RNase P branches out from RNP to protein: organelle-triggered diversification?

RNase P is the enzyme that removes 5' leader sequences from precursor tRNAs. Remarkably, in most organisms, RNase P is a ribonucleoprotein particle where the RNA component is responsible for catalysis. In this issue of Genes & Development, Gutmann and colleagues (pp. 1022-1027) report the first organism, Arabidopsis thaliana, to employ protein-only RNase P in both its nucleus and organelles. An intriguing possibility is that replacement of RNase P ribonucleoprotein particles (RNPs) by proteins may have been triggered by the acquisition of organelles.

Tracking expression and subcellular localization of RNA and protein species using high-throughput single cell imaging flow cytometry.

We report a high-throughput application of multispectral imaging flow cytometry (MIFC) for analyzing the expression and localization of both RNA and protein molecules in a heterogeneous population of cells. The approach was developed using polyadenylated nuclear (PAN) RNA, an abundant, noncoding RNA expressed by Kaposi's sarcoma-associated herpesvirus (KSHV) during the lytic phase of infection. High levels of PAN RNA are, in part, dependent on its interaction with poly(A)-binding protein C1 (PABPC1), which relocalizes from the cytoplasm to the nucleus of lytically infected cells. We quantitatively tracked the cytoplasmic to nuclear translocation of PABPC1 and examined how this translocation relates to the expression and localization of viral RNA and protein molecules in KSHV-infected cells. This high-throughput approach will be useful for other systems in which changes in subcellular localization of RNA and protein molecules need to be monitored simultaneously.

A viral nuclear noncoding RNA binds re-localized poly(A) binding protein and is required for late KSHV gene expression.

During the lytic phase of infection, the gamma herpesvirus Kaposi's Sarcoma-Associated Herpesvirus (KSHV) expresses a highly abundant, 1.1 kb nuclear noncoding RNA of unknown function. We observe that this polyadenylated nuclear (PAN) RNA avidly binds host poly(A)-binding protein C1 (PABPC1), which normally functions in the cytoplasm to bind the poly(A) tails of mRNAs, regulating mRNA stability and translation efficiency. During the lytic phase of KSHV infection, PABPC1 is re-localized to the nucleus as a consequence of expression of the viral shutoff exonuclease (SOX) protein; SOX also mediates the host shutoff effect in which host mRNAs are downregulated while viral mRNAs are selectively expressed. We show that whereas PAN RNA is not required for the host shutoff effect or for PABPC1 re-localization, SOX strongly upregulates the levels of PAN RNA in transient transfection experiments. This upregulation is destroyed by the same SOX mutation that ablates the host shutoff effect and PABPC1 nuclear re-localization or by removal of the poly(A) tail of PAN. In cells induced into the KSHV lytic phase, depletion of PAN RNA using RNase H-targeting antisense oligonucleotides reveals that it is necessary for the production of late viral proteins from mRNAs that are themselves polyadenylated. Our results add to the repertoire of functions ascribed to long noncoding RNAs and suggest a mechanism of action for nuclear noncoding RNAs in gamma herpesvirus infection.

Drosophila hnRNP A1 homologs Hrp36/Hrp38 enhance U2-type versus U12-type splicing to regulate alternative splicing of the prospero twintron.

During Drosophila embryogenesis, the transcription factor Prospero is critical for neuronal differentiation and axonal outgrowth. The prospero pre-mRNA undergoes alternative splicing, but is unique in that it harbors a rare twintron whereby one intron lies embedded within another. The innermost intron is excised by the major U2-type spliceosome and the outermost is excised by the minor U12-type spliceosome. Previously, an intronic purine-rich element (PRE) was identified as an enhancer of both U2- and U12-type splicing, with a greater effect on the U2-type pathway. We find that the PRE binds Drosophila homologs of heterogeneous nuclear ribonucleoprotein (hnRNP) A1, Hrp38 and Hrp36. RNAi-mediated knockdown of these proteins in S2 cells specifically decreases U2-type splicing of the twintron, which is surprising because hnRNPs usually are repressive. Conversely, tethering Hrp38 to the twintron increases U2-type splicing. Thus, developmentally regulated alternative splicing of the prospero twintron can be explained by documented changes in the abundance of these hnRNP A1-like proteins during embryogenesis.

Detection of Aberrant TERT Promoter Methylation by Combined Bisulfite Restriction Enzyme Analysis for Cancer Diagnosis.

Aberrant CpG dinucleotide methylation in a specific region of the telomerase reverse transcriptase (TERT) promoter is associated with increased TERT mRNA levels and malignancy in several cancer types. However, routine screening of this region to aid cancer diagnosis can be challenging because i) several established methylation assays may inaccurately report on hypermethylation of this particular region, ii) interpreting the results of methylation assays can sometimes be difficult for clinical laboratories, and iii) use of high-throughput methylation assays for a few patient samples can be cost prohibitive. Herein, we describe the use of combined bisulfite restriction enzyme analysis (COBRA) as a diagnostic tool for detecting the hypermethylated TERT promoter using in vitro methylated and unmethylated genomic DNA as well as genomic DNA from four melanomas and two benign melanocytic lesions. We compare COBRA with MassARRAY, a more commonly used high-throughput approach, in screening for promoter hypermethylation in 28 formalin-fixed, paraffin-embedded neuroblastoma samples. COBRA sensitively and specifically detected samples with hypermethylated TERT promoter and was as effective as MassARRAY at differentiating high-risk from benign or low-risk tumors. This study demonstrates the utility of this low-cost, technically straightforward, and easily interpretable assay for cancer diagnosis in tumors of an ambiguous nature.

Genetic and Epigenetic Alterations of TERT Are Associated with Inferior Outcome in Adolescent and Young Adult Patients with Melanoma.

Progression of melanoma to distant sites in adolescents and young adults (AYAs) is not reliably predicted by clinicopathologic criteria. TERT promoter mutations when combined with BRAF/NRAS mutations correlate with adverse outcome in adult melanoma. To determine the prognostic value of TERT alterations in AYA melanoma, we investigated the association of TERT promoter mutations, as well as promoter methylation, an epigenetic alteration also linked to TERT upregulation, with TERT mRNA expression and outcome using a well-characterized cohort of 27 patients with melanoma (ages 8-25, mean 20). TERT mRNA expression levels were significantly higher in tumors harboring TERT promoter mutation and/or hypermethylation than those without either aberration (P = 0.046). TERT promoter mutations alone did not predict adverse outcomes (P = 0.50), but the presence of TERT promoter methylation, alone or concurrent with promoter mutations, correlated with reduced recurrence-free survival (P = 0.001). These data suggest that genetic and epigenetic alterations of TERT are associated with TERT upregulation and may predict clinical outcomes in AYA melanoma. A more exhaustive understanding of the different molecular mechanisms leading to increased TERT expression may guide development of prognostic assays to stratify AYA melanoma patients according to clinical risk.

Cancer. TERT promoter mutations and telomerase reactivation in urothelial cancer.

Reactivation of telomerase, the chromosome end-replicating enzyme, drives human cell immortality and cancer. Point mutations in the telomerase reverse transcriptase (TERT) gene promoter occur at high frequency in multiple cancers, including urothelial cancer (UC), but their effect on telomerase function has been unclear. In a study of 23 human UC cell lines, we show that these promoter mutations correlate with higher levels of TERT messenger RNA (mRNA), TERT protein, telomerase enzymatic activity, and telomere length. Although previous studies found no relation between TERT promoter mutations and UC patient outcome, we find that elevated TERT mRNA expression strongly correlates with reduced disease-specific survival in two independent UC patient cohorts (n = 35; n = 87). These results suggest that high telomerase activity may be a better marker of aggressive UC tumors than TERT promoter mutations alone.

Nuclear translocation and regulation of intranuclear distribution of cytoplasmic poly(A)-binding protein are distinct processes mediated by two Epstein Barr virus proteins.

Many viruses target cytoplasmic polyA binding protein (PABPC) to effect widespread inhibition of host gene expression, a process termed viral host-shutoff (vhs). During lytic replication of Epstein Barr Virus (EBV) we observed that PABPC was efficiently translocated from the cytoplasm to the nucleus. Translocated PABPC was diffusely distributed but was excluded from viral replication compartments. Vhs during EBV infection is regulated by the viral alkaline nuclease, BGLF5. Transfection of BGLF5 alone into BGLF5-KO cells or uninfected 293 cells promoted translocation of PAPBC that was distributed in clumps in the nucleus. ZEBRA, a viral bZIP protein, performs essential functions in the lytic program of EBV, including activation or repression of downstream viral genes. ZEBRA is also an essential replication protein that binds to viral oriLyt and interacts with other viral replication proteins. We report that ZEBRA also functions as a regulator of vhs. ZEBRA translocated PABPC to the nucleus, controlled the intranuclear distribution of PABPC, and caused global shutoff of host gene expression. Transfection of ZEBRA alone into 293 cells caused nuclear translocation of PABPC in the majority of cells in which ZEBRA was expressed. Co-transfection of ZEBRA with BGLF5 into BGLF5-KO cells or uninfected 293 cells rescued the diffuse intranuclear pattern of PABPC seen during lytic replication. ZEBRA mutants defective for DNA-binding were capable of regulating the intranuclear distribution of PABPC, and caused PABPC to co-localize with ZEBRA. One ZEBRA mutant, Z(S186E), was deficient in translocation yet was capable of altering the intranuclear distribution of PABPC. Therefore ZEBRA-mediated nuclear translocation of PABPC and regulation of intranuclear PABPC distribution are distinct events. Using a click chemistry-based assay for new protein synthesis, we show that ZEBRA and BGLF5 each function as viral host shutoff factors.

Conservation of a triple-helix-forming RNA stability element in noncoding and genomic RNAs of diverse viruses.

Abundant expression of the long noncoding (lnc) PAN (polyadenylated nuclear) RNA by the human oncogenic gammaherpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) depends on a cis-element called the expression and nuclear retention element (ENE). The ENE upregulates PAN RNA by inhibiting its rapid nuclear decay through triple-helix formation with the poly(A) tail. Using structure-based bioinformatics, we identified six ENE-like elements in evolutionarily diverse viral genomes. Five are in double-stranded DNA viruses, including mammalian herpesviruses, insect polydnaviruses, and a protist mimivirus. One is in an insect picorna-like positive-strand RNA virus, suggesting that the ENE can counteract cytoplasmic as well as nuclear RNA decay pathways. Functionality of four of the ENEs was demonstrated by increased accumulation of an intronless polyadenylated reporter transcript in human cells. Identification of these ENEs enabled the discovery of PAN RNA homologs in two additional gammaherpesviruses, RRV and EHV2. Our findings demonstrate that searching for structural elements can lead to rapid identification of lncRNAs.

Noncoding RNPs of viral origin.

Like their host cells, many viruses produce noncoding (nc)RNAs. These show diversity with respect to time of expression during viral infection, length and structure, protein-binding partners and relative abundance compared with their host-cell counterparts. Viruses, with their limited genomic capacity, presumably evolve or acquire ncRNAs only if they selectively enhance the viral life cycle or assist the virus in combating the host's response to infection. Despite much effort, identifying the functions of viral ncRNAs has been extremely challenging. Recent technical advances and enhanced understanding of host-cell ncRNAs promise accelerated insights into the RNA warfare mounted by this fascinating class of RNPs.

ORF73 of herpesvirus saimiri, a viral homolog of Kaposi's sarcoma-associated herpesvirus, modulates the two cellular tumor suppressor proteins p53 and pRb.

All known DNA tumor viruses are known to target and inactivate two main cell cycle regulatory proteins, retinoblastoma protein (pRb) and p53. Inactivation of pRb promotes host cell cycle progression into S phase, and inactivation of p53 promotes cell immortalization. The DNA tumor virus Kaposi's sarcoma associated herpesvirus (KSHV)-encoded latency-associated nuclear antigen (LANA) was shown to target and inactivate pRb as well as p53. In this report we provide evidence that these functions are conserved in the homologous protein encoded by the related gammaherpesvirus herpesvirus saimiri (HVS). ORF73, the HVS homologue of LANA, is shown to bind both p53 and pRb in vitro and in vivo, to colocalize with p53 in human T cells infected with HVS, and in cells overexpressing both ORF73 and p53, as well as to adversely influence pRB/E2F and p53 transcriptional regulation. The C terminus of LANA, the region most highly conserved in ORF73, is shown to be responsible for both pRb and p53 interactions, supporting the hypothesis that these functions are conserved in both homologues. Finally, the region of p53 targeted by LANA (and ORF73) maps to the domain required for tetramerization. However, preliminary cross-linking studies do not detect disruption of p53 tetramerization by either LANA or HVS-encoded ORF73, suggesting that p53 inactivation may be by a mechanism independent of tetramer disruption.

Latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus up-regulates transcription of human telomerase reverse transcriptase promoter through interaction with transcription factor Sp1.

Telomerase is required for the maintenance of telomere length and is an important determinant for cell immortalization. In human cells, telomerase activity is due to the expression of its enzymatic subunit, human telomerase reverse transcriptase (hTERT). The expression of hTERT is not typically detectable in healthy somatic human cells but is present in cancerous tissues and immortalized cells. We have previously shown that hTERT promoter activity is up-regulated by the Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded latency-associated nuclear antigen (LANA). LANA is expressed in all forms of human malignancies associated with KSHV. The hTERT promoter sequence located at positions -130 to +5 contains several Sp1 binding motifs and was shown be important for up-regulation by LANA. In this report, we demonstrate that hTERT promoter activity is due to the direct interaction of LANA with Sp1. The interaction of LANA with Sp1 was demonstrated through in vitro binding experiments and coimmunoprecipitation and is supported by the colocalization of these two molecules in the nuclei of KSHV-infected cells. Moreover, LANA modulates Sp1-mediated transcription in transient GAL4 fusion reporter assays. Mapping of the regions involved in binding and transcriptional activation showed that the amino terminus of LANA is the major site for interaction and up-regulation but that it can cooperate with the carboxy terminus to enhance these functions. An analysis of Sp1 binding to its cognate sequence corroborated the binding data. Together, our results suggest that the interaction of LANA with Sp1 up-regulates the telomerase promoter, potentially contributing to the immortalization of KSHV-infected cells.