An oncogenic form of p53 confers a dominant, gain-of-function phenotype that disrupts spindle checkpoint control

Abstract

Although it is well-established that p53 functions as a tumor suppressor gene, certain mutations exhibit gain-of-function activities that increase oncogenic transformation. We have found a common class of p53 missense mutation that exhibits a dominant, gain-of-function activity that generates genomic instability. Fibroblasts from LiFraumeni syndrome heterozygotes with such mutations generate polyploid cells when exposed to spindle depolymerizing agents. Expression of such mutant alleles in normal fibroblasts yields the same phenotype. This class of dominant, gain-of-function p53 mutation (p53 RSC , relaxed spindle checkpoint allele) does not require the transcriptional activation function of p53 for this behavior. Thus p53 mutations can contribute to progression of a cancer cell not only by absence of p53 tumor suppressor activity but also by the presence of an activity that promotes genetic instability.

Transcription regulation by mutant p53

“…Mutp53 was also reported to protect hepatocytes from a combination of HBV-X protein and TNFa (Lee et al, 2000). Moreover, Li-Fraumeni syndrome-derived fibroblasts, endogenously expressing mutp53, exhibited increased resistance to apoptosis in response to mitomycin C, UV and IR, relative to p53-null fibroblasts (Gualberto et al, 1998).…”

“…Analysis of additional cell systems exogenously expressing mutp53 further demonstrated the ability of mutp53 to promote cancersupporting phenotypes. Mutp53 was shown to increase genomic instability in Li-Fraumeni syndrome-derived fibroblasts in conjunction with disruption of the mitotic spindle checkpoint (Gualberto et al, 1998), in Jurkat cells following X-irradiation as measured by altered T-cell receptor surface expression (Iwamoto et al, 1996), in mammary mouse fibroblasts following ultraviolet (UV) and ionizing radiation (IR) as reflected by aberrant centrosome numbers (Murphy et al, 2000) and in Saos-2 human osteosarcoma cells as assessed by gene amplification (El-Hizawi et al, 2002). Mutp53 was also shown to enhance colony formation when overexpressed in p53-null mouse fibroblasts and human lung cancer-derived cells (Murphy et al, 2000;Deb et al, 2002;Weisz et al, 2004;Scian et al, 2004a).…”

Identification of genomic DNA sequences bound by mutant p53 protein (Gly245→Ser) in vivo

“…Satellite sequences frequently cluster at centromeres and play an important role in the correct assembly of kinetochores, required for an accurate segregation of chromosomes during mitosis (Murphy and Karpen, 1998). In this respect, it is of interest that a`gain of function' for mut p53 has been suggested by contributing to the loss of spindle checkpoint controls (Gualberto et al, 1998). Our ®nding that mut p53 in Onda 11 cells in vivo binds to satellite sequences may relate to this observation, as it suggests that mut p53 interacts with centromere DNA.…”

Mutant p53: an oncogenic transcription factor

“…The attempt to verify whether the structural classification of p53 mutants reveals similar differences in terms of biological activities has been quite difficult to tackle. In vitro studies have shown that the overexpression of conformational p53 mutants renders cell cultures more resistant to etoposide, whereas DNA defective mutants increases the resistance to cisplatin (Gualberto et al, 1998;Li et al, 1998;Blandino et al, 1999;Sigal and Rotter, 2000). The presence of conformational mutants has been associated with breast cancer patients, whose relapse after conventional chemotherapy occurred more frequently when compared with patients carrying DNA contact defective p53 mutants (Aas et al, 1996;Berns et al, 2000;Geisler et al, 2003).…”