A single nucleotide polymorphism in the HDM-2 gene regulates the p53 apoptotic response and influences the age of onset of cancers in humans: the SNP 309 HDM-2 polymorphism
© BioMed Central 2002
Published: 17 June 2005
The HDM-2 gene in humans has two promoters for transcription. 5' to the first exon is a maintenance promoter providing low levels of HDM-2 in the cell. In the first intron are the P53 DNA binding sites and the p53 inducible promoter that yields threefold to 10-fold more HDM-2 mRNA after a p53 activation and response. When this intronic promoter is employed, transcriptional initiation starts at the second exon and this mRNA is translated more efficiently than mRNA that starts at the first exon. The coding region of the HDM-2 protein starts in the third exon. At residue 309 in this first intron is a single nucleotide polymorphism, with 12% of people being a G/G homozygote, 40% being a G/T heterozygote and 48% of people being T/T wild-type homozygotes (the G/G genotype is lower in black Americans and the sample size is now over 300 people). We have found that the G/G genotype creates a better SP-1 transcription factor binding site, raises the level of m-RNA in unstressed cells and produces threefold to sixfold more HDM-2 protein in cells (cancer cells in culture) with the G/G genotype. This mRNA starts at the second exon, and is probably translated better in unstressed cells. After DNA damage or other stresses, P53 activity in cells with the G/G genotype is lower and the percentage of cells undergoing apoptosis is lower when compared with cells in culture with T/T genotypes. We have reproduced these observations with lymphocytes taken from human volunteers and placed in culture, with EBV-immortalized B cells in culture, with primary fibroblasts in cell culture and with cancer cell lines in culture. In 92 individuals that have donated lymphocytes we see individuals forming a distribution of apoptotic responses between 20% and 60% after gamma radiation, with individuals being quite reproducible in repeated experiments. The lower half of the distribution is heavily weighted with the G/G genotype, while the upper half of the distribution has mainly the T/T genotype. The higher HDM-2 levels in cells thus result in a lower apoptotic index in cells from these volunteers. It has become clear in recent studies that SNP 309 has a clinical impact. We have genotyped two cancer cohorts, one at MD Anderson and one in Germany, containing patients with sarcomas and breast cancers. The results have been statistically significant (P = 0.01-0.02) and clear in both cohorts, and the average age of onset of these cancers is 10–15 years earlier in people with the G/G genotype than in people with the same cancer with the T/T genotype. The interpretation is then that the probability of eliminating pre-cancerous clones of cells via a p53 mechanism is lower in people with a G/G genotype (high HDM-2 levels) and the probability of developing a cancer at an earlier time in life is higher. In addition, in patients that have a germline mutation in the p53 gene (this yields one-half of the p53 protein level in a cell) those individuals that have a G/G genotype or a G/T genotype develop multiple cancers (three, four or five cancers) over their lifetimes, while no T/T homozygotes develop that many independent cancers.