Distribution of Ha-ras alleles in patients with colorectal cancer and Crohn's disease

N-Acetyltransferase (NAT) 2 acetylator status and age of tumour onset in patients with sporadic and familial, microsatellite stable (MSS) colorectal cancer

INTRODUCTION

N-Acetyltransferase (NAT) 2 is an important enzyme involved in the metabolism of different xenobiotics, including potential carcinogens. Allelic variants of the NAT2 gene are determined by a pattern of single nucleotide polymorphisms (SNPs) resulting in slow (SA), intermediate (IA) or rapid acetylator (RA) phenotypes and causing the individual differences in the NAT2 metabolic capacity. To clarify the potential modifying role of the NAT2 acetylator status in microsatellite stable (MSS) colorectal cancer (CRC), we studied 140 patients with sporadic CRC (group 1) and 69 patients with CRC who met at least one criterion of the revised Bethesda guidelines (group 2).

OBSERVATIONS

We did not observe any significant difference in the NAT2 acetylator status frequency between patients in both groups and 100 healthy controls (P=0.486). Regardless of a younger median age of tumour onset (AO) of 41 years in group 2 patients compared to 64 years in group 1 patients, no significant difference in AO was found between RA and SA status patients in both groups. The median AO in group 1 was 65 years in patients with RA and 63 years with SA status (P=0.065). The median AO in group 2 was 40 years in patients with RA and 42 years with SA status (P=0.814). Multivariate Cox regression analysis revealed that neither the NAT2 acetylator status (P=0.064 and 0.810, respectively) nor the gender (P=0.165 and 0.918, respectively) was a risk factor for the AO in both groups. These data do not support the hypothesis that the NAT2 acetylatorship acts as a modifying factor on the AO in sporadic and familial, microsatellite stable CRC.

HRAS1 variable number of tandem repeats polymorphism and risk of bladder cancer

The HRAS1 variable number of tandem repeats (VNTR) polymorphism, 1 kb downstream from the HRAS1 gene, has been reported to be associated with risk of various cancers. To examine whether individuals with rare HRAS1 VNTR alleles are at increased risk of bladder cancer we carried out a case control study with 230 bladder cancer cases and 203 hospital-based controls frequency-matched on ethnicity, gender and age. For genotyping we used a PCR-based long-gel electrophoretic assay that provides precise allele size discrimination. We did not find evidence of a strong overall effect of the HRAS1 VNTR on bladder cancer risk. Genotype data for whites and blacks were analyzed separately, but the number of black subjects was too small to estimate meaningful odds ratios. Compared to white subjects with 2 common alleles, the odds ratio (OR) for white subjects with 1 rare allele was 0.9 (95% confidence interval (CI) = 0.5-1.4) and for those with 2 rare alleles OR = 1.7 (95% CI = 0.6-5.4). HRAS1 genotype may be related to the prognosis of bladder cancer, however, because incident cases, i.e., newly diagnosed cases had a higher frequency of rare alleles than did prevalent cases, i.e., cases already existing at the time of recruitment. Repeating the analyses with incident cases only (n = 53), the OR for subjects with 1 rare allele was 1.2 (95% CI = 0.6-2.4) and for those with 2 rare alleles 3.2 (95% CI = 0.8-13.7). The number of incident cases was too small to draw firm conclusions on a possible association with a subgroup of tumors with a poor prognosis. Published 2002 Wiley-Liss, Inc.

Polymorphisms and colorectal tumor risk

BACKGROUND & AIMS

Increasingly, studies of the relationship between common genetic variants and colorectal tumor risk are being proposed. To assess the evidence that any of these confers a risk, a systematic review and meta-analysis of published studies was undertaken.

METHODS

Fifty studies of the effect of common alleles of 13 genes on risk were identified. To clarify the impact of individual polymorphisms on risk, pooled analyses were performed.

RESULTS

Of the 50 studies identified, significant associations were seen in 16, but only 3 were reported in more than one study. Pooling studies, significant associations were only seen for 3 of the polymorphisms: adenomatosis polyposis coli (APC)-I1307K (odds ratio [OR] = 1.58, 95% confidence interval [CI]: 1.21-2.07); Harvey ras-1 variable number tandem repeat polymorphism (HRAS1-VNTR; OR = 2.50, 95% CI: 1.54-4.05); and methylenetetrahydrofolate reductase (MTHFR)(Val/Val) (OR = 0.76, 95% CI: 0.62-0.92). For tumor protein 53 (TP53), N-acetyl transferase 1 (NAT1), NAT2, glutathione-S transferase Mu (GSTM1), glutathione-S transferase Theta (GSTT1), and glutathione-S transferase Pi (GSTP1) polymorphisms, the best estimates are sufficient to exclude a 1.7-fold increase in risk of colorectal cancer.

CONCLUSIONS

APC-I1307K, HRAS1-VNTR, and MTHFR variants represent the strongest candidates for low penetrance susceptibility alleles identified to date. Although their genotypic risks are modest, their high frequency in the population implies that they may well have considerable impact on colorectal cancer incidence. Determining precise risk estimates associated with other variants and gene-gene and gene-environment interactions will be contingent on further studies with sample sizes larger than typically used to date.

Minisatellite variant repeat (MVR) analysis of the HRAS1 minisatellite locus

Two alternative electrophoretic strategies were used to study the internal variation of the HRAS1 minisatellite after minisatellite variant repeat mapping (MVR-PCR) was carried out. While the use of automated sequencers with fluorescent based technology is ideal for analyzing fragment size, and therefore, for analyzing the repeat number, the use of polyacrylamide gels and silver staining is more appropriate for the analysis of internal variation. Thirteen different fragments ranging from 27 to 80 repeats were found in a sample from 80 healthy Caucasian individuals. By using MVR mapping we were able to detect heterozygotes which appear as homozygotes when fragment length analysis was used. As a result of this, the 13 alleles, which we had detected, increased to 16 alleles when MVR sequences were analyzed. The extremely conservative arrays of repeats allow us to infer the theoretical origin of rare alleles from a major group of specific alleles. The HRAS1 minisatellite has been extensively studied due to its association with cancer. However, the methodology used up to now has limited the scope of previous research. Our approach permits the identification of alleles in a fast and reliable way using their MVR codes, thus allowing association studies with cancer.

Analysis of Ha-ras 1 allele frequencies in hereditary non-polyposis colorectal cancer

Restriction enzyme digests of genomic DNA show multiple alleles of common, intermediate, and rare frequencies at the minisatellite locus of Ha-ras. It has been suggested that a higher frequency of rare alleles is associated with the presence of colorectal and other types of cancer. This study investigated the distribution of Ha-ras alleles in 40 members of hereditary non-polyposis colorectal carcinoma (HNPCC) families and in 34 cancer free subjects (spouses). There was no difference in rare allele frequency between the cancer group and cancer free group (chi2 = 0.25, not significant).

Dysplasia and early carcinoma in inflammatory bowel disease and colorectal adenomas

The control of colorectal cancer is currently dependent on early detection, and its prevention requires the recognition and treatment of its precursor lesions. The adenoma has been established as the precursor of colorectal carcinoma in the general population. Among patients with inflammatory bowel disease (IBD), dysplasia is associated with, and precedes, invasive carcinoma. In this section criteria are described for the histological detection of preinvasive and early invasive neoplasia in the large intestine of patients with and without IBD. The therapeutic implications of these diagnoses are stressed. A brief review of subcellular changes, including genetic alterations, in colorectal neoplasia is included.

Population genetics of the HRAS1 minisatellite locus

Several years ago it was reported that rare HRAS1 VNTR alleles occurred more frequently in U.S. Caucasian cancer patients than in unaffected controls. Such an association, in theory, could be caused by undetected population heterogeneity. Also, in a study clearly relevant to this issue, it was recently reported that significant deviations from Hardy-Weinberg equilibrium exist at this locus in a sample of U.S. Caucasians. These considerations motivate our population genetic analysis of the HRAS1 locus. From published studies of the HRAS1 VNTR locus, which classified alleles into types, we found only small differences in the allele frequency distributions of samples from various European nations, although there were larger differences among ethnic groups (African American, Caucasian, and Oriental). In an analysis of variation of rare-allele frequencies among samples from four European nations, most of the variance was attributable to molecular methodology, and very samples from four European nations, most of the variance was attributable to molecular methodology, and very little of the variance was accounted for by nationality. In addition, we showed that mixture of European subpopulations should result in only minor deviations from expected genotype proportions in a Caucasian database and demonstrated that there was no significant deviation from Hardy-Weinberg equilibrium in our HRAS1 data.

An association between the risk of cancer and mutations in the HRAS1 minisatellite locus

BACKGROUND

The role of mutations in protooncogenes and their regulatory sequences in the pathogenesis of cancer is under close scrutiny. Minisatellites are unstable repetitive sequences of DNA that are present throughout the human genome. The highly polymorphic HRAS1 minisatellite locus just downstream from the protooncogene H-ras-1 consists of four common progenitor alleles and several dozen rare alleles, which apparently derive from mutations of the progenitors. We previously observed an association of the rare mutant alleles with many forms of cancer, and we undertook the present study to pursue this observation further.

METHODS

We conducted a case-control study, typing 736 HRAS1 alleles from patients with cancer and 652 from controls by Southern blotting of leukocyte DNA. We also carried out a meta-analysis of this study and 22 other published studies, estimating the relative risk of cancer (such as bladder, breast, or colorectal cancer) when one of the rare HRAS1 alleles was present.

RESULTS

Both the present case-control study (odds ratio, 1.83; 95 percent confidence interval, 1.28 to 2.67; P = 0.002) and the present study combined with our previous study (odds ratio, 2.07; 95 percent confidence interval, 1.47 to 2.92; P < 0.001), as well as the meta-analysis of all 23 studies (odds ratio, 1.93; 95 percent confidence interval, 1.63 to 2.30; chi-square = 57.58; P < 0.001), replicated our original finding and demonstrated a significant association of rare HRAS1 alleles with cancer. We found significant associations for four types of cancer: carcinomas of the breast, colorectum, and urinary bladder and acute leukemia. We also identified suggestive but not statistically significant associations for cancers of the lung and prostate and for non-Hodgkin's lymphoma.

CONCLUSIONS

Mutant alleles of the HRAS1 minisatellite locus represent a major risk factor for common types of cancer. Although the relative risk associated with the presence of one rare allele is moderate, the aggregate prevalence of one rare allele is moderate, the aggregate prevalence of this class of mutant alleles implies an extremely important attributable risk: 1 in 11 cancers of the breast, colorectum, and bladder.