In this study, for the first time, we assessed the prognostic value of the codon 72 polymorphism of p53 in African American and Caucasian patients with colorectal cancers and showed that this polymorphism, specifically the Pro/Pro phenotype, is an independent factor of prognosis for African American patients. Similar frequencies of p53 mutations were observed in both African American and Caucasian patients. However, the incidence of the homozygous mutant variant Pro/Pro was higher in African American patients as compared with Caucasians; in contrast, the wild-type Arg/Arg variants were more common in Caucasians than in African Americans. In African American patients, colorectal cancers with Pro/Pro mutant phenotypes exhibited a higher incidence of p53 mutations, specifically of the disruptive type, and were associated with nodal metastasis and short overall survival. These aggressive phenotypic features of the Pro/Pro mutant phenotype were not observed in Caucasian patients. Thus, these findings suggest that the Pro/Pro mutant phenotype is associated with advanced tumor stage and short survival of African American patients.
In the United States, African Americans have the highest incidence of colorectal cancer, a younger mean age at colorectal cancer diagnosis (47
), a higher proportion of proximal colon cancers (48
), and a lower survival rate compared with Caucasian patients (49
). Consistent with these studies, our present study also showed that there was a preponderance of proximal tumors in African Americans and that the proximal tumor site is an independent prognostic indicator for this group. Relating to the differences in the distribution of adenocarcinomas within the colorectum in these two racial groups, there are various hypotheses relating to differences in diet, alcohol consumption, hormone status, socioeconomic status, and physical activity (51
). It is likely that there are differences throughout the colon/rectum regarding sensitivity to dietary carcinogen exposure. Indeed, in experimental studies in vivo
, a diet high in fat increased the incidence of tumor development in the proximal colon compared with the distal colon (54
). A subsequent study showed that the difference in the incidence of tumors between the proximal and distal colon was attributable to the greater capacity of the distal colon to cope with initial damage to DNA caused by carcinogens (55
). These findings suggest the importance of studies focused on understanding the underlying mechanisms involved in tumor development in relation to anatomic tumor site and the consideration of tumor location in assessing the aggressiveness and the outcome of patients based on race/ethnicity.
Although the differences in colorectal cancer incidence and survival between Caucasian and African American patients might be due to contributions of various factors, including dietary habits, physical activities, access to screening/health care, or distinct genetic or molecular features, there are no established biological explanations for these differences. The differences in frequency or in mutational spectra at different structural and functional domains of oncogenes/tumor suppressor genes or differences in incidence of genetic or epigenetic events among African American and Caucasian patients suggest that genetic susceptibility, in terms of allelic loss, methylation, and mutagenic events or environmental exposure, could be different. Alternatively, genetic traits that protect against mutations in vital genes in different pathways could be present. Indeed, in endometrial cancers, the incidence of p53 overexpression as a result of point mutations is higher in African American than in Caucasian patients, suggesting that alterations in p53
might be one of the key contributing factors to the racial disparity in disease outcome (56
). Further, such alterations, including the codon 72 polymorphism of p53
as observed in this study, might be helpful in understanding the biological differences related to tumor aggressiveness between races. Other factors may, of course, contribute to the racial disparity in patient clinical outcome.
More than 85% of known, cancer-related p53
mutations are missense mutations. For several cancers, including colorectal cancers, mutations of p53
and overexpression of mutant p53 protein (nuclear accumulation) are associated with advanced tumor stage and poor survival (57
). Missense mutations occurring in vivo
lead to single amino acid substitutions, which result in the disruption of p53
conformation and in loss of function (59
). The consequences of these changes might contribute to tumor progression and to a poor prognosis. In agreement with these results, there was, in the present study, no racial disparity in the missense mutation frequency or prognostic importance of p53 mutations in African American and Caucasian patients; in both racial groups, p53
missense mutations were associated with poor patient survival.
Several single-nucleotide polymorphisms have been identified in the coding region of the p53
gene. Because of the silent nature of codons 34 and 36 in exon 4, and codon 213 in exon 6 polymorphisms of p53
, their functional characteristics were not studied. However, an additional single-nucleotide polymorphism at codon 47 in exon 4 is a rare germ-line polymorphism that results in an amino acid change, proline > serine (16
). The functional characterization studies show that the mutant p53Ser47 phenotype has decreased capacity to induce apoptosis; to transactivate two p53
target genes, p53AIP1
; and to bind the MAPK1 protein as compared with the wild-type p53Pro47 phenotype (60
). Additionally, a recent report indicated that the p53Pro47Ser polymorphism was neither involved in susceptibility to developing malignancy (glioma) nor helpful in predicting patient survival (61
). Nevertheless, future studies on larger populations of different ethnic groups are essential to determine a definitive role of the p53Pro47Ser polymorphism, alone or combination with other alterations within the p53
gene, in tumor development and clinical outcomes.
In addition to missense mutations and other known polymorphisms within the coding region of the p53
gene, polymorphism at codon 72 located in a proline-rich region (residues 64-92) of the p53 protein are functionally important in the growth suppression and apoptosis mediated by p53 but not for cell cycle arrest (30
). Indeed, the Arg72 form of p53 has 15-fold enhanced capacity to induce apoptosis, compared with Pro72, and this is due to more efficient localization of the Arg72 form to mitochondria than the Pro72 form (28
). Moreover, several earlier studies have reported a significant association between Pro72 and the risk of developing colorectal cancers (31
). Other studies have noted different biological and biochemical activity between the p53 mutant protein containing arginine at codon 72 and mutant p53 protein containing proline at this position (63
). However, a recent study with lung cancer has shown that polymorphism at codon 72 (CGC to CCC; substitution of an arginine residue with a proline residue) is a risk factor for lung cancer development and that tumors with the Pro/Pro phenotype have an increased incidence of missense point mutations in the p53
gene, indicating a poor prognosis (33
). Consistent with these studies, our results show a higher incidence of p53
mutations associated with the Pro/Pro phenotype. This phenotype correlates with aggressive tumor behavior and a poor prognosis in colorectal cancers compared with the Arg/Arg or Arg/Pro phenotype, specifically in African American patients. As outlined above, this study shows that differences in p53
mutation rates and differential p53
mutational spectra in relation to important structural elements (h sheets and helix motifs) and functional domains (L2, L3, and LSH) relate to linkage disequilibrium among codon 72 phenotypes. Such disequilibrium could be a basis for differences among Arg
alleles in relation to tumor aggressiveness and poor patient survival.
DNA polymorphisms and their incidences, susceptibility for occurrence of genetic alterations, and the risk of tumor progression for patients with cancer can vary substantially between different racial groups (30
). Although most polymorphisms are functionally neutral, some affect regulation of gene expression or the function of the coded protein. These functional polymorphisms, including the codon 72 polymorphisms of p53
, despite being of low occurrence, could contribute to the differences between individuals or races in susceptibility and severity of disease (30
). Thus, it is important to determine if common structural polymorphisms are in linkage disequilibrium with genetic (mutations and allele loss) and epigenetic events (methylation and acetylation) and their differential profiles between races. Indeed, the effects of these genetic alterations, alone or in combination, or through interaction with environmental factors, have been implicated in angiogenic pathways and in lung cancer susceptibility and/or severity of disease in African Americans and Caucasians (30
). In another study, a higher prevalence of the Pro/Pro phenotype of p53 and its association with high risk of developing lung cancer in African American patients was reported, suggesting that there is substantial interindividual variation in susceptibility to genetic events and to tumor development (30
). It is evident from our data that an increased rate of p53
mutations, specifically disruptive mutations or mutations of outside evolutionary conserved regions, and decreased survival were noted among African American patients who exhibit the Pro/Pro phenotype. These findings suggest that the Pro/Pro phenotype of p53 is a race-specific molecular prognostic marker for African American patients with colorectal cancer.
The findings of this study also have implications in the interpretation of molecular variability in colorectal cancer in relation to patient race/ethnicity. An earlier colorectal cancer study in a German patient population, which included all stages of tumor, reported a preferential loss of the Pro72
allele and an increased frequency of missense mutations of p53
in the retained Arg72
allele both in primary as well as liver metastatic lesions, and correlated with aggressive tumor behavior (65
). A similar increase in Arg72 status of p53 with advanced disease was reported in urinary tract cancers (transitional cell carcinoma) in a Japanese patient population (66
). These as well as several other population genetics studies suggest that race/ethnicity categories aid in identifying unique germline alleles and that allelic combinations can modify both cancer risk and/or progression in some molecular subsets of human malignancies. Furthermore, as the lifestyles and/or environmental exposures (e.g., dietary carcinogens or exposure to infections) vary according to race/ethnicity (51
), studies based on race/ethnicity could be useful for understanding how differences among different populations affect the pathobiology of colorectal cancer.
Despite the increased interest of oncology medical practitioners to use molecular biomarkers in predicting the aggressiveness of colorectal cancer and clinical outcomes, markers that are indicative of prognosis are still lacking. The significant correlations we report in this article between germline alleles of p53 leading to Pro/Pro phenotypes and increased incidence of p53 mutations, advanced tumor stage, and short survival are clear indications that the codon 72 polymorphism of the p53 gene is involved in colorectal cancer progression specifically in African American patients. Although these correlations need to be validated in future large prospective studies, our findings suggest that together with other confounding factors of disease progression, analysis of the codon 72 polymorphism of the p53 gene might aid in understanding racial differences in determining aggressiveness of colorectal cancer and in designing optimal treatment regimens.