The mutagenic properties of ionizing radiation are well known, but the presence of specific mutations in human radiation-induced tumours is not established. We have studied a series of 36 secondary sarcomas arising in the irradiation field of a primary tumour following radiotherapy. The allelic status and the presence of mutations of the TP53 gene were investigated. The mutation pattern was compared with data from sporadic sarcomas recorded in the IARC TP53 somatic mutations database. A high proportion (58%) of the radiation-induced sarcomas exhibited a somatic inactivating mutation for one allele of TP53, systematically associated with a loss of the other allele. The high frequency (52%) of short deletions observed in the mutation pattern of radiation-induced sarcomas may be related to the induction of DNA breaks by ionizing radiation. The lack of hyper-reactivity of CpG dinucleotides and the presence of recurrent sites of mutation at codons 135 and 237 seem also to be specific for radiation tumorigenesis.

Human herpesvirus 8 (HHV-8) is the etiological agent of Kaposi's sarcoma (KS).

Interferon γ (IFN-γ) has recently been implicated in cancer immunosurveillance. Among the most abundant proteins induced by IFN-γ are guanylate binding proteins (GBPs), which belong to the superfamily of large GTPases and are widely expressed in various species. Here, we investigated whether the well-known human GBP-1 (hGBP-1), which has been shown to exert antiangiogenic activities and was described as a prognostic marker in colorectal carcinomas, may contribute to an IFN-γ-mediated tumor defense. To this end, an IFN-independent, inducible hGBP-1 expression system was established in murine mammary carcinoma (TS/A) cells, which were then transplanted into syngeneic immune-competent Balb/c mice. Animals carrying TS/A cells that had been given doxycycline for induction of hGBP-1 expression revealed a significantly reduced tumor growth compared with mock-treated mice. Immunohistochemical analysis of the respective tumors demonstrated a tightly regulated, high-level expression of hGBP-1. No signs of an enhanced immunosurveillance were observed by investigating the number of infiltrating B and T cells. However, hemoglobin levels as well as the number of proliferating tumor cells were shown to be significantly reduced in hGBP-1-expressing tumors. This finding corresponded to reduced amounts of vascular endothelial growth factor A (VEGF-A) released by hGBP-1-expressing TS/A cells in vitro and reduced VEGF-A protein levels in the corresponding mammary tumors in vivo. The results suggest that hGBP-1 may contribute to IFN-γ-mediated antitumorigenic activities by inhibiting paracrine effects of tumor cells on angiogenesis. Consequently, owing to these activities GBPs might be considered as potent members in an innate, IFN-γ-induced antitumoral defense system.

The tumour suppressor genes, TP53 and RB1, and four genes involved in their regulation, INK4a, ARF, MDM2 and MDMX, were analysed in a series of 36 postradiotherapy radiation-induced sarcomas. One-third of the tumours developed in patients carrying a germline mutation of RB1 that predisposed them to retinoblastoma and radiation-induced sarcomas. The genetic inactivation of RB1 and/or TP53 genes was frequently observed in these sarcomas. These inactivations were owing to an interplay between point mutations and losses of large chromosome segments. Radiation-induced somatic mutations were observed in TP53, but not in RB1 or in the four other genes, indicating an early role of TP53 in the radio-sarcomagenesis. RB1 and TP53 genes were biallelically coinactivated in all sarcomas developing in the context of the predisposition, indicating that both genes played a major role in the formation of these sarcomas. In the absence of predisposition, TP53 was biallelically inactivated in one-third of the sarcomas, whereas at least one allele of RB1 was wild type. In both genetic contexts, the TP53 pathway was inactivated by genetic lesions and not by the activation of the ARF/MDM2/MDMX pathway, as recently shown in retinoblastomas. Together, these findings highlight the intricate tissue-and aetiologyspecific relationships between TP53 and RB1 pathways in tumorigenesis.