Fbxw7/Cdc4 is a p53-dependent haploinsufficient tumour suppressor gene

Allan Balmain
abalmain@cc.ucsf.edu
Cancer Research Institute, University of California at San Francisco,
San Francisco CA 94143, USA
Website: http://cc.ucsf.edu/balmain/index.html

Mice carrying inactivated alleles of the p53 gene (p53-/-) frequently develop lymphomas and sarcomas, even though epithelial tumors are relatively uncommon. Heterozygous mice with one functional p53 allele (p53+/-) develop tumors after a considerably longer latency period than their homozygous null counterparts, even after acceleration of tumour appearance by exposure to ionizing radiation. It is normally assumed that the latency time between exposure and tumor appearance is longer in p53 heterozygous (p53+/-) mice because of the requirement for loss of the remaining wild type p53 allele. This project is focused on investigating the possibility that other genetic events occur prior to somatic loss of the remaining p53 allele. If additional events are required for growth or survival of somatic cells that suffer a second hit in the p53 gene, this would result in certain genetic changes would be expected exclusively in tumors from the p53 heterozygous mice.

Project Goals

1. To compare the genetic alterations in tumors developed in (p53-/-) and (p53+/) mice
2. To demonstrate that several mouse chromosomes show loss of heterozygosity only in tumors that developed from somatic cells that contain at least one functional p53 allele.
3. Expression of Aurora-A kinase and Notch4 is specifically increased by RNAi-mediated suppression of Fbxw7/Cdc4 expression in mouse embryonic fibroblasts with functional p53, and in cells from Fbxw7/Cdc4+/- mice. We propose to investigate the p53-dependent loss of Fbxw7/Cdc4 and determine if it leads to genetic instability by activation of Aurora-A. This will provide a rationale for the early occurrence of these mutations in human cancers.

Experimental Approach

One of the genes in this class of tumors that show high frequency loss of heterozygosity and, more rarely, complete loss of function is the Fbxw7/Cdc4 gene. The h Fbxw7/Cdc4 gene encodes an ubiquitin ligase implicated in control of chromosome stability and with tumor suppressor activity . Using a mammalian genetic screen for p53-dependent genes involved in tumorigenesis, the mouse Fbxw7/Cdc4 gene has been identified as a p53-dependent, haploinsufficient tumour suppressor gene. The spectrum of genetic alterations in p53 null tumors that had developed in animals with one functional p53 allele was compared with similar tumors from p53 null mice. If loss of the wild type p53 allele in heterozygous mice is the first somatic genetic event, the overall pattern of genetic changes would be expected to be very similar to that of tumors from p53 null mice.

Outcomes

It has been demonstrated that r adiation-induced lymphomas from p53+/- mice, but not those from p53-/- mice, show frequent loss of heterozygosity and a 10% mutation rate of the Fbxw7/Cdc4 gene. Although p53 deficient mice develop predominantly lymphomas and sarcomas, mice that carry inactivated alleles of both p53 and Fbxw7/Cdc4 develop tumours in a range of epithelial tissues such as the ovary, lung and liver. Haploinsufficiency for Fbxw7/Cdc4 therefore changes the spectrum of tumours normally seen in p53 heterozygous mice 1.