Effects of heterozygosity for the Rb-1t19neo allele in the mouse.

We describe the pathology of a cohort of 80 mice heterozygous for an inactive allele of the Rb-1 tumour suppressor gene. The majority of these mice developed locally invasive tumours, arising from the pituitary gland. The time of onset of overt signs of disease in mice known to have inherited their mutant allele paternally shows a small but statistically significant shift to the lower end of the spectrum, suggesting that tumorigenesis is influenced by gametic imprinting. In situ hybridisation analysis demonstrates the presence in the tumours of pro-opiomelanocortin mRNA, which is normally found both in corticotroph and melanotroph cells. Mice within this cohort also develop systemic defects. Most notably, there is increased siderosis in the spleen indicating the possibility of an abnormality in red blood cell turnover. This is consistent with the abnormalities of erythropoiesis described previously in homozygous Rb-1-deficient mice. In addition, a proportion of mice developed liver steatosis, probably representing the end organ effects of hormonal imbalance as a direct consequence of tumour presence. A significant proportion showed C cell hyperplasia in the thyroid. The spectrum of pathology in mice differs from that in the human but does provide a useful model of site-specific tumour predisposition.