initiator of OS. Deletion of Rb does not initiate sarcoma formation in mice, but it accelerates formation of both soft tissue sarcomas and OS.

Ewing’s sarcoma is characterized by the t(11;22)(q24:q12) reciprocal translocation. To study the effects of the fusion gene EWS-FLI1 on development and tumor formation, a transgenic mouse model was created. A strategy of conditional expression was used to limit the potentially deleterious effects of EWS-FLI1 to certain tissues. In the absence of Cre recombinase, EWS-FLI1 was not expressed in the EWS-FLI1 transgenic mice, and they had a normal phenotype. When crossed to the Prx1-Cre transgenic mouse, which expresses Cre recombinase in the primitive mesenchymal cells of the embryonic limb bud, the EF mice were noted to have a number of developmental defects of the limbs. These included shortening of the limbs, muscle atrophy, cartilage dysplasia, and immature bone. By itself, EWS-FLI1 did not induce the formation of tumors in the EF transgenic mice. However, in the setting of p53 deletion, EWS-FLI1 accelerated the formation of sarcomas from a median time of 50 weeks to 21 weeks. Furthermore, EWS-FLI1 altered the type of tumor that formed. Conditional deletion of p53 in mesenchymal cells (Prx1-Cre p53lox/lox) produced osteosarcomas as the predominant tumor. The presence of EWS-FLI1 shifted the tumor phenotype to a poorly differentiated sarcoma. The results taken together suggest that EWS-FLI1 inhibits normal limb development and accelerates the formation of poorly differentiated sarcomas.