The murine gene p27Kip1 is haplo-insufficient for tumour suppression

Abstract

Abstractp27 Kip1 is a candidate human tumour-suppressor protein, because it is able to inhibit cyclindependent kinases and block cell proliferation [1][2][3][4][5] . Abnormally low levels of the p27 protein are frequently found in human carcinomas, and these low levels correlate directly with both histological aggressiveness and patient mortality [6][7][8][9][10] . However, it has not been possible to establish a causal link between p27 and tumour suppression, because only rare instances of homozygous inactivating mutations of the p27 gene have been found in human tumours [11][12][13][14] . Thus, p27 Kip1 does not fulfill Knudson's 'two-mutation' criterion for a tumour suppressor gene 15 . Here we show that both p27 nullizygous and p27 heterozygous mice are predisposed to tumours in multiple tissues when challenged with g-irradiation or a chemical carcinogen. Therefore p27 is a multipletissue tumour suppressor in mice. Molecular analyses of tumours in p27 heterozygous mice show that the remaining wild-type allele is neither mutated nor silenced. Hence, p27 is haplo-insufficient for tumour suppression. The assumption that null mutations in tumour-suppressor genes are recessive excludes those genes that exhibit haplo-insufficiency.Tumour-suppressor proteins affect several cellular pathways, such as those controlling proliferation, apoptosis, differentiation and genomic integrity. Consequently, the identification of a tumour-suppressor gene is usually established genetically rather than by any particular functional criterion. The common operational definition of a tumoursuppressor gene requires the demonstration of mutations of both copies of a candidate gene in tumours 15 . Genetic and physical mapping of bi-allelic tumour specific mutations allows localization of tumour-suppressor genes, and provides evidence for a causal link between mutations in those genes and tumorigenesis. This approach has successfully demonstrated the tumour-suppressor function of the retinoblastoma protein (Rb), p53, INK4a, and others [16][17][18] . However, we show here that this definition is too restrictive because it excludes tumour suppressors such as p27 Kip1 that exhibit haplo-insufficiency.We studied the tumour-suppressor function of p27 Kip1 by examining the susceptibility of p27-deficient mice to tumorigenesis induced by two different DNA-damaging agents, gCorrespondence and requests for materials should be addressed to M.L.F. (mfero@fhcrc.org). -ethyl-N-nitrosourea (ENU). p27 −/− mice showed decreased tumour-free survival following g-irradiation compared with p27 +/+ controls (Fig. 1a). p27 +/− mice showed an intermediate sensitivity (see below). Thymic lymphomas were the most frequent tumours in wild-type mice (incidence 0.07), but their frequency was not significantly increased in p27 −/− animals (incidence 0.17). The increased mortality of p27-null mice was mainly due to the development of pituitary tumours and intestinal adenomas (Fig. 2). A separate group of mice was injected with a single dose of ENU, and again p27-nu...

Mutant B-RAF signaling and cyclin D1 regulate Cks1/S-phase kinase-associated protein 2-mediated degradation of p27Kip1 in human melanoma cells

“…Alterations in p27 Kip1 warrant particular attention as p27 Kip1 protein levels are reduced in several types of cancer including melanoma (Catzavelos et al, 1997;Loda et al, 1997;Mori et al, 1997;Florenes et al, 1998;Kawana et al, 1998), and p27 Kip1 heterozygous mice are prone to induced tumor formation (Fero et al, 1998). B-RAF is mutated in approximately 70% of melanomas (Davies et al, 2002).…”

Deregulated degradation of the cdk inhibitor p27 and malignant transformation

“…When Rb +/− and p27 −/− are crossed, Rb +/− and p27 −/− offspring develop more aggressive tumors and at an earlier age than Rb +/− alone, indicating that Rb and p27 cooperate in tumor suppression [33]. p27 is classified as a haploinsufficient tumor suppressor because mutation in just one allele of Cdkn1b, the gene encoding mouse p27, renders mice more susceptible to tumorigenesis by DNA-damaging agents compared to wild-type mice [34]. Furthermore, concomitant inactivation of one allele of the tumor suppressor gene Pten and both or even one of the Cdkn1b alleles accelerates spontaneous neoplastic transformation and incidence of tumors of various histological origins [35].…”

p27Kip1-deficient mice exhibit accelerated growth hormone-releasing hormone (GHRH)-induced somatotrope proliferation and adenoma formation

“…In contrast to the tumorigenic e ect of p27-de®ciency on intermediate lobe cells, hyperplasia of the spleen, thymus, testis and ovary does not progress to neoplasia under controlled environmental conditions. However, Fero and coworkers (Fero et al, 1998) recently reported that p27+/7 mice, as well as p277/7 mice, are more susceptible than are p27+/+ mice to both radiationand chemical-induced tumors of the lung, ovary and endometrium. These results indicate that a partial reduction in p27 can sensitize cells, in a tissue-speci®c manner, to external tumorigenic factors.…”

“…These results indicate that a partial reduction in p27 can sensitize cells, in a tissue-speci®c manner, to external tumorigenic factors. Therefore, p27 is considered haplo-insu cient for tumor suppression (Fero et al, 1998).…”