Global Deletion of Trp53 Reverts Ovarian Tumor Phenotype of the Germ Cell–Deficient White Spotting Variant (Wv) Mice

Abstract

White spotting variant (Wv) mice are spontaneous mutants attributed to a point mutation in the c-Kit gene, which reduces the tyrosine kinase activity to around 1% and affects the development of melanocytes, mast cells, and germ cells. Homozygous mutant mice are sterile but can live nearly a normal life span. The female Wv mice have a greatly reduced ovarian germ cell and follicle reserve at birth, and the remaining follicles are largely depleted soon after the females reach reproductive stage at around 7 weeks of age. Consequently, ovarian epithelial tumors develop in 100% of Wv females by 3 to 4 months of age. These tumors, called tubular adenomas, are benign but can become invasive in older Wv mice.We tested if additional genetic mutation(s) could convert the benign ovarian epithelial tumors to malignant tumors by crossing the Wv mutant into the Trp53 knockout background. Surprisingly, we found that global deletion of Trp53 suppressed the development of ovarian tubular adenomas in Wv mice. The ovaries of Wv/Wv; Trp53 (−/−) mice were covered by a single layer of surface epithelium and lacked excessive epithelial proliferation. Rather, the ovaries contained a small number of follicles. The presence of ovarian follicles and granulosa cells, as indicated by Pgc7 and inhibin-alpha expression, correlated with the absence of epithelial lesions. A reduction of Pten gene dosage, as in Wv/Wv; Pten (+/−) mice, produced a similar, though less dramatic, phenotype. We conclude that deletion of Trp53 prolongs the survival of ovarian follicles in Wv mice and consequently prevents the proliferation of ovarian epithelial cells and development of ovarian tubular adenomas. The results suggest that various cell types within the ovary communicate and mutually modulate, and an intact tissue environment is required to ensure homeostasis of ovarian surface epithelial cells. Especially, the current finding emphasizes the importance of ovarian follicles in suppressing the hyperplastic growth of ovarian epithelial cells, dominating over the loss of p53.

Follicle Depletion Provides a Permissive Environment for Ovarian Carcinogenesis

“…Thus, a strict inverse correlation between cytokeratin 8 and inhibin ␣ exists in the tumors from these Wv/Wv; p27 ϩ/Ϫ and Wv/Wv; p27 Ϫ/Ϫ mice. The inverse correlation supports the suggestion that a paracrine factor(s) produced by follicles or granulosa cells suppresses epithelial growth in ovaries (45).…”

“…Previously, we had observed that in ovarian tubular adenomas from Wv/Wv mice, the presence of inhibin ␣-positive granulosa cells was inversely correlated with the presence of cytokeratin-positive epithelial cells (45). Here, we found that the same correlation existed in the ovarian tumors from Wv/Wv; p27 ϩ/Ϫ and the Wv/Wv; p27 Ϫ/Ϫ mice (Fig.…”

“…An immunohistochemistry procedure was performed to analyze paraffin-embedded tissues according to the standard procedure (41,45). Antigen retrieval was performed by boiling the slides submerged in antigen retrieval buffer (Dako) for 20 min.…”

“…However, global deletion of p53 did not enhance, but rather prevented, the development of tubular adenomas in the Wv/Wv; p53 Ϫ/Ϫ mice. Investigation of the mechanism revealed that p53 deletion rescued and prolonged the survival of the ovarian follicles, which in turn suppressed the development of ovarian tubular adenomas (45). This surprising finding indicates that the presence of ovarian follicles suppresses the growth and remodeling of epithelial cells in the ovaries.…”

“…Previously, in an attempt to convert these benign lesions into more malignant tumors, we added a homozygous p53 deletion in Wv mice (45). However, global deletion of p53 did not enhance, but rather prevented, the development of tubular adenomas in the Wv/Wv; p53 Ϫ/Ϫ mice.…”