p15Ink4b is a critical tumour suppressor in the absence of p16Ink4a.

The CDKN2b-CDKN2a locus on chromosome 9p21 in human (chromosome 4 in mouse) is frequently lost in cancer. The locus encodes three cell cycle inhibitory proteins: p15INK4b encoded by CDKN2b, p16INK4a encoded by CDKN2a and p14ARF (p19Arf in mice) encoded by an alternative reading frame of CDKN2a (ref. 1). Whereas the tumour suppressor functions for p16INK4a and p14ARF have been firmly established, the role of p15INK4b remains ambiguous. However, many 9p21 deletions also remove CDKN2b, so we hypothesized a synergistic effect of the combined deficiency for p15INK4b, p14ARF and p16INK4a. Here we report that mice deficient for all three open reading frames (Cdkn2ab-/-) are more tumour-prone and develop a wider spectrum of tumours than Cdkn2a mutant mice, with a preponderance of skin tumours and soft tissue sarcomas (for example, mesothelioma) frequently composed of mixed cell types and often showing biphasic differentiation. Cdkn2ab-/- mouse embryonic fibroblasts (MEFs) are substantially more sensitive to oncogenic transformation than Cdkn2a mutant MEFs. Under conditions of stress, p15Ink4b protein levels are significantly elevated in MEFs deficient for p16Ink4a. Our data indicate that p15Ink4b can fulfil a critical backup function for p16Ink4a and provide an explanation for the frequent loss of the complete CDKN2b-CDKN2a locus in human tumours.

Genetic interactions between the Wilms' tumor 1 gene and the p53 gene.

In recent years, a number of proteins have been identified that can modify the activities of the Wilms' Tumor 1 (WT1) proteins. One of these modifiers is the p53 protein. To investigate a genetic interaction between the p53 gene and the wt1 gene, we have crossed their respective knockout mice. The absence of p53 appears to have no gross effect on the phenotype of wt1-null mice. Both wt1-null and double-null embryos develop pericardial bleeding and die in utero. In adult p53-null mice, wt1-heterozygosity (wt1het) predisposes to an earlier onset of lymphomagenesis and the development of kidney abnormalities resembling oncocytoma in humans. wt1-heterozygosity alone predisposes to the development of glomerular sclerosis.

The Wilms' tumor gene WT1 is a common marker of progenitor cells in fetal liver.

It is well known that the Wilms' tumor gene WT1 plays an important role in cell proliferation and differentiation, and in organ development. In this study, to examine the role of the WT1 gene in lineage determination, fetal liver cells from LacZ-transgenic mice, in which WT1 expression was marked by the expression of the LacZ gene driven by WT1 promoter, were FACS-sorted according to LacZ expression of high (LacZ(++)) or undetectable (LacZ(-)) levels, which paralleled endogenous WT1 expression levels. LacZ(++) fetal liver cells were enriched by hepatocyte and endothelial progenitor cells. These results indicated that WT1 expression is a common marker of both hepatocyte and endothelial progenitors. These results also implied a role of the WT1 gene in lineage determination.

In vivo activation of PPAR target genes by RXR homodimers.

The ability of a retinoid X receptor (RXR) to heterodimerize with many nuclear receptors, including LXR, PPAR, NGF1B and RAR, underscores its pivotal role within the nuclear receptor superfamily. Among these heterodimers, PPAR:RXR is considered an important signalling mediator of both PPAR ligands, such as fatty acids, and 9-cis retinoic acid (9-cis RA), an RXR ligand. In contrast, the existence of an RXR/9-cis RA signalling pathway independent of PPAR or any other dimerization partner remains disputed. Using in vivo chromatin immunoprecipitation, we now show that RXR homodimers can selectively bind to functional PPREs and induce transactivation. At the molecular level, this pathway requires stabilization of the homodimer-DNA complexes through ligand-dependent interaction with the coactivator SRC1 or TIF2. This pathway operates both in the absence and in the presence of PPAR, as assessed in cells carrying inactivating mutations in PPAR genes and in wild-type cells. In addition, this signalling pathway via PPREs is fully functional and can rescue the severe hypothermia phenotype observed in fasted PPARalpha-/- mice. These observations have important pharmacological implications for the development of new rexinoid-based treatments.

The wt1-heterozygous mouse; a model to study the development of glomerular sclerosis.

In the present study, it is shown that mice heterozygous for wt1 develop glomerular sclerosis and the nature and time course of events leading to the glomerular scarring are determined. Wt1-heterozygous (wt1het) mice and their wild-type littermates were closely monitored from birth and plasma levels of urea, creatinine, and albumin were compared with histological data and clinical features. One of the first indications of nephropathy in the wt1het mouse was the development of proteinuria, accompanied by progressive elevation of the plasma levels of urea and creatinine. Subsequently, the mice developed albuminuria, which correlated with thickening of the glomerular basement membrane and fusion of the podocyte foot processes. Glomerulosclerosis was a relatively late event, accompanied by severe albuminuria and loss of WT1, nephrin, CD2AP, and alpha-actinin-4.