Anna Saran

Oncogenic bystander radiation effects in Patched heterozygous mouse cerebellum

The central dogma of radiation biology, that biological effects of ionizing radiation are a direct consequence of DNA damage occurring in irradiated cells, has been challenged by observations that genetic/epigenetic changes occur in unexposed “bystander cells” neighboring directly-hit cells, due to cell-to-cell communication or soluble factors released by irradiated cells. To date, the vast majority of these effects are described in cell-culture systems, while in vivo validation and assessment of biological consequences within an organism remain uncertain. Here, we describe the neonatal mouse cerebellum as an accurate in vivo model to detect, quantify, and mechanistically dissect radiation-bystander responses. DNA double-strand breaks and apoptotic cell death were induced in bystander cerebellum in vivo. Accompanying these genetic events, we report bystander-related tumor induction in cerebellum of radiosensitive Patched-1 (Ptch1) heterozygous mice after x-ray exposure of the remainder of the body. We further show that genetic damage is a critical component of in vivo oncogenic bystander responses, and provide evidence supporting the role of gap-junctional intercellular communication (GJIC) in transmission of bystander signals in the central nervous system (CNS). These results represent the first proof-of-principle that bystander effects are factual in vivo events with carcinogenic potential, and implicate the need for re-evaluation of approaches currently used to estimate radiation-associated health risks.

Basal Cell Carcinoma and Its Development: Insights from Radiation-Induced Tumors in Ptch1-Deficient Mice

Loss-of-function mutations in Patched (Ptch1) are implicated in constitutive activation of the Sonic hedgehog pathway in human basal cell carcinomas (BCCs), and inherited Ptch1 mutations underlie basal cell nevus syndrome in which a typical feature is multiple BCC occurring with greater incidence in portals of radiotherapy. Mice in which one copy of Ptch1 is inactivated show increased susceptibility to spontaneous tumor development and hypersensitivity to radiation-induced tumorigenesis, providing an ideal in vivo model to study the typical pathologies associated with basal cell nevus syndrome. We therefore examined BCC development in control and irradiated Ptch1neo67/+ mice. We show that unirradiated mice develop putative BCC precursor lesions, i.e., basaloid hyperproliferation areas arising from both follicular and interfollicular epithelium, and that these lesions progress to nodular and infiltrative BCCs only in irradiated mice. Data of BCC incidence, multiplicity, and latency support the notion of epidermal hyperproliferations, nodular and infiltrative BCC-like tumors representing different stages of tumor development. This is additionally supported by the pattern of p53 protein expression observed in BCC subtypes and by the finding of retention of the normal remaining Ptch1 allele in all nodular, circumscribed BCCs analyzed compared with its constant loss in infiltrative BCCs. Our data suggest chronological tumor progression from basaloid hyperproliferations to nodular and then infiltrative BCC occurring in a stepwise fashion through the accumulation of sequential genetic alterations.

High incidence of medulloblastoma following X-ray-irradiation of newborn Ptc1 heterozygous mice

Individuals affected with the Gorlin syndrome inherit a germ-line mutation of the patched (Ptc1) developmental gene and, analogously to Ptc1 heterozygous mice, show an increased susceptibility to spontaneous tumor development. Human and mouse Ptc1 heterozygotes (Ptc1+/−) are also hypersensitive to ionizing radiation (IR)-induced tumorigenesis in terms of basal cell carcinoma (BCC) induction. We have analysed the involvement of Ptc1 in the tumorigenic response to a single dose of 3 Gy X-rays in neonatal and adult Ptc1 heterozygous and wild type mice. We report that irradiation dramatically increased the incidence of medulloblastoma development (51%) over the spontaneous rate (7%) in neonatal but not adult Ptc1 heterozygotes, indicating that medulloblastoma induction by IR is subjected to temporal restriction. Analysis of Ptc1 allele status in the tumors revealed loss of the wild type allele in 17 of 18 medulloblastomas from irradiated mice and in two of three spontaneous medulloblastomas. To our knowledge, irradiated newborn Ptc1+/− heterozygous mice constitute the first mouse model of IR-induced medulloblastoma tumorigenesis, providing a useful tool to elucidate the molecular basis of medulloblastoma development.

Pharmacological activation of mGlu4 metabotropic glutamate receptors inhibits the growth of medulloblastomas.

Moving from the evidence that activation of type 4 metabotropic glutamate (mGlu4) receptors inhibits proliferation and promotes differentiation of cerebellar granule cell neuroprogenitors, we examined the expression and function of mGlu4 receptors in medulloblastoma cells. mGlu4 receptors were expressed in 46 of 60 human medulloblastoma samples. Expression varied in relation to the histotype (nodular desmoplastic>classic≫large-cell anaplastic) and was inversely related to tumor severity, spreading, and recurrence. mGlu4 receptors were also found in D283med, D341med, and DAOY medulloblastoma cell lines, where receptor activation with the selective enhancer PHCCC inhibited adenylyl cyclase and the phosphatidylinositol-3-kinase pathway without affecting the mitogen-activated protein kinase, Sonic Hedgehog, and Wnt pathways. Interestingly, mGlu4 receptor activation reduced DNA synthesis and cell proliferation in all three cell lines. This effect was abrogated by the phosphatidylinositol-3-kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one]. In in vivo experiments, repeated subcutaneous injections of N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) reduced the growth of D283med and DAOY cell xenografts in nude mice. More remarkably, subcutaneous or intracranial injections of PHCCC during the first week of life prevented the development of medulloblastomas in mice lacking one Patched-1 allele and x-irradiated 1 d after birth. These data suggest that mGlu4 receptor enhancers are promising drugs for the treatment of medulloblastomas.

Inhibition of medulloblastoma tumorigenesis by the antiproliferative and pro-differentiative gene PC3

Medulloblastoma, the most common brain tumor in childhood, appears to originate from cerebellar granule cell precursors (GCPs), located in the external granular layer (EGL) of the cerebellum. The antiproliferative gene PC3 (Tis21 /BTG2) promotes cerebellar neurogenesis by inducing GCPs to shift from proliferation to differentiation. To assess whether PC3 can prevent the neoplastic transformation of GCPs and medulloblastoma development, we crossed transgenic mice conditionally expressing PC3 (TgPC3) in GCPs with Patchedl heterozygous mice (Ptc+/−), a model of medulloblastoma pathogenesis characterized by hyper‐activation of the Sonic Hedgehog pathway. Perinatal up‐regulation of PC3 in Ptc+/−/TgPC3 mice results in a decrease of medulloblastoma incidence of ~40% and in a marked reduction of preneoplastic abnormalities, such as hyperplastic EGL areas and lesions. Moreover, overexpression of cyclin D1, hyperproliferation, and defective differentiation—observed in Ptc+/− GCPs— are restored to normality in Ptc+/−/TgPC3 mice. The PC3‐mediated inhibition of cyclin D1 expression correlates with recruitment of PC3 to the cyclin D1 promoter, which is accompanied by histone deacetylation. Remarkably, down‐regulation of PC3 is observed in pre‐neoplastic lesions, as well as in human and murine medulloblastomas. As a whole, this indicates that PC3 may prevent medulloblastoma development by controlling cell cycle and promoting differentiation of GCPs.–Farioli‐Vecchioli, S., Tanori, M., Micheli, L., Mancuso, M., Leonardi, L., Saran, A., Ciotti, M. T., Ferretti, E., Gulino, A., Pazzaglia, S., Tirone, F. Inhibition of medulloblastoma tumorigenesis by the antiprolifera‐tive and pro‐differentiative gene PC3. FASEB J. 21, 2215‐2225 (2007)

Role of connexin43 and ATP in long-range bystander radiation damage and oncogenesis in vivo

Ionizing radiation is a genotoxic agent and human carcinogen. Recent work has questioned long-held dogmas by showing that cancer-associated genetic alterations occur in cells and tissues not directly exposed to radiation, questioning the robustness of the current system of radiation risk assessment. In vitro, diverse mechanisms involving secreted soluble factors, gap junction intercellular communication (GJIC) and oxidative metabolism are proposed to mediate these indirect effects. In vivo, the mechanisms behind long-range ‘bystander’ responses remain largely unknown. Here, we investigate the role of GJIC in propagating radiation stress signals in vivo, and in mediating radiation-associated bystander tumorigenesis in mouse central nervous system using a mouse model in which intercellular communication is downregulated by targeted deletion of the connexin43 (Cx43) gene. We show that GJIC is critical for transmission of oncogenic radiation damage to the non-targeted cerebellum, and that a mechanism involving adenosine triphosphate release and upregulation of Cx43, the major GJIC constituent, regulates transduction of oncogenic damage to unirradiated tissues in vivo. Our data provide a novel hypothesis for transduction of distant bystander effects and suggest that the highly branched nervous system, similar to the vascular network, has an important role.

Two-hit model for progression of medulloblastoma preneoplasia in Patched heterozygous mice

Inactivation of one Ptc1 allele predisposes humans and mice to spontaneous medulloblastoma development, and irradiation of newborn Ptc1 heterozygous mice results in dramatic increase of medulloblastoma incidence. While a role for loss of wild-type (wt) Ptc1 (LOH) in radiation-induced medulloblastomas from Ptc1neo67/+ mice is well established, the importance of this event in spontaneous medulloblastomas is still unclear. Here, we demonstrate that biallelic Ptc1 loss plays a crucial role in spontaneous medulloblastomas, as shown by high rate of wt Ptc1 loss in spontaneous tumors. In addition, remarkable differences in chromosomal events involving the Ptc1 locus in spontaneous and radiation-induced medulloblastomas suggest distinct mechanisms for Ptc1 loss. To assess when, during tumorigenesis, Ptc1 loss occurs, we characterized cerebellar abnormalities that precede tumor appearance in Ptc1neo67/+ mice. We show that inactivation of only one copy of Ptc1 is sufficient to give rise to abnormal cerebellar proliferations with different degree of altered cell morphology, but lacking potential to progress to neoplasia. Furthermore, we identify biallelic Ptc1 loss as the event causally related to the transition from the preneoplastic stage to full blown medulloblastoma. These results underscore the utility of the Ptc1neo67/+ mouse model for studies on the mechanisms of medulloblastoma and for development of new therapeutic strategies.

Modulation of basal and squamous cell carcinoma by endogenous estrogen in mouse models of skin cancer

Patched1 heterozygous mice (Ptch1(+/-)) are useful for basal cell carcinoma (BCC) studies, being remarkably susceptible to BCC induction by ultraviolet or ionizing radiation. Analogously, skin carcinogenesis-susceptible (Car-S) mice are elective for studies of papilloma and squamous cell carcinoma (SCC) induction. We previously reported a striking effect of gender on BCC induction in Ptch1(+/-) mice, with total resistance of females; likewise, Car-S females show increased skin tumor resistance relative to males. Here, we investigated the protective role of endogenous estrogen in skin keratinocyte tumorigenesis. Control (CN) and ovariectomized Ptch1(+/-) or Car-S females were irradiated for BCC induction or topically treated with chemical carcinogens for SCC induction. Susceptibility to BCC or SCC was dramatically increased in ovariectomized Ptch1(+/-) and Car-S females and restored to levels observed in males. Remarkably, progression of initially benign papillomas to malignant SCC occurred only in ovariectomized Car-S females. We explored the mechanisms underlying tumor progression and report overexpression of estrogen receptor (ER)-alpha, downregulation of ERbeta and upregulation of cyclin D1 in papillomas from ovariectomized Car-S relative to papillomas from CN females. Thus, an imbalanced ERalpha/ERbeta expression may be associated with estrogen-mediated modulation of non-melanoma skin carcinogenesis, with a key role played by cyclin D1. Our findings underscore a highly protective role of endogenous estrogen against skin tumorigenesis by diverse agents in two independent mouse models of skin cancer.

Hair Cycle–Dependent Basal Cell Carcinoma Tumorigenesis in Ptc1neo67/+ Mice Exposed to Radiation

We examined the effects of hair cycle phase on basal cell carcinoma (BCC) tumorigenesis induced by radiation in mice lacking one Patched allele (Ptc1(neo67/+)). Our results show that Ptc1(neo67/+) mouse skin irradiated in early anagen is highly susceptible to tumor induction, as a 3.2-fold incidence of visible BCC-like tumors was observed in anagen-irradiated compared with telogen-irradiated mice. Microscopic nodular BCC-like tumors were also enhanced by irradiation during active hair-follicle growth phases. Interestingly, histologic examination of the tumors revealed a qualitative difference in BCC tumorigenesis depending on hair growth phase at the time of exposure. In fact, in addition to typical BCC-like tumors, we observed development of a distinct basal cell tumor subtype characterized by anti-cytokeratin 14 and anti-smooth muscle actin reactivity. These tumors showed relatively short latency and rapid growth and were strictly dependent on age at irradiation, as they occurred only in mice irradiated in early anagen phase. Examination of anatomic and immunohistochemical relationships revealed a close relation of these tumors with the follicular outer root sheath of anagen skin. In contrast, there are strong indications for the derivation of typical, smooth muscle actin-negative BCC-like tumors from cell progenitors of interfollicular epidermis. These results underscore the role of follicular bulge stem cells and their progeny with high self-renewal capacity in the formation of basal cell tumors and contribute to clarify the relationship between target cell and tumor phenotype in BCC tumorigenesis induced by radiation.

A cancer modifier role for parathyroid hormone-related protein.

The parathyroid hormone-related protein (PTHrP) gene (Pthlh) maps in the distal region of mouse chromosome 6 that contains a quantitative trait locus associated with genetic predisposition to skin tumorigenesis. Here, we report a genetic polymorphism located in the osteostatin encoding region of the Pthlh gene and that produces Thr/Pro PTHrP variants. PthlhThr and PthlhPro alleles were significantly linked with resistance and susceptibility to skin carcinogenesis in phenotypically selected Car-R and Car-S outbred mice. Transfection of human NCI-H520 squamous cell carcinoma cells with the PthlhPro allele resulted in cells growing in clusters, tending to pile up, and growing at a significantly faster rate in nude mice than non-transfected and PthlhThr-transfected cells. These results point to the role of the Pthlh gene as a cancer modifier gene in skin tumorigenesis.