No abstract

From the archives of the National Toxicology Program, National Institutes of Health, kidney sections from twenty-four carcinogenicity studies (representing twenty-three chemicals) in male and female F344 rats were histopathologically re-evaluated to grade the severity of chronic progressive nephropathy (CPN) on an expanded scale of 0-8, and to record the presence of renal tubule tumors (RTT) and their precursor, atypical tubule hyperplasia (ATH). The data were statistically analyzed using SAS software for logistic regression analysis. This histopathological survey of 2,436 F344 rats showed clear evidence of a qualitative and statistically significant association between advanced stages of CPN severity and the development of low-grade RTT and ATH. Advanced CPN severity therefore represents a risk factor for the development of RTT and appears to be an underlying basis for spontaneous occurrence of RTT in the F344 rat. The difference in incidence and severity of CPN between the sexes also explains the 9:1 male-to-female sex difference in the spontaneous occurrence of ATH and RTT observed here. The regulatory significance of this finding is that chemicals exacerbating CPN as their only renal effect are likely to show a numerical increase in RTT with dose, which does not represent a direct tumorigenic effect of the chemical.

The Toxicology Data Management System (TDMS) of the National Toxicology Program, National Institutes of Environmental Health Sciences, National Institutes of Health, was surveyed for occurrence and distribution of a distinctive renal tubule tumor type in rats. The hallmark features of this tumor included eosinophilic/amphophilic staining, large finely granular cells, and numerous vacuoles and/or minilumens. It is referred to here as the amphophilic-vacuolar (AV) variant of renal tubule tumor. Of 154 studies in which renal tubule tumors had been recorded in the standard single sections of kidney in the TDMS, there were collectively 1012 rats with renal adenomas, carcinomas, or adenocarcinomas, and of these, 100 displayed the distinctive AV morphology, representing 74 studies involving mostly the F344 rat, but also the Sprague-Dawley and Wistar strains. The AV tumors (mainly adenomas but also some carcinomas) occurred usually as solitary lesions in the affected animals. However, they were multiple and bilateral in a few cases. They were equally distributed between the sexes, did not metastasize (at least to the lung), and were not associated with chronic progressive nephropathy. The distribution of this renal tumor type was random across studies and dose groups, underscoring the likelihood that it was of spontaneous origin and not chemically induced. Accordingly, it is suggested that this distinctive renal tumor phenotype be recorded as a separate category from conventional RTT when assessing the carcinogenic potential of a test compound.

Mutations in both p53 and BRCA2 are commonly seen together in human tumors suggesting that the loss of both genes enhances tumor development. To elucidate this interaction in an animal model, mice lacking the carboxy terminal domain of Brca2 were crossed with p53 heterozygous mice. Females from this intercross were then irradiated with an acute dose of 5 Gy ionizing radiation at 5 weeks of age and compared to nonirradiated controls. We found decreased survival and timing of tumor onsets, and significantly higher overall tumor incidences and prevalence of particular tumors, including stomach tumors and squamous cell carcinomas, associated with the homozygous loss of Brca2, independent of p53 status. The addition of a p53 mutation had a further impact on overall survival, incidence of osteosarcomas and stomach tumors, and tumor latency. The spectrum of tumors observed for this Brca2 germline mouse model suggest that it faithfully recapitulates some human disease phenotypes associated with BRCA2 loss. In addition, these findings include extensive in vivo data demonstrating that germline Brca2 and p53 mutations cooperatively affect animal survivals, tumor susceptibilities, and tumor onsets.