David Ramsey

The CpG Island Methylator Phenotype Correlates with Long-Range Epigenetic Silencing in Colorectal Cancer

The CpG island methylator phenotype (CIMP), characterized by an exceptionally high frequency of methylation of discrete CpG islands, is observed in 18% to 25% of sporadic colorectal cancers. Another hypermethylation pattern found in colorectal cancers, termed long-range epigenetic silencing, is associated with DNA/histone methylation in three distinct gene clusters at chromosome 2q14.2, showing that DNA hypermethylation can span larger chromosomal domains and lead to the silencing of flanking, unmethylated genes. We investigated whether these two phenotypes are interrelated in colorectal cancers. The CIMP status of 148 sporadic colorectal cancers was determined by methylation-specific PCR. We determined the BRAF V600E mutation by mutant allele–specific PCR amplification. The methylation status of the MLH1 gene and of three CpG islands (EN1, SCTR, and INHBB), corresponding to three distinct clusters along 2q14.2, was determined by methylation-specific PCR. The average number of sites showing methylation in CIMP+ tumors was 2.21, compared with 1.22 for CIMP− individuals, and this difference was highly significant (P = 3.6 × 10−8, Mann-Whitney test). Moreover, all CIMP+ tumors showed hypermethylation of at least one of these loci, in contrast to CIMP− tumors, where 18 (16%) samples remained unmethylated. The mean number of simultaneously hypermethylated CpG islands at 2q14.2 differs significantly between CIMP− and CIMP+ tumors, suggesting varying effects of domain silencing in this region. Given that the number of hypermethylated loci at 2q14.2 likely affects the range of silenced flanking genes, high frequency of simultaneous hypermethylation of three CpG islands (EN1, SCTR, and INHBB) may have potential influence on specific characteristics of CIMP+ colorectal cancers. (Mol Cancer Res 2008;6(4):585–91)

Protein tyrosine phosphatase receptor-like genes are frequently hypermethylated in sporadic colorectal cancer

The activity of phosphatases could be influenced by genetic, as well as epigenetic alterations. In our study, we have investigated the methylation status of four PTPRs: PTPRM, PTPRT, PTPRR and PTPRZ1, which were pre-selected using microarray techniques as being alternatively methylated in sporadic colorectal cancer (CRC). The analyses were carried out on 131 surgical specimens obtained from sporadic CRC patients. The methylation status of the four genes was examined using methyl specific PCR (MSP). The analysis of promoter methylation using an Illumina 27K microarray revealed four protein tyrosine phosphatases PTPRM, PTPRT, PTPRR and PTPRZ1 as being hypermethylated with β-value ⩾0.2 and P⩽0.05. Subsequent analysis using MSP confirmed these observations—the frequency of promoter methylation was significantly higher in tumor cells compared with matched normal tissue for each of the analyzed genes. There was no association observed between the methylation status of PTPRs and either CIMP, K-ras (codon 12) and BRAF (exon 15, V600E) mutations or tumor localization (proximal/distal). The results of our study show a statistically significant difference between promoter methylation in cancerous and healthy tissue. This result supports the hypothesis that the PTPR family has an important role in the etiology of CRC.

Detection of viral DNA sequences in sporadic colorectal cancers in relation to CpG island methylation and methylator phenotype

There is evidence that insertion of viral DNA into a mammalian genome can lead to alterations of methylation patterns. The aim of the present study was to examine the presence of DNA sequences of five human DNA viruses (assessed by PCR): JC polyoma virus (JCV), human adenovirus (AdV), Epstein–Barr virus (EBV), Kaposi sarcoma-associated herpesvirus (KSHV/HHV8) and human papillomavirus (HPV) in a cohort of 186 sporadic colorectal cancers (CRCs) and related these data with the methylation status of six CpG island methylator phenotype (CIMP)-specific genes (MLH1, CACNA1G, NEUROG1, IGF2, SOCS1, RUNX3) and seven cancer-related genes markers (p16, MINT1, MINT2, MINT31, EN1, SCTR and INHBB) assessed by methylation-specific PCR in 186 and 134 CRC cases, respectively. The AdV, KSHV and HPV were detected in four (2%), two (1%) and zero CRC cases, respectively, and thus were excluded from further analyses. Although 19% and 9% of the CRCs were positive for EBV and JCV, respectively, no associations between virus presence and CpG island methylation were found after correction for multiple testing. Our results demonstrate that the presence of DNA sequences of JCV and EBV in CRC is unrelated to the methylation of the 13 cancer-related CpG islands and CIMP.

Intermediate- and Low-Methylation Epigenotypes Do Not Correspond to CpG Island Methylator Phenotype (Low and -Zero) in Colorectal Cancer

Background: Most recent genome-wide studies on the CpG island methylation in colorectal cancer (CRC) have led to the discovery of at least 3 distinct methylation clusters. However, there remains an uncertainty whether the CRC clusters identified in these studies represent compatible phenotypes. Methods: We carried out comprehensive genome-scale DNA methylation profiling by Illumina Infinium HumanMethylation27 of 21 DNA pools that represent 84 CRC samples divided according to their high-, intermediate-, and low-methylation epigenotypes (HME, IME, and LME, respectively) and 70 normal-adjacent colonic tissues. We have also examined the relationship among 3 epigenotypes and chromosomal gains and deletions (assessed by Comparative Genomic Hybridization) in a group of 100 CRC samples. Results: The HME subgroup showed features associated with CpG island methylator phenotype – high (CIMP-high) including methylation of specific CpG sites (CpGs) as well as significantly lower mean number of chromosomal imbalances when compared with other epigenotypes. The IME subgroup displayed the lowest number of methylated CpGs (717 vs. 2,399 and 2,679 in HME and LME, respectively) and highest mean number of chromosomal imbalances when compared with HME (P, 0.001) and LME (P, 0.004). A comparison between the methylation profiles of 3 epigenotypes revealed more similarities between the HME and LME (1,669 methylated CpGs overlapped) than HME and IME (673 methylated CpGs overlapped). Conclusion: Our results provide evidence that IME and LME CRCs show opposite features to those that have been previously attributed to CIMP-low and CIMP-0 CRCs. Impact: These discrepancies should be considered when interpreting the data from a particular epigenotyping method. Cancer Epidemiol Biomarkers Prev; 22(2); 201–8. ©2012 AACR.

Assessment of three epigenotypes in colorectal cancer by combined bisulfite restriction analysis

Recent investigations have demonstrated the clear heterogeneity of sporadic colorectal cancer (CRC) with regard to CpG island methylation. Two unsupervised cluster analyses revealed that CRCs form three distinct DNA methylation subsets, which are referred to as the high‐, intermediate‐, and low‐methylation epigenotypes (HME, IME, and LME, respectively). A recent study by Yagi et al. found a fairly sensitive and specific identification of HME, IME, and LME using two marker panels analyzed by MALDI‐TOF mass spectrometry (MassARRAY). However, the expensive equipment required for this method substantially increases the cost and complexity of the assay. In this article, we demonstrate the assessment of HME, IME, and LME in a group of 233 sporadic CRCs using seven markers proposed by Yagi et al. The DNA methylation of each marker was quantified using combined bisulfite restriction analysis (COBRA) and analyzed along with various genetic factors associated with CRC [the BRAF and KRAS mutations, MLH1 methylation and microsatellite instability (MSI)]. The baseline methylation of each marker was generated from pooled DNA isolated from 50 normal colon tissues. We demonstrate that the correlation of HME, IME, and LME epigenotyped by COBRA using different molecular classifiers is similar to that achieved by MassARRAY. Therefore, epigenotyping CRCs using COBRA is a simple, specific, and cost‐effective method that has the potential to be widely used in CRC research.

Assessment of chromosomal imbalances in CIMP-high and CIMP-low/CIMP-0 colorectal cancers.

Data presented in a number of recent studies have revealed a negative correlation between CpG island methylator phenotype (CIMP) and chromosomal instability (CIN) measured by a loss of heterozygosity (LOH) of selected loci, suggesting that CIN and CIMP represent two independent mechanisms in sporadic colorectal cancer (CRC) carcinogenesis. However, CIN is a heterogeneous phenomenon, which may be studied not only by employing LOH analysis but also by observing chromosomal imbalances (gains and deletions). The current study aimed to investigate the relationship between CIMP and chromosomal gains and deletions (assessed by comparative genomic hybridization) in a group of 20 CIMP-high and 79 CIMP-low/CIMP-0 CRCs. Our results revealed that the mean numbers of gains and of total chromosomal imbalances were significantly greater (p = 0.004 and p = 0.007, respectively) in the CIMP-low/CIMP-0 group compared to the CIMP-high group, while no significant difference was observed between the mean numbers of losses (p = 0.056). The analysis of copy number changes of 41 cancer-related genes by multiplex ligation-dependent probe amplification showed that CRK gene was exclusively deleted in CIMP-low/CIMP-0 tumors (p = 0.02). Given that chromosomal losses play an important role in tumor suppressor inactivation and chromosomal gains, in the activation of proto-oncogenes, we hypothesize that tumor suppressor inactivation plays similar roles in both CIMP-high and CIMP-low/CIMP-0 CRCs, while the predominance of chromosomal gains in CIMP-low/CIMP-0 tumors may suggest that the activation of proto-oncogenes is the underlying mechanism of CIMP-low/CIMP-0 CRC progression.

DNA pooling and statistical tests for the detection of single nucleotide polymorphisms.

Abstract The development of next generation genome sequencers gives the opportunity of learning more about the genetic make-up of human and other populations. One important question involves the location of sites at which variation occurs within a population. Our focus will be on the detection of rare variants. Such variants will often not be present in smaller samples and are hard to distinguish from sequencing errors in larger samples. This is particularly true for pooled samples which are often used as part of a cost saving strategy. The focus of this article is on experiments that involve DNA pooling. We derive experimental designs that optimize the power of statistical tests for detecting single nucleotide polymorphisms (SNPs, sites at which there is variation within a population). We also present a new simple test that calls a SNP, if the maximum number of reads of a prospective variant across lanes exceeds a certain threshold. The value of this threshold is defined according to the number of available lanes, the parameters of the genome sequencer and a specified probability of accepting that there is variation at a site when no variation is present. On the basis of this test, we derive pool sizes which are optimal for the detection of rare variants. This test is compared with a likelihood ratio test, which takes into account the number of reads of a prospective variant from all the lanes. It is shown that the threshold based rule achieves a comparable power to this likelihood ratio test and may well be a useful tool in determining near optimal pool sizes for the detection of rare alleles in practical applications.

Mutual Mate Choice with Multiple Criteria

This article presents a model of mutual mate search based on two trait measures. One measure describes the attractiveness of an individual and preferences are common according to this measure i.e., each female prefers highly attractive males and all females agree as to which males are attractive. Preferences are homotypic with respect to the second measure, referred to as character i.e., all individuals prefer mates of a similar character. It is assumed that attractiveness is easy to measure, but to observe the character of a prospective partner, it is necessary to court. Hence, on meeting a prospective partner an individual must decide whether to try and court the other. Courtship only occurs by mutual consent. During courtship, individuals observe the character of the prospective partner and then decide whether to mate or not. Mutual acceptance is required for mating to occur. This paper presents the model and outlines a procedure for finding a Nash equilibrium which satisfies a set of criteria based on the concept of subgame perfection. Two examples are presented and it is shown that multiple equilibria may exist.

Global DNA methylation status in laryngeal cancer

The purpose of our study was to evaluate if DNA methylation level in leukocytes may be used as a surrogate marker of genome methylation status in laryngeal cancer tissues.

Partnership formation with age-dependent preferences

We analyze a model of partnership formation in which players’ preferences are based on the age of a prospective partner. There are two classes of individuals, called for convenience here male and female. Males and females are fertile for the same length of time, normalized to one unit. A male enters the mating pool at age 0 and meets prospective partners according to a Poisson process. At equilibrium, he accepts a female if the utility from mating exceeds the expected utility from future search, which depends on the acceptance strategies of all males and females and the corresponding steady-state distribution of age in the pool of unmated individuals. Females face an analogous problem. Mating pairs are only formed by mutual consent and individuals leave the pool of unmated individuals on finding a mating partner or reaching the age of 1. A policy iteration algorithm is used to determine the equilibrium acceptance strategies and the corresponding steady-state distribution of the age of individuals in the mating pool. Two examples are presented.