Malignant mesothelioma (MM) is an aggressive tumor with a poor prognosis mainly linked to past asbestos exposure. Murine models of MM based on fiber exposure have been developed to elucidate the mechanism of mesothelioma formation. Genomic alterations in murine MM have now been partially characterized. To gain insight into the pathophysiology of mesothelioma, 16 murine and 35 human mesotheliomas were characterized by array-comparative genomic hybridization and were screened for common genomic alterations. Alteration of the 9p21 human region, often by biallelic deletion, was the most frequent alteration in both species, in agreement with the CDKN2A/CDKN2B locus deletion in human disease and murine models. Other shared aberrations were losses of 1p36.3-p35 and 13q14-q33 and gains of 5p15.3-p13 regions. However, some differences were noted, such as absence of recurrent alterations in mouse regions corresponding to human chromosome 22. Comparison between altered recurrent regions in asbestos-exposed and non-asbestos-exposed patients showed a significant difference in the 14q11.2-q21 region, which was also lost in fiber-induced murine mesothelioma. A correlation was also demonstrated between genomic instability and tumorigenicity of human mesothelioma xenografts in nude mice. Overall, these data show similarities between murine and human disease, and contribute to the understanding of the influence of fibers in the pathogenesis of mesothelioma and validation of the murine model for preclinical testing.

Although human malignant mesothelioma (HMM) is mainly caused by asbestos exposure, refractory ceramic fibres (RCFs) have been classified as possibly carcinogenic to humans on the basis of their biological effects in rodents' lung and pleura and in cultured cells. Hence, further investigations are needed to clarify the mechanism of fibre-induced carcinogenicity and to prevent use of harmful particles. In a previous study, mesotheliomas were found in hemizygous Nf2 (Nf2(+/-)) mice exposed to asbestos fibres, and showed similar alterations in genes at the Ink4 locus and in Trp53 as described in HMM. Here we found that Nf2(+/-) mice developed mesotheliomas after intra-peritoneal inoculation of a RCF sample (RCF1). Clinical features in exposed mice were similar to those observed in HMM, showing association between ascite and mesothelioma. Early passages of 12 mesothelioma cell cultures from ascites developed in RCF1-exposed Nf2(+/-) mice demonstrated frequent inactivation by deletion of genes at the Ink4 locus, and low rate of Trp53 point and insertion mutations. Nf2 gene was inactivated in all cultures. In most cases, co-inactivation of genes at the Ink4 locus and Nf2 was found and, at a lower rate, of Trp53 and Nf2. These results are the first to identify mutations in RCF-induced mesothelioma. They suggest that nf2 mutation is complementary of p15(Ink4b), p16(Ink4a) and p19(Arf) or p53 mutations and show similar profile of gene alterations resulting from exposure to ceramic or asbestos fibres in Nf2(+/-) mice, also consistent with the one found in HMM. These somatic genetic changes define different pathways of mesothelial cell transformation.

The status of tumor suppressor genes (TSGs) relevant to human malignant mesothelioma (HMM) pathogenesis was examined in cultures of mesothelioma cells from tumoral ascites developed in mice exposed to asbestos (asb) fibers. The status of the respective hortologous human genes was also investigated in 12 HMM cell cultures. Eleven primary cultures from mice hemizygous for Nƒ2 (asb-Nf2KO3/+) and 4 wild type counterparts (asb-Nf2+/+) were analyzed for mutations in Nf2, p16/Cdkn2a, p19/Arf and Trp53 genes and protein expression of p15/Cdkn2b and Cdk4. TSG alterations in both mouse and human mesothelioma cells consisted in frequent inactivation of p16/Cdkn2a, p19/Arf (or P14/ARF) and p15/Cdkn2b, co-inactivation of p16/Cdkn2a and p15/Cdkn2b and low rate of Trp53 mutations in both asb-Nf2KO3/+ and asb-Nf2+/+ mesothelioma cells. In both mouse and human mesothelioma cells, inactivation of the hortologous genes p16/Cdkn2a or P16/CDKN2A was due to deletions at the Ink4/Arf locus encompassing p19/Arf or P14/ARF, respectively. Loss of heterozygosity at the Nf2 locus was detected in 10 of 11 asb-Nf2KO3/+ cultures and Nf2 gene rearrangement in one asb-Nf2+/+ culture. These data show that the profile of TSG alterations in asbestos-induced mesothelioma is similar in mice and humans. Thus, the mouse mesothelioma model could be useful for human risk assessment, taking into account interindividual variations in genetic sensitivity to carcinogens.

Biallelic NF2 gene inactivation is frequently found in human malignant mesothelioma. In order to assess whether NF2 hemizygosity may enhance susceptibility to asbestos fibres, we investigated the Nf2 status in mesothelioma developed in mice presenting a heterozygous mutation of the Nf2 gene (Nf2(KO3/+)), after intraperitoneal inoculation of crocidolite fibres. Asbestos-exposed Nf2(KO3/+) mice developed tumoural ascites and mesothelioma at a higher frequency than their wild-type (WT) counterparts (P&<0.05). Six out of seven mesothelioma cell lines established from neoplastic ascitic fluids of Nf2(KO3/+) mice exhibited loss of the WT Nf2 allele and no neurofibromatosis type 2 protein expression was found in these cells. The results show the importance of the NF2 gene in mesothelial oncogenesis, the potential association of asbestos exposure and tumour suppressor gene inactivation, and suggest that NF2 gene mutation may be a susceptibility factor to asbestos.