Key role of PIN1 in telomere maintenance and oncogenic behavior in a human glioblastoma model.

PIN1 is the only known enzyme capable of recognizing and isomerizing the phosphorylated Serine/Threonine­Proline motif. Through this mechanism, PIN1 controls diverse cellular functions, including telomere maintenance. Both PIN1 overexpression and its involvement in oncogenic pathways are involved in several cancer types, including glioblastoma (GBM), a lethal disease with poor therapeutic resources. However, knowledge of the role of PIN1 in GBM is limited. Thus, the present work aimed to study the role of PIN1 as a telomere/telomerase regulator and its contribution to tumor biology. PIN1 knockout (KO) LN­229 cell variant using CRISPR/Cas9 was developed and compared with PIN1 LN­229 expressing cells. To study the effect of PIN1 absence, status of NF­κB pathway was evaluated by luciferase reporter gene assay and quantitative PCR. Results revealed that PIN1 deletion in GBM cells diminished the active levels of NF­κB and decrease the transcription of il­8 and htert genes. Then, telomere/telomerase related processes were studied by RQ­TRAP assay and telomere length determination by qPCR, obtaining a reduction both in telomerase activity as in telomere length in PIN1 KO cells. In addition, measurement of SA ß­galactosidase and caspase­3 activities revealed that loss of PIN1 triggers senescence and apoptosis. Finally, migration, cell cycle progression and tumorigenicity were studied by flow cytometry/western blot, Transwell assay and in vivo experiments, respectively. PIN1 deletion decreased migration as well as cell cycle progression by increasing doubling time and also resulted in the loss of LN­229 cell ability to form tumors in mice. These results highlight the role of PIN1 in telomere homeostasis and GBM progression, which supports PIN1 as a potential molecular target for the development of novel therapeutic agents for GBM treatment.

Diagnóstico tardío de atrofia muscular espinal en paciente con síndrome de Down / Late diagnosis of spinal muscular atrophy in a patient with Down syndrome

El síndrome de Down (SD) es la anomalía cromosómica más frecuente entre los recién nacidos vivos. La atrofia muscular espinal (AME), por su parte, es una enfermedad neuromuscular caracterizada por la degeneración progresiva de las motoneuronas del asta anterior de la médula espinal que produce atrofia muscular, debilidad y parálisis. Presentamos el caso de una niña de 6 años con síndrome de Down derivada a nuestro centro para estudio por cuadro de debilidad muscular generalizada de evolución crónica con falta de adquisición de la marcha. Realizamos una revisión bibliográfica enfocándonos en el compromiso neurológico esperable en el síndrome de Down, la evolución de los hitos del desarrollo motor grueso estipulado para este grupo de pacientes y en los hallazgos que deben sugerir la presencia de una enfermedad neuromuscular.

A case of a 6-year-old girl with Down's syndrome is presented. She was referred to our center due to a history of generalized muscle weakness of chronic evolution, associated to her inability to walk. Her mother claimed that the girl's muscle weakness always called her attention as well as the difficulties to the development of motor skills shown by her daughter compared to other children, whether they were healthy or with Down's syndrome. There was information in her medical record and physical exam that strongly suggested the possibility of suffering a neuromuscular disorder. We asked for a molecular study that confirmed the spinal muscular atrophy diagnosis. We carried out a bibliographical revision focusing on the expected neurological impairment in Down's syndrome, the retardation of the gross motor skills development determined for this kind of patients and on the findings that must suggest a neuromuscular disorder.

[Late diagnosis of spinal muscular atrophy in a patient with Down syndrome]. / Diagnóstico tardío de atrofia muscular espinal en paciente con síndrome de Down.

A case of a 6-year-old girl with Down's syndrome is presented. She was referred to our center due to a history of generalized muscle weakness of chronic evolution, associated to her inability to walk. Her mother claimed that the girl's muscle weakness always called her attention as well as the difficulties to the development of motor skills shown by her daughter compared to other children, whether they were healthy or with Down's syndrome. There was information in her medical record and physical exam that strongly suggested the possibility of suffering a neuromuscular disorder. We asked for a molecular study that confirmed the spinal muscular atrophy diagnosis. We carried out a bibliographical revision focusing on the expected neurological impairment in Down's syndrome, the retardation of the gross motor skills development determined for this kind of patients and on the findings that must suggest a neuromuscular disorder.

El síndrome de Down (SD) es la anomalía cromosómica más frecuente entre los recién nacidos vivos. La atrofia muscular espinal (AME), por su parte, es una enfermedad neuromuscular caracterizada por la degeneración progresiva de las motoneuronas del asta anterior de la médula espinal que produce atrofia muscular, debilidad y parálisis. Presentamos el caso de una niña de 6 años con síndrome de Down derivada a nuestro centro para estudio por cuadro de debilidad muscular generalizada de evolución crónica con falta de adquisición de la marcha. Realizamos una revisión bibliográfica enfocándonos en el compromiso neurológico esperable en el síndrome de Down, la evolución de los hitos del desarrollo motor grueso estipulado para este grupo de pacientes y en los hallazgos que deben sugerir la presencia de una enfermedad neuromuscular.

In vitro characterization and rational analog design of a novel inhibitor of telomerase assembly in MDA MB 231 breast cancer cell line.

Tumor cells have unlimited replicative potential, principally due to telomerase activity, which requires assembly of components such as dyskerin (hDKC1), human telomerase reverse transcriptase and human telomerase RNA (hTR). The present study aimed to develop novel inhibitors of telomerase to target the interaction between hTR and hDKC1. Based on docking­based virtual screening, the candidates R1D2­10 and R1D2­15, which exert an in vitro inhibitory effect on telomerase activity, were selected. Human mammary adenocarcinoma MDA­MB 231 cell line was selected to evaluate the treatment with the aforementioned compounds; the effect on telomere length was evaluated by qPCR, where both compounds caused telomere shortening. Furthermore, expression of genes related to apoptosis and senescence process, as well SA ß galactosidase staining and caspase 3 activity. We determine that only compound R1D2­10 showed and effect on the induction of these cellular processes. To identify a lead compound from R1D2­10, 100 analogs were designed by LigDream server and then analyzed by AutoDock Vina and Protein­Ligand Interaction Profile to calculate their docking energy and target interaction. Those with the best values and specific residue interactions were selected for in silico prediction of absorption, distribution, metabolism, excretion (ADME), off­target interaction, toxicity and chemical diversity. A total of nine chemically different analogs was identified with higher docking affinity to the target, suitable ADME properties and not off­target interaction and side effects. These results indicated R1D2­10 and its analogs may serve as potential novel inhibitors of telomerase and antitumoral drugs in clinical use.

Rational design of PIN1 inhibitors for cancer treatment based on conformational diversity analysis and docking based virtual screening.

The parvulin PIN1 (peptidyl-prolyl cis-trans isomerase NIMA-interacting 1), is the only enzyme capable of isomerizing prolines of phospho-Serine/Threonine-Proline motifs. PIN1 binds to a subset of proteins and plays an essential role in regulating protein function post-phosphorylation control. Furthermore, the activity of PIN1 regulates the outcome of the signalling of proline-directed kinases (e.g. MAPK, CDK, or GSK3) and thus regulates cell proliferation and cell survival. For these reasons, PIN1 inhibitors are interesting since they may have therapeutic implications for cancer. Several authors have already reported that the non-structural point mutation Trp34Ala prevents PIN1 from interacting with its downstream effector proteins. In this work, we characterized PIN1 structurally, intending to explore new inhibition targets for the rational design of pharmacological activity compounds. Through a conformational diversity analysis of PIN1, we identified and characterized a highly specific druggable pocket around the residue Trp34. This pocket was used in a high-throughput docking screening of 450,000 drug-like compounds, and the top 10 were selected for re-docking studies on the previously used conformers. Finally, we evaluated the binding of each compound by thermal shift assay and found four molecules with a high affinity for PIN1 and potential inhibitory activity. Through this strategy, we achieved novel drug candidates with the ability to interfere with the phosphorylation-dependent actions of PIN1 and with potential applications in the treatment of cancer.Communicated by Ramaswamy H. Sarma.

Implementation of chromosomal microarrays in a cohort of patients with intellectual disability at the Argentinean public health system.

Intellectual disability is a neurodevelopmental disorder in which genetic, epigenetic and environmental factors are involved. In consequence, the determination of its etiology is usually complex. Though many countries have migrated from conventional cytogenetic analysis to chromosomal microarrays as the first-tier genetic test for patients with this condition, this last technique was implemented in our country a few years ago. We report on the results of the implementation of chromosomal microarrays in a cohort of 133 patients with intellectual disability and dysmorphic features, normal karyotype and normal subtelomeric MLPA results in an Argentinean public health institution. Clinically relevant copy number variants were found in 12% of the patients and one or more copy number variants classified as variants of uncertain significance were found in 5.3% of them. Although the diagnostic yield of chromosomal microarrays is greater than conventional cytogenetics for these patients, there are financial limitations to adopt this technique as a first-tier test in our country, especially in the public health system.

New drugs are not enough­drug repositioning in oncology: An update.

Drug repositioning refers to the concept of discovering novel clinical benefits of drugs that are already known for use treating other diseases. The advantages of this are that several important drug characteristics are already established (including efficacy, pharmacokinetics, pharmacodynamics and toxicity), making the process of research for a putative drug quicker and less costly. Drug repositioning in oncology has received extensive focus. The present review summarizes the most prominent examples of drug repositioning for the treatment of cancer, taking into consideration their primary use, proposed anticancer mechanisms and current development status.

A homozygous mutation in the highly conserved Tyr60 of the mature IGF1 peptide broadens the spectrum of IGF1 deficiency.

BACKGROUND: IGF1 is a key factor in fetal and postnatal growth. To date, only three homozygous IGF1 gene defects leading to complete or partial loss of IGF1 activity have been reported in three short patients born small for gestational age. We describe the fourth patient with severe short stature presenting a novel homozygous IGF1 gene mutation. RESULTS: We report a boy born from consanguineous parents at 40 weeks of gestational age with intrauterine growth restriction and severe postnatal growth failure. Physical examination revealed proportionate short stature, microcephaly, facial dysmorphism, bilateral sensorineural deafness and mild global developmental delay. Basal growth hormone (GH) fluctuated from 0.2 to 29 ng/mL, while IGF1 levels ranged from -1.15 to 2.95 SDS. IGFBP3 was normal-high. SNP array delimited chromosomal regions of homozygosity, including 12q23.2 where IGF1 is located. IGF1 screening by HRM revealed a homozygous missense variant NM_000618.4(IGF1):c.322T>C, p.(Tyr108His). The change of the highly conserved Tyr60 in the mature IGF1 peptide was consistently predicted as pathogenic by multiple bioinformatic tools. Tyr60 has been described to be critical for IGF1 interaction with type 1 IGF receptor (IGF1R). In vitro, HEK293T cells showed a marked reduction of IGF1R phosphorylation after stimulation with serum from the patient as compared to sera from age-matched controls. Mutant IGF1 was also less efficient in inducing cell growth. CONCLUSION: The present report broadens the spectrum of clinical and biochemical presentation of homozygous IGF1 defects and underscores the variability these patients may present depending on the IGF/IGF1R pathway activity.

Aplicación de Hibridación In Situ Fluorescente (FISH) en pacientes con sospecha clínica de Síndrome de Deleción 22q11. 2 (SD22q11. 2) / Applying Fluorescent in situ hy-bridiza on (FISH) in patients with suspected clinical signs of 22q11. 2 dele on syndrome (DS22q11. 2)

El SD22q11.2 está asociado a síndromes de DiGeorge, velocardiofacial, facioconotruncal y Cayler, reconocidos con la misma etiología: microdeleción 22q11.2. El fenotipo es variable y presenta cardiopatía conotruncal (CC), dismorfias faciales, anomalías palatinas, inmunodeficiencias y trastornos del neurodesarrollo. Las manifestaciones endocrinológicas que predominan son talla baja, hipocalcemia neonatal asociada a hipoparatiroidismo y disfunción tiroidea. El 90% de los afectados presenta una deleción típica de 3-Mb, mientras que el resto tiene deleciones de menor tamaño o deleción localizada más distal a la región crítica . El objetivo del trabajo es identificar en una cohorte de 63 pacientes con sospecha clínica de SD22q11.2, la presencia de la microdeleción 22q11.2 empleando como método diagnóstico la técnica de FISH y describir brevemente las características clínicas más prevalentes que presentan los pacientes con resultado de FISH positivo y negativo. La microdeleción 22q11.2 se identificó en el 38% (24/63) de los pacientes estudiados. Las características clínicas más prevalentes en este grupo fueron las cardiopatías congénitas conotruncales (95,6%), microcefalia (50%), inmunodeficiencias (50%), hipocalcemia (48,8%), anomalías del paladar (45,8%), retraso del desarrollo y déficit cognitivo (41,5%). En nuestro hospital, el algoritmo diagnóstico para la detección de la microdeleción 22q11.2 es el cariotipo de alta resolución y el estudio por la técnica de FISH.

DS22q11.2 is associated with a wide spectrum of clinical disorders (DiGeorge, velocardiofacial, facioconotrunal and Cayler syndromes) known to arise from the same etiology 22q11.2 microdeletion The phenotype is variable and includes conotruncal cardiac defect (CCD), facial phenotype, palate anomalies, inmunodeficiency and developmental disorders. The endocrine manifestations are short stature (ST), neonatal hypocalcemia due to hypoparathyroidism, and thyroid dysfunction. In 90% of patients with 22q11.1 deletion a common 3-Mb deletion has been found, whereas the rest of cases share a smaller deletion or more distal atypical deletions. The aim of the present study was to identify the 22q11.2 microdeletion by FISH in 63 patients from the Genetic and Endocrinology Division between 2002 and 2017 who had more than one clinical feature of DS22q11. 2. High resolution karyotype and fluorescent in situ hybridization (FISH) were performed with different commercial probes. The 22q11.2 microdeletion was demonstrated in 24/63 patients (38%). The more relevant clinical features in this group were: conotruncal cardiac defect (95.6%), microcephaly (50%), immunodeficiency (50%), hypocalcaemia (48.8%) palate anomalies (45.8%), development delay and cognitive deficit (41.5%). In our hospital, the diagnostic algorithm for the detection of the 22q11.2 microdeletion is the high resolution karyotype and the study by the FISH technique.

Telomeropathies: Etiology, diagnosis, treatment and follow-up. Ethical and legal considerations.

Telomeropathies involve a wide variety of infrequent genetic diseases caused by mutations in the telomerase maintenance mechanism or the DNA damage response (DDR) system. They are considered a family of rare diseases that often share causes, molecular mechanisms and symptoms. Generally, these diseases are not diagnosed until the symptoms are advanced, diminishing the survival time of patients. Although several related syndromes may still be unrecognized this work describes those that are known, highlighting that because they are rare diseases, physicians should be trained in their early diagnosis. The etiology and diagnosis are discussed for each telomeropathy and the treatments when available, along with a new classification of this group of diseases. Ethical and legal issues related to this group of diseases are also considered.

Homology Model and Docking-Based Virtual Screening for Ligands of Human Dyskerin as New Inhibitors of Telomerase for Cancer Treatment.

Immortality is one of the main features of cancer cells. Tumor cells have an unlimited replicative potential, principally due to the holoenzyme telomerase. Telomerase is composed mainly by dyskerin (DKC1), a catalytic retrotranscriptase (hTERT) and an RNA template (hTR). The aim of this work is to develop new inhibitors of telomerase, selecting the interaction between hTR⁻DKC1 as a target. We designed two models of the human protein DKC1: homology and ab initio. These models were evaluated by different procedures, revealing that the homology model parameters were the most accurate. We selected two hydrophobic pockets contained in the PUA (pseudouridine synthase and archaeosine transglycosylase) domain, using structural and stability analysis. We carried out a docking-based virtual screen on these pockets, using the reported mutation K314 as the center of the docking. The hDKC1 model was tested against a library of 450,000 drug-like molecules. We selected the first 10 molecules that showed the highest affinity values to test their inhibitory activity on the cell line MDA MB 231 (Monroe Dunaway Anderson Metastasis Breast cancer 231), obtaining three compounds that showed inhibitory effect. These results allowed us to validate our design and set the basis to continue with the study of telomerase inhibitors for cancer treatment.

AZT exerts its antitumoral effect by telomeric and non-telomeric effects in a mammary adenocarcinoma model.

Limitless replicative potential is one of the hallmarks of cancer that is mainly due to the activity of telomerase. This holoenzyme maintains telomere length, adding TTAGGG repetitions at the end of chromosomes in each cell division. In addition to this function, there are extratelomeric roles of telomerase that are involved in cancer promoting events. It has been demonstrated that TERT, the catalytic component of telomerase, acts as a transcriptional modulator in many signaling pathways. Taking into account this evidence and our experience on the study of azidothymidine (AZT) as an inhibitor of telomerase activity, the present study analyzes the effect of AZT on some telomeric and extratelomeric activities. To carry out the present study, we evaluated the transcription of genes that are modulated by the Wnt/ß-catenin pathway, such as c-Myc and cyclin-D1 (Cyc-D1) and cell processes related with their expression, such as, proliferation, modifications of the actin cytoskeleton, cell migration and cell cycle in a mammary carcinoma cell line (F3II). Results obtained after treatment with AZT (600 µM) for 15 passages confirmed the inhibitory effect on telomerase. Regarding extratelomeric activities, our results showed a decrease of 64, 38 and 25% in the transcription of c-Myc, Cyc-D1 and TERT, respectively (p<0.05) after AZT treatment. Furthermore, we found an effect on cell migration, reaching an inhibition of 48% (p<0.05) and a significant passage-dependent increase on cell doubling time during treatment. Finally, we evaluated the effect on cell cycle, obtaining a decline in G0/G1 in AZT-treated cells. These results allow us to postulate that AZT is not only an inhibitor of telomerase activity, but also a potential modulator of extratelomeric processes involved in cancer promotion.

[Telomerase and telomere: their structure and dynamics in health and disease]. / Telomerasa y telómero: su estructura y dinámica en salud y enfermedad.

Telomerase is the enzyme responsible for the maintenance of telomere length by adding guanine-rich repetitive sequences. Its activity can be seen in gametes, stem cells and tumor cells. In human somatic cells the proliferative potential is limited, reaching senescence after 50-70 cell divisions, because the DNA polymerase is not able to copy the DNA at the ends of chromosomes. By contrast, in most tumor cells the replicative potential is unlimited due to the maintenance of the telomeric length given by telomerase. Telomeres have additional proteins that regulate the binding of telomerase, likewise telomerase associates, with a protein complex that regulates its activity. This work focuses on the structure and function of the telomere/telomerase complex and how changes in its behavior lead to the development of different diseases, mainly cancer. Development of inhibitors of the telomere/telomerase complex could be a target with promising possibilities.

Telomerasa y telómero: su estructura y dinámica en salud y enfermedad / Telomerase and telomere: their structure and dynamics in health and disease

La telomerasa es la enzima responsable del mantenimiento de la longitud de los telómeros mediante la adición de secuencias repetitivas ricas en guanina, y su actividad se observa principalmente en gametos, células madre y células tumorales. En las células somáticas humanas el potencial de proliferación es limitado, alcanzando la senescencia luego de 50-70 divisiones celulares, debido a que la ADN polimerasa no es capaz de copiar el ADN en los extremos de los cromosomas. Por el contrario, en la mayoría de las células tumorales el potencial de replicación es ilimitado debido al mantenimiento de la longitud telomérica dado por la telomerasa. Los telómeros tienen proteínas adicionales que regulan la unión de la telomerasa. De la misma manera la telomerasa también se asocia con un complejo de proteínas que regulan su actividad. Este trabajo se centra en la estructura y función del complejo telómero/telomerasa y a cómo las alteraciones en su comportamiento conducen al desarrollo de diversas enfermedades, principalmente cáncer. El desarrollo de inhibidores del sistema telómero / telomerasa podría ser un blanco con posibilidades prometedoras.

Telomerase is the enzyme responsible for the maintenance of telomere length by adding guanine-rich repetitive sequences. Its activity can be seen in gametes, stem cells and tumor cells. In human somatic cells the proliferative potential is limited, reaching senescence after 50-70 cell divisions, because the DNA polymerase is not able to copy the DNA at the ends of chromosomes. By contrast, in most tumor cells the replicative potential is unlimited due to the maintenance of the telomeric length given by telomerase. Telomeres have additional proteins that regulate the binding of telomerase, likewise telomerase associates, with a protein complex that regulates its activity. This work focuses on the structure and function of the telomere/telomerase complex and how changes in its behavior lead to the development of different diseases, mainly cancer. Development of inhibitors of the telomere/telomerase complex could be a target with promising possibilities.

Telomerasa y telómero: su estructura y dinámica en salud y enfermedad / Telomerase and telomere: their structure and dynamics in health and disease

La telomerasa es la enzima responsable del mantenimiento de la longitud de los telómeros mediante la adición de secuencias repetitivas ricas en guanina, y su actividad se observa principalmente en gametos, células madre y células tumorales. En las células somáticas humanas el potencial de proliferación es limitado, alcanzando la senescencia luego de 50-70 divisiones celulares, debido a que la ADN polimerasa no es capaz de copiar el ADN en los extremos de los cromosomas. Por el contrario, en la mayoría de las células tumorales el potencial de replicación es ilimitado debido al mantenimiento de la longitud telomérica dado por la telomerasa. Los telómeros tienen proteínas adicionales que regulan la unión de la telomerasa. De la misma manera la telomerasa también se asocia con un complejo de proteínas que regulan su actividad. Este trabajo se centra en la estructura y función del complejo telómero/telomerasa y a cómo las alteraciones en su comportamiento conducen al desarrollo de diversas enfermedades, principalmente cáncer. El desarrollo de inhibidores del sistema telómero / telomerasa podría ser un blanco con posibilidades prometedoras.(AU)

Telomerase is the enzyme responsible for the maintenance of telomere length by adding guanine-rich repetitive sequences. Its activity can be seen in gametes, stem cells and tumor cells. In human somatic cells the proliferative potential is limited, reaching senescence after 50-70 cell divisions, because the DNA polymerase is not able to copy the DNA at the ends of chromosomes. By contrast, in most tumor cells the replicative potential is unlimited due to the maintenance of the telomeric length given by telomerase. Telomeres have additional proteins that regulate the binding of telomerase, likewise telomerase associates, with a protein complex that regulates its activity. This work focuses on the structure and function of the telomere/telomerase complex and how changes in its behavior lead to the development of different diseases, mainly cancer. Development of inhibitors of the telomere/telomerase complex could be a target with promising possibilities.(AU)

Telomerasa y telómero: su estructura y dinámica en salud y enfermedad. / [Telomerase and telomere: their structure and dynamics in health and disease].

Telomerase is the enzyme responsible for the maintenance of telomere length by adding guanine-rich repetitive sequences. Its activity can be seen in gametes, stem cells and tumor cells. In human somatic cells the proliferative potential is limited, reaching senescence after 50-70 cell divisions, because the DNA polymerase is not able to copy the DNA at the ends of chromosomes. By contrast, in most tumor cells the replicative potential is unlimited due to the maintenance of the telomeric length given by telomerase. Telomeres have additional proteins that regulate the binding of telomerase, likewise telomerase associates, with a protein complex that regulates its activity. This work focuses on the structure and function of the telomere/telomerase complex and how changes in its behavior lead to the development of different diseases, mainly cancer. Development of inhibitors of the telomere/telomerase complex could be a target with promising possibilities.

Telomere structure and telomerase in health and disease (review).

Telomerase is the enzyme responsible for maintenance of the length of telomeres by addition of guanine-rich repetitive sequences. Telomerase activity is exhibited in gametes and stem and tumor cells. In human somatic cells, proliferation potential is strictly limited and senescence follows approximately 50-70 cell divisions. In most tumor cells, on the contrary, replication potential is unlimited. The key role in this process of the system of the telomere length maintenance with involvement of telomerase is still poorly studied. Undoubtedly, DNA polymerase is not capable of completely copying DNA at the very ends of chromosomes; therefore, approximately 50 nucleotides are lost during each cell cycle, which results in gradual telomere length shortening. Critically short telomeres cause senescence, following crisis and cell death. However, in tumor cells the system of telomere length maintenance is activated. Much work has been done regarding the complex telomere/telomerase as a unique target, highly specific in cancer cells. Telomeres have additional proteins that regulate the binding of telomerase. Telomerase, also associates with a number of proteins forming the sheltering complex having a central role in telomerase activity. This review focuses on the structure and function of the telomere/telomerase complex and its altered behavior leading to disease, mainly cancer. Although telomerase therapeutics are not approved yet for clinical use, we can assume that based on the promising in vitro and in vivo results and successful clinical trials, it can be predicted that telomerase therapeutics will be utilized soon in the combat against malignancies and degenerative diseases. The active search for modulators is justified, because the telomere/telomerase system is an extremely promising target offering possibilities to decrease or increase the viability of the cell for therapeutic purposes.

AZT as a telomerase inhibitor.

Telomerase is a highly specialized reverse transcriptase (RT) and the maintenance of telomeric length is determined by this specific enzyme. The human holoenzyme telomerase is a ribonucleoprotein composed by a catalytic subunit, hTERT, an RNA component, hTR, and a group of associated proteins. Telomerase is normally expressed in embryonic cells and is repressed during adulthood. The enzyme is reexpressed in around 85% of solid tumors. This observation makes it a potential target for developing drugs that could be developed for therapeutic purposes. The identification of the hTERT as a functional catalytic RT prompted studies of inhibiting telomerase with the HIV RT inhibitor azidothymidine (AZT). Previously, we have demonstrated that AZT binds preferentially to telomeres, inhibits telomerase and enhances tumor cell senescence, and apoptosis after AZT treatment in breast mammary adenocarcinoma cells. Since then, several studies have considered AZT for telomerase inhibition and have led to potential clinical strategies for anticancer therapy. This review covers present thinking of the inhibition of telomerase by AZT and future treatment protocols using the drug.