Mutations in the Hedgehog receptor, Patched 1 (Ptch1), have been linked to both familial and sporadic forms of basal cell carcinoma (BCC), leading to the hypothesis that loss of Ptch1 function is sufficient for tumor progression. By combining conditional knockout technology with the inducible activity of the Keratin6 promoter, we provide in vivo evidence that loss of Ptch1 function from the basal cell population of mouse skin is sufficient to induce rapid skin tumor formation, reminiscent of human BCC. Elimination of Ptch1 does not promote the nuclear translocation of B-catenin and does not induce ectopic activation or expression of Notch pathway constituents. In the absence of Ptch1, however, a large proportion of basal cells exhibit nuclear accumulation of the cell cycle regulators cyclin D1 and B1. Collectively, our data suggest that Ptch1 likely functions as a tumor suppressor by inhibiting G 1 -S phase and G 2 -M phase cell cycle progression, and the rapid onset of tumor progression clearly indicates Ptch1 functions as a ''gatekeeper.'' In addition, we note the high frequency and rapid onset of tumors in this mouse model makes it an ideal system for testing therapeutic strategies, such as Patched pathway inhibitors. (Cancer Res 2006; 66(4): 2081-8)
Tumours arising in BRCA1 mutation carriers have a characteristic phenotype, the molecular and cellular basis of which is unknown. To address the hypothesis that this phenotype reflects a role for BRCA1 in either in the basal or the stem cell compartments of the mammary epithelia, we have targeted its disruption to K14 and K6a expressing cells of the mouse. Unlike MMTV and WAP driven conditional knockout models of Brca1, these two models did not result in any observable changes in the mammary gland. Our results suggest that BRCA1-associated tumours arise either in K14 and K6a negative basal cells of the mammary gland, or possibly from transdifferentiation of luminal epithelia.
Activation of the Hedgehog (Hh) signalling pathway by loss of function mutations of the Ptch1 receptor promotes stem or progenitor cell proliferation in several cell types, most notably the basal cells of the skin and granule cells of the cerebellum. We have intercrossed MxCre transgenic mice with conditional Ptch1 knockout mice to study the effects of deleting Ptch1 on adult hematopoiesis, with the hypothesis that loss of Ptch1 would activate the Hh pathway leading to increased hematopoietic stem cells (HSC). Within 4 weeks after deletion of Ptch1 with administration of poly(I:C), MxCrePtch1-null mice developed apoptosis of bone marrow pre-B cells and double positive thymocytes. Overall, MxCrePtch1-null mice have 10-fold less pre-B cells and thymocytes. MxCrePtch1-null mice also develop a 3-fold increase in lineage negative c-kit+ Sca-1+ (LKS) bone marrow cells, a cell fraction enriched for HSCs. Despite increased numbers of LKS, loss of Ptch1 did not increase the numbers of HSCs as measured by competitive repopulation assays. MxCrePtch1-null mice also developed typical Ptch1-related tumours including basal cell carcinomas and cerebellar tumours, which was consistent with the ability of the MxCre transgene to delete loxP-flanked genes in cell types other than hematopoietic cells. To determine if the hematopoietic changes observed in the MxCrePtch1-null mice were cell intrinsic or due to loss of Ptch1 on cells of the microenvironment, we intercrossed conditional Ptch1 mice with hematopoietic specific Cre transgenic mice. Surprisingly, HSC-specific deletion of Ptch1 using tamoxifen-inducible SCLert(2)Cre mice did not lead to any increase in LKS numbers. Similarly, lymphoid specific deletion of Ptch1 with the B-cell specific CD19Cretransgene or the T-cell specific LckCre transgene did not lead to any lymphoid defects. The lack phenotype in hematopoietic-specific Ptch1-null mice indicates that Ptch1 is redundant on hematopoietic cells including HSCs. Furthermore, the lack of phenotype also suggests that the defects observed in the MxCrePtch1-null mice were due to loss of Ptch1 in the microenvironment. To prove that Ptch1 regulates the hematopoietic microenvironment, we performed reciprocal transplant experiments whereby lethally irradiated MxCrePtch1- null mice were reconstituted with wild-type bone marrow cells. Remarkably, wild-type hematopoiesis grown within the MxCrePtch1-null microenvironment developed the identical hematopoietic defects with increased LKS and apoptosis of pre-B cells and thymocytes. Conversely, the MxCrePtch1-null hematopoietic defects could be completely rescued by transplant into lethally irradiated wild-type mice. Histological examination of bones from MxCrePtch1-null mice showed marked alterations in trabecular and cortical bone. Given the recent demonstration that the Hh pathway regulates adult bone homeostasis, we hypothesize that the increased LKS and loss of pre-B cells observed in Ptch1-null mice are secondary to changes within the bone marrow cell niche.
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