A Legionella toxin exhibits tRNA mimicry and glycosyl transferase activity to target the translation machinery and trigger a ribotoxic stress response

A widespread strategy employed by pathogens to establish infection is to inhibit host-cell protein synthesis. Legionella pneumophila, an intracellular bacterial pathogen and the causative organism of Legionnaires' disease, secretes a subset of protein effectors into host cells that inhibit translation elongation. Mechanistic insights into how the bacterium targets translation elongation remain poorly defined. We report here that the Legionella effector SidI functions in an unprecedented way as a transfer-RNA mimic that directly binds to and glycosylates the ribosome. The 3.1 Å cryo-electron microscopy structure of SidI reveals an N-terminal domain with an 'inverted L' shape and surface-charge distribution characteristic of tRNA mimicry, and a C-terminal domain that adopts a glycosyl transferase fold that licenses SidI to utilize GDP-mannose as a sugar precursor. This coupling of tRNA mimicry and enzymatic action endows SidI with the ability to block protein synthesis with a potency comparable to ricin, one of the most powerful toxins known. In Legionella-infected cells, the translational pausing activated by SidI elicits a stress response signature mimicking the ribotoxic stress response, which is activated by elongation inhibitors that induce ribosome collisions. SidI-mediated effects on the ribosome activate the stress kinases ZAKα and p38, which in turn drive an accumulation of the protein activating transcription factor 3 (ATF3). Intriguingly, ATF3 escapes the translation block imposed by SidI, translocates to the nucleus and orchestrates the transcription of stress-inducible genes that promote cell death, revealing a major role for ATF3 in the response to collided ribosome stress. Together, our findings elucidate a novel mechanism by which a pathogenic bacterium employs tRNA mimicry to hijack a ribosome-to-nuclear signalling pathway that regulates cell fate.

Transcription factor UBF depletion in mouse cells results in downregulation of both downstream and upstream elements of the rRNA transcription network

Transcription/processing of the ribosomal RNA (rRNA) precursor, as part of ribosome biosynthesis, is intensively studied and characterized in eukaryotic cells. Here, we constructed shRNA-based mouse cell lines partially silenced for the Upstream Binding Factor UBF, the master regulator of rRNA transcription and organizer of open rDNA chromatin. Full Ubf silencing in vivo is not viable, and these new tools allow further characterization of rRNA transcription and its coordination with cellular signaling. shUBF cells display cell cycle G1 delay and reduced 47S rRNA precursor and 28S rRNA at baseline and serum-challenged conditions. Growth-related mTOR signaling is downregulated with the fractions of active phospho-S6 Kinase and pEIF4E translation initiation factor reduced, similar to phosphorylated cell cycle regulator retinoblastoma, pRB, positive regulator of UBF availability/rRNA transcription. Additionally, we find transcription-competent pUBF (Ser484) severely restricted and its interacting initiation factor RRN3 reduced and responsive to extracellular cues. Furthermore, fractional UBF occupancy on the rDNA unit is decreased in shUBF, and expression of major factors involved in different aspects of rRNA transcription is severely downregulated by UBF depletion. Finally, we observe reduced RNA Pol1 occupancy over rDNA promoter sequences and identified unexpected regulation of RNA Pol1 expression, relative to serum availability and under UBF silencing, suggesting that regulation of rRNA transcription may not be restricted to modulation of Pol1 promoter binding/elongation rate. Overall, this work reveals that UBF depletion has a critical downstream and upstream impact on the whole network orchestrating rRNA transcription in mammalian cells.

HMG-boxes, ribosomopathies and neurodegenerative disease

The UBTF E210K neuroregression syndrome is a predominantly neurological disorder caused by recurrent de novo dominant variants in Upstream Binding Factor, that is, essential for transcription of the ribosomal RNA genes. This unusual form of ribosomopathy is characterized by a slow decline in cognition, behavior, and sensorimotor functioning during the critical period of development. UBTF (or UBF) is a multi-HMGB-box protein that acts both as an epigenetic factor to establish "open" chromatin on the ribosomal genes and as a basal transcription factor in their RNA Polymerase I transcription. Here we review the possible mechanistic connections between the UBTF variants, ribosomal RNA gene transcription and the neuroregression syndrome, and suggest that DNA topology may play an important role.

The human RNA polymerase I structure reveals an HMG-like docking domain specific to metazoans

We characterize the human RNA polymerase I by evolutionary biochemistry and cryo-EM revealing a built-in structural domain that apparently serves as transcription factor–binding platform in metazoans.

Transcription of the ribosomal RNA precursor by RNA polymerase (Pol) I is a major determinant of cellular growth, and dysregulation is observed in many cancer types. Here, we present the purification of human Pol I from cells carrying a genomic GFP fusion on the largest subunit allowing the structural and functional analysis of the enzyme across species. In contrast to yeast, human Pol I carries a single-subunit stalk, and in vitro transcription indicates a reduced proofreading activity. Determination of the human Pol I cryo-EM reconstruction in a close-to-native state rationalizes the effects of disease-associated mutations and uncovers an additional domain that is built into the sequence of Pol I subunit RPA1. This “dock II” domain resembles a truncated HMG box incapable of DNA binding which may serve as a downstream transcription factor–binding platform in metazoans. Biochemical analysis, in situ modelling, and ChIP data indicate that Topoisomerase 2a can be recruited to Pol I via the domain and cooperates with the HMG box domain–containing factor UBF. These adaptations of the metazoan Pol I transcription system may allow efficient release of positive DNA supercoils accumulating downstream of the transcription bubble.

Control of ribosomal RNA synthesis by hematopoietic transcription factors

Ribosomal RNAs (rRNAs) are the most abundant cellular RNAs, and their synthesis from rDNA repeats by RNA polymerase I accounts for the bulk of all transcription. Despite substantial variation in rRNA transcription rates across cell types, little is known about cell-type-specific factors that bind rDNA and regulate rRNA transcription to meet tissue-specific needs. Using hematopoiesis as a model system, we mapped about 2,200 ChIP-seq datasets for 250 transcription factors (TFs) and chromatin proteins to human and mouse rDNA and identified robust binding of multiple TF families to canonical TF motifs on rDNA. Using a 47S-FISH-Flow assay developed for nascent rRNA quantification, we demonstrated that targeted degradation of C/EBP alpha (CEBPA), a critical hematopoietic TF with conserved rDNA binding, caused rapid reduction in rRNA transcription due to reduced RNA Pol I occupancy. Our work identifies numerous potential rRNA regulators and provides a template for dissection of TF roles in rRNA transcription.

Behavioral and molecular effects of Ubtf knockout and knockdown in mice

The UBTF E210K neuroregression syndrome is caused by de novo dominant mutations in UBTF (NM_014233.3:c.628G > A, p.Glu210Lys). In humans, onset is typically at 2.5 to 3 years and characterized by slow progression of global motor, cognitive and behavioral dysfunction. Other potentially pathogenic UBTF variants have been reported in humans with severe neurological disease and it remains undetermined if the UBTF E210K mutation operates via gain- and/or loss-of-function. Here we examine the behavioral, cognitive, motor, and molecular effects of Ubtf knockout and knockdown in mice as a means of gauging the role of loss-of-function in humans. Ubtf+/- mice show progression of behavioral (dominance tube), cognitive (cross maze), and mild motor abnormalities from 3 to 18 months. At 18 months, Ubtf+/- mice had more slips on a raised 9-mm round beam task, shorter latencies to fall on the accelerated rotarod, reduced open field vertical and jump counts, and significant deficits in spatial learning and memory. Via crosses to Nestin-Cre (NesCre) mice we found that homozygous Ubtf deletion limited to the central nervous system was embryonic lethal. Tamoxifen-induced homozygous knockdown of Ubtf in adult mice with the Cre-ERT2 system was associated with precipitous deterioration in neurological functioning. At the molecular level, 18-month-old Ubtf+/- mice showed mild increases in cerebellar 53BP1 immunoreactivity. These findings show that UBTF is essential for embryogenesis and survival in adults, and the deleterious effects of UBTF haploinsufficiency progress with age. Loss-of-function mechanisms may contribute, in part, to the human UBTF E210K neuroregression syndrome.

Ribosomal DNA promoter recognition is determined in vivo by cooperation between UBTF1 and SL1 and is compromised in the UBTF-E210K neuroregression syndrome

Transcription of the ~200 mouse and human ribosomal RNA genes (rDNA) by RNA Polymerase I (RPI/PolR1) accounts for 80% of total cellular RNA, around 35% of all nuclear RNA synthesis, and determines the cytoplasmic ribosome complement. It is therefore a major factor controlling cell growth and its misfunction has been implicated in hypertrophic and developmental disorders. Activation of each rDNA repeat requires nucleosome replacement by the architectural multi-HMGbox factor UBTF to create a 15.7 kbp nucleosome free region (NFR). Formation of this NFR is also essential for recruitment of the TBP-TAFI factor SL1 and for preinitiation complex (PIC) formation at the gene and enhancer-associated promoters of the rDNA. However, these promoters show little sequence commonality and neither UBTF nor SL1 display significant DNA sequence binding specificity, making what drives PIC formation a mystery. Here we show that cooperation between SL1 and the longer UBTF1 splice variant generates the specificity required for rDNA promoter recognition in cell. We find that conditional deletion of the TAF1B subunit of SL1 causes a striking depletion of UBTF at both rDNA promoters but not elsewhere across the rDNA. We also find that while both UBTF1 and -2 variants bind throughout the rDNA NFR, only UBTF1 is present with SL1 at the promoters. The data strongly suggest an induced-fit model of RPI promoter recognition in which UBTF1 plays an architectural role. Interestingly, a recurrent UBTF-E210K mutation and the cause of a pediatric neurodegeneration syndrome provides indirect support for this model. E210K knock-in cells show enhanced levels of the UBTF1 splice variant and a concomitant increase in active rDNA copies. In contrast, they also display reduced rDNA transcription and promoter recruitment of SL1. We suggest the underlying cause of the UBTF-E210K syndrome is therefore a reduction in cooperative UBTF1-SL1 promoter recruitment that may be partially compensated by enhanced rDNA activation.

The chemotherapeutic agent CX-5461 irreversibly blocks RNA polymerase I initiation and promoter release to cause nucleolar disruption, DNA damage and cell inviability

In the search for drugs to effectively treat cancer, the last 10 years have seen a resurgence of interest in targeting ribosome biogenesis. CX-5461 is a potential inhibitor of ribosomal RNA synthesis that is now showing promise in phase I trials as a chemotherapeutic agent for a range of malignancies. Here, we show that CX-5461 irreversibly inhibits ribosomal RNA transcription by arresting RNA polymerase I (RPI/Pol1/PolR1) in a transcription initiation complex. CX-5461 does not achieve this by preventing formation of the pre-initiation complex nor does it affect the promoter recruitment of the SL1 TBP complex or the HMGB-box upstream binding factor (UBF/UBTF). CX-5461 also does not prevent the subsequent recruitment of the initiation-competent RPI–Rrn3 complex. Rather, CX-5461 blocks promoter release of RPI–Rrn3, which remains irreversibly locked in the pre-initiation complex even after extensive drug removal. Unexpectedly, this results in an unproductive mode of RPI recruitment that correlates with the onset of nucleolar stress, inhibition of DNA replication, genome-wide DNA damage and cellular senescence. Our data demonstrate that the cytotoxicity of CX-5461 is at least in part the result of an irreversible inhibition of RPI transcription initiation and hence are of direct relevance to the design of improved strategies of chemotherapy.

AB1321-HPR DEVELOPING A SELF-MANAGEMENT INTERVENTION TO MANAGE JOINT HYPERMOBILITY SYNDROME AND EHLERS-DANLOS SYNDROME HYPERMOBILITY TYPE: AN ANALYSIS INFORMED BY BEHAVIOUR CHANGE THEORY

Background:

Joint Hypermobility Syndrome (JHS) and Ehlers-Danlos Syndrome Hypermobility Type (EDS-HT) are heritable disorders of connective tissue that can cause joint instability and pain and are associated with increased anxiety and depression. There is currently little UK guidance for supporting patients with JHS/EDS-HT1. The analysis presented here used the Behaviour Change Wheel (made up of the Theoretical Domains Framework (TDF) and Capability, Opportunity, Motivation and Behaviour (COM-B) model2) to identify possible intervention options to improve self-management in people with JHS/EDS-HT.

Objectives:

To determine recommendations for the components of a behaviour change intervention for people with JHS or EDS-HT.

Methods:

Data from: 1) A systematic review and thematic synthesis of the literature examining adults’ lived experiences of JHS/EDS3and 2) A thematic analysis of interview data where UK adults with JHS/EDS-HT (n=17, 14 women, 3 men) discussed the psychosocial impact of the condition on their lives4, were mapped onto the TDF and COM-B in a behavioural analysis. A modified Nominal Group Technique focus group (n=9, all women) explored which interventions identified by the TDF/COM-B mapping exercise were most important to them.

Results:

Participants prioritised a range of potential self-management interventions, including:

Education: Participants wanted greater support to improve their knowledge of JHS/EDS-HT, including self-help strategies for coping with injury, fatigue and overexertion, and how to evaluate information about their condition.

Training: In activity pacing, assertiveness and communication skills, and what to expect during pregnancy, when symptoms of JHS/EDS-HT can worsen.

Environmental restructuring and enablement:Support from occupational therapists to maintain independence at home. Enablement of access to CBT, mindfulness and emotional support.

Modelled behaviour:Positive first-person narratives that address how other patients with JHS/EDS-HT have coped with anxiety, depression, distress, fear, frustration and feelings of loss.

Conclusion:

This study is the first to apply theoretically-informed approaches to the management of JHS/EDS-HT. Through a modified nominal group technique, potential behaviour change interventions for addressing barriers to self-management have been prioritised. Discussion with participants indicated poor access to psychological support, occupational therapy and a lack of knowledge of JHS/EDS-HT. Future research with healthcare professional and patient stakeholder groups will further evaluate which intervention options would be most acceptable and feasible for the management of JHS/EDS-HT.

References:

[1]Palmer, S., Terry, R. Rimes, K.A., Clark, C., Simmonds, J. & Horwood, J. (2016). Physiotherapy management of joint hypermobility syndrome – a focus group study of patient and health professional perspectives. Physiotherapy,http://dx.doi.org/10.1016/j.physio.2015.05.001.

[2]Michie, S., van Stralen, M.M. & West, R. (2011). The behaviour change wheel: A new method for characterising and designing behaviour change interventions. Implementation Science, 6(42).

[3]Bennett, S.E., Walsh, N., Moss, T. and Palmer, S. (2019a) “The lived experience of joint hypermobility and Ehlers-Danlos Syndromes: A systematic review and thematic synthesis.”Physical Therapy Reviews, 24 (1-2). pp. 12-28. ISSN 1083-3196

[4]Bennett, S.E., Walsh, N., Moss, T. and Palmer, S. (2019b) “Understanding the psychosocial impact of Joint Hypermobility Syndrome and Ehlers-Danlos Syndrome Hypermobility Type: A qualitative interview study.”Disability and Rehabilitation. ISSN 0963-8288

Disclosure of Interests:

None declared

The Short N-Terminal Repeats of Transcription Termination Factor 1 Contain Semi-Redundant Nucleolar Localization Signals and P19-ARF Tumor Suppressor Binding Sites

The p14/p19^ARF (ARF) tumor suppressor provides an important link in the activation of p53 (TP53) by inhibiting its targeted degradation via the E3 ligases MDM2/HDM2. However, ARF also limits tumor growth by directly inhibiting ribosomal RNA synthesis and processing. Initial studies of the ARF tumor suppressor were compounded by overlap between the INK4A and ARF genes encoded by the CDKN2A locus, but mouse models of pure ARF-loss and its inactivation in human cancers identified it as a distinct tumor suppressor even in the absence of p53. We previously demonstrated that both human and mouse ARF interact with Transcription Termination Factor 1 (TTF1, TTF-I), an essential factor implicated in transcription termination and silencing of the ribosomal RNA genes. Accumulation of ARF upon oncogenic stress was shown to inhibit ribosomal RNA synthesis by depleting nucleolar TTF1. Here we have mapped the functional nucleolar localization sequences (NoLS) of mouse TTF1 and the sequences responsible for interaction with ARF. We find that both sequences lie within the 25 amino acid N-terminal repeats of TTF1. Nucleolar localization depends on semi-redundant lysine-arginine motifs in each repeat and to a minor extent on binding to target DNA sequences by the Myb homology domain of TTF1. While nucleolar localization of TTF1 predominantly correlates with its interaction with ARF, NoLS activity and ARF binding are mediated by distinct sequences within each N-terminal repeat. The data suggest that the N-terminal repeats of mouse TTF1, and by analogy those of human TTF1, cooperate to mediate both nucleolar localization and ARF binding.

MECP2 Mutation Interrupts Nucleolin-mTOR-P70S6K Signaling in Rett Syndrome Patients

Rett syndrome (RTT) is a severe and rare neurological disorder that is caused by mutations in the X-linked MECP2 (methyl CpG-binding protein 2) gene. MeCP2 protein is an important epigenetic factor in the brain and in neurons. In Mecp2-deficient neurons, nucleoli structures are compromised. Nucleoli are sites of active ribosomal RNA (rRNA) transcription and maturation, a process mainly controlled by nucleolin and mechanistic target of rapamycin (mTOR)-P70S6K signaling. Currently, it is unclear how nucleolin-rRNA-mTOR-P70S6K signaling from RTT cellular model systems translates into human RTT brain. Here, we studied the components of nucleolin-rRNA-mTOR-P70S6K signaling in the brain of RTT patients with common T158M and R255X mutations. Immunohistochemical examination of T158M brain showed disturbed nucleolin subcellular localization, which was absent in Mecp2-deficient homozygous male or heterozygote female mice, compared to wild type (WT). We confirmed by Western blot analysis that nucleolin protein levels are altered in RTT brain, but not in Mecp2-deficient mice. Further, we studied the expression of rRNA transcripts in Mecp2-deficient mice and RTT patients, as downstream molecules that are controlled by nucleolin. By data mining of published ChIP-seq studies, we showed MeCP2-binding at the multi-copy rRNA genes in the mouse brain, suggesting that rRNA might be a direct MeCP2 target gene. Additionally, we observed compromised mTOR-P70S6K signaling in the human RTT brain, a molecular pathway that is upstream of rRNA-nucleolin molecular conduits. RTT patients showed significantly higher phosphorylation of active mTORC1 or mTORC2 complexes compared to age- and sex-matched controls. Correlational analysis of mTORC1/2-P70S6K signaling pathway identified multiple points of deviation from the control tissues that may result in abnormal ribosome biogenesis in RTT brain. To our knowledge, this is the first report of deregulated nucleolin-rRNA-mTOR-P70S6K signaling in the human RTT brain. Our results provide important insight toward understanding the molecular properties of human RTT brain.

A recurrent de novo missense mutation in UBTF causes developmental neuroregression

UBTF (upstream binding transcription factor) exists as two isoforms; UBTF1 regulates rRNA transcription by RNA polymerase 1, whereas UBTF2 regulates mRNA transcription by RNA polymerase 2. Herein, we describe 4 patients with very similar patterns of neuroregression due to recurrent de novo mutations in UBTF (GRCh37/hg19, NC_000017.10: g.42290219C > T, NM_014233.3: c.628G > A) resulting in the same amino acid change in both UBTF1 and UBTF2 (p.Glu210Lys [p.E210K]). Disease onset in our cohort was at 2.5 to 3 years and characterized by slow progression of global motor, cognitive and behavioral dysfunction. Notable early features included hypotonia with a floppy gait, high-pitched dysarthria and hyperactivity. Later features included aphasia, dystonia, and spasticity. Speech and ambulatory ability were lost by the early teens. Magnetic resonance imaging showed progressive generalized cerebral atrophy (supratentorial > infratentorial) with involvement of both gray and white matter. Patient fibroblasts showed normal levels of UBTF transcripts, increased expression of pre-rRNA and 18S rRNA, nucleolar abnormalities, markedly increased numbers of DNA breaks, defective cell-cycle progression, and apoptosis. Expression of mutant human UBTF1 in Drosophila neurons was lethal. Although no loss-of-function variants are reported in the Exome Aggregation Consortium (ExAC) database and Ubtf-/- is early embryonic lethal in mice, Ubtf+/- mice displayed only mild motor and behavioral dysfunction in adulthood. Our data underscore the importance of including UBTF E210K in the differential diagnosis of neuroregression and suggest that mainly gain-of-function mechanisms contribute to the pathogenesis of the UBTF E210K neuroregression syndrome.

A Deconvolution Protocol for ChIP-Seq Reveals Analogous Enhancer Structures on the Mouse and Human Ribosomal RNA Genes

The combination of Chromatin Immunoprecipitation and Massively Parallel Sequencing, or ChIP-Seq, has greatly advanced our genome-wide understanding of chromatin and enhancer structures. However, its resolution at any given genetic locus is limited by several factors. In applying ChIP-Seq to the study of the ribosomal RNA genes, we found that a major limitation to resolution was imposed by the underlying variability in sequence coverage that very often dominates the protein-DNA interaction profiles. Here, we describe a simple numerical deconvolution approach that, in large part, corrects for this variability, and significantly improves both the resolution and quantitation of protein-DNA interaction maps deduced from ChIP-Seq data. This approach has allowed us to determine the in vivo organization of the RNA polymerase I preinitiation complexes that form at the promoters and enhancers of the mouse (Mus musculus) and human (Homo sapiens) ribosomal RNA genes, and to reveal a phased binding of the HMG-box factor UBF across the rDNA. The data identify and map a "Spacer Promoter" and associated stalled polymerase in the intergenic spacer of the human ribosomal RNA genes, and reveal a very similar enhancer structure to that found in rodents and lower vertebrates.

A unique enhancer boundary complex on the mouse ribosomal RNA genes persists after loss of Rrn3 or UBF and the inactivation of RNA polymerase I transcription

Transcription of the several hundred of mouse and human Ribosomal RNA (rRNA) genes accounts for the majority of RNA synthesis in the cell nucleus and is the determinant of cytoplasmic ribosome abundance, a key factor in regulating gene expression. The rRNA genes, referred to globally as the rDNA, are clustered as direct repeats at the Nucleolar Organiser Regions, NORs, of several chromosomes, and in many cells the active repeats are transcribed at near saturation levels. The rDNA is also a hotspot of recombination and chromosome breakage, and hence understanding its control has broad importance. Despite the need for a high level of rDNA transcription, typically only a fraction of the rDNA is transcriptionally active, and some NORs are permanently silenced by CpG methylation. Various chromatin-remodelling complexes have been implicated in counteracting silencing to maintain rDNA activity. However, the chromatin structure of the active rDNA fraction is still far from clear. Here we have combined a high-resolution ChIP-Seq protocol with conditional inactivation of key basal factors to better understand what determines active rDNA chromatin. The data resolve questions concerning the interdependence of the basal transcription factors, show that preinitiation complex formation is driven by the architectural factor UBF (UBTF) independently of transcription, and that RPI termination and release corresponds with the site of TTF1 binding. They further reveal the existence of an asymmetric Enhancer Boundary Complex formed by CTCF and Cohesin and flanked upstream by phased nucleosomes and downstream by an arrested RNA Polymerase I complex. We find that the Enhancer Boundary Complex is the only site of active histone modification in the 45kbp rDNA repeat. Strikingly, it not only delimits each functional rRNA gene, but also is stably maintained after gene inactivation and the re-establishment of surrounding repressive chromatin. Our data define a poised state of rDNA chromatin and place the Enhancer Boundary Complex as the likely entry point for chromatin remodelling complexes.

A randomized phase 2 study of MK-2206 versus everolimus in refractory renal cell carcinoma

Background

Activation of the phosphoinisitide-3 kinase (PI3K) pathway through mutation and constitutive upregulation has been described in renal cell carcinoma (RCC), making it an attractive target for therapeutic intervention. We performed a randomized phase II study in vascular endothelial growth factor (VEGF) therapy refractory patients to determine whether MK-2206, an allosteric inhibitor of AKT, was more efficacious than the mammalian target of rapamycin inhibitor everolimus.

Patients and methods

A total of 43 patients were randomized in a 2:1 distribution, with 29 patients assigned to the MK-2206 arm and 14 to the everolimus arm. Progression-free survival (PFS) was the primary endpoint.

Results

The trial was closed at the first futility analysis with an observed PFS of 3.68 months in the MK-2206 arm and 5.98 months in the everolimus arm. Dichotomous response rate profiles were seen in the MK-2206 arm with one complete response and three partial responses in the MK-2206 arm versus none in the everolimus arm. On the other hand, progressive disease was best response in 44.8% of MK2206 versus 14.3% of everolimus-treated patients. MK-2206 induced significantly more rash and pruritis than everolimus, and dose reduction occurred in 37.9% of MK-2206 versus 21.4% of everolimus-treated patients. Genomic analysis revealed that 57.1% of the patients in the PD group had either deleterious TP53 mutations or ATM mutations or deletions. In contrast, none of the patients in the non-PD group had TP53 or ATM defects. No predictive marker for response was observed in this small dataset.

Conclusions

Dichotomous outcomes are observed when VEGF therapy refractory patients are treated with MK-2206, and MK-2206 does not demonstrate superiority to everolimus. Additionally, mutations in DNA repair genes are associated with early disease progression, indicating that dysregulation of DNA repair is associated with a more aggressive tumor phenotype in RCC.

Depletion of the cisplatin targeted HMGB-box factor UBF selectively induces p53-independent apoptotic death in transformed cells

Cisplatin-DNA adducts act as strong decoys for the Upstream Binding Factor UBF (UBTF) and have been shown to inhibit transcription of the ribosomal RNA genes by RNA polymerase I. However, it is unclear if this plays a significant role in the chemotherapeutic activity of cis- or carboplatin. We find that cisplatin in fact induces a very rapid displacement of UBF from the ribosomal RNA genes and strong inhibition of ribosomal RNA synthesis, consistent with this being an important factor in its cytotoxicity. Using conditional gene deletion, we recently showed that UBF is an essential factor for transcription of the ribosomal RNA genes and for ribosome biogenesis. We now show that loss of UBF arrests cell proliferation and induces fully penetrant, rapid and synchronous apoptosis, as well as nuclear disruption and cell death, specifically in cells subjected to oncogenic stress. Apoptosis is not affected by homozygous deletion of the p53 gene and occurs equally in cells transformed by SV40 T antigens, by Myc or by a combination of Ras & Myc oncogenes. The data strongly argue that inhibition of UBF function is a major factor in the cytotoxicity of cisplatin. Hence, drug targeting of UBF may be a preferable approach to the use of the highly toxic platins in cancer therapy.

Loss of all 3 Extended Synaptotagmins does not affect normal mouse development, viability or fertility

The extended synaptotagmins, E-Syt1, 2 and 3, are multiple C2 domain membrane proteins that are tethered to the endoplasmic reticulum and interact in a calcium dependent manner with plasma membrane phospholipids to form endoplasmic reticulum - plasma membrane junctions. These junctions have been implicated in the exchange of phospholipids between the 2 organelles. The E-Syts have further been implicated in receptor signaling and endocytosis and can interact directly with fibroblast growth factor and other cell surface receptors. Despite these multiple functions, the search for a requirement in vivo has been elusive. Most recently, we found that the genes for E-Syt2 and 3 could be inactivated without effect on mouse development, viability, fertility or morphology. We have now created insertion and deletion mutations in the last of the mouse E-Syt genes. We show that E-Syt1 is specifically expressed throughout the embryonic skeleton during the early stages of chrondrogenesis in a pattern quite distinct from that of E-Syt2 or 3. Despite this, E-Syt1 is also not required for mouse development and propagation. We further show that even the combined inactivation of all 3 E-Syt genes has no effect on mouse viability or fertility in the laboratory. However, this inactivation induces an enhancement in the expression of the genes encoding Orp5/8, Orai1, STIM1 and TMEM110, endoplasmic reticulum - plasma membrane junction proteins that potentially could compensate for E-Syt loss. Given the multiple functions suggested for the E-Syts and their evolutionary conservation, our unexpected findings suggest that they may only provide a survival advantage under specific conditions that have as yet to be identified.

Loss of Extended Synaptotagmins ESyt2 and ESyt3 does not affect mouse development or viability, but in vitro cell migration and survival under stress are affected

The Extended Synaptotagmins (Esyts) are a family of multi-C2 domain membrane proteins with orthologs in organisms from yeast to human. Three Esyt genes exist in mouse and human and these have most recently been implicated in the formation of junctions between endoplasmic reticulum and plasma membrane, as well as the Ca²⁺ dependent replenishment of membrane phospholipids. The data are consistent with a function in extracellular signal transduction and cell adhesion, and indeed Esyt2 was previously implicated in both these functions in Xenopus. Despite this, little is known of the function of the Esyts in vivo. We have generated mouse lines carrying homozygous deletions in one or both of the genes encoding the highly homologous Esyt2 and Esyt3 proteins. Surprisingly, esyt2^−/−/esyt3^−/− mice develop normally and are both viable and fertile. In contrast, esyt2^−/−/esyt3^−/− mouse embryonic fibroblasts display a reduced ability to migrate in standard in vitro assays, and are less resistant to stringent culture conditions and to oxidative stress than equivalent wild type fibroblasts.

Extended Synaptotagmin Interaction with the Fibroblast Growth Factor Receptor Depends on Receptor Conformation, Not Catalytic Activity

We previously demonstrated that ESyt2 interacts specifically with the activated FGF receptor and is required for a rapid phase of receptor internalization and for functional signaling via the ERK pathway in early Xenopus embryos. ESyt2 is one of the three-member family of Extended Synaptotagmins that were recently shown to be implicated in the formation of endoplasmic reticulum (ER)-plasma membrane (PM) junctions and in the Ca(2+) dependent regulation of these junctions. Here we show that ESyt2 is directed to the ER by its putative transmembrane domain, that the ESyts hetero- and homodimerize, and that ESyt2 homodimerization in vivo requires a TM adjacent sequence but not the SMP domain. ESyt2 and ESyt3, but not ESyt1, selectively interact in vivo with activated FGFR1. In the case of ESyt2, this interaction requires a short TM adjacent sequence and is independent of receptor autophosphorylation, but dependent on receptor conformation. The data show that ESyt2 recognizes a site in the upper kinase lobe of FGFR1 that is revealed by displacement of the kinase domain activation loop during receptor activation.

A novel role for the Pol I transcription factor UBTF in maintaining genome stability through the regulation of highly transcribed Pol II genes

Mechanisms to coordinate programs of highly transcribed genes required for cellular homeostasis and growth are unclear. Upstream binding transcription factor (UBTF, also called UBF) is thought to function exclusively in RNA polymerase I (Pol I)-specific transcription of the ribosomal genes. Here, we report that the two isoforms of UBTF (UBTF1/2) are also enriched at highly expressed Pol II-transcribed genes throughout the mouse genome. Further analysis of UBTF1/2 DNA binding in immortalized human epithelial cells and their isogenically matched transformed counterparts reveals an additional repertoire of UBTF1/2-bound genes involved in the regulation of cell cycle checkpoints and DNA damage response. As proof of a functional role for UBTF1/2 in regulating Pol II transcription, we demonstrate that UBTF1/2 is required for recruiting Pol II to the highly transcribed histone gene clusters and for their optimal expression. Intriguingly, lack of UBTF1/2 does not affect chromatin marks or nucleosome density at histone genes. Instead, it results in increased accessibility of the histone promoters and transcribed regions to micrococcal nuclease, implicating UBTF1/2 in mediating DNA accessibility. Unexpectedly, UBTF2, which does not function in Pol I transcription, is sufficient to regulate histone gene expression in the absence of UBTF1. Moreover, depletion of UBTF1/2 and subsequent reduction in histone gene expression is associated with DNA damage and genomic instability independent of Pol I transcription. Thus, we have uncovered a novel role for UBTF1 and UBTF2 in maintaining genome stability through coordinating the expression of highly transcribed Pol I (UBTF1 activity) and Pol II genes (UBTF2 activity).

The Cruciform DNA Mobility Shift Assay: A Tool to Study Proteins That Recognize Bent DNA

So-called architectural DNA-binding proteins such as those of the HMGB-box family induce DNA bending and kinking. However, these proteins often display only a weak sequence preference, making the analysis of their DNA-binding characteristics difficult if not impossible in a standard electrophoretic mobility shift assay (EMSA). In contrast, such proteins often bind prebent DNAs with high affinity and specificity. A synthetic cruciform DNA structure will often provide an ideal binding site for such proteins, allowing their affinities for both bent and linear DNAs to be directly and simply determined by a modified form of EMSA.

In Vitro DNase I Footprinting

The association of proteins with the DNA double helix can interfere with the accessibility of the latter to nucleases. This is particularly true when using bulky nucleases such as DNase I. The DNase I footprinting method was developed to take advantage of this fact in the study of DNA-protein interactions: it consists in comparing the pattern of fragments generated by the partial digestion of a DNA sequence in the absence of a protein to that produced by its partial digestion in the presence of said protein. Normally, when the two sets of fragments are separated side by side on a gel, the ladder of DNase I-generated fragments produced in the presence of the protein will feature blank regions (devoid of fragments, indicating protection) and/or enhanced cleavage sites (indicating increased availability to the nuclease). This technique can furthermore reveal if multiple sites for a DNA-binding protein are present on a same fragment and in such a case will also allow the comparison of their respective affinities.

Conditional inactivation of Upstream Binding Factor reveals its epigenetic functions and the existence of a somatic nucleolar precursor body

Upstream Binding Factor (UBF) is a unique multi-HMGB-box protein first identified as a co-factor in RNA polymerase I (RPI/PolI) transcription. However, its poor DNA sequence selectivity and its ability to generate nucleosome-like nucleoprotein complexes suggest a more generalized role in chromatin structure. We previously showed that extensive depletion of UBF reduced the number of actively transcribed ribosomal RNA (rRNA) genes, but had little effect on rRNA synthesis rates or cell proliferation, leaving open the question of its requirement for RPI transcription. Using gene deletion in mouse, we now show that UBF is essential for embryo development beyond morula. Conditional deletion in cell cultures reveals that UBF is also essential for transcription of the rRNA genes and that it defines the active chromatin conformation of both gene and enhancer sequences. Loss of UBF prevents formation of the SL1/TIF1B pre-initiation complex and recruitment of the RPI-Rrn3/TIF1A complex. It is also accompanied by recruitment of H3K9me₃, canonical histone H1 and HP1α, but not by de novo DNA methylation. Further, genes retain penta-acetyl H4 and H2A.Z, suggesting that even in the absence of UBF the rRNA genes can maintain a potentially active state. In contrast to canonical histone H1, binding of H1.4 is dependent on UBF, strongly suggesting that it plays a positive role in gene activity. Unexpectedly, arrest of rRNA synthesis does not suppress transcription of the 5S, tRNA or snRNA genes, nor expression of the several hundred mRNA genes implicated in ribosome biogenesis. Thus, rRNA gene activity does not coordinate global gene expression for ribosome biogenesis. Loss of UBF also unexpectedly induced the formation in cells of a large sub-nuclear structure resembling the nucleolar precursor body (NPB) of oocytes and early embryos. These somatic NPBs contain rRNA synthesis and processing factors but do not associate with the rRNA gene loci (NORs).

Upstream Binding Factor (UBF) is multi-HMGB-box protein found in all vertebrates. Although this protein has been implicated in transcription of the ribosomal RNA (rRNA) gene in vitro, little is known of its function in vivo. We previously found that UBF creates a nucleosome-like structure on DNA, and that this structure is remodeled by MAP-kinase phosphorylation. Using conditional gene deletion in mouse and mouse cells we show that UBF defines the active chromatin domains of the rRNA genes and is essential for transcription of these genes. Using this system we show that, contrary to expectation, rRNA gene activity does not coordinate ribosome production. We further show that in the complete absence of rRNA synthesis a somatic nucleolar precursor body is formed. Our data show that UBF determines a dynamic transition between the active and inactive rRNA gene states that is independent of changes in DNA methylation.

The endocytic adapter E-Syt2 recruits the p21 GTPase activated kinase PAK1 to mediate actin dynamics and FGF signalling

Fibroblast growth factor (FGF) signalling plays an essential role in early vertebrate development. However, the response to FGF requires endocytosis of the activated FGF receptor (FGFR) that is in part dependent on remodelling of the actin cytoskeleton. Recently we showed that the extended synaptotagmin family plasma membrane protein, E-Syt2, is an essential endocytic adapter for FGFR1. Here we show E-Syt2 is also an interaction partner for the p21-GTPase Activated Kinase PAK1. The phospholipid binding C2C domain of E-Syt2 specifically binds a site adjacent to the CRIB/GBD of PAK1. PAK1 and E-Syt2 selectively complex with FGFR1 and functionally cooperate in the FGF signalling. E-Syt2 binding suppresses actin polymerization and inhibits the activation of PAK1 by the GTPases Cdc42 and Rac. Interestingly, the E-Syt2 binding site on PAK1 extensively overlaps a site recently suggested to bind phospholipids. Our data suggest that PAK1 interacts with phospholipid membrane domains via E-Syt2, where it may cooperate in the E-Syt2-dependent endocytosis of activated FGFR1 by modulating cortical actin stability.

The cellular abundance of the essential transcription termination factor TTF-I regulates ribosome biogenesis and is determined by MDM2 ubiquitinylation

The ARF tumour suppressor stabilizes p53 by negatively regulating the E3 ubiquitin ligase MDM2 to promote cell cycle arrest and cell death. However, ARF is also able to arrest cell proliferation by inhibiting ribosome biogenesis. In greater part this is achieved by targeting the transcription termination factor I (TTF-I) for nucleolar export, leading to an inhibition of both ribosomal RNA synthesis and processing. We now show that in the absence of ARF, TTF-I is ubiquitinylated by MDM2. MDM2 interacts directly with TTF-I and regulates its cellular abundance by targeting it for degradation by the proteasome. Enhanced TTF-I levels inhibit ribosome biogenesis by suppressing ribosomal RNA synthesis and processing, strongly suggesting that exact TTF-I levels are critical for efficient ribosome biogenesis. We further show that concomitant with its ability to displace TTF-I from the nucleolus, ARF inhibits MDM2 ubiquitinylation of TTF-I by competitively binding to a site overlapping the MDM2 interaction site. Thus, both the sub-nuclear localization and the abundance of TTF-I are key regulators of ribosome biogenesis.

A model for the topology of active ribosomal RNA genes

The Christmas tree view of active ribosomal RNA (rRNA) genes suggests a gene topology in which a large number of nascent rRNA transcripts are prevented from intertwining. The way in which this is achieved has remained unclear. By using a combination of chromatin immunoprecipitation and chromosome conformation capture techniques, we show that the promoter, upstream region and terminator R3 of active rRNA genes are held together spatially throughout the cell cycle, forming a stable core around which the transcribed region is organized. We suggest a new core-helix model for the topology of rRNA genes, that provides a structural basis for the productive synthesis or rRNA.

DNA methyltransferase inhibition may limit cancer cell growth by disrupting ribosome biogenesis

"Mutations" in the pattern of CpG methylation imprinting of the human genome have been correlated with a number of diseases including cancer. In particular, aberrant imprinting of tumor suppressor genes by gain of CpG methylation has been observed in many cancers and thus represents an important alternative pathway to gene "mutation" and tumor progression. Inhibitors of DNA methylation display therapeutic effects in the treatment of certain cancers, and it has been assumed these effects are due to the reversal of "mutant" gene imprinting. However, significant reactivation of imprinted tumor suppressor genes is rarely observed in vivo following treatment with DNA methylation inhibitors. A recent study revealed an unexpected requirement for CpG methylation in the synthesis and assembly of the ribosome, an essential function for cell growth and proliferation. As such, the data provide an unforeseen explanation of the action of DNA methylation inhibitors in restricting cancer cell growth.

Extended-synaptotagmin-2 mediates FGF receptor endocytosis and ERK activation in vivo

Targeting of activated plasma membrane receptors to endocytic pathways is important in determining the outcome of growth factor signaling. However, the molecular mechanisms are still poorly understood. Here, we show that the synaptotagmin-related membrane protein E-Syt2 is essential for rapid endocytosis of the activated FGF receptor and for functional signal transduction during Xenopus development. E-Syt2 depletion prevents an early phase of activated FGF receptor endocytosis that we show is required for ERK activation and the induction of the mesoderm. E-Syt2 interacts selectively with the activated FGF receptor and with Adaptin-2, and is required upstream of Ras activation and of receptor autophosphorylation for ERK activation and the induction of the mesodermal marker Xbra. The data identify E-Syt2 as an endocytic adaptor for the clathrin-mediated pathway whose function is conserved in human and suggest a broader role for the E-Syt subfamily in growth factor signaling.

Role of p21-activated kinase in cell polarity and directional mesendoderm migration in the Xenopus gastrula

The p21 activated kinases (Paks) are prominently involved in the regulation of cell motility. Using a kinase-dead mutant of xPak1, we show that during Xenopus gastrulation, the kinase activity of Pak1 is required upstream of Cdc42 for the establishment of cell polarity in the migrating mesendoderm. Overactivation of Pak1 function by the expression of constitutively active xPak1 compromises the maintenance of cell polarity, by indirectly inhibiting RhoA function. Inhibition of cell polarization does not affect the migration of single mesendoderm cells. However, Pak1 inhibition interferes with the guidance of mesendoderm migration by directional cues residing in the extracellular matrix of the blastocoel roof, and with mesendoderm translocation in the embryo.

Loss of human ribosomal gene CpG methylation enhances cryptic RNA polymerase II transcription and disrupts ribosomal RNA processing

Epigenetic methyl-CpG silencing of the ribosomal RNA (rRNA) genes is thought to downregulate rRNA synthesis in mammals. In contrast, we now show that CpG methylation in fact positively influences rRNA synthesis and processing. Human HCT116 cells, inactivated for DNMT1 and DNMT3b or treated with aza-dC, lack CpG methylation and reactivate a large fraction of normally silent rRNA genes. Unexpectedly, these cells display reduced rRNA synthesis and processing and accumulate unprocessed 45S rRNA. Reactivation of the rRNA genes is associated with their cryptic transcription by RNA polymerase II. Ectopic expression of cryptic rRNA gene transcripts recapitulates the defects associated with loss of CpG methylation. The data demonstrate that rRNA gene silencing prevents cryptic RNA polymerase II transcription of these genes. Lack of silencing leads to the partial disruption of rRNA synthesis and rRNA processing, providing an explanation for the cytotoxic effects of loss of CpG methylation.

UBF levels determine the number of active ribosomal RNA genes in mammals

In mammals, the mechanisms regulating the number of active copies of the approximately 200 ribosomal RNA (rRNA) genes transcribed by RNA polymerase I are unclear. We demonstrate that depletion of the transcription factor upstream binding factor (UBF) leads to the stable and reversible methylation-independent silencing of rRNA genes by promoting histone H1-induced assembly of transcriptionally inactive chromatin. Chromatin remodeling is abrogated by the mutation of an extracellular signal-regulated kinase site within the high mobility group box 1 domain of UBF1, which is required for its ability to bend and loop DNA in vitro. Surprisingly, rRNA gene silencing does not reduce net rRNA synthesis as transcription from remaining active genes is increased. We also show that the active rRNA gene pool is not static but decreases during differentiation, correlating with diminished UBF expression. Thus, UBF1 levels regulate active rRNA gene chromatin during growth and differentiation.

The splice variants of UBF differentially regulate RNA polymerase I transcription elongation in response to ERK phosphorylation

The mammalian architectural HMGB-Box transcription factor UBF is ubiquitously expressed in two variant forms as the result of a differential splicing event, that in the UBF2 deletes 37 amino acid from the second of six HMGB-boxes. Several attempts to define a function for this shorter UBF2 protein have been less than satisfactory. However, since all mammals appear to display similar levels of the longer and shorter UBF variants, it is unlikely that UBF2 is simply nonfunctional. Previously we showed that phosphorylation of UBF by the MAP-kinase ERK regulates chromatin folding and transcription elongation, explaining the rapid response of the ribosomal RNA genes to growth factors. Here we have investigated the roles the UBF variants play in the response of these genes to ERK activity. We demonstrate that the variant HMGB-box 2 of UBF2 has lost the ability to bind bent DNA and hence to induce chromatin folding. As a result it is significantly less effective than UBF1 at arresting RNAPI elongation but at the same time is more responsive to ERK phosphorylation. Thus, UBF2 functionally simulates a hemi-phosphorylated UBF whose expression may provide a means by which to tune the response of the ribosomal RNA genes to growth factor stimulation.

Mice lacking both mixed-lineage kinase genes Mlk1 and Mlk2 retain a wild type phenotype

The mitogen-activated protein kinase kinase kinases of the mixed-lineage kinase (MLK) family have been shown to activate the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) pathway, and to regulate the other two principal MAPK cascades, p38 and extracellular signal-regulated kinase (ERK). Although there is growing evidence for their involvement in neuronal cell death leading to neurodegenerative disorders, little in vivo data is available for the members of this family of kinases. Here, we report that the inactivation of mouse Mlk1 and Mlk2 genes. Mlk1(-/-) and Mlk2(-/-) mice were found to be viable and healthy. Surprisingly, mice carrying the compound Mlk1/Mlk2 null mutations were also found to be viable, fertile and to have a normal life span. The nervous system, testis and kidney, the major sites of MLK1 and 2 expression, all appear normal, as do other organs where these kinases were found to be more weakly expressed. Surprisingly, developmental neuronal programmed cell death, another potential target for MLK family members, was also found to be unaffected. Our results suggest that there is extensive functional redundancy between MLK1/MLK2 and the other member of the family, MLK3, which is also not required for survival in mouse.

EphA4 signaling regulates blastomere adhesion in the Xenopus embryo by recruiting Pak1 to suppress Cdc42 function

The control of cell adhesion is an important mechanism by which Eph receptors regulate cell sorting during development. Activation of EphA4 in Xenopus blastulae induces a reversible, cell autonomous loss-of-adhesion and disruption of the blastocoel roof. We show this phenotype is rescued by Nckbeta (Grb4) dependent on its interaction with EphA4. Xenopus p21(Cdc42/Rac)-activated kinase xPAK1 interacts with Nck, is activated in embryo by EphA4 in an Nck-dependent manner, and is required for EphA4-induced loss-of-adhesion. Ectopic expression of xPAK1 phenocopies EphA4 activation. This does not require the catalytic activity of xPAK1, but it does require its GTPase binding domain and is enhanced by membrane targeting. Indeed, membrane targeting of the GTPase binding domain (GBD) of xPAK1 alone is sufficient to phenocopy EphA4 loss-of-adhesion. Both EphA4 and the xPAK1-GBD down-regulate RhoA-GTP levels, and consistent with this, loss-of-adhesion can be rescued by activated Cdc42, Rac, and RhoA and can be epistatically induced by dominant-negative RhoA. Despite this, neither Cdc42 nor Rac activities are down-regulated by EphA4 activation or by the xPAK1-GBD. Together, the data suggest that EphA4 activation sequesters active Cdc42 and in this way down-regulates cell-cell adhesion. This novel signaling pathway suggests a mechanism for EphA4-guided migration.

A new paradigm for the regulation of the mammalian ribosomal RNA genes

Ribosome assembly occurs co-transcriptionally on the rRNA genes. This process requires the co-ordinated expression and assembly of many hundreds of proteins and is finely tuned to cell and organism growth. Co-ordinate regulation of the rRNA genes and the ribosomal protein genes is therefore essential for high-fidelity ribosome assembly. Recent work shows that rRNA gene transcription is regulated at the level of elongation via the mitogen-activated protein kinase pathway. We argue that this may provide an explanation for the high fidelity of ribosome assembly.

Immunization information systems progress - United States, 2005

Immunization registries are confidential, computerized information systems that collect and consolidate vaccination data from multiple health-care providers, generate reminder and recall notifications, and assess vaccination coverage within a defined geographic area. A registry with added capabilities, such as vaccine management, adverse event reporting, lifespan vaccination histories, and linkages with electronic data sources, is called an immunization information system (IIS). This report summarizes data from CDC's 2005 Immunization Information System Annual Report (IISAR), a survey of grantees in 50 states, five cities, and the District of Columbia (DC) that receive funding under section 317b of the Public Health Service Act. These data indicated that approximately 56% of U.S. children aged <6 years participated in an IIS, an increase from 48% in 2004. Moreover, 75% percent of public vaccination provider sites and 44% of private vaccination provider sites submitted vaccination data to an IIS during July-December 2005. These findings underscore the need to increase the number of participating children, from the current 13 million to approximately 21 million, to assure 95% participation of children aged <6 years and improve the effectiveness of U.S. immunization programs.

A housekeeper with power of attorney: the rRNA genes in ribosome biogenesis

Ribosome biogenesis centres both physically and functionally on the activity of the ribosomal RNA (rRNA) genes. Ribosome assembly occurs co-transcriptionally on these genes, requires the coordinated expression and assembly of many hundreds of proteins and is finely tuned to cell and organism growth. This review presents contemporary understanding of the mode and the means of rRNA gene transcription and how growth factors, oncogenes and tumour suppressors regulate this transcription. It is argued that transcription elongation is a key mechanism regulating rRNA gene transcription. This unorthodox view provides a logical framework to explain the co-transcriptional phase of ribosome biogenesis.

Malignant transformation of posterior fossa epidermoid cyst

The authors report the case of a 45-year-old man who presented with a short duration of a painful ophthalmoparesis. Initial magnetic resonance imaging revealed an extraaxial petroclival mass characteristic of an epidermoid cyst, with the exception of a contiguous contrast-enhancing lobule. A subtotal resection was performed with the histopathological diagnosis revealing malignant transformation of an epidermoid cyst. Despite aggressive postoperative adjuvant therapy, the patient developed leptomeningeal metastasis and died shortly thereafter. The presence of contrast enhancement at the site of an epidermoid cyst combined with an acute, progressive neurological deficit should alert the treating physician to the possibility of a malignant transformation. When transformation does occur, the clinical and radiological course is quite aggressive as compared with the indolent growth of epidermoid cysts. Treatment options include surgery with adjuvant chemotherapy or radiotherapy. We review the pertinent features of this case along with the relevant literature regarding primary intracranial squamous cell carcinomas.

Cellular senescence in naevi and immortalisation in melanoma: a role for p16?

Cellular senescence, the irreversible proliferative arrest seen in somatic cells after a limited number of divisions, is considered a crucial barrier to cancer, but direct evidence for this in vivo was lacking until recently. The best-known form of human cell senescence is attributed to telomere shortening and a DNA-damage response through p53 and p21. There is also a more rapid form of senescence, dependent on the p16-retinoblastoma pathway. p16 (CDKN2A) is a known melanoma susceptibility gene. Here, we use retrovirally mediated gene transfer to confirm that the normal form of senescence in cultured human melanocytes involves p16, since disruption of the p16/retinoblastoma pathway is required as well as telomerase activation for immortalisation. Expression (immunostaining) patterns of senescence mediators and markers in melanocytic lesions provide strong evidence that cell senescence occurs in benign melanocytic naevi (moles) in vivo and does not involve p53 or p21 upregulation, although p16 is widely expressed. In comparison, dysplastic naevi and early (radial growth-phase, RGP) melanomas show less p16 and some p53 and p21 immunostaining. All RGP melanomas expressed p21, suggesting areas of p53-mediated senescence, while most areas of advanced (vertical growth-phase) melanomas lacked both p16 and p21, implying escape from both forms of senescence (immortalisation). Moreover, nuclear p16 but not p21 expression can be induced in human melanocytes by oncogenic BRAF, as found in around 80% of naevi. We conclude that cell senescence can form a barrier to melanoma development. This also provides a potential explanation of why p16 is a melanoma suppressor gene.

ERK modulates DNA bending and enhancesome structure by phosphorylating HMG1-boxes 1 and 2 of the RNA polymerase I transcription factor UBF

Transcription of the ribosomal RNA genes of mammals by RNA polymerase I is rapidly activated by epidermal growth factor via the MAP-kinase (ERK) signaling cascade. This activation is mediated by direct phosphorylation of the HMG box DNA binding domains of the architectural transcription factor UBF. Mutation of the ERK sites of UBF inhibits its normal function and blocks growth factor activation of ribosomal transcription. UBF has little or no DNA sequence selectivity and binds throughout the ribosomal genes, defining a specialized chromatin. Indeed, the HMG boxes of UBF induce looping of the ribosomal DNA to create the enhancesome, a structure somewhat reminiscent of the nucleosome. Here, we show that both ERK phosphorylation and mutations that simulate this phosphorylation decrease the affinity of the individual HMG boxes of UBF for linear ribosomal DNA but have little or no effect on the capacity of these HMG boxes to bind to pre-bent DNA and do not affect the overall binding constant of UBF for the DNA. Electron spectroscopic imaging showed that ERK site UBF mutants do not induce the characteristic DNA looping of the enhancesome and associate with no more than half of the enhancesomal DNA. The data demonstrate that ERK phosphorylation of UBF prevents DNA bending by its first two HMG boxes, leading to a cooperative unfolding of the enhancesome.