The chicken telomerase RNA gene: conservation of sequence, regulatory elements and synteny among viral, avian and mammalian genomes

The first chicken oocyte nucleus whole transcriptomic profile defines the spectrum of maternal mRNA and non-coding RNA genes transcribed by the lampbrush chromosomes

Lampbrush chromosomes, with their unusually high rate of nascent RNA synthesis, provide a valuable model for studying mechanisms of global transcriptome up-regulation. Here, we obtained a whole-genomic profile of transcription along the entire length of all lampbrush chromosomes in the chicken karyotype. With nuclear RNA-seq, we obtained information about a wider set of transcripts, including long non-coding RNAs retained in the nucleus and stable intronic sequence RNAs. For a number of protein-coding genes, we visualized their nascent transcripts on the lateral loops of lampbrush chromosomes by RNA-FISH. The set of genes transcribed on the lampbrush chromosomes is required for basic cellular processes and is characterized by a broad expression pattern. We also present the first high-throughput transcriptome characterization of miRNAs and piRNAs in chicken oocytes at the lampbrush chromosome stage. Major targets of predicted piRNAs include CR1 and long terminal repeat (LTR) containing retrotransposable elements. Transcription of tandem repeat arrays was demonstrated by alignment against the whole telomere-to-telomere chromosome assemblies. We show that transcription of telomere-derived RNAs is initiated at adjacent LTR elements. We conclude that hypertranscription on the lateral loops of giant lampbrush chromosomes is required for synthesizing large amounts of transferred to the embryo maternal RNA for thousands of genes.

Satellite cell-mediated breast muscle regeneration decreases with broiler size

Satellite cells (SCs) reside between the sarcolemma and basal lamina of muscle fibers and are the primary contributor of DNA for post-hatch muscle growth and repair. Alterations in SC content or properties by intrinsic and extrinsic factors can have detrimental effects on muscle health and function, and ultimately meat quality. We hypothesized that disrupted SC homeostasis may account in part for the increased breast myopathies observed in growing broilers. To test this hypothesis, we selected broilers with different body weights at comparable ages and studied SC characteristics in vitro and in vivo. Data shows that SC numbers in the breast muscles decrease (P < 0.001) and their inherent abilities to proliferate and differentiate diminish (P < 0.001) with age and size. Further, when breast muscle is presented with an insult, muscle of larger broilers regenerates more slowly than their smaller, age-matched counterparts arguing that SC quality changes with size and age. Together, our studies show that birds with greater muscle hypertrophy have less SCs with diminished ability to function, and suggest that aggressive selection for breast growth in broilers may exhaust SC pools when birds are grown to heavier processing weights. These findings provide new insights into a possible mechanism leading to breast myopathies in the poultry industry and provide targets for mitigating adverse fresh breast quality.

Virus and host genomic, molecular, and cellular interactions during Marek's disease pathogenesis and oncogenesis

Marek's Disease Virus (MDV) is a chicken alphaherpesvirus that causes paralysis, chronic wasting, blindness, and fatal lymphoma development in infected, susceptible host birds. This disease and its protective vaccines are highly relevant research targets, given their enormous impact within the poultry industry. Further, Marek's disease (MD) serves as a valuable model for the investigation of oncogenic viruses and herpesvirus patterns of viral latency and persistence--as pertinent to human health as to poultry health. The objectives of this article are to review MDV interactions with its host from a variety of genomic, molecular, and cellular perspectives. In particular, we focus on cytogenetic studies, which precisely assess the physical status of the MDV genome in the context of the chicken host genome. Combined, the cytogenetic and genomic research indicates that MDV-host genome interactions, specifically integration of the virus into the host telomeres, is a key feature of the virus life cycle, contributing to the viral achievement of latency, transformation, and reactivation of lytic replication. We present a model that outlines the variety of virus-host interactions, at the multiple levels, and with regard to the disease states.

Comparative cytogenomics of poultry: mapping of single gene and repeat loci in the Japanese quail (Coturnix japonica)

Well-characterized molecular and cytogenetic maps are yet to be established in Japanese quail (Coturnix japonica). The aim of the current study was to cytogenetically map and determine linkage of specific genes and gene complexes in Japanese quail through the use of chicken (Gallus gallus) and turkey (Meleagris gallopavo) genomic DNA probes and conduct a comparative study among the three genomes. Chicken and turkey clones were used as probes on mitotic metaphase and meiotic pachytene stage chromosomes of the three species for the purpose of high-resolution fluorescence in situ hybridization (FISH). The genes and complexes studied included telomerase RNA (TR), telomerase reverse transcriptase (TERT), 5S rDNA, 18S-5.8S-28S rDNA (i.e., nucleolus organizer region (NOR)), and the major histocompatibility complex (MHC). The telomeric profile of Japanese quail was investigated through the use of FISH with a TTAGGG-PNA probe. A range of telomeric array sizes were confirmed as found for the other poultry species. Three NOR loci were identified in Japanese quail, and single loci each for TR, TERT, 5S rDNA and the MHC-B. The MHC-B and one NOR locus were linked on a microchromosome in Japanese quail. We confirmed physical linkage of 5S rDNA and the TR gene on an intermediate-sized chromosome in quail, similar to both chicken and turkey. TERT localized to CJA 2 in quail and the orthologous chromosome region in chicken (GGA 2) and in turkey (MGA 3). The cytogenetic profile of Japanese quail was further developed by this study and synteny was identified among the three poultry species.

[Telomere biology of the chicken]

Telomeres, the nucleoprotein "caps" protecting the ends of linear chromosomes, are maintained by telomerase. Telomeres have important roles in maintaining genomic stability and preventing senescence or oncogenesis. Chicken is a classical model animal for genetic and developmental studies. With further development of chicken genomics, great progress has been made in research of chicken telomere and telomerase. This review describes recent advances and future research directions in chicken telomere biology.

Chromosomal integration of an avian oncogenic herpesvirus reveals telomeric preferences and evidence for lymphoma clonality

BACKGROUND

Herpesviruses are a major health concern for numerous organisms, including humans, causing both acute and chronic infections recurrent over an individual's lifespan. Marek's disease virus (MDV) is a highly contagious herpesvirus which causes a neoplastic condition in chicken populations. Several vertebrate-infecting herpesviruses have been shown to exist in an integrated state during latent periods of infection. However the status of MDV during latency has been a topic of debate.

RESULTS

Here we employed high-resolution multi-color fluorescence in situ hybridization (FISH) to show integration of MDV at the telomeres of chicken chromosomes. Cytogenomic mapping of the chromosomal integrations allowed us to examine the clonal relationships among lymphomas within individuals, whereas analysis of tumors from multiple individuals indicated the potential for chromosomal preferences.

CONCLUSIONS

Our data highlight that substantive genome-level interactions between the virus and host exist, and merit consideration for their potential impact and role in key aspects of herpesvirus pathobiology including infection, latency, cellular transformation, latency-breaks and viral evolution.

Effects of ectopic expression of human telomerase reverse transcriptase on immortalization of feather keratinocyte stem cells

Normal somatic cells possess a finite life span owing to replicative senescence. Telomerase functions as a potential regulator of senescence in various cells. Expression level of human telomerase reverse transcriptase (hTERT) is correlated with telomerase activity and cellular immortalization. In this study, we investigated the effects of ectopic expression of hTERT on proliferation potential of chicken feather keratinocyte stem cells (FKSCs). We established FKSCs transduced with hTERT catalytic subunit fused with EGFP marker gene (hTERT-EGFP-FKSCs). hTERT-EGFP-FKSCs had the great potential of proliferation in vitro and expressed kerainocyte stem cell markers integrin beta1 and CD49c. Keratin 15 and keratin 19, as native FKSCs, were also detected in hTERT-EGFP-FKSCs. By the analysis of fluorescent RT-PCR, western blotting and TRAP assay, hTERT-EGFP-FKSCs were positive for telomerase activity, in comparison with native FKSCs showing no telomerase activity. We demonstrated that ectopic expression of hTERT could result in immortalization of FKSCs. Tumorigenecity of hTERT-EGFP-FKSCs were examined by soft agar assay and transplantation into NOD-SCID mice. Results showed that hTERT-EGFP-FKSCs sustained the cellular characteristics of native FKSCs and had no transforming activity. In vivo differentiation multipotentials of hTERT-EGFP-FKSCs were confirmed by transplantation into developing chicken embryos and in situ hybridization analysis. These data provide a novel framework for understanding human telomerase activity in different species and suggest a new insight for manipulating hTERT for therapeutic purposes in treating tissue injury and aging.

Genetic variation exists for telomeric array organization within and among the genomes of normal, immortalized, and transformed chicken systems

This study investigated telomeric array organization of diverse chicken genotypes utilizing in vivo and in vitro cells having phenotypes with different proliferation potencies. Our experimental objective was to characterize the extent and nature of array variation present to explore the hypothesis that mega-telomeres are a universal and fixed feature of chicken genotypes. Four different genotypes were studied including normal (UCD 001, USDA-ADOL Line 0), immortalized (DF-1), and transformed (DT40) cells. Both cytogenetic and molecular approaches were utilized to develop an integrated view of telomeric array organization. It was determined that significant variation exists within and among chicken genotypes for chromosome-specific telomeric array organization and total genomic-telomeric sequence content. Although there was variation for mega-telomere number and distribution, two mega-telomere loci were in common among chicken genetic lines (GGA 9 and GGA W). The DF-1 cell line was discovered to maintain a complex derivative karyotype involving chromosome fusions in the homozygous and heterozygous condition. Also, the DF-1 cell line was found to contain the greatest amount of telomeric sequence per genome (17%) as compared to UCD 001 (5%) and DT40 (1.2%). The chicken is an excellent model for studying unique and universal features of vertebrate telomere biology, and characterization of the telomere length variation among genotypes will be useful in the exploration of mechanisms controlling telomere length maintenance in different cell types having unique phenotypes.

Vaccination against Marek's disease reduces telomerase activity and viral gene transcription in peripheral blood leukocytes from challenged chickens

We investigated whether telomerase activity and viral gene transcription were associated with protection against the RB-1B strain of Marek's disease virus (MDV) in chickens vaccinated with Rispens CVI988 or the herpes virus of turkey (HVT). Telomerase activity in peripheral blood leukocytes (PBLs) seemed to be an appropriate marker of lymphoma and levels of viral transcription were correlated with the virulence of MDV strains. Vaccinated protected birds had lower levels of telomerase activity and RB-1B viral gene transcription than unvaccinated chickens infected with RB-1B. The decrease in RB-1B viral transcription was more marked in chickens vaccinated with CVI988 than in those vaccinated with HVT. Indeed, RB-1B viral transcription was not detectable after 14 days post-challenge. In conclusion, telomerase activity and gene transcription in challenge MDV strains are potential new reliable criteria of protection in vaccinated chickens.

Telomerase reverse transcriptase expression elevated by avian leukosis virus integration in B cell lymphomas

Simple retroviruses induce tumors by integrating into the host genome, activating cellular oncogenes and microRNAs, or inactivating tumor suppressor genes. The identification of these genes elucidates molecular mechanisms of tumorigenesis. In this study, we identified avian leukosis virus (ALV) proviral integration sites in rapid-onset B cell lymphomas arising <12 weeks after infection of chicken embryos. By using inverse PCR, 28 unique viral integration sites were identified in rapid-onset tumors. Integrations in the telomerase reverse transcriptase (TERT) promoter/enhancer region were observed in four different tumors, suggesting that this is a common integration site. These provirus integrations ranged from 217 to 2,584 bp upstream of the TERT transcription initiation site and were all in the opposite transcriptional orientation to TERT. Southern blots of tumor samples demonstrated that these integrations are clonal and therefore occurred early in the process of tumorigenesis. Real-time RT-PCR showed overexpression of TERT mRNA in tumors harboring viral integrations in the TERT promoter. Telomerase activity was also up-regulated in these tumors; however, telomere-length alterations were not detected. Furthermore, viral LTR sequences directly enhanced the expression of luciferase reporters containing the TERT promoter sequences. This study documents retroviral up-regulation of cellular TERT by insertional activation to initiate or enhance tumor progression.

Chromosomal mapping of chicken mega-telomere arrays to GGA9, 16, 28 and W using a cytogenomic approach

Four mega-telomere loci were mapped to chicken chromosomes 9, 16, 28, and the W sex chromosome by dual-color fluorescence in situ hybridization using a telomeric sequence probe and BAC clones previously assigned to chicken chromosomes. The in-common features of the mega-telomere chromosomes are that microchromosomes are involved rather than macrochromosomes; in three cases (9, 16, 28) acrocentrics are involved with the mega-telomeres mapping to the p arms. Three of the four chromosomes (9, 16, W) encode tandem repeats which in two cases (9 and 16) involve the ribosomal DNA arrays (the 5S and 18S-5.8S-28S gene repeats, respectively). All involved chromosomes have a typical-sized telomere on the opposite terminus. Intra- and interindividual variation for mega-telomere distribution are discussed in terms of karyotype abnormalities and the potential for mitotic instability of some telomeres. The diversity and distribution of telomere array quantity in the chicken genome should be useful in contributing to research related to telomere length regulation - how and by what mechanism genomes and individual chromosomes establish and maintain distinct sets of telomere array sizes, as well as for future studies related to stability of the chicken genome affecting development, growth, cellular lifespan and disease. An additional impact of this study includes the listing of BAC clones (26 autosomal and six W BACs tested) that were cytogenetically verified; this set of BACs provide a useful tool for future cytogenetic analyses of the microchromosomes.

A virus-encoded telomerase RNA promotes malignant T cell lymphomagenesis

Telomerase is a ribonucleoprotein complex consisting of two essential core components: a reverse transcriptase and an RNA subunit (telomerase RNA [TR]). Dysregulation of telomerase has been associated with cell immortalization and oncogenesis. Marek's disease herpesvirus (MDV) induces a malignant T cell lymphoma in chickens and harbors in its genome two identical copies of a viral TR (vTR) with 88% sequence identity to chicken TR. MDV mutants lacking both copies of vTR were significantly impaired in their ability to induce T cell lymphomas, although lytic replication in vivo was unaffected. Tumor incidences were reduced by >60% in chickens infected with vTR⁻ viruses compared with animals inoculated with MDV harboring at least one intact copy of vTR. Lymphomas in animals infected with the vTR⁻ viruses were also significantly smaller in size and less disseminated. Constitutive expression of vTR in the chicken fibroblast cell line DF-1 resulted in a phenotype consistent with transformation as indicated by morphological alteration, enhanced anchorage-independent cell growth, cell growth beyond saturation density, and increased expression levels of integrin αv. We concluded that vTR plays a critical role in MDV-induced T cell lymphomagenesis. Furthermore, our results provide the first description of tumor-promoting effects of TR in a natural virus–host infection model.

Telomerase gene expression in the chicken: Telomerase RNA (TR) and reverse transcriptase (TERT) transcript profiles are tissue-specific and correlate with telomerase activity

Telomerase is the specialized enzyme which replicates the telomeres, thus maintaining the integrity of the chromosome ends; in absence of enzyme activity telomere lengths decrease, ultimately impacting genome stability. In this study, we examined the mRNA expression of both enzyme components, the RNA template (TR) and catalytic subunit (TERT) during growth and development of the chicken to better understand mechanisms which regulate telomerase activity in vertebrates. Quantitative real-time PCR was used to establish transcript profiles for six ages ranging from pre-blastula to two-year old adults. Organ-specific profiles were established for brain, heart, liver, intestine, spleen and gonad. The pre-blastula and gastrula stages exhibited very high transcript levels of both telomerase components; organs from the embryos and adult showed transcript levels either similar or down-regulated relative to the early differentiation embryo stages. Organs which are known to become negative for telomerase activity between the embryo and adult stages (brain, heart, liver) exhibited down-regulation of TR and either no change or an increase in TERT transcripts. Whereas, organs which maintain high telomerase activity even in adults (intestine, spleen, gonad), generally exhibited up-regulation of transcripts for both components. However, there were some tissue-specific differences between telomerase-positive tissues. These results show that TERT and TR transcript levels correlate with telomerase activity profiles and suggest that TR is the rate-limiting component in telomerase-negative tissues.

Differential expression of genes associated with telomere length homeostasis and oncogenesis in an avian model

Telomere-binding proteins, their interaction partners and transcription factors play a prominent role in telomere maintenance and telomerase activation. We examined mRNA expression levels of tankyrase 1 and 2, TRF1 and 2, c-myc, TERT and TR in Gallus domesticus, the domestic chicken, by quantitative real-time PCR, establishing expression profiles for three contrasting cell systems: the pluripotent gastrula, differentiated embryo fibroblasts and transformed DT40 cells. All seven genes were up-regulated in DT40 cells compared to telomerase-negative CEFs and a majority of the genes were also up-regulated in the gastrula relative to CEFs. Surprisingly, we found TERT and TR transcripts in CEFs, albeit at low levels. TRF1 was down-regulated in the six CEF cultures by the time of culture growth arrest. A marked increase in the TRF2:TRF1 ratio occurred at or near senescence in all of the CEF cultures studied, with the most elevated ratio found in a short-lived culture in which TRF1 mRNA levels decreased two-fold and TRF2 levels increased 21-fold. This culture also showed highly reduced, degraded telomeres by Southern blot analysis. These data suggest that genes involved in telomere maintenance and telomerase induction are expressed differentially in pluripotent, differentiated and transformed cell systems.

Meiotic instability of chicken ultra-long telomeres and mapping of a 2.8 megabase array to the W-sex chromosome

The objective of this research was to study the meiotic stability of a subset of chicken telomere arrays, which are the largest reported for any vertebrate species. Inheritance of these ultra-long telomere arrays (200 kb to 3 mb) was studied in a highly homozygous inbred line, UCD 003 (F >or= 99.9). Analysis of array transmission in four families indicated unexpected heterogeneity and non-Mendelian segregation including high-frequency-generation of novel arrays. Additionally, the largest array detected (2.8 Mb) was female-specific and correlated to the most intense telomeric DNA signal on the W-sex chromosome by fluorescence in situ hybridization (FISH). These results are discussed in regard to the potential functions of the ultra-long telomere arrays in the chicken genome including generation of genetic variation through enhanced recombination, protection against erosion by providing a buffer for gene-dense regions, and sex-chromosome organization.

Telomerase activity and differential expression of telomerase genes and c-myc in chicken cells in vitro

This study examined telomerase activity and gene expression profiles for three genes in Gallus gallus domesticus: telomerase reverse transcriptase (chTERT), telomerase RNA (chTR), and c-myc. Expression of these genes was studied in chicken embryonic stem (chES) cells, chicken embryo fibroblasts (CEFs), and DT40 cells using quantitative real-time polymerase chain reaction. Our results establish that, relative to transcription levels in telomerase-negative CEFs, chTERT and chTR are up-regulated in telomerase-positive chES cells. Transcription levels of chTERT, chTR, and c-myc are dramatically up-regulated in telomerase-positive DT40 cells, relative to CEFs and chES cells. These results are consistent with a model in which telomerase activity is up-regulated in proliferating embryonic stem cells requiring stable telomeres to endure multiple rounds of cell division; down-regulated in differentiated, lifespan-limited cells; and dramatically up-regulated in immortalized, transformed cells for which uncontrolled proliferation is correlated with c-myc dysregulation and telomerase activity.

The chicken telomerase reverse transcriptase (chTERT): molecular and cytogenetic characterization with a comparative analysis

Telomerase activity is essential for maintaining the termini of linear chromosomes. Telomerase consists of both a RNA and a specialized reverse transcriptase. Our objective for this study was to determine the molecular and cytogenetic features of the chicken telomerase reverse transcriptase (chTERT) gene and protein. The TERT mRNA from gastrula stage embryos was found to be 4497 bp in length, translating into a protein of 1346 amino acids (aa). The chTERT protein shares 45% aa identity with human TERT (hTERT). A distinctive feature of chTERT, as compared to human and other vertebrate TERTs, is the larger size of the protein due mainly to a considerably longer N-terminal flexible linker region (144 aa longer than in human). Chicken TERT was mapped to chromosome 2q21 near an interstitial telomere site. Several transcription factor binding motifs in the 5' flanking/promoter region of chTERT were similar to those found associated with hTERT (E-box, Ik1, MAZ, Sp1 sites), whereas several c-Myb sites were found associated with chTERT only and c-Ets-2 and WT1 were associated with hTERT only. Results presented here should promote structure-function studies of chTERT, as well as contribute to the comparative analysis of TERT regulation and function in vertebrates utilizing the telomere clock mechanism to different degrees.