ß-Cell and renal transplantation options for diabetes.

Despite major advances in structured education, insulin delivery and glucose monitoring, diabetes self-management remains an unremitting challenge. Insulin therapy is inextricably linked to risk of dangerous hypoglycaemia and sustained hyperglycaemia remains a leading cause of renal failure. This review sets out to demystify transplantation for diabetes multidisciplinary teams, facilitating consideration and incorporation within holistic overall person-centred management. Deceased and living donor kidney, whole pancreas and isolated islet transplant procedures, indications and potential benefits are described, in addition to outcomes within the integrated UK transplant programme.

Ral activation promotes melanomagenesis.

Up to one-third of human melanomas are characterized by an oncogenic mutation in the gene encoding the small guanosine triphosphatase (GTPase) NRAS. Ras proteins activate three primary classes of effectors, namely, Rafs, phosphatidyl-inositol-3-kinases (PI3Ks) and Ral guanine exchange factors (RalGEFs). In melanomas lacking NRAS mutations, the first two effectors can still be activated through an oncogenic BRAF mutation coupled with a loss of the PI3K negative regulator PTEN. This suggests that Ras effectors promote melanoma, regardless of whether they are activated by oncogenic NRas. The only major Ras effector pathway not explored for its role in melanoma is the RalGEF-Ral pathway, in which Ras activation of RalGEFs converts the small GTPases RalA and RalB to an active guanosine triphosphate-bound state. We report that RalA is activated in several human melanoma cancer cell lines harboring an oncogenic NRAS allele, an oncogenic BRAF allele or wild-type NRAS and BRAF alleles. Furthermore, short hairpin RNA (shRNA)-mediated knockdown of RalA, and to a lesser extent of RalB, variably inhibited the tumorigenic growth of melanoma cell lines having these three genotypes. Thus, as is the case for Raf and PI3 K signaling, Rals also contribute to melanoma tumorigenesis.

How safe is it to transplant organs from deceased donors with primary intracranial malignancy? An analysis of UK Registry data.

Patients dying from primary intracranial malignancy are a potential source of organs for transplantation. However, a perceived risk of tumor transfer to the organ recipient has limited their use. We evaluated the risk of tumor transmission by reviewing the incidence in patients transplanted in the UK. Information from the UK Transplant Registry was combined with that from the national cancer registries of England, Wales and Northern Ireland to identify all organ donors between 1985 and 2001 inclusive with a primary intracranial malignancy and to identify the occurrence of posttransplant malignancy in the recipients of the organs transplanted. Of 11,799 organ donors in the study period, 179 were identified as having had a primary intracranial malignancy, including 33 with high-grade malignancy (24 grade IV gliomas and 9 medulloblastomas). A total of 448 recipients of 495 organs from 177 of these donors were identified. No transmission of donor intracranial malignancy occurred. Organs from patients dying from primary intracranial malignancy, including those with high-grade tumors, should be considered for transplantation and the small risk of tumor transmission should be balanced against the likely mortality for potential recipients who remain on the transplant waiting list.

DNA-based animal models of human disease: from genotype to phenotype.

Biomedical research utilizes animal models to elucidate human disease processes at the cellular and molecular level and for the development of new therapies. Traditionally, mammalian models have been limited to the mouse, primarily because of well characterized genetic lines and the ability to manipulate the genome to directly test hypotheses regarding causal mutations and disease phenotypes. The emerging availability of genome sequences of other mammals (bovine, canine, equine, feline, and porcine) now permits utilization of the mammal in which the phenotype best approximates the human condition. Equally important is the use of somatic cell nuclear cloning (SCNT) coupled with targeted germline manipulation to create animals to resolve the molecular mechanisms of the disease state. Our efforts have focused on the pig, which has emerged as an important biomedical mammalian model due to its closer physiology to humans. The utility of porcine genetically-defined tumour, cardiovascular and neurological disease models is described.

Genetic induction of tumorigenesis in swine.

The transition from basic to clinical cancer research for a number of experimental therapeutics is hampered by the lack of a genetically malleable, large animal model. To this end, we genetically engineered primary porcine cells to be tumorigenic by expression of proteins known to perturb pathways commonly corrupted in human cancer. Akin to human cells, these porcine cells were quite resistant to transformation, requiring multiple genetic changes. Moreover, the transformed porcine cells produced tumors when returned to the isogenic host animal. The ability to now rapidly and reproducibly genetically induce tumors of sizes similar to those treated clinically in a large mammal similar to humans in many respects will provide a robust cancer model for preclinical studies dependent on generating large tumors.

N-terminal domains of the human telomerase catalytic subunit required for enzyme activity in vivo.

Most tumor cells depend upon activation of the ribonucleoprotein enzyme telomerase for telomere maintenance and continual proliferation. The catalytic activity of this enzyme can be reconstituted in vitro with the RNA (hTR) and catalytic (hTERT) subunits. However, catalytic activity alone is insufficient for the full in vivo function of the enzyme. In addition, the enzyme must localize to the nucleus, recognize chromosome ends, and orchestrate telomere elongation in a highly regulated fashion. To identify domains of hTERT involved in these biological functions, we introduced a panel of 90 N-terminal hTERT substitution mutants into telomerase-negative cells and assayed the resulting cells for catalytic activity and, as a marker of in vivo function, for cellular proliferation. We found four domains to be essential for in vitro and in vivo enzyme activity, two of which were required for hTR binding. These domains map to regions defined by sequence alignments and mutational analysis in yeast, indicating that the N terminus has also been functionally conserved throughout evolution. Additionally, we discovered a novel domain, DAT, that "dissociates activities of telomerase," where mutations left the enzyme catalytically active, but was unable to function in vivo. Since mutations in this domain had no measurable effect on hTERT homomultimerization, hTR binding, or nuclear targeting, we propose that this domain is involved in other aspects of in vivo telomere elongation. The discovery of these domains provides the first step in dissecting the biological functions of human telomerase, with the ultimate goal of targeting this enzyme for the treatment of human cancers.

Inhibition of telomerase is related to the life span and tumorigenicity of human prostate cancer cells.

PURPOSE: Telomerase, the enzyme that catalyzes the elongation of telomeres, is illegitimately activated in the majority of cancers, including that of the prostate, where it may greatly extend the life span of malignant cells. The inhibition of telomerase by molecular intervention has been shown to lead eventually to cell death in several tumor or in vitro immortalized cell lines and in 1 case prevent tumor growth in vivo. Therefore, we tested whether a similar strategy may be used to limit the tumorigenic potential of late stage prostate cancer cells. MATERIALS AND METHODS: PC-3, LNCaP and DU-145 human prostate cancer cells were infected with a retrovirus encoding a dominant-negative version of the catalytic subunit of telomerase (DN-hTERT). Subclones or polyclonal populations were assayed for DN-hTERT expression, telomerase activity, telomere length, cell life span and in most cases tumorigenicity in nude mice. RESULTS: DN-hTERT expression levels directly correlated with cell life span and tumorigenic growth. PC-3 cells expressing high levels of DN-hTERT died rapidly and failed to form tumors in nude mice, whereas cells expressing the lowest levels proliferated the longest and generated tumors that later spontaneously regressed. Similarly the inhibition of telomerase activity in LNCaP cells was greater than in DU-145 cells and correspondingly LNCaP cells had a shorter life span. CONCLUSIONS: DN-hTERT expression limits the life span and tumorigenic potential of human prostate cancer cells, although the onset of these effects appears to be dictated by the expression level of DN-hTERT. Therefore, telomerase represents an attractive target for potentially managing prostate cancer. Nevertheless, effective means of inhibiting the enzyme may be required for a therapeutically useful outcome.

A genetically tractable model of human glioma formation.

Gliomas remain one of the deadliest forms of cancer. Improved therapeutics will require a better understanding of the molecular nature of these tumors. We, therefore, mimicked the most common genetic changes found in grade III-IV gliomas, disruption of the p53 and RB pathways and activation of telomere maintenance and independence from growth factors, through the ectopic expression of the SV40 T/t-Ag oncogene, an oncogenic form of H-ras (H-ras(V12G)), and the human telomerase catalytic subunit hTERT in normal human astrocytes. The resulting cells displayed many of the hallmarks of grade III-IV gliomas, including greatly expanded life span and growth in soft agar and, most importantly, were tumorigenic with pathology consistent with grade III-IV neuroectodermal tumors in mice. This model system will, for the first time, allow the biological significance of selected genetic alterations to be studied in human gliomas.

The MEK pathway is required for stimulation of p21(WAF1/CIP1) by transforming growth factor-beta.

Transforming growth factor-beta (TGF-beta)can induce the cyclin-dependent kinase inhibitors p21 and p15 in a variety of cell types. We have shown previously that Smad3 is required for the growth inhibitory activity of TGF-beta, whereas overexpression of Smads is not sufficient to activate the expression of p21 in HaCaT cells. These data suggest that an additional signaling pathway may be involved in stimulating p21 in HaCaT cells. Given the recent finding that the mitogen-activated protein kinase (MAPK) pathway can cause p21 induction and arrest cells, we examined the involvement of this pathway for p21 and p15 induction by TGF-beta. We found that TGF-beta can regulate the MAPK pathway, leading to the increased transactivation ability of transcription factor Elk. Constitutively active components in the MAPK pathway activate p21 expression, and inhibitors or dominant negative constructs for the MAPK pathway significantly decrease p21 induction by TGF-beta. Both constitutively active MEK and inhibitors for MEK have no effect on Smad activity, including DNA binding, localization, and interaction with coactivator p300/CBP. These findings suggest that the MAPK pathway may be an independent pathway that is involved in p21 and p15 induction by TGF-beta.

Creation of human tumour cells with defined genetic elements.

During malignant transformation, cancer cells acquire genetic mutations that override the normal mechanisms controlling cellular proliferation. Primary rodent cells are efficiently converted into tumorigenic cells by the coexpression of cooperating oncogenes. However, similar experiments with human cells have consistently failed to yield tumorigenic transformants, indicating a fundamental difference in the biology of human and rodent cells. The few reported successes in the creation of human tumour cells have depended on the use of chemical or physical agents to achieve immortalization, the selection of rare, spontaneously arising immortalized cells, or the use of an entire viral genome. We show here that the ectopic expression of the telomerase catalytic subunit (hTERT) in combination with two oncogenes (the simian virus 40 large-T oncoprotein and an oncogenic allele of H-ras) results in direct tumorigenic conversion of normal human epithelial and fibroblast cells. These results demonstrate that disruption of the intracellular pathways regulated by large-T, oncogenic ras and telomerase suffices to create a human tumor cell.

Characterization of the repeat-tract instability and mutator phenotypes conferred by a Tn3 insertion in RFC1, the large subunit of the yeast clamp loader.

The RFC1 gene encodes the large subunit of the yeast clamp loader (RFC) that is a component of eukaryotic DNA polymerase holoenzymes. We identified a mutant allele of RFC1 (rfc1::Tn3) from a large collection of Saccharomyces cerevisiae mutants that were inviable when present in a rad52 null mutation background. Analysis of rfc1::Tn3 strains indicated that they displayed both a mutator and repeat-tract instability phenotype. Strains bearing this allele were characterized in combination with mismatch repair (msh2Delta, pms1Delta), double-strand break repair (rad52), and DNA replication (pol3-01, pol30-52, rth1Delta/rad27Delta) mutations in both forward mutation and repeat-tract instability assays. This analysis indicated that the rfc1::Tn3 allele displays synthetic lethality with pol30, pol3, and rad27 mutations. Measurement of forward mutation frequencies in msh2Delta rfc1:Tn3 and pms1Delta rfc1:Tn3 strains indicated that the rfc1::Tn3 mutant displayed a mutation frequency that appeared nearly multiplicative with the mutation frequency exhibited by mismatch-repair mutants. In repeat-tract instability assays, however, the rfc1::Tn3 mutant displayed a tract instability phenotype that appeared epistatic to the phenotype displayed by mismatch-repair mutants. From these data we propose that defects in clamp loader function result in DNA replication errors, a subset of which are acted upon by the mismatch-repair system.

Dissociation among in vitro telomerase activity, telomere maintenance, and cellular immortalization.

The immortalization of human cells is a critical step during tumorigenesis. In vitro, normal human somatic cells must overcome two proliferative blockades, senescence and crisis, to become immortal. Transformation with viral oncogenes extends the life span of human cells beyond senescence. Such transformed cells eventually succumb to crisis, a period of widespread cellular death that has been proposed to be the result of telomeric shortening. We now show that ectopic expression of the telomerase catalytic subunit (human telomerase reverse transcriptase or hTERT) and subsequent activation of telomerase can allow postsenescent cells to proliferate beyond crisis, the last known proliferative blockade to cellular immortality. Moreover, we demonstrate that alteration of the carboxyl terminus of human telomerase reverse transcriptase does not affect telomerase enzymatic activity but impedes the ability of this enzyme to maintain telomeres. Telomerase-positive cells expressing this mutant enzyme fail to undergo immortalization, further tightening the connection between telomere maintenance and immortalization.

Expression of TERT in early premalignant lesions and a subset of cells in normal tissues.

Activation of telomerase, the enzyme that synthesizes the telomere ends of linear chromosomes, has been implicated in human cell immortalization and cancer cell pathogenesis. Enzyme activity is undetectable in most normal cells and tissues, but present in immortal cells and cancer tissues. While expression of TERC, the RNA component of telomerase, is widespread, the restricted expression pattern of TERT, the telomerase catalytic subunit gene, is correlated with telomerase activity, and its ectopic expression in telomerase-negative cells is sufficient to reconstitute telomerase activity and extend cellular lifespan. We have used in situ hybridization to study TERT expression at the single-cell level in normal tissues and in various stages of tumour progression. In normal tissues, including some that are known to be telomerase-negative, TERT mRNA was present in specific subsets of cells thought to have long-term proliferative capacity. This included mitotically inactive breast lobular epithelium in addition to some actively regenerating cells such as the stratum basale of the skin. TERT expression appeared early during tumorigenesis in vivo, beginning with early pre-invasive changes in human breast and colon tissues and increasing gradually during progression, both in the amount of TERT mRNA present within individual cells and in the number of expressing cells within a neoplastic lesion. The physiological expression of TERT within normal epithelial cells that retain proliferative potential and its presence at the earliest stages of tumorigenesis have implications for the regulation of telomerase expression and for the identification of cells that may be targets for malignant transformation.

Telomerase activity is restored in human cells by ectopic expression of hTERT (hEST2), the catalytic subunit of telomerase.

The expression of telomerase, the enzyme that synthesizes telomeric DNA de novo, is suppressed in normal somatic human cells but is reactivated during tumorigenesis. This reactivation appears to arrest the normal loss of telomeric DNA incurred as human cells divide. Since continual loss of telomeric DNA is predicted to eventually limit cell proliferation, activation of telomerase in cancer cells may represent an important step in the acquisition of the cell immortalization which occurs during tumor progression. The telomerase holoenzyme is composed of both RNA and protein subunits. In humans, mRNA expression of hTERT (hEST2), the candidate telomerase catalytic subunit gene, appears to parallel the levels of telomerase enzyme activity, suggesting that induction of hTERT is necessary and perhaps sufficient for expression of telomerase activity in tumor cells. To test this model directly, we ectopically expressed an epitope-tagged version of hTERT in telomerase-negative cells and show that telomerase activity was induced to levels comparable to those seen in immortal telomerase-positive cells and that the expressed hTERT protein was physically associated with the cellular telomerase activity. We conclude that synthesis of the hTERT telomerase subunit represents the rate-limiting determinant of telomerase activity in these cells and that this protein, once expressed, becomes part of the functional telomerase holoenzyme.

The catalytic subunit of yeast telomerase.

Telomerase is an RNA-directed DNA polymerase, composed of RNA and protein subunits, that replicates the telomere ends of linear eukaryotic chromosomes. Using a genetic strategy described here, we identify the product of the EST2 gene, Est2p, as a subunit of telomerase in the yeast Saccharomyces cerevisiae. Est2p is required for enzyme catalysis, as mutations in EST2 were found to result in the absence of telomerase activity. Immunochemical experiments show that Est2p is an integral subunit of the telomerase enzyme. Critical catalytic residues present in RNA-directed DNA polymerases are conserved in Est2p; mutation of one such residue abolishes telomerase activity, suggesting a direct catalytic role for Est2p.

hEST2, the putative human telomerase catalytic subunit gene, is up-regulated in tumor cells and during immortalization.

Telomerase, the ribonucleoprotein enzyme that elongates telomeres, is repressed in normal human somatic cells but is reactivated during tumor progression. We report the cloning of a human gene, hEST2, that shares significant sequence similarity with the telomerase catalytic subunit genes of lower eukaryotes. hEST2 is expressed at high levels in primary tumors, cancer cell lines, and telomerase-positive tissues but is undetectable in telomerase-negative cell lines and differentiated telomerase-negative tissues. Moreover, the message is up-regulated concomitant with the activation of telomerase during the immortalization of cultured cells and down-regulated during in vitro cellular differentiation. Taken together, these observations suggest that the induction of hEST2 mRNA expression is required for the telomerase activation that occurs during cellular immortalization and tumor progression.

The roles of telomeres and telomerase in cell life span.

Telomeres cap and protect the ends of chromosomes from degradation and illegitimate recombination. The termini of a linear template cannot, however, be completely replicated by conventional DNA-dependent DNA polymerases, and thus in the absence of a mechanisms to counter this effect, telomeres of eukaryotic cells shorten every round of DNA replication. In humans and possibly other higher eukaryotes, telomere shortening may have been adopted to limit the life span of somatic cells. Human somatic cells have a finite proliferative capacity and enter a viable growth arrested state called senescence. Life span appears to be governed by cell division, not time. The regular loss of telomeric DNA could therefore serve as a mitotic clock in the senescence programme, counting cell divisions. In most eukaryotic organisms, however, telomere shortening can be countered by the de novo addition of telomeric repeats by the enzyme telomerase. Cells which are "immortal' such as the human germ line or tumour cell lines, established mouse cells, yeast and ciliates, all maintain a stable telomere length through the action of telomerase. Abolition of telomerase activity in such cells nevertheless results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Therefore, loss of terminal DNA sequences may limit cell life span by two mechanisms: by acting as a mitotic clock and by denuding chromosomes of protective telomeric DNA necessary for cell viability.

Telomerase activity in normal leukocytes and in hematologic malignancies.

Telomeres are essential for function and stability of eukaryotic chromosomes. In the absence of telomerase, the enzyme that synthesizes telomeric DNA, telomeres shorten with cell division, a process thought to contribute to cell senescence and the proliferative crisis of transformed cells. We reported telomere stabilization concomitant with detection of telomerase activity in cells immortalized in vitro and in ovarian carcinoma cells, and suggested that telomerase is essential for unlimited cell proliferation. We have now examined the temporal pattern of telomerase expression in selected hematologic malignancies. We found that, unlike other somatic tissues, peripheral, cord blood, and bone marrow leukocytes from normal donors expressed low levels of telomerase activity. In leukocytes from chronic lymphocytic leukemia (CLL) patients, activity was lower than in controls in early disease, and comparable with controls in late disease. Relative to bone marrow, telomerase activity was enhanced in myelodysplastic syndrome (MDS) and more significantly so in acute myeloid leukemia (AML). Regardless of telomerase levels, telomeres shortened with progression of the diseases. Our results suggest that early CLL and MDS cells lack an efficient mechanism of telomere maintenance and that telomerase is activated late in the progression of these cancers, presumably when critical telomere loss generates selective pressure for cell immortality.

Telomeres and telomerase in human cancer (review).

Telomerase has recently come into the limelight as one of the most prevalent tumour markers, due to its nearly ubiquitous presence in malignant tissues and absence from most somatic tissues. The essential role of telomeres in unlimited cell proliferation and that of the enzyme in telomere maintenance have suggested that telomerase inhibitors may be effective in cancer therapy. We provide here a compendium and an evaluation of the available data relating to this hypothesis.

Stabilization of short telomeres and telomerase activity accompany immortalization of Epstein-Barr virus-transformed human B lymphocytes.

We have measured telomere length and telomerase activity throughout the life span of clones of human B lymphocytes transformed by Epstein-Barr virus. Shortening of telomeres occurred at similar rates in all populations and persisted until chromosomes had little telomeric DNA remaining. At this stage, some of the clones entered a proliferative crisis and died. Only clones in which telomeres were stabilized, apparently by activation of telomerase, continued to proliferate indefinitely, i.e., became immortal. Since loss of telomeres impairs chromosome function, and may thus affect cell survival, we propose that telomerase activity is required for immortality. We have now detected this enzyme in a variety of immortal human cells transformed by different viruses, indicating that telomerase activation may be a common step in immortalization.