Differential effects of genes of the Rb1 signalling pathway on osteosarcoma incidence and latency in alpha-particle irradiated mice.

Osteosarcoma is the most frequent secondary malignancy following radiotherapy of patients with bilateral retinoblastoma. This suggests that the Rb1 tumour suppressor gene might confer genetic susceptibility towards radiation-induced osteosarcoma. To define the contribution of the Rb1 pathway in the multistep process of radiation carcinogenesis, we evaluated somatic allelic changes affecting the Rb1 gene itself as well as its upstream regulator p16 in murine osteosarcoma induced by (227)Th incorporation. To distinguish between the contribution of germline predisposition and the effect of a 2-hit allelic loss, two mouse models harbouring heterozygote germline Rb1 and p16 defects were tested for the incidence and latency of osteosarcoma following irradiation. We could show that all tumours arising in BALB/c×CBA/CA hybrid mice (wild-type for Rb1 and for p16) carried a somatic allelic loss of either the Rb1 gene (76.5%) or the p16 gene (59%). In none of the tumours, we found concordant retention of heterozygosity at both loci. Heterozygote knock-out mice for Rb1 exhibit a significant increase in the incidence of osteosarcoma following (227)Th incorporation (11/24 [corrected] in Rb1+/- vs. 2/18 in Rb1+/+, p=4×10(-5)), without affecting tumour latency. In contrast, heterozygote knock-out mice for p16 had no significant change in tumour incidence, but a pronounced reduction of latency (LT(50%) =355 days in p16+/- vs. 445 days in p16+/+, p=8×10(-3)). These data suggest that Rb1 germline defects influence early steps of radiation osteosarcomagenesis, whereas alterations in p16 mainly affect later stages of tumour promotion and growth.

Hemizygosity for Atm and Brca1 influence the balance between cell transformation and apoptosis.

BACKGROUND: In recent years data from both mouse models and human tumors suggest that loss of one allele of genes involved in DNA repair pathways may play a central role in genomic instability and carcinogenesis. Additionally several examples in mouse models confirmed that loss of one allele of two functionally related genes may have an additive effect on tumor development. To understand some of the mechanisms involved, we examined the role of monoallelic loss or Atm and Brca1 on cell transformation and apoptosis induced by radiation. METHODS: Cell transformation and apoptosis were measured in mouse embryo fibroblasts (MEF) and thymocytes respectively. Combinations of wild type and hemizygous genotypes for ATM and BRCA1 were tested in various comparisons. RESULTS: Haploinsufficiency of either ATM or BRCA1 resulted in an increase in the incidence of radiation-induced transformation of MEF and a corresponding decrease in the proportion of thymocytes dying an apoptotic death, compared with cells from wild-type animals. Combined haploinsufficiency for both genes resulted in an even larger effect on apoptosis. CONCLUSIONS: Under stress, the efficiency and capacity for DNA repair mediated by the ATM/BRCA1 cell signalling network depends on the expression levels of both proteins.

The genetics of radiation-induced and sporadic osteosarcoma: a unifying theory?

Cancer is a disease of the genome, with the neoplastic phenotype being passed from one cell generation to the other. Radiation-induced cancer has often been considered to represent a unique entity amongst neoplasia, with the energy deposition being held responsible for both direct (gene mutations) and indirect (bystander effects, induced instability etc) alterations to the cellular genome. However, radiogenic tumours in man and experimental animals appear to be physiologically and genetically indistinguishable from their sporadic counterparts, suggesting that the aetiologies of these two tumour types are in fact closely related. We have conducted a general screen of the genetic alterations in radiation-induced mouse osteosarcoma, a tumour that is histopathologically indistinguishable from human sporadic osteosarcoma. Comparison of the two tumour types indicates the existence of a common set of genetic changes, providing additional evidence to support the concept that the molecular pathology of radiation-induced malignancy is no different to that of sporadic cancers.

Allelic imbalance in hMLH1 or BRCA2 loci associated with response of cervical and endometrial cancer to radiotherapy.

Effectiveness of radiotherapy is influenced by several genetic properties of the targeted cells. The aim of this study was the identification of prognostic indicators of tumor response to radiation in cervical and endometrial cancer. Using microsatellite DNA analysis, we investigated 31 markers, located on 1p, 2p, 2q, 3p, 9p, 9q, 13q, 17p and 17q for genomic alterations in 37 cervical and 21 endometrial cancer cases, with complete follow-up data. Genetic alterations of the initial tumor genotypes were observed after radiation in 86.5% of cervical and 81.0% of endometrial cases. Reversions to the original normal genotype were observed in 40.5 and 28.6% respectively, predominantly in cured patients rather than in recurred cases. Survival curves by the Kaplan-Meier method showed a worse prognosis for cervical cancer patients whose tumors harbor allelic imbalance (AI) on 3p or 13q, and for endometrial cancer patients whose tumors harbor AI on 13q. Our data suggest a possible association of the hMLH1 or BRCA2 genes, implicated in distinct DNA repair pathways and located on 3p and 13q respectively, with response of cervical and endometrial cancer to radiotherapy. Moreover, microsatellite DNA analysis before and after radiation treatment could be used as a marker of the clinical outcome of patients.