Telomerase activation in colorectal carcinogenesis

Real-time PCR quantification of human telomerase reverse transcriptase (hTERT) in colorectal cancer

AIMS

Human telomerase reverse transcriptase (hTERT) is believed to a reliable marker for telomerase activity. The expression of telomerase activity has not been investigated in a consecutive series of patients with colorectal adenocarcinoma in North Queensland. The objective of this study was to evaluate the significance of hTERT mRNA expression in colorectal adenocarcinoma in North Queensland.

METHODS

Matched samples of tumour and adjacent non-tumorous mucosa samples from 53 colorectal carcinomas and nine colorectal adenomas were collected. In all these samples, RNA was extracted and then transcribed to cDNA. Real-time polymerase chain reaction (RT-PCR) was used to quantitate expression the level of hTERT mRNA. The findings were correlated with the clinicopathological features of patients with these tumours prospectively collected into a computerised database.

RESULTS

hTERT mRNA was expressed in all tumour samples. The level of expression in the colorectal adenocarcinomas was significantly higher than the corresponding non-tumorous mucosa (p = 0.009, t-test). The level of expression in the adenocarcinomas was slightly higher than those of adenomas, but the difference was not statistically significant. A higher level of hTERT expression was often noted in the adenocarcinomas arising from the left colon and rectum when compared with those from the right colon (p = 0.029).

CONCLUSIONS

Colorectal adenocarcinoma revealed expression of telomerase hTERT mRNA, which was detected quantitatively by real-time PCR. hTERT could be a potential biomarker for colorectal cancer. The difference between proximal and distal colorectum in hTERT expression could account for their known different clinical behaviour.

Telomere length, telomeric proteins and genomic instability during the multistep carcinogenic process

Telomeres form specialized structures at the ends of eukaryotic chromosomes, preventing them from being wrongly recognized as DNA damage. The human telomere DNA sequence is a tandem repetition of the sequence TTAGGG. In normal cells, the DNA replication machinery is unable to completely duplicate the telomeric DNA; thus, telomeres are shortened after every cell division. Having reached a critical length, telomeres may be recognized as double strand break DNA lesions, and cells eventually enter senescence. Carcinogenesis is a multistep process involving multiple mutations and chromosomal aberrations. One of the most prevalent aberrations in pre-cancerous lesions is telomere shortening and telomerase activation. We discuss the role and homeostasis of telomeres in normal cells and their implication in the early steps of carcinogenesis. We also discuss various techniques used, and their limitations, in the study of telomeres and genome instability and their role in carcinogenesis and related genomic modifications.

Indefinite for non-invasive neoplasia lesions in gastric intestinal metaplasia: the immunophenotype

BACKGROUND

In the Padova International Classification, gastric precancerous lesions are labelled as "indefinite for non-invasive neoplasia" (Indef-NiN) cytohistological alterations mimicking non-invasive neoplasia (NiN), but lacking all the attributes required for a definite NiN categorisation.

AIM

To apply a panel of immunohistochemical (IHC) markers of cell proliferation (Mib1), intestinal differentiation (Cdx2), apoptosis (pro-caspase 3) and cell immortalisation (hTERT) to compare the IHC profiles of a series of precancerous lesions arising in gastric intestinalised (ie, IM-positive) glands.

MATERIALS AND METHODS

By applying the histological criteria consistently provided by both the Padova Classification and the World Health Organization International Agency, 112 consecutive cases were considered: intestinal metaplasia (IM; n = 54), Indef-NiN in IM-positive gastric glands (n = 28) and low-grade (LG) NiN (n = 30). In each histological category, the expression of the marker was separately scored in superficial, proliferative and coil compartments.

RESULTS

In all glandular compartments, Mib1, Cdx2, hTERT and pro-caspase 3 were consistently more expressed in LG-NiN than in either IM or Indef-NiN lesions (analysis of variance: p<0.001). Significant ORs (calculated by ordinal logistic regression analysis for each glandular compartment) associated IM, Indef-NiN and LG-NiN with the expression of the considered markers.

CONCLUSIONS

A consistent overexpression (unrestricted to the proliferative zone) of IHC markers of cell proliferation, intestinal differentiation, decreased apoptosis and cell immortalisation differentiates LG-NiN from both (simple) IM and Indef-NiN (arising in IM). An increased proliferative activity in the proliferative zone discriminates Indef-NiN lesions (ie, hyperproliferative IM) from IM. Such divergent IHC profiles may provide a rationale for scheduling follow-up protocols more properly tailored on the patient's risk for cancer.

Telomerase activity as a prognostic factor in colorectal cancer

BACKGROUND

As the enzyme telomerase extends the life of the cell through its ability to lengthen telomeres, its activity in different types of tumor has been evaluated as a possible factor involved in tumorigenesis. The aim of this study was to assess the prognostic significance of telomerase activity in patients with colorectal carcinoma.

PATIENTS AND METHODS

Telomerase activity was determined in 103 patients undergoing surgery for colorectal cancer between 2001 and 2003. Telomerase activity was determined by an enzyme-linked immunoassay based on the amplification of telomeric repeat sequences (TRAP assay).

RESULTS

90% of our study population showed telomerase activity. Telomerase activity was related to tumor stage and site: a lower proportion of patients with stage A tumors showed telomerase activity compared to more advanced stages; and more patients with colon than with rectal carcinomas were telomerase positive. Multivariate analysis revealed that by adjusting for tumor stage, telomerase activity could be used to predict the risk of death or recurrence (p < 0.001).

CONCLUSIONS

Activation of telomerase seems to be a frequent event related to the stage of the tumor in colorectal tumorigenesis. Our findings suggest that telomerase activity can predict a greater risk of death or recurrence, irrespective of the more conventional prognostic factors.

Senescence and immortalization: role of telomeres and telomerase

Telomere dynamics are a critical component of both aging and cancer. Telomeres progressively shorten in almost all dividing cells and most human cells do not express or maintain sufficient telomerase activity to fully maintain telomeres. There is accumulating evidence that when only a few telomeres are short, they form end-associations, leading to a DNA damage signal resulting in replicative senescence (a cellular growth arrest, also called the M1 stage). In the absence of cell-cycle checkpoint pathways (e.g. p53 and or p16/Rb), cells bypass M1 senescence and telomeres continue to shorten eventually resulting in crisis (also called the M2 stage). M2 is characterized by many 'uncapped' chromosome ends, end-fusions, chromosome breakage fusion-bridge cycles, mitotic catastrophe and a high fraction of apoptotic cells. In a rare M2 cell, telomerase (a cellular reverse transcriptase) can be reactivated or up-regulated, resulting in indefinite cell proliferation. This cellular immortalization is a potentially rate-limiting step in carcinogenesis that is important for the continuing evolution of most advanced cancers. In this perspective we will present our views on the evidence for telomere dysfunction in aging and in cancer progression. We will argue that telomere shortening in the absence of other alterations may be a potent tumor suppressor mechanism and we will discuss the evidence for and against the major molecular mechanisms proposed to initiate replicative senescence.

Telomeres, stem cells, senescence, and cancer

Mammalian aging occurs in part because of a decline in the restorative capacity of tissue stem cells. These self-renewing cells are rendered malignant by a small number of oncogenic mutations, and overlapping tumor suppressor mechanisms (e.g., p16(INK4a)-Rb, ARF-p53, and the telomere) have evolved to ward against this possibility. These beneficial antitumor pathways, however, appear also to limit the stem cell life span, thereby contributing to aging.

Demethylation of the human telomerase catalytic subunit (hTERT) gene promoter reduced hTERT expression and telomerase activity and shortened telomeres

Telomerase is the ribonucleoproteic complex involved in maintaining telomere size. It is expressed in germ and stem cells but not in normal somatic cells. In most tumors, telomerase is reactivated. In humans, telomerase activity is tightly regulated by expression of the hTERT gene. In a previous study, we found a direct correlation between methylation of the hTERT promoter and hTERT gene expression. In order to demonstrate this correlation, demethylation experiments were performed with the demethylating agent 5aza-2'-deoxycytidine (5azadC). Three telomerase-positive tumor cell lines (Lan-1, HeLa, and Co115), presenting a hypermethylated hTERT promoter, were treated with different doses and types of treatment for a long period. Analysis of methylation revealed a final hTERT promoter demethylation up to 95%. Quantification of hTERT mRNA showed that transcription was strongly repressed during drug exposure. In contrast, expression of c-Myc, an activator of hTERT promoter, was barely down-regulated or increased by the treatment. Using a TRAP assay, telomerase activity was semiquantified in all experiments. It strongly decreased or was suppressed after two to four passages. Finally, telomere length was measured by Southern blot. Their averages were not modified, but ranges concentrated around the mean. Thus, it is likely that hTERT promoter hypermethylation would be necessary for its expression.

Equine telomeres and telomerase in cellular immortalisation and ageing

To determine the role of telomeres in cellular ageing in equids, we analysed telomere lengths in peripheral blood derived DNA samples from a panel of donkeys (Equus asinus) ranging from 2 to 30 years of age. The average telomere lengths ranged from 7 to 21 kbp and a statistically significant inverse correlation between telomere lengths and donor age was demonstrated. Similarly, telomere lengths in primary fibroblasts isolated from a horse (Equus equus) demonstrated telomeric loss with in vitro ageing when cultured to senescence. We extended this study to evaluate activity of the enzyme telomerase in various equine cell cultures, normal equine tissues and equine benign tumour samples. Initially a panel of equine immortalised and primary cell cultures were evaluated for telomerase activity using a standard telomere repeat amplification protocol (TRAP) assay. High levels of telomerase activity were detected in equine immortalised cells with no activity evident in primary cell cultures. Similarly, no telomerase activity could be detected in normal equine tissues or equine benign tumour samples of the sarcoid or papilloma type. We conclude that telomere attrition may contribute to ageing in equids. However, it would appear that telomerase does not play a major role in the development of the most common benign tumours of the horse.

Separation of long-range human TERT gene haplotypes by transformation-associated recombination cloning in yeast

The hTERT gene encoding a catalytic subunit of human telomerase contains four blocks of variable number of tandem repeats (VNTRs)--two in intron 2 and two in intron 6. The segregation of hTERT VNTRs was analysed in families, revealing that all of them were transmitted through meiosis following a Mendelian inheritance. The work reports a further characterization of the minisatellites in hTERT. We employed transformation-associated recombination (TAR) cloning to isolate parental hTERT alleles and determined the specific combination of minisatellites at each of the polymorphic sites. A long-range haplotyping of hTERT determined by TAR cloning was verified by classical Mendelian analysis. Since such a strategy can be applied for any chromosomal locus, we conclude that recombinational gene capture could greatly facilitate haplotypes analysis.

Connecting chromosomes, crisis, and cancer

Cancer is a disease of impaired genome stability. The molecular forces that maintain genome integrity and sense altered chromosome structure are invariably subverted in cancer cells. Here, we explore the contrasting contributions of telomeres in the initiation and suppression of cancer and review the evidence supporting a role for telomere dysfunction as a mechanism driving the radical chromosomal aberrations that typify cancer genomes. Recent work suggests that passage of cells through crisis in the setting of deactivated DNA damage checkpoints provides a mutational mechanism that can generate the diverse genetic alterations required for cancer initiation. A greater understanding of telomere-induced crisis and the cell's crisis management mechanisms should guide the rational development of new therapeutics for cancer and other disorders.

Modeling chromosomal instability and epithelial carcinogenesis in the telomerase-deficient mouse

Human carcinomas are intimately linked to advancing age. These cancers have complex cytogenetic profiles, including aneuploidy and chromosomal structural aberrations. While aged humans sustain a high rate of carcinomas, mice bearing common tumor suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. One marked species distinction between human and mouse that bears on the predisposition to carcinogenesis lies in the radical differences in length and regulation of the telomere, nucleoprotein complexes that cap the ends of eukaryotic chromosomes. Recent cancer modeling studies in the telomerase knockout p53 mutant mice revealed that telomere dynamics might be relevant to carcinogenesis. In these mice, there is a shift in the tumor spectrum towards epithelial carcinomas, and these cancers emerge with complex cytogenetic profiles classical for human carcinomas. In this review, we suggest that the mechanism of fusion-bridge-breakage-translocation, triggered by critically short telomeres, may be one of the generators of genomic instability commonly seen in human carcinomas.

Clinical implications of telomerase detection

In 1994 a sensitive method for the detection of telomerase was described. This assay, which was based on the polymerase chain reaction, suggested that telomerase activity was associated with immortal and cancer cells. Since then more than a thousand studies have documented the expression and activity of the enzyme in diseased tissues, primarily tumours. This review gives an overview of the biological significance of telomerase expression and methods for detecting its activity. This is followed by an organ system-based discussion of expression in normal tissues and disease states. We finish with speculation as to the future role of telomerase detection in diagnostic histopathology.

Rearrangements of minisatellites in the human telomerase reverse transcriptase gene are not correlated with its expression in colon carcinomas

Telomerase activation is crucial in human carcinogenesis. The limiting component of telomerase, the catalytic subunit (hTERT), is undetectable in normal somatic cells but present in most tumor cells, including the earliest stages of colon carcinoma. The mechanisms involved in the differential expression in normal and tumor cells are not understood. In normal cells hTERT expression is shut down by a repressor, and upregulation could be a consequence of cis-acting changes in the hTERT gene, making it resistant to repression. We have identified a polymorphic and a monomorphic minisatellite in the second intron of the hTERT gene, and polymorphic one in intron 6. The polymorphic minisatellite in intron 2 contains binding sites for c-Myc, which has been shown to upregulate hTERT transcription. Screening colon carcinoma DNAs for rearrangements of hTERT minisatellites we detected no changes in 33 samples from tumors, most of which express hTERT. This indicates that size rearrangements of the hTERT minisatellites are not required for telomerase expression in colon carcinomas. Minor changes and one LOH were seen in five tumors.

Expression of telomerase genes correlates with telomerase activity in human colorectal carcinogenesis

The human telomerase enzyme is composed of two essential components, hTR, which acts as a template for reverse transcription, and hTERT, which is the putative catalytic subunit for the enzyme. Recent studies have demonstrated a good correlation between hTERT expression and telomerase activation, whereas RT-PCR results seemed to reveal that hTR is ubiquitously expressed in all cells. These observations left unclear the role of hTR, and to a lesser extent hTERT, in the regulation of telomerase activation. In the present study, the correlation of telomerase activity and the expression of these genes was examined in a total of 70 colorectal tissues (25 adenocarcinomas, 30 adenomas, and 15 samples of normal colorectal mucosa). Total RNA for RT-PCR analysis and cell extracts for TRAP assay were obtained from consecutive sections and histological control was simultaneously performed. To avoid false-positive results, due to the fact that hTR cDNA and genomic hTR DNA are identical (the gene has no introns), extensive DNase digestion was performed before cDNA synthesis. RT-PCR analysis revealed that hTERT mRNA was expressed in all cancers and in 13 of 14 telomerase-positive adenomas, but never in telomerase-negative colorectal tissues. hTR transcripts were observed in all telomerase-positive samples but also in three telomerase-negative samples, two adenomas, and one normal colonic mucosa. It is concluded that hTERT and hTR expression is strongly correlated with telomerase activity. hTR transcripts, however, also occur in some telomerase-negative tissues and these results are in keeping with the concept that hTERT expression is a major regulator of telomerase activity.