Hodgkin and Reed-Sternberg cells of classical Hodgkin's disease overexpress the telomerase RNA template (hTR)

Precision detection of hepatocellular carcinoma-associated telomerase RNA with SA@Comb-HCR nanosystem

Accurate intracellular visualization of human telomerase RNA (hTR) is imperative for early diagnosis and treatment monitoring of hepatocellular carcinoma (HCC). While isothermal amplification-based DNA cascade strategies are promising, challenges persist in achieving great intake efficiency of detection probes within tumor cells and enhancing intracellular reaction efficiency. This study introduces a SA@Comb-HCR nanosystem, a highly effective approach for in situ hTR detection in HCC cells. Sodium alginate-coated liposomes ensures efficient nanoprobe delivery, which are then combined with proximity effect-inspired signal amplification. The coating of sodium alginate facilitates receptor-mediated endocytosis, prevents serum protein adhesion, and mitigates cationic liposome cytotoxicity. The designed Comb-like consolidated hairpin probe enhances the concentration of the local reactant, resulting in cascade amplification upon hTR activation. This technique achieves precision detection of intracellularly overexpressed hTR in HCC cells with a remarkable detection limit of 0.7 pM. This approach holds great promise for advancing targeted and sensitive early clinical diagnosis of HCC.

Ratiometric fluorescence resonance energy transfer for reliable and sensitive detection of intracellular telomerase RNA via strand displacement reaction amplification

Human telomerase RNA (hTR) is one essential component of telomerase and is overexpressed in tumor cells. Therefore, the reliable and sensitive detection of hTR is essential for the early cancer diagnosis. Herein, to avoid the false positive signals caused by co-existing components in the cell, a ratiometric fluorescence resonance energy transfer (FRET) strategy was developed to achieve reliable detection of intracellular hTR. Manganese dioxide nanosheets (MnO₂NS) with good biocompatibility carry two fluorophore-labelled hairpin DNA probes into the cancer cell and then release the probes via decomposition of MnO₂NS by intracellular L-glutathione reduced (GSH). Then, hTR triggered the cyclic strand displacement reaction (SDR) between two hairpin DNA probes to continuously form DNA duplexes, which made two fluorophores close to each other and led to an effective FRET. Fluorescence imaging demonstrated a higher expression level of hTR in HeLa cells than that in normal HL-7702 cells. The high specificity of hairpin DNA probes and SDR make it easy to discriminate the single-base mutation. Therefore, it provides a highly sensitive, simple and reliable method for the extracellular and intracellular detection of hTR.

A facile DNA/RNA nanoflower for sensitive imaging of telomerase RNA in living cells based on "zipper lock-and-key" strategy

The sensitive imaging of telomerase RNA (TR) in living cells is crucial for improved guidance in cancer clinical diagnosis because its expression level is closely related to malignant diseases. The efficient delivery of multiple nucleic acid probes to target cells is critical for nucleic acid-based methods to successfully image low-abundance TR in living cells. While novel nanomaterials enhance delivery efficiency, uncontrolled loading and slow intracellular release remain major challenges for multiple-probe delivery. Here, we designed a facile DNA/RNA nanoflower (NF) to perform the controlled loading of multiple probes and rapid intracellular release based on the "zipper lock-and-key" strategy. First, a long RNA generated by rolling circle transcription acts as both the "smart zipper lock" and the delivery carrier to alternately lock multiple functional DNAs through DNA-RNA base pairing, and the resulting RNA/DNA hybrids self-assemble into packed NFs. The functional DNAs include the fluorescence molecular beacon H₁ for TR recognition, H₂ for hybrid chain reaction (HCR) and DNA-cholesterol for size control. After NF internalization by the cells, the intracellular RNase H acts as the "key" to specifically open the DNA/RNA NFs by cleaving the RNA in the DNA/RNA hybrid, releasing high amounts of H₁ and H₂ in a confined space and thereby facilitating the HCR amplification analysis of cytoplasmic TR. With the addition of a DNA-nuclear localization peptide component in the same NF, nuclear TR can also be sensitively detected. Compared with the regular H₁/H₂ mixture, the DNA/RNA NFs produced a higher-contrast fluorescence signal. This indicated that the proposed strategy allowed the side arms of H₁/H₂ to be sealed into the RNA sequence-programmed "zipper lock" by controlled loading, avoiding mutual nonspecific H₁/H₂ hybridization. In addition, due to the fast kinetics of the RNase endonuclease reaction, the loaded H₁/H₂ was quickly released. Furthermore, the strategy was successfully used to assay the expression levels of TR in HeLa, HepG2 and HL-7702 cells, demonstrating that this approach holds the potential for the sensitive detection of low-abundance biomarkers in living cells.

Sensitive detection of intracellular RNA of human telomerase by using graphene oxide as a carrier to deliver the assembly element of hybridization chain reaction

Since the level of human telomerase RNA (hTR) in tumor cells is higher than that in normal somatic cells, the quantitative assay of hTR is of significant importance in tumor diagnosis. Herein, graphene oxide (GO) was simultaneously exploited as a fluorescence quencher and a carrier of nucleic acid to successfully deliver two hairpin DNA probes of hybridization chain reaction (HCR) into the cancer cell for detecting telomerase RNA based on DNA nanoassembly of HCR. The sticky end of HCR probes could tightly absorb on the surface of GO, resulting in fluorescence quenching of the dye which was tagged at the sticky end of two hairpin probes. When faced with hTR, the fluorescence of DNA probes is subsequently recovered because hTR could trigger HCR to autonomous assembly of a DNA polymer which released from the GO and led to fluorescence recovery. Taking advantage of nucleic acid nanoassembly of HCR, this intracellular HCR strategy creates enormous signal amplification, and enables ultra-sensitive fluorescence imaging of hTR expression. By monitoring fluorescence change, human telomerase RNA could be specifically studied and this method can also be used for detecting single-base mutation. The GO-aided HCR strategy allowed us to sensitively detect hTR in a living cell, which holds great potential for analyzing other low-abundance biomolecules in living cells via HCR.

Sticky-flares for in situ monitoring of human telomerase RNA in living cells

Human telomerase RNA (hTR), a template of telomerase for telomeric repeat synthesis, was used to reflect the telomerase activity and act as a potential target of antitumor therapy. Here, we report a novel DNA-conjugated AuNP probe termed sticky-flares for the in situ detection of intracellular human telomerase RNA. The sticky-flares probe is capable of entering living cells directly without any auxiliary and recognizing the binding domain of human telomerase RNA. On recognition, the fluorophore-modified recognition flares can specifically bind to the target, separate from the sticky-flares and act as a fluorescent reporter to quantify and dynamically profile human telomerase RNA in living cells. We envision that the sticky-flares probe would be a valuable platform to investigate the function and regulation of hTR in antitumor therapy and hTR-related drug invention.

Disruption of direct 3D telomere-TRF2 interaction through two molecularly disparate mechanisms is a hallmark of primary Hodgkin and Reed-Sternberg cells

In classical Hodgkin's lymphoma (cHL), specific changes in the 3D telomere organization cause progression from mononuclear Hodgkin cells (H) to multinucleated Reed-Sternberg cells (RS). In a post-germinal center B-cell in vitro model, permanent latent membrane protein 1 (LMP1) expression, as observed in Epstein-Barr virus (EBV)-associated cHL, results in multinuclearity and complex chromosomal aberrations through downregulation of key element of the shelterin complex, the telomere repeat binding factor 2 (TRF2). Thus, we hypothesized that the three-dimensional (3D) telomere-TRF2 interaction was progressively disturbed during transition from H to RS cells. To this end, we developed and applied for the first time a combined quantitative 3D TRF2-telomere immune fluorescent in situ hybridization (3D TRF2/Telo-Q-FISH) technique to monolayers of primary H and RS cells, and adjacent benign internal control lymphocytes of lymph node biopsy suspensions from diagnostic lymph node biopsies of 14 patients with cHL. We show that H and RS cells are characterized by two distinct patterns of disruption of 3D telomere-TRF2 interaction. Disruption pattern A is defined by massive attrition of telomere signals and a considerable increase of TRF2 signals not associated with telomeres. This pattern is restricted to EBV-negative cHL. Disruption pattern B is defined by telomere de-protection due to an impressive loss of TRF2 signals, physically linked to telomeres. This pattern is typical of, but is not restricted to, LMP1+EBV-associated cHL. In the disruption pattern B group, so-called 'ghost' end-stage RS cells, void of both TRF2 and telomere signals, were identified, whether or not associated with EBV. Our findings demonstrate that two molecularly disparate mechanisms converge on the level of 3D telomere-TRF2 interaction in the formation of RS cells.

The Use of 3D Telomere FISH for the Characterization of the Nuclear Architecture in EBV-Positive Hodgkin's Lymphoma

The 3D nuclear architecture is closely related to cellular functions and chromosomes are organized in distinct territories. Quantitative 3D telomere FISH analysis (3D Q-FISH) and 3D super-resolution imaging (3D-SIM) at a resolution up to 80 nm as well as the recently developed combined quantitative 3D TRF2-telomere immune FISH technique (3D TRF2/Telo-Q-FISH) have substantially contributed to elucidate molecular pathogenic mechanisms of hematological diseases. Here we report the methods we applied to uncover major molecular steps involved in the pathogenesis of EBV-associated Hodgkin's lymphoma. These methods allowed us to identify the EBV-encoded oncoprotein LMP1 as a key element in the formation of Hodgkin (H-cell) and multinucleated Reed-Sternberg cells (RS-cell), the diagnostic tumor cell of classical Hodgkin's lymphoma (cHL). LMP1 mediates multinuclearity through downregulation of shelterin proteins, in particular telomere repeat binding factor 2 (TRF2).

Tumor viruses and replicative immortality--avoiding the telomere hurdle

Tumor viruses promote cell proliferation in order to gain access to an environment suitable for persistence and replication. The expression of viral products that promote growth transformation is often accompanied by the induction of multiple signs of telomere dysfunction, including telomere shortening, damage of telomeric DNA and chromosome instability. Long-term survival and progression to full malignancy require the bypassing of senescence programs that are triggered by the damaged telomeres. Here we review different strategies by which tumor viruses interfere with telomere homeostasis during cell transformation. This frequently involves the activation of telomerase, which assures both the integrity and functionality of telomeres. In addition, recent evidence suggests that oncogenic viruses may activate a recombination-based mechanism for telomere elongation known as Alternative Lengthening of Telomeres (ALT). This error-prone strategy promotes genomic instability and could play an important role in viral oncogenesis.

Sister chromatid cohesion defects are associated with chromosome instability in Hodgkin lymphoma cells

Background

Chromosome instability manifests as an abnormal chromosome complement and is a pathogenic event in cancer. Although a correlation between abnormal chromosome numbers and cancer exist, the underlying mechanisms that cause chromosome instability are poorly understood. Recent data suggests that aberrant sister chromatid cohesion causes chromosome instability and thus contributes to the development of cancer. Cohesion normally functions by tethering nascently synthesized chromatids together to prevent premature segregation and thus chromosome instability. Although the prevalence of aberrant cohesion has been reported for some solid tumors, its prevalence within liquid tumors is unknown. Consequently, the current study was undertaken to evaluate aberrant cohesion within Hodgkin lymphoma, a lymphoid malignancy that frequently exhibits chromosome instability.

Methods

Using established cytogenetic techniques, the prevalence of chromosome instability and aberrant cohesion was examined within mitotic spreads generated from five commonly employed Hodgkin lymphoma cell lines (L-1236, KM-H2, L-428, L-540 and HDLM-2) and a lymphocyte control. Indirect immunofluorescence and Western blot analyses were performed to evaluate the localization and expression of six critical proteins involved in the regulation of sister chromatid cohesion.

Results

We first confirmed that all five Hodgkin lymphoma cell lines exhibited chromosome instability relative to the lymphocyte control. We then determined that each Hodgkin lymphoma cell line exhibited cohesion defects that were subsequently classified into mild, moderate or severe categories. Surprisingly, ~50% of the mitotic spreads generated from L-540 and HDLM-2 harbored cohesion defects. To gain mechanistic insight into the underlying cause of the aberrant cohesion we examined the localization and expression of six critical proteins involved in cohesion. Although all proteins produced the expected nuclear localization pattern, striking differences in RAD21 expression was observed: RAD21 expression was lowest in L-540 and highest within HDLM-2.

Conclusion

We conclude that aberrant cohesion is a common feature of all five Hodgkin lymphoma cell lines evaluated. We further conclude that aberrant RAD21 expression is a strong candidate to underlie aberrant cohesion, chromosome instability and contribute to the development of the disease. Our findings support a growing body of evidence suggesting that cohesion defects and aberrant RAD21 expression are pathogenic events that contribute to tumor development.

Three-dimensional Telomere Signatures of Hodgkin- and Reed-Sternberg Cells at Diagnosis Identify Patients with Poor Response to Conventional Chemotherapy

In classic Hodgkin lymphoma (HL) the malignant mononuclear Hodgkin (H) and multinuclear Reed-Sternberg (RS) cells are characterized by a distinct three-dimensional nuclear telomere organization with shortening of the telomere length and the formation of telomeric aggregates. We asked if the severity of these telomere changes correlates with the clinical behavior of the disease. We retrospectively evaluated three-dimensional telomere organization by quantitative fluorescent in situ hybridization (Q-FISH) of diagnostic biopsies from 16 patients who were good responders and compared them with 16 diagnostic biopsies of 10 patients with refractory or relapsing HL (eight initial biopsies, four confirming progressions, and four confirming relapses). The H cells from patients with refractory/relapsing disease contained a significantly higher percentage of very small telomeres (P = .027) and telomere aggregates (P = .032) compared with H cells of patients entering rapid remission. These differences were even more significant (P = .002 and P = .013, respectively) when comparing the eight initial diagnostic biopsies of refractory/relapsing HL with diagnostic biopsies of eight patients with ongoing long-lasting remission (mean of 47 months). This specific three-dimensional telomere Q-FISH signature identifies these highly aggressive mononuclear H cells at the first diagnostic biopsy and thus may offer a new molecular marker to optimize initial treatment.

3D Telomere FISH defines LMP1-expressing Reed-Sternberg cells as end-stage cells with telomere-poor 'ghost' nuclei and very short telomeres

In Epstein-Barr virus (EBV) negative Hodgkin's cell lines and classical EBV-negative Hodgkin's lymphoma (HL), Reed-Sternberg cells (RS cells) represent end-stage tumor cells, in which further nuclear division becomes impossible because of sustained telomere loss, shortening and aggregation. However, the three-dimensional (3D) telomere organization in latent membrane protein 1 (LMP1)-expressing RS cells of EBV-associated HL is not known. We performed a 3D telomere analysis after quantitative fluorescent in situ hybridization on 5 mum tissue sections on two LMP1-expressing HL cases and showed highly significant telomere shortening (P<0.0001) and formation of telomere aggregates in RS cells (P<0.0001), when compared with the mononuclear precursor Hodgkin cells (H cells). Telomere-poor or telomere-free 'ghost' nuclei were a regular finding in these RS cells. These nuclei and their telomere content strongly contrasted with the corona of surrounding lymphocytes showing numerous midsized telomere hybridization signals. Both H cells and RS cells of two EBV-negative HL cases analyzed in parallel showed 3D telomere patterns identical to those of LMP1-expressing cases. As a major advance, our 3D nuclear imaging approach allows the visualization of hitherto unknown profound changes in the 3D nuclear telomere organization associated with the transition from LMP1-positive H cells to LMP1-positive RS cells. We conclude that RS cells irrespective of LMP1 expression are end-stage tumor cells in which the extent of their inability to divide further is proportional to the increase of very short telomeres, telomere loss, aggregate formation and the generation of 'ghost' nuclei.

The 3D nuclear organization of telomeres marks the transition from Hodgkin to Reed-Sternberg cells

To get an insight into the transition from mononuclear Hodgkin cells (H cells) to diagnostic multinuclear Reed-Sternberg cells (RS cells), we performed an analysis of the three-dimensional (3D) structure of the telomeres in the nuclei of the Hodgkin cell lines HDLM-2, L-428, L-1236 and lymph node biopsies of patients with Hodgkin's disease. Cellular localization of key proteins of the telomere-localized shelterin complex, the mitotic spindle and double-stranded DNA breaks was also analyzed. RS cells show significantly shorter and significantly fewer telomeres in relation to the total nuclear volume when compared with H cells; in particular, telomere-poor 'ghost' nuclei are often adjacent to one or two nuclei displaying huge telomeric aggregates. Shelterin proteins are mainly cytoplasmic in both H and RS cells, whereas double-stranded DNA breaks accumulate in the nuclei of RS cells. In RS cells, multipolar spindles prevent proper chromosome segregation. In conclusion, a process of nuclear disorganization seems to initiate in H cells and further progresses when the cells turn into RS cells and become end-stage tumor cells, unable to divide further because of telomere loss, shortening and aggregate formation, extensive DNA damage and aberrant mitotic spindles that may no longer sustain chromosome segregation. Our findings allow a mechanistic 3D understanding of the transition of H to RS cells.

The neoplastically transformed (CD30hi) Marek's disease lymphoma cell phenotype most closely resembles T-regulatory cells

INTRODUCTION

Marek's disease (MD), a herpesvirus-induced lymphoma of chickens is a unique natural model of CD30-overexpressing (CD30hi) lymphoma. We have previously proposed that the CD30hi neoplastically transformed CD4+ T cells in MD lymphomas have a phenotype antagonistic to cell mediated immunity. Here were test the hypothesis that the CD30hi neoplastically transformed MD lymphoma cells have a phenotype more closely resembling T-helper (Th)-2 or regulatory T (T-reg) cells.

MATERIALS AND METHODS

We separated ex vivo-derived CD30hi, from the CD30lo/- (non-transformed), MD lymphoma cells and then quantified the relative amounts of mRNA and proteins for cytokines and other genes that define CD4+ Th-1, Th-2 or T-reg phenotypes.

RESULTS AND DISCUSSION

Gene Ontology-based modeling of our data shows that the CD30hi MD lymphoma cells having a phenotype more similar to T-reg. Sequences that could be bound by the MD virus putative oncoprotein Meq in each of these genes' promoters suggests that the MD herpesvirus may play a direct role in maintaining this T-reg-like phenotype.

In situ detection of non-polyadenylated RNA molecules using Turtle Probes and target primed rolling circle PRINS

BACKGROUND

In situ detection is traditionally performed with long labeled probes often followed by a signal amplification step to enhance the labeling. Whilst short probes have several advantages over long probes (e.g. higher resolution and specificity) they carry fewer labels per molecule and therefore require higher amplification for detection. Furthermore, short probes relying only on hybridization for specificity can result in non-specific signals appearing anywhere the probe attaches to the target specimen. One way to obtain high amplification whilst minimizing the risk of false positivity is to use small circular probes (e.g. Padlock Probes) in combination with target primed rolling circle DNA synthesis. This has previously been used for DNA detection in situ, but not until now for RNA targets.

RESULTS

We present here a proof of principle investigation of a novel rolling circle technology for the detection of non-polyadenylated RNA molecules in situ, including a new probe format (the Turtle Probe) and optimized procedures for its use on formalin fixed paraffin embedded tissue sections and in solid support format applications.

CONCLUSION

The method presented combines the high discriminatory power of short oligonucleotide probes with the impressive amplification power and selectivity of the rolling circle reaction, providing excellent signal to noise ratios in combination with exact target localization due to the target primed reaction. Furthermore, the procedure is easily multiplexed, allowing visualization of several different RNAs.

Modeling the proteome of a Marek's disease transformed cell line: a natural animal model for CD30 overexpressing lymphomas

Marek's disease (MD) in the chicken, caused by the highly infectious MD alpha-herpesvirus (MDV), is both commercially important and a unique, naturally occurring model for human T-cell lymphomas overexpressing the Hodgkin's disease antigen, CD30. Here, we used proteomics as a basis for modeling the molecular functions and biological processes involved in MDV-induced lymphomagenesis. Proteins were extracted from an MDV-transformed cell line and were then identified using 2-D LC-ESI-MS/MS. From the resulting 3870 cellular and 21 MDV proteins we confirm the existence of 3150 "predicted" and 12 "hypothetical" chicken proteins. The UA-01 proteome is proliferative, differentiated, angiogenic, pro-metastatic and pro-immune-escape but anti-programmed cell death, -anergy, -quiescence and -senescence and is consistent with a cancer phenotype. In particular, the pro-metastatic integrin signaling pathway and the ERK/MAPK signaling pathways were the two predominant signaling pathways represented. The cytokines, cytokine receptors, and their related proteins suggest that UA-01 has a regulatory T-cell phenotype.

The importance of the telomere and telomerase system in hematological malignancies

Telomeres are specialized chromosomal end structures composed of repeat TTAGGG sequences in humans. They shorten with each cell division and thus serve as the "mitotic clock" of the cell. One of their main functions is the maintenance of chromosomal integrity and their excessive shortening is associated with DNA instability. Telomerase, a unique reverse transcriptase, is inactive in most somatic human cells and is up-regulated in most cancer cells. Recently, the biology of the telomere/telomerase system has attracted much attention because of its possible role in carcinogenesis and aging. In this article we review the biology of this system and its relevance to normal and malignant hematopoietic cells. The biological, diagnostic and prognostic value of telomere/telomerase biology is discussed, as well as its potential future applications in cancer therapeutics.

Expression-patterns of the RNA component (hTR)and the catalytic subunit (hTERT) of human telomerase in nonneoplastic prostate tissue, prostatic intraepithelial neoplasia, and prostate cancer

BACKGROUND

Telomerase, a ribonucleoprotein, is composed of a RNA component (hTR) and two protein subunits. One of these subunits, the catalytic subunit (human telomerase reverse transcriptase, hTERT), represents a reverse transcriptase. hTERT-expression is closely correlated with telomerase activity. The telomerase is thought to be involved in immortalization process. By adding hexamic repeats to the end of chromosomal DNA, the telomeres, the enzyme is able to stop progresssive telomeric DNA loss that occurs during cell division due to the end replication problem that stops the mitotic clock.

METHODS

Expression-patterns of hTR using radioactive in situ hybridization with (35)S-labelled RNA probes were compared with immunhistochemical staining for hTERT in 14 cases of archival paraffin-embedded samples of normal prostatic tissue, high-grade prostatic intraepithelial neoplasia (PIN), prostatic cancer, and one atypical adenomatous hyperplasia (AAH). Beside the expression-patterns each telomerase component was evaluated semiquantitatively.

RESULTS

hTERT and hTR can be found in nonneoplstic tissue and are upregulated in premalignant transformated lesions. AAH showed no hTERT-expression and low hTR-expression. There is a heterogenous expression within prostatic carcinomas.

CONCLUSIONS

No association was observed between the grade of the tumour differentiation and semiquantitative levels of hTR- or hTERT-expression. Telomerase is of limited value for the diagnostic of malignant or benign lesions in prostate.

Telomerase as a prognostic marker in breast cancer: high-throughput tissue microarray analysis of hTERT and hTR

Telomerase activity (TA) has been shown to correlate with poor clinical outcome in various tumour entities, indicating that tumours expressing this enzyme may be more aggressive and that TA may be a useful prognostic marker. For breast cancer, however, TA is a controversial prognostic marker; whereas some studies suggest an association between TA and disease outcome, others do not find this association. This study used tissue microarrays (breast carcinoma prognosis arrays) containing 611 samples (each 0.6 mm in diameter) from the tumour centre of paraffin-embedded breast carcinomas to analyse the catalytic subunit of telomerase, human telomerase reverse-transcriptase (hTERT), and the internal RNA component (hTR), which are the core components of the telomerase holoenzyme complex. hTERT protein expression was obtained by immunohistochemistry (human anti-telomerase antibody Ab-2, Calbiochem), and hTR RNA was measured by radioactive in situ hybridization. hTERT and hTR expression were determined semi-quantitatively and graded (scores 1-4). Clinical data, such as histological subtype, pT stage, tumour diameter, pN stage, BRE grade, tumour-specific survival (in months), patient's age and others, were available for statistical analysis. A statistically significant correlation was found between tumour-specific survival (overall survival) and hTERT expression (p < 0.0001) or hTR expression (p = 0.00110). Tumours with higher scores (scores 3, 4) for hTR and/or hTERT were associated with a worse prognosis. In multivariate analysis, hTERT expression was an independent prognostic factor. Previous studies, focusing on analysis of TA in smaller numbers of fresh-frozen breast carcinomas by the TRAP assay, gave controversial results with respect to TA as a prognostic marker. Using tissue microarrays from 611 breast carcinomas, this study has demonstrated that increased expression levels of the telomerase core components, hTERT and hTR, are associated with lower overall survival. These findings suggest that TA should be included in future validation studies as a prognostic marker in breast cancer.

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.