Gain-Of-Function Mutations in the Tumor Suppressor Gene P53

2138 Vol. 6, 2138–2145, June 2000 Clinical Cancer Research Review Gain-of-Function Mutations in the Tumor Suppressor Gene p53 Monique G. C. T. van Oijen and transcription regulation of MDM2, which targets p53 for ubiq- Pieter J. Slootweg1 uitination. The p53 pathways have been reviewed extensively (7–10). Department of Pathology, University Medical Center Utrecht, 3508 GA Utrecht, the Netherlands Mutations in the p53 Gene Loss of p53 activity predisposes cells to the acquisition of Abstract oncogenic mutations and may favor genetic instability. Inacti- The tumor suppressor protein p53 is a multifunctional vation of p53 occurs mainly through point mutations, although transcription factor involved in the control of cell cycle small deletions/insertions in the gene also have been detected. progression, DNA integrity, and cell survival. p53 is mutated About 10,000 p53 mutations have already been identified in in half of all tumors and has a wide spectrum of mutation human tumors and are gathered in databases (2–4). The p53 types. p53 mutants show different degrees of dominance gene has a wide spectrum of mutations in human tumors (2–4). over coexpressed wild-type p53, and loss of the wild-type p53 The great majority of the mutations are clustered (Fig. 1) in the allele has been observed frequently. Several p53 mutants can core domain (120–292 bp). This domain is important for DNA- exert oncogenic functions beyond their negative domination specific binding and is essential for p53 function. Despite the over the wild-type p53 tumor suppressor functions. These wide mutation spectrum, a few hot spots for mutations are found so-called gain-of-function effects, such as enhancement of in the most conserved areas of the gene (2, 3, 11, 12). tumorigenicity and therapy resistance, were investigated in In the last decade, many studies, which yielded inconsistent p53-null cells. The possible mechanisms by which p53 mu- results, have tried to ascribe prognostic significance to the tants exert their gain-of-function effects are reviewed. The presence of mutated p53 (13). One explanation is that p53 existence of functional gains of certain p53 mutants has mutations might be missed by analyzing only exons 5–8, by important ramifications for tumor prognosis and cancer sequencing only genomic DNA, or by using only immunohis- therapies. tochemistry (5, 14). Another explanation for the variable results is that a lot of studies did not included the consequences of Tumor Suppressor p53 different p53 mutations. The key molecular changes in the multistep progression of cancer are still unknown, and a better understanding of this Various Types of p53 Mutations process might lead to more rational therapies and improved The DNA-binding structure of the p53 gene (Fig. 2) con- survival of patients. The development of tumors is generally tains a sandwich of two anti-parallel ␤-sheets that have four and accepted to be a multistep process in which alterations in on- five ␤-strands and a loop-sheet-helix motif that packs tightly cogenes and tumor suppressor genes play an important role (1). against one end of the ␤-sandwich. Furthermore, there are two The tumor suppressor gene p53 is mutated in 50% of all large loops (L2 and L3) that are held together in part by a tumors (2–4), and it plays a role in the carcinogenesis of many tetrahedrally coordinated zinc atom (12). Although the ␤-sand- different malignancies. The gene is mutated in more than 90% wich comprises a major part of the core domain structure, it is of head and neck squamous cell carcinomas (5). In contrast, the not directly involved in DNA binding. Instead, the core domain incidence of p53 mutations is very low in hematological malig- uses the loop-sheet-helix motif and one of the two large loops to nancies (6). bind DNA (12). The wild-type protein p53 controls cell cycle progression Several categories of p53 mutation can be distinguished by by acting as transcription factor for many genes. All these genes taking into account the impact of the mutation on either the contain a p53 consensus sequence in their promoter region. p53 protein structure/stabilization or interaction with DNA: (a) type controls cell cycle arrest and apoptosis via the transcription I, missense mutations that affect residues of the DNA-binding regulation of genes such as the cyclin-dependent kinase inhib- surface and disrupt the protein-DNA contact points (such as itor p21Waf1/Cip1, the protein GADD45, and the apoptosis pro- p53-Trp248 and p53-His273); (b) type II, missense mutations that teins Bax and Bcl-2. p53 controls its own functionality via disrupt the protein conformation (such as p53-Ala143, p53- His175, p53-His179, and p53-Gly281); and (c) type III, null mutations that completely destroy the functionality of the protein [insertions/deletions (frameshift mutations), nonsense muta- Received 9/29/99; revised 3/13/00; accepted 3/13/00. tions, and splicing junction mutations]. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to Loss- and Gain-of-Function p53 Mutants indicate this fact. It is generally believed that p53 loses its tumor suppressor 1 To whom correspondence should be addressed, at Department of Pathology (H04-312), University Medical Center Utrecht, P. O. Box function as a consequence of a mutation in p53. Most p53 85500, 3508 GA Utrecht, the Netherlands. Phone: 00-31-30-2506561; mutants have impaired sequence-specific transactivation activ- Fax: 00-31-30-2544990; E-mail: [email protected]. ity, which means that p21Waf1 expression, for example, is not Downloaded from clincancerres.aacrjournals.org on October 1, 2021. © 2000 American Association for Cancer Research. Clinical Cancer Research 2139 Fig. 1 p53 gene structure and mutations. A, the bars represent the percentage of mutations found in tumors that were pri- marily investigated for core domain mutations (2, 3). B, the p53 gene consists of 11 exons encoding a protein that comprises a transactivation domain, a core domain/sequence-specific DNA-binding domain, and a COOH-terminal domain with a tetramerization domain and a nonspecific DNA-binding domain (12). Boxes, conserved do- mains. up-regulated, and cell cycle arrest or apoptosis after DNA dam- mice, these transgenic mice exhibited increased susceptibility to age will not occur. However, several studies indicate that certain chemical carcinogenesis, with greatly accelerated benign papil- types of p53 mutations, so called gain-of-function mutants, exert loma formation, malignant conversion, and metastasis. The pap- functions that the wild-type p53 does not. Known p53 gain-of- illomas in the transgenic mice showed centrosome abnormalities function effects are summarized in Table 1 (15–29). Because at high frequencies (75% of the cells), whereas the p53-null most p53 mutants exert dominant negative effects on coex- tumors exhibited abnormal centrosomes less often (4% of the pressed wild-type p53 (30), gain-of-function effects of several cells; Ref. 20). p53 mutants had to be investigated by introduction of the p53 These studies show that certain p53 mutants not only lose mutants into cells lacking wild-type p53. their tumor suppressor function but gain oncogenic functions. Mutant human p53 alleles (p53-Ala143, p53-His175, p53- Trp248, p53-His273, and p53-Gly281) expressed in cell lines lacking p53 resulted in either enhanced tumorigenic potential in Loss of Heterozygosity nude mice or enhanced plating efficiency in agar cell culture Loss of the wild-type p53 allele is frequently detected in (17). In another study, nude mice injected with 103 murine tumors (31, 32). The p53 protein functions optimally when it fibroblast null cells transfected with the p53 mutant p53-Gly281 binds to DNA as a wild-type p53 tetramer (33). One mutant p53 developed tumors in contrast to mice injected with the 103 protein can disturb a functional tetramer and is therefore able to untransfected null cells (27). p53-null, leukemic T cells trans- override the function of three wild-type p53 proteins. However, fected with certain p53 mutants (p53-His175, p53-Gln213, and some biochemical factors and binding regulators can modulate a p53-Gln248) showed metastatic capacities when they were in- genotypically mutant p53 into an equilibrium with the wild-type jected into severe combined immunodeficient mice, in contrast conformation (34). to some other mutants (p53-Cys273 and p53-His234). As a con- p53 mutants show different degrees of dominance over sequence, the mice from the first group showed a shorter sur- wild-type p53 (30, 35). In a study of Li-Fraumeni tumor pa- vival (29). tients, loss of the wild-type allele was observed in about half of A transgenic mouse model was developed with the murine the cases carrying p53 germ-line mutations. This loss apparently mutant p53-His172 under the control of human keratin-1-based was associated with mutation types occurring outside the core vector (20). In contrast to the wild-type p53 and p53 knockout domain or truncating the protein (type III; Ref. 36). An associ- Downloaded from clincancerres.aacrjournals.org on October 1, 2021. © 2000 American Association for Cancer Research. 2140 Gain-of-Function Mutations in p53 Fig. 2 Topological diagram of the secondary structure ele- ments of p53. The core domain of p53 is depicted. The ␤-strands (S), ␣-helices (H), three of the loops (L), and the zinc atom (Zn) are labeled, and the residues at the beginning and the end of each secondary structure element are indicated. The boundaries of the two ␤-sheets that make up the ␤-sandwich are shaded. The conserved regions are colored yellow for region II, blue for region III, orange for region IV, and pink for region V (reprinted from Ref.

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