Functional characterization of miR-708 microRNA in telomerase positive and negative human cancer cells.

Activation of a telomere length maintenance mechanism (TMM), including telomerase and alternative lengthening of telomeres (ALT), is essential for replicative immortality of tumor cells, although its regulatory mechanisms are incompletely understood. We conducted a microRNA (miRNA) microarray analysis on isogenic telomerase positive (TEP) and ALT cancer cell lines. Amongst nine miRNAs that showed difference in their expression in TEP and ALT cancer cells in array analysis, miR-708 was selected for further analysis since it was consistently highly expressed in a large panel of ALT cells. miR-708 in TEP and ALT cancer cells was not correlated with C-circle levels, an established feature of ALT cells. Its overexpression induced suppression of cell migration, invasion, and angiogenesis in both TEP and ALT cells, although cell proliferation was inhibited only in TEP cells suggesting that ALT cells may have acquired the ability to escape inhibition of cell proliferation by sustained miR-708 overexpression. Further, cell proliferation regulation in TEP cells by miR708 appears to be through the CARF-p53 pathway. We demonstrate here that miR-708 (i) is the first miRNA shown to be differentially regulated in TEP and ALT cancer cells, (ii) possesses tumor suppressor function, and (iii) deregulates CARF and p21WAF1-mediated signaling to limit proliferation in TEP cells.

Heterozygous germline BLM mutations increase susceptibility to asbestos and mesothelioma.

Rare biallelic BLM gene mutations cause Bloom syndrome. Whether BLM heterozygous germline mutations (BLM+/-) cause human cancer remains unclear. We sequenced the germline DNA of 155 mesothelioma patients (33 familial and 122 sporadic). We found 2 deleterious germline BLM+/- mutations within 2 of 33 families with multiple cases of mesothelioma, one from Turkey (c.569_570del; p.R191Kfs*4) and one from the United States (c.968A>G; p.K323R). Some of the relatives who inherited these mutations developed mesothelioma, while none with nonmutated BLM were affected. Furthermore, among 122 patients with sporadic mesothelioma treated at the US National Cancer Institute, 5 carried pathogenic germline BLM+/- mutations. Therefore, 7 of 155 apparently unrelated mesothelioma patients carried BLM+/- mutations, significantly higher (P = 6.7E-10) than the expected frequency in a general, unrelated population from the gnomAD database, and 2 of 7 carried the same missense pathogenic mutation c.968A>G (P = 0.0017 given a 0.00039 allele frequency). Experiments in primary mesothelial cells from Blm+/- mice and in primary human mesothelial cells in which we silenced BLM revealed that reduced BLM levels promote genomic instability while protecting from cell death and promoted TNF-α release. Blm+/- mice injected intraperitoneally with asbestos had higher levels of proinflammatory M1 macrophages and of TNF-α, IL-1ß, IL-3, IL-10, and IL-12 in the peritoneal lavage, findings linked to asbestos carcinogenesis. Blm+/- mice exposed to asbestos had a significantly shorter survival and higher incidence of mesothelioma compared to controls. We propose that germline BLM+/- mutations increase the susceptibility to asbestos carcinogenesis, enhancing the risk of developing mesothelioma.

Tumour predisposition and cancer syndromes as models to study gene-environment interactions.

Cell division and organismal development are exquisitely orchestrated and regulated processes. The dysregulation of the molecular mechanisms underlying these processes may cause cancer, a consequence of cell-intrinsic and/or cell-extrinsic events. Cellular DNA can be damaged by spontaneous hydrolysis, reactive oxygen species, aberrant cellular metabolism or other perturbations that cause DNA damage. Moreover, several environmental factors may damage the DNA, alter cellular metabolism or affect the ability of cells to interact with their microenvironment. While some environmental factors are well established as carcinogens, there remains a large knowledge gap of others owing to the difficulty in identifying them because of the typically long interval between carcinogen exposure and cancer diagnosis. DNA damage increases in cells harbouring mutations that impair their ability to correctly repair the DNA. Tumour predisposition syndromes in which cancers arise at an accelerated rate and in different organs - the equivalent of a sensitized background - provide a unique opportunity to examine how gene-environment interactions influence cancer risk when the initiating genetic defect responsible for malignancy is known. Understanding the molecular processes that are altered by specific germline mutations, environmental exposures and related mechanisms that promote cancer will allow the design of novel and effective preventive and therapeutic strategies.

Chromatin-associated APC regulates gene expression in collaboration with canonical WNT signaling and AP-1.

Mutation of the APC gene occurs in a high percentage of colorectal tumors and is a central event driving tumor initiation in the large intestine. The APC protein performs multiple tumor suppressor functions including negative regulation of the canonical WNT signaling pathway by both cytoplasmic and nuclear mechanisms. Published reports that APC interacts with ß-catenin in the chromatin fraction to repress WNT-activated targets have raised the possibility that chromatin-associated APC participates more broadly in mechanisms of transcriptional control. This screening study has used chromatin immunoprecipitation and next-generation sequencing to identify APC-associated genomic regions in colon cancer cell lines. Initial target selection was performed by comparison and statistical analysis of 3,985 genomic regions associated with the APC protein to whole transcriptome sequencing data from APC-deficient and APC-wild-type colon cancer cells, and two types of murine colon adenomas characterized by activated Wnt signaling. 289 transcripts altered in expression following APC loss in human cells were linked to APC-associated genomic regions. High-confidence targets additionally validated in mouse adenomas included 16 increased and 9 decreased in expression following APC loss, indicating that chromatin-associated APC may antagonize canonical WNT signaling at both WNT-activated and WNT-repressed targets. Motif analysis and comparison to ChIP-seq datasets for other transcription factors identified a prevalence of binding sites for the TCF7L2 and AP-1 transcription factors in APC-associated genomic regions. Our results indicate that canonical WNT signaling can collaborate with or antagonize the AP-1 transcription factor to fine-tune the expression of shared target genes in the colorectal epithelium. Future therapeutic strategies for APC-deficient colorectal cancers might be expanded to include agents targeting the AP-1 pathway.

A tri-serine cluster within the topoisomerase IIα-interaction domain of the BLM helicase is required for regulating chromosome breakage in human cells.

The recQ-like helicase BLM interacts directly with topoisomerase IIα to regulate chromosome breakage in human cells. We demonstrate that a phosphosite tri-serine cluster (S577/S579/S580) within the BLM topoisomerase IIα-interaction region is required for this function. Enzymatic activities of BLM and topoisomerase IIα are reciprocally stimulated in vitro by ten-fold for topoisomerase IIα decatenation/relaxation activity and three-fold for BLM unwinding of forked DNA duplex substrates. A BLM transgene encoding alanine substitutions of the tri-serine cluster in BLM-/- transfected cells increases micronuclei, DNA double strand breaks and anaphase ultra-fine bridges (UFBs), and decreases cellular co-localization of BLM with topoisomerase IIα. In vitro, these substitutions significantly reduce the topoisomerase IIα-mediated stimulation of BLM unwinding of forked DNA duplexes. Substitution of the tri-serine cluster with aspartic acids to mimic serine phosphorylation reverses these effects in vitro and in vivo. Our findings implicate the modification of this BLM tri-serine cluster in regulating chromosomal stability.

Functions of the APC tumor suppressor protein dependent and independent of canonical WNT signaling: implications for therapeutic targeting.

The acquisition of biallelic mutations in the APC gene is a rate-limiting step in the development of most colorectal cancers and occurs in the earliest lesions. APC encodes a 312-kDa protein that localizes to multiple subcellular compartments and performs diverse functions. APC participates in a cytoplasmic complex that promotes the destruction of the transcriptional licensing factor ß-catenin; APC mutations that abolish this function trigger constitutive activation of the canonical WNT signaling pathway, a characteristic found in almost all colorectal cancers. By negatively regulating canonical WNT signaling, APC counteracts proliferation, promotes differentiation, facilitates apoptosis, and suppresses invasion and tumor progression. APC further antagonizes canonical WNT signaling by interacting with and counteracting ß-catenin in the nucleus. APC also suppresses tumor initiation and progression in the colorectal epithelium through functions that are independent of canonical WNT signaling. APC regulates the mitotic spindle to facilitate proper chromosome segregation, localizes to the cell periphery and cell protrusions to establish cell polarity and appropriate directional migration, and inhibits DNA replication by interacting directly with DNA. Mutations in APC are often frameshifts, insertions, or deletions that introduce premature stop codons and lead to the production of truncated APC proteins that lack its normal functions and possess tumorigenic properties. Therapeutic approaches in development for the treatment of APC-deficient tumors are focused on the inhibition of canonical WNT signaling, especially through targets downstream of APC in the pathway, or on the restoration of wild-type APC expression.

Mutational Mechanisms That Activate Wnt Signaling and Predict Outcomes in Colorectal Cancer Patients.

APC biallelic loss-of-function mutations are the most prevalent genetic changes in colorectal tumors, but it is unknown whether these mutations phenocopy gain-of-function mutations in the CTNNB1 gene encoding ß-catenin that also activate canonical WNT signaling. Here we demonstrate that these two mutational mechanisms are not equivalent. Furthermore, we show how differences in gene expression produced by these different mechanisms can stratify outcomes in more advanced human colorectal cancers. Gene expression profiling in Apc-mutant and Ctnnb1-mutant mouse colon adenomas identified candidate genes for subsequent evaluation of human TCGA (The Cancer Genome Atlas) data for colorectal cancer outcomes. Transcriptional patterns exhibited evidence of activated canonical Wnt signaling in both types of adenomas, with Apc-mutant adenomas also exhibiting unique changes in pathways related to proliferation, cytoskeletal organization, and apoptosis. Apc-mutant adenomas were characterized by increased expression of the glial nexin Serpine2, the human ortholog, which was increased in advanced human colorectal tumors. Our results support the hypothesis that APC-mutant colorectal tumors are transcriptionally distinct from APC-wild-type colorectal tumors with canonical WNT signaling activated by other mechanisms, with possible implications for stratification and prognosis.Significance: These findings suggest that colon adenomas driven by APC mutations are distinct from those driven by WNT gain-of-function mutations, with implications for identifying at-risk patients with advanced disease based on gene expression patterns. Cancer Res; 78(3); 617-30. ©2017 AACR.

Differential requirements for DNA repair proteins in immortalized cell lines using alternative lengthening of telomere mechanisms.

Cancer cells require telomere maintenance to enable uncontrolled growth. Most often telomerase is activated, although a subset of human cancers are telomerase-negative and depend on recombination-based mechanisms known as ALT (Alternative Lengthening of Telomeres). ALT depends on proteins that are essential for homologous recombination, including BLM and the MRN complex, to extend telomeres. This study surveyed the requirement for requisite homologous recombination proteins, yet to be studied in human ALT cell lines, by protein depletion using RNA interference. Effects on ALT were evaluated by measuring C-circle abundance, a marker of ALT. Surprisingly, several proteins essential for homologous recombination, BARD1, BRCA2, and WRN, were dispensable for C-circle production, while PALB2 had varying effects on C-circles among ALT cell lines. Depletion of homologous recombination proteins BRCA1 and BLM, which have been previously studied in ALT, decreased C-circles in all ALT cell lines. Depletion of the non-homologous end joining proteins 53BP1 and LIG4 had no effect on C-circles in any ALT cell line. Proteins such as chromatin modifiers that recruit double-strand break proteins, RNF8 and RNF168, and other proteins loosely grouped into excision DNA repair processes, XPA, MSH2, and MPG, reduced C-circles in some ALT cell lines. MSH2 depletion also reduced recombination at telomeres as measured by intertelomeric exchanges. Collectively, the requirement for DNA repair proteins varied between the ALT cell lines compared. In sum, our study suggests that ALT proceeds by multiple mechanisms that differ between cell lines and that some of these depend on DNA repair proteins not associated with homologous recombination pathways.

Identification of endometrial cancer methylation features using combined methylation analysis methods.

BACKGROUND: DNA methylation is a stable epigenetic mark that is frequently altered in tumors. DNA methylation features are attractive biomarkers for disease states given the stability of DNA methylation in living cells and in biologic specimens typically available for analysis. Widespread accumulation of methylation in regulatory elements in some cancers (specifically the CpG island methylator phenotype, CIMP) can play an important role in tumorigenesis. High resolution assessment of CIMP for the entire genome, however, remains cost prohibitive and requires quantities of DNA not available for many tissue samples of interest. Genome-wide scans of methylation have been undertaken for large numbers of tumors, and higher resolution analyses for a limited number of cancer specimens. Methods for analyzing such large datasets and integrating findings from different studies continue to evolve. An approach for comparison of findings from a genome-wide assessment of the methylated component of tumor DNA and more widely applied methylation scans was developed. METHODS: Methylomes for 76 primary endometrial cancer and 12 normal endometrial samples were generated using methylated fragment capture and second generation sequencing, MethylCap-seq. Publically available Infinium HumanMethylation 450 data from The Cancer Genome Atlas (TCGA) were compared to MethylCap-seq data. RESULTS: Analysis of methylation in promoter CpG islands (CGIs) identified a subset of tumors with a methylator phenotype. We used a two-stage approach to develop a 13-region methylation signature associated with a "hypermethylator state." High level methylation for the 13-region methylation signatures was associated with mismatch repair deficiency, high mutation rate, and low somatic copy number alteration in the TCGA test set. In addition, the signature devised showed good agreement with previously described methylation clusters devised by TCGA. CONCLUSION: We identified a methylation signature for a "hypermethylator phenotype" in endometrial cancer and developed methods that may prove useful for identifying extreme methylation phenotypes in other cancers.

Regulation of BLM Nucleolar Localization.

Defects in coordinated ribosomal RNA (rRNA) transcription in the nucleolus cause cellular and organismal growth deficiencies. Bloom's syndrome, an autosomal recessive human disorder caused by mutated recQ-like helicase BLM, presents with growth defects suggestive of underlying defects in rRNA transcription. Our previous studies showed that BLM facilitates rRNA transcription and interacts with RNA polymerase I and topoisomerase I (TOP1) in the nucleolus. The mechanisms regulating localization of BLM to the nucleolus are unknown. In this study, we identify the TOP1-interaction region of BLM by co-immunoprecipitation of in vitro transcribed and translated BLM segments and show that this region includes the highly conserved nuclear localization sequence (NLS) of BLM. Biochemical and nucleolar co-localization studies using site-specific mutants show that two serines within the NLS (S1342 and S1345) are critical for nucleolar localization of BLM but do not affect the functional interaction of BLM with TOP1. Mutagenesis of both serines to aspartic acid (phospho-mimetic), but not alanine (phospho-dead), results in approximately 80% reduction in nucleolar localization of BLM while retaining the biochemical functions and nuclear localization of BLM. Our studies suggest a role for this region in regulating nucleolar localization of BLM via modification of the two serines within the NLS.

Manipulation of DNA Repair Proficiency in Mouse Models of Colorectal Cancer.

Technical and biological innovations have enabled the development of more sophisticated and focused murine models that increasingly recapitulate the complex pathologies of human diseases, in particular cancer. Mouse models provide excellent in vivo systems for deciphering the intricacies of cancer biology within the context of precise experimental settings. They present biologically relevant, adaptable platforms that are amenable to continual improvement and refinement. We discuss how recent advances in our understanding of tumorigenesis and the underlying deficiencies of DNA repair mechanisms that drive it have been informed by using genetically engineered mice to create defined, well-characterized models of human colorectal cancer. In particular, we focus on how mechanisms of DNA repair can be manipulated precisely to create in vivo models whereby the underlying processes of tumorigenesis are accelerated or attenuated, dependent on the composite alleles carried by the mouse model. Such models have evolved to the stage where they now reflect the initiation and progression of sporadic cancers. The review is focused on mouse models of colorectal cancer and how insights from these models have been instrumental in shaping our understanding of the processes and potential therapies for this disease.

Loss-of-function screening to identify miRNAs involved in senescence: tumor suppressor activity of miRNA-335 and its new target CARF.

Significance of microRNAs (miRs), small non-coding molecules, has been implicated in a variety of biological processes. Here, we recruited retroviral insertional mutagenesis to obtain induction of an arbitrary noncoding RNAs, and coupled it with a cell based loss-of-function (5-Aza-2'-deoxycytidine (5Aza-dC)-induced senescence bypass) screening system. Cells that escaped 5-Aza-dC-induced senescence were subjected to miR-microarray analysis with respect to the untreated control. We identified miR-335 as one of the upregulated miRs. In order to characterize the functional significance, we overexpressed miR-335 in human cancer cells and found that it caused growth suppression. We demonstrate that the latter accounted for inhibition of 5-Aza-dC incorporation into the cell genome, enabling them to escape from induction of senescence. We also report that CARF (Collaborator of ARF) is a new target of miR-335 that regulates its growth suppressor function by complex crosstalk with other proteins including p16(INK4A), pRB, HDM2 and p21(WAF1).

Genetic Manipulation of Homologous Recombination In Vivo Attenuates Intestinal Tumorigenesis.

Although disruption of DNA repair capacity is unquestionably associated with cancer susceptibility in humans and model organisms, it remains unclear if the inherent tumor phenotypes of DNA repair deficiency syndromes can be regulated by manipulating DNA repair pathways. Loss-of-function mutations in BLM, a member of the RecQ helicase family, cause Bloom's syndrome (BS), a rare, recessive genetic disorder that predisposes to many types of cancer. BLM functions in many aspects of DNA homeostasis, including the suppression of homologous recombination (HR) in somatic cells. We investigated whether BLM overexpression, in contrast with loss-of-function mutations, attenuated the intestinal tumor phenotypes of Apc(Min/+) and Apc(Min/+);Msh2(-/-) mice, animal models of familial adenomatous polyposis coli (FAP). We constructed a transgenic mouse line expressing human BLM (BLM-Tg) and crossed it onto both backgrounds. BLM-Tg decreased adenoma incidence in a dose-dependent manner in our Apc(Min/) (+) model of FAP, although levels of GIN were unaffected and concomitantly increased animal survival over 50%. It did not reduce intestinal tumorigenesis in Apc(Min/) (+);Msh2(-/-) mice. We used the pink-eyed unstable (p(un)) mouse model to demonstrate that increasing BLM dosage in vivo lowered endogenous levels of HR by 2-fold. Our data suggest that attenuation of the Min phenotype is achieved through a direct effect of BLM-Tg on the HR repair pathway. These findings demonstrate that HR can be manipulated in vivo to modulate tumor formation at the organismal level. Our data suggest that lowering HR frequencies may have positive therapeutic outcomes in the context of specific hereditary cancer predisposition syndromes, exemplified by FAP.

Association of BLM and BRCA1 during Telomere Maintenance in ALT Cells.

Fifteen percent of tumors utilize recombination-based alternative lengthening of telomeres (ALT) to maintain telomeres. The mechanisms underlying ALT are unclear but involve several proteins involved in homologous recombination including the BLM helicase, mutated in Bloom's syndrome, and the BRCA1 tumor suppressor. Cells deficient in either BLM or BRCA1 have phenotypes consistent with telomere dysfunction. Although BLM associates with numerous DNA damage repair proteins including BRCA1 during DNA repair, the functional consequences of BLM-BRCA1 association in telomere maintenance are not completely understood. Our earlier work showed the involvement of BRCA1 in different mechanisms of ALT, and telomere shortening upon loss of BLM in ALT cells. In order to delineate their roles in telomere maintenance, we studied their association in telomere metabolism in cells using ALT. This work shows that BLM and BRCA1 co-localize with RAD50 at telomeres during S- and G2-phases of the cell cycle in immortalized human cells using ALT but not in cells using telomerase to maintain telomeres. Co-immunoprecipitation of BRCA1 and BLM is enhanced in ALT cells at G2. Furthermore, BRCA1 and BLM interact with RAD50 predominantly in S- and G2-phases, respectively. Biochemical assays demonstrate that full-length BRCA1 increases the unwinding rate of BLM three-fold in assays using a DNA substrate that models a forked structure composed of telomeric repeats. Our results suggest that BRCA1 participates in ALT through its interactions with RAD50 and BLM.

MicroRNA-135b promotes cancer progression by acting as a downstream effector of oncogenic pathways in colon cancer.

MicroRNA deregulation is frequent in human colorectal cancers (CRCs), but little is known as to whether it represents a bystander event or actually drives tumor progression in vivo. We show that miR-135b overexpression is triggered in mice and humans by APC loss, PTEN/PI3K pathway deregulation, and SRC overexpression and promotes tumor transformation and progression. We show that miR-135b upregulation is common in sporadic and inflammatory bowel disease-associated human CRCs and correlates with tumor stage and poor clinical outcome. Inhibition of miR-135b in CRC mouse models reduces tumor growth by controlling genes involved in proliferation, invasion, and apoptosis. We identify miR-135b as a key downsteam effector of oncogenic pathways and a potential target for CRC treatment.

WRN loss induces switching of telomerase-independent mechanisms of telomere elongation.

Telomere maintenance can occur in the presence of telomerase or in its absence, termed alternative lengthening of telomeres (ALT). ALT adds telomere repeats using recombination-based processes and DNA repair proteins that function in homologous recombination. Our previous work reported that the RecQ-like BLM helicase is required for ALT and that it unwinds telomeric substrates in vitro. WRN is also a RecQ-like helicase that shares many biochemical functions with BLM. WRN interacts with BLM, unwinds telomeric substrates, and co-localizes to ALT-associated PML bodies (APBs), suggesting that it may also be required for ALT processes. Using long-term siRNA knockdown of WRN in three ALT cell lines, we show that some, but not all, cell lines require WRN for telomere maintenance. VA-13 cells require WRN to prevent telomere loss and for the formation of APBs; Saos-2 cells do not. A third ALT cell line, U-2 OS, requires WRN for APB formation, however WRN loss results in p53-mediated apoptosis. In the absence of WRN and p53, U-2 OS cells undergo telomere loss for an intermediate number of population doublings (50-70), at which point they maintain telomere length even with the continued loss of WRN. WRN and the tumor suppressor BRCA1 co-localize to APBs in VA-13 and U-2 OS, but not in Saos-2 cells. WRN loss in U-2 OS is associated with a loss of BRCA1 from APBs. While the loss of WRN significantly increases telomere sister chromatid exchanges (T-SCE) in these three ALT cell lines, loss of both BRCA1 and WRN does not significantly alter T-SCE. This work demonstrates that ALT cell lines use different telomerase-independent maintenance mechanisms that variably require the WRN helicase and that some cells can switch from one mechanism to another that permits telomere elongation in the absence of WRN. Our data suggest that BRCA1 localization may define these mechanisms.

The HsRAD51B-HsRAD51C stabilizes the HsRAD51 nucleoprotein filament.

There are six human RAD51 related proteins (HsRAD51 paralogs), HsRAD51B, HsRAD51C, HsRAD51D, HsXRCC2, HsXRCC3 and HsDMC1, that appear to enhance HsRAD51 mediated homologous recombinational (HR) repair of DNA double strand breaks (DSBs). Here we model the structures of HsRAD51, HsRAD51B and HsRAD51C and show similar domain orientations within a hypothetical nucleoprotein filament (NPF). We then demonstrate that HsRAD51B-HsRAD51C heterodimer forms stable complex on ssDNA and partially stabilizes the HsRAD51 NPF against the anti-recombinogenic activity of BLM. Moreover, HsRAD51B-HsRAD51C stimulates HsRAD51 mediated D-loop formation in the presence of RPA. However, HsRAD51B-HsRAD51C does not facilitate HsRAD51 nucleation on a RPA coated ssDNA. These results suggest that the HsRAD51B-HsRAD51C complex plays a role in stabilizing the HsRAD51 NPF during the presynaptic phase of HR, which appears downstream of BRCA2-mediated HsRAD51 NPF formation.

Collaborating functions of BLM and DNA topoisomerase I in regulating human rDNA transcription.

Bloom's syndrome (BS) is an inherited disorder caused by loss of function of the recQ-like BLM helicase. It is characterized clinically by severe growth retardation and cancer predisposition. BLM localizes to PML nuclear bodies and to the nucleolus; its deficiency results in increased intra- and inter-chromosomal recombination, including hyper-recombination of rDNA repeats. Our previous work has shown that BLM facilitates RNA polymerase I-mediated rRNA transcription in the nucleolus (Grierson et al., 2012 [18]). This study uses protein co-immunoprecipitation and in vitro transcription/translation (IVTT) to identify a direct interaction of DNA topoisomerase I with the C-terminus of BLM in the nucleolus. In vitro helicase assays demonstrate that DNA topoisomerase I stimulates BLM helicase activity on a nucleolar-relevant RNA:DNA hybrid, but has an insignificant effect on BLM helicase activity on a control DNA:DNA duplex substrate. Reciprocally, BLM enhances the DNA relaxation activity of DNA topoisomerase I on supercoiled DNA substrates. Our study suggests that BLM and DNA topoisomerase I function coordinately to modulate RNA:DNA hybrid formation as well as relaxation of DNA supercoils in the context of nucleolar transcription.

Alternative mechanisms of telomere lengthening: permissive mutations, DNA repair proteins and tumorigenic progression.

Telomeres protect chromosome termini to maintain genomic stability and regulate cellular lifespan. Maintenance of telomere length is required for neoplastic cells after the acquisition of mutations that deregulate cell cycle control and increase cellular proliferation, and can occur through expression of the enzyme telomerase or in a telomerase-independent manner termed alternative lengthening of telomeres (ALT). The precise mechanisms that govern the activation of ALT or telomerase in tumor cells are unknown, although cellular origin may favor one or the other mechanisms. ALT pathways are incompletely understood to date; however, recent publications have increasingly broadened our understanding of how ALT is activated, how it proceeds, and how it influences tumor growth. Specific mutational events influence ALT activation, as mutations in genes that suppress recombination and/or alterations in the regulation of telomerase expression are associated with ALT. Once engaged, ALT uses DNA repair proteins to maintain telomeres in the absence of telomerase; experiments that manipulate the expression of specific proteins in cells using ALT are illuminating some of its mechanisms. Furthermore, ALT may influence tumor growth, as experimental and clinical data suggest that telomerase expression may favor tumor progression. This review summarizes recent findings in mammalian cells and models, as well as clinical data, that identify the genetic mutations permissive to ALT, the DNA repair proteins involved in ALT mechanisms and the importance of telomere maintenance mechanisms for tumor progression. A comprehensive understanding of the mechanisms that permit tumor cell immortalization will be important for identifying novel therapeutic targets in cancer.