P53 INFLUENCES MICE SKELETAL DEVELOPMENT

The p53 tumor suppressor gene encodes a transcriptional activator whos e targets include genes that regulate cell cycle progression and apopt osis. Since we have shown that a critical event in the life history of the chondrocyte is programmed cell death, we asked the question: does loss of the p53 gene influence skeletogenesis? Female p53(+/-) mice w ere mated with p53(+/-) male mice and 17-day-old fetal mice were studi ed. Exencephaly was the most profound skeletal defect of the p53 null mutation. This defect was due to failure of formation of the bones tha t comprise the mouse calvarium. There was also loss of the hyoid bone, and defective mineralization of the manubrium sternum and the termina l phalanges. In the homozygous state (-/-), in the absence of exenceph aly, the number of skeletal deformities was markedly reduced. Aside fr om the gross changes associated with null status, the mutants exhibite d alterations in bone length and width. Small differences in the size and orientation of the mineral crystals in embryonic bone, as evaluate d by small-angle X-ray scattering, were found to disappear after birth . To explain these observations, we evaluated the extent of apoptosis in the tibial growth plates using the TUNEL stain. In the growth plate of the p53(-/-) homozygote, there was minimal labeling of the hypertr ophic layer. Since the p53(-/-) TUNEL stain pattern at 17 days was ver y similar to the pattern of labeling of the p53(+/+) at 15 days, we co ncluded that the growth defect reflected a delay in cartilage maturati on father than a change in chondrocyte phenotype. On this basis, we pr edict that after birth, in mice that survive, differences in bone leng th would become minimal, and at maturity, the length of the long bones of (+/+) and (-/-) mice would be similar.