Inherited P53 Mutations Research Articles

Genetic and stochastic influences upon tumor formation and tumor types in Li-Fraumeni mouse models.

p53 is the most frequently mutated gene in human cancers. Li-Fraumeni syndrome patients inheriting heterozygous p53 mutations often have a much-increased risk to develop cancer(s) at early ages. Recent studies suggest that some individuals inherited p53 mutations do not have the early onset or high frequency of cancers. These observations suggest that other genetic, environmental, immunological, epigenetic, or stochastic factors modify the penetrance of the cancerous mutant Tp53 phenotype. To test this possibility, this study explored dominant genetic modifiers of Tp53 mutations in heterozygous mice with different genetic backgrounds. Both genetic and stochastic effects upon tumor formation were observed in these mice. The genetic background of mice carrying Tp53 mutations has a strong influence upon the tissue type of the tumor produced and the number of tumors formed in a single mouse. The onset age of a tumor is correlated with the tissue type of that tumor, although identical tumor tissue types can occur at very different ages. These observations help to explain the great diversity of cancers in different Li-Fraumeni patients over lifetimes.

The Evolution of Tumor Formation in Humans and Mice with Inherited Mutations in the p53 Gene.

While tumors are very heterogeneous in their origins, mutations in the p53 gene and inactivation of p53 gene functions are the most common feature that predispose to the formation of cancers in humans. Inherited p53 mutations lead to different tumor types at very different frequencies and at very different ages than somatic p53 mutations. The reasons for this are explored. When the first mutation arises in a stem cell (a gatekeeper mutation) it selects for a specific subset of second mutations which in turn select for mutations in a third subset of genes. The nature of the first mutation in a tumor determines, by selection, the functional types of subsequent mutations. Inherited mutations occur at different developmental times and in different orders of mutational sequences than somatic mutations. The excess risk of developing a cancer with an inherited p53 mutation is two- to three-fold in endodermal derived tissues compared with 100- to 1000-fold for ectodermal and mesenchymal derived tissues. By contrast, endodermal derived tumors with somatic p53 mutations occur at very high frequencies (70-100%). These evolutionary restrictions upon the mutational path that tumor development may take could open up new avenues for therapy and prevention.

The Inherited p53 Mutation in the Brazilian Population

A common criticism of studying rare diseases is the often-limited relevance of the findings to human health. Here, we review ∼15 years of research into an unusual germline TP53 mutation (p.R337H) that began with its detection in children with adrenocortical carcinoma (ACC), a remarkably rare childhood cancer that is associated with poor prognosis. We have come to learn that the p.R337H mutation exists at a very high frequency in Southern and Southeastern Brazil, occurring in one of 375 individuals within a total population of ∼100 million. Moreover, it has been determined that carriers of this founder mutation display variable tumor susceptibility, ranging from isolated cases of pediatric ACC to Li-Fraumeni or Li-Fraumeni-like (LFL) syndromes, thus representing a significant medical issue for this country. Studying the biochemical and molecular consequences of this mutation on p53 tumor-suppressor activity, as well as the putative additional genetic alterations that cooperate with this mutation, is advancing our understanding of how p53 functions in tumor suppression in general. These studies, which originated with a rare childhood tumor, are providing important information for guiding genetic counselors and physicians in treating their patients and are already providing clinical benefit.

Brazilian story of the R337H p53 mutation

The p53 tumor suppressor is an evergreen of molecular oncology. Since its discovery in 1979, it has been subjected to intensive investigation. The p53 protein is composed of "only" 393 amino acid residues, and function of almost each of them has been addressed in detail. Somatic mutations are extremely frequent, they can be found almost in each of the p53 codons and in all types of tumors. Inherited p53 mutations are rare but very penetrant, and they are typically associated with development of a broad spectrum of tumors. However, in 2001, the p53 research provided an unexpected discovery: the R337H allele was found in southern Brazil. This allele was atypically associated with only one type of tumor - childhood adrenocortical carcinoma and it exhibited low penetrance. Therefore, new data on functioning and impact of the R337H mutation were highly desired. The results obtained during a few following years helped to elucidate not only this specific p53 variant but also provided insight into general principles of mutant p53 variants function. It also turned out that all R337H alleles that are very frequent in southern Brazil originate from one common ancestor.

Association of Polymorphism in MDM-2 and p53 Genes with Breast Cancer Risk in Indian Women

Single nucleotide polymorphism (SNP) at position -309 (T309G) in MDM-2 promoter induces tumor formation in the individuals possessing inherited p53 mutations. The present study was undertaken to investigate the association of MDM-2 SNP309, p53 Arg72Pro, and p53 intron-6 G/A polymorphism with total, premenopausal, and postmenopausal breast cancer risks in Indian women. Genotyping of MDM-2 SNP309, p53 Arg72Pro, and p53 intron-6 G/A in 104 patients and 105 controls was performed either by ARMS-PCR or by polymerase chain reaction and direct sequencing. The p53 Arg72Pro heterozygous variant and in combination with its homozygous variant exhibited a significant protective association with total (odds ratio [95% confidence interval]: 0.42 [0.22-0.81] and 0.46 [0.25-0.85], p value; 0.007 and 0.012) and postmenopausal breast cancer risk (odds ratio [95% confidence interval]: 0.25 [0.07-0.73] and 0.27 [0.08-0.77], p value; 0.009 and 0.013]. Neither combined nor homozygous/heterozygous MDM-2 SNP309G was associated with total, premenopausal, or postmenopausal breast cancer risk; however, MDM-2 SNP309G, along with p53 Arg72Pro heterozygous variant, showed a significant protective association with premenopausal breast cancer risk (odds ratio [95% confidence interval]: 0.18 [0.02-1.20], p value; 0.041 for homozygous + heterozygous MDM-2 SNP309G). The results indicate protective associations of p53 Arg72Pro heterozygous variant with postmenopausal and MDM-2 SNP309G along with p53 Arg72Pro heterozygous variant with premenopausal breast cancer risk.

Transcriptional Functionality of Germ Line p53 Mutants Influences Cancer Phenotype

The TP53 tumor suppressor gene encodes a sequence-specific transcription factor that is able to transactivate several sets of genes, the promoters of which include appropriate response elements. Although human cancers frequently contain mutated p53, the alleles as well as the clinical expression are often heterogeneous. Germ line mutations of TP53 result in cancer proneness syndromes known as Li-Fraumeni, Li-Fraumeni--like, and nonsyndromic predisposition with or without family history. p53 mutants can be classified as partial deficiency alleles or severe deficiency alleles depending on their ability to transactivate a set of human target sequences, as measured using a standardized yeast-based assay (see http://www.umd.be:2072/index.html). We have investigated the extent to which the functional features of p53 mutant alleles determine clinical features in patients who have inherited these alleles and have developed cancer. We retrieved clinical data from the IARC database (see http://www.p53.iarc.fr/Germline.html) for all cancer patients with germ line p53 mutations and applied stringent statistical evaluations to compare the functional classification of p53 alleles with clinical phenotypes. Our analyses reveal that partial deficiency alleles are associated with a milder family history (P = 0.007), a lower numbers of tumors (P = 0.007), and a delayed disease onset (median, 31 versus 15 years; P = 0.007) which could be related to distinct tumor spectra. These findings establish for the first time significant correlations between the residual transactivation function of individual TP53 alleles and clinical variables in patients with inherited p53 mutations who develop cancer.

Influence of the MDM2 single nucleotide polymorphism SNP309 on tumour development in BRCA1 mutation carriers

BackgroundThe MDM2 gene encodes a negative regulator of the p53 tumour suppressor protein. A single nucleotide polymorphism (SNP) in the MDM2 promoter (a T to G exchange at nucleotide 309) has been reported to produce accelerated tumour formation in individuals with inherited p53 mutations. We have investigated the effect of the MDM2 SNP309 on clinical outcome in a cohort of patients with germline mutations of BRCA1.MethodsGenomic DNA was obtained for 102 healthy controls and 116 patients with established pathogenic mutations of BRCA1 and Pyrosequencing technology™ was used to determine the genotype at the MDM2 SNP309 locus.ResultsThe polymorphism was present in 52.9% of the controls (G/T in 37.3% and G/G in 15.6%) and 58.6% of the BRCA1 mutation carriers (47.4% G/T and 11.2% G/G). Incidence of malignancy in female BRCA1 carriers was not significantly higher in SNP309 carriers than in wildtype (T/T) individuals (72.7% vs. 75.6%, p = 1.00). Mean age of diagnosis of first breast cancer was 41.2 years in the SNP309 G/G genotype carriers, 38.6 years in those with the SNP309 G/T genotype and 39.0 years in wildtype subjects (p = 0.80).ConclusionWe found no evidence that the MDM2 SNP309 accelerates tumour development in carriers of known pathogenic germline mutations of BRCA1.

Genetic predisposition and treatment-related leukemia.

Treatment-related leukemias are one of the most devastating late complications of cancer therapy. Patients with rare cancer predisposition syndromes including neurofibromatosis type 1 and inherited p53 mutations are at an increased risk for this complication. Other patients may have increased susceptibility because they possess common genetic polymorphisms in drug-metabolizing enzymes that result in impaired detoxification of chemotherapy or inefficient repair of drug-induced genetic damage. We review studies that have identified a potential role for polymorphisms in the genes encoding the glutathione-S-transferases (GSTs), NAD(P) H: quinone oxidoreductase, myeloperoxidase, N-acetyltransferase (NATs), cytochrome P450 (CYP) 1A1 and 3A4, methylenetetrahydrofolate reductase (MTHFR), cystathionine-beta-synthase (CBS), and others in the etiology of primary or secondary acute leukemias, and therapy-related complications. The identification of high risk polymorphisms and use of pharmacogenetically-guided therapies holds promise to improve the outcome of cancer therapy and reduce the risk of treatment-related leukemias.

Cytogenetic Response to G 2 -Phase X Irradiation in Relation to DNA Repair and Radiosensitivity in a Cancer-Prone Family with Li-Fraumeni Syndrome

Noncancerous skin fibroblasts from six family members with Li-Fraumeni syndrome, five with cancer of diverse tissue origin and one with a premalignant neoplasm, showed a high frequency of chromatid aberrations, 94 to 119 breaks and 58 to 95 gaps per 100 metaphase cells arrested with colcemid 0.5 to 1.5 h after X irradiation (1.75 x 10(-2) C/kg). This response results from deficient repair of the radiation-induced DNA damage. In contrast, skin fibroblasts from two unrelated normal controls and a spouse showed 19 breaks and 17 to 19 gaps per 100 cells. Whereas all six members of the cancer-prone family had a radioresistant phenotype, only four had an inherited p53 mutation. Fibroblasts from a radioresistant family member showed the same extent of chromatid damage directly (0 to 0.5 h) after G2-phase X irradiation as those from the radiosensitive control spouse. We conclude, therefore, that radiosensitivity, as determined by cell killing in asynchronous populations of skin fibroblasts, is unrelated to chromosomal sensitivity to G2-phase X irradiation. However, the persistence of a high frequency of chromatid breaks and gaps at 0.5 to 1.5 h after G2-phase X irradiation, a manifestation of deficient DNA repair, is associated with proneness to cancer in this family.

Germ-line transmission of a mutated p53 gene in a cancer-prone family with Li–Fraumeni syndrome

Tumour suppressor genes, whose usual function seems to be controlling normal cell proliferation, have been implicated in many inherited and sporadic forms of malignancies Much evidence supports the concept of tumour formation by loss-of-function mutations in suppressor genes, as predicted by the two-hit model of Knudson and DeMars. The suppressor gene, p53, is affected in such a manner by numerous mutations, which occur in a variety of human tumours. These mutations usually represent the loss of one allele and the substitution of a single base in the other. We have now analysed the p53 gene in a family affected by Li-Fraumeni syndrome, a rare autosomal dominant syndrome characterized by the occurrence of diverse mesenchymal and epithelial neoplasms at multiple sites. In some instances the neoplasms seem to be related to exposure to carcinogens, including ionizing radiation. The Li-Fraumeni family that we studied had noncancerous skin fibroblasts (NSF) with an unusual radiation-resistant phenotype. DNA derived from the NSF cells of four family members, spanning two generations, had the same point mutation in codon 245 (GGC----GAC) of the p53 gene. This mutation leads to substitution of aspartic acid for glycine in one of the regions identified as a frequent target of point mutations in p53. The NSF cell lines with the mutation also retained the normal p53 allele. This inherited p53 mutation may predispose the members of this family to increased susceptibility to cancer.