p53 tumor suppressor protein regulates the levels of huntingtin gene expression.

The p53 protein is a transcription factor that integrates various cellular stress signals. The accumulation of the mutant huntingtin protein with an expanded polyglutamine tract plays a central role in the pathology of human Huntington's disease. We found that the huntingtin gene contains multiple putative p53-responsive elements and p53 binds to these elements both in vivo and in vitro. p53 activation in cultured human cells, either by a temperature-sensitive mutant p53 protein or by gamma-irradiation (gamma-irradiation), increases huntingtin mRNA and protein expression. Similarly, murine huntingtin also contains multiple putative p53-responsive elements and its expression is induced by p53 activation in cultured cells. Moreover, gamma-irradiation, which activates p53, increases huntingtin gene expression in the striatum and cortex of mouse brain, the major pathological sites for Huntington's disease, in p53+/+ but not the isogenic p53-/- mice. These results demonstrate that p53 protein can regulate huntingtin expression at transcriptional level, and suggest that a p53 stress response could be a modulator of the process of Huntington's disease.

Obstruction of BRAFV600E transcription by complementary PNA oligomers as a means to inhibit BRAF-mutant melanoma growth.

Peptide nucleic acid (PNA) oligomers are DNA mimics, which are capable of binding gene sequences 1000-fold more avidly than complementary native DNA by strand invasion and effectively obstruct transcription. Irreversibly obstructing the transcription or replication of a gene sequence, such as BRAFV600E, offers a potential route to specifically target the cancer cell itself. We have employed PNA oligomers to target BRAFV600E in a sequence-specific complementary manner. These PNAs have been modified by appending configurationally stabilizing cationic peptides in order to improve their cellular delivery and target avidity. Our results indicate that exposure of the melanoma cell lines to a modified PNA-peptide conjugate complementary to BRAFV600E mutation sequence results in a concentration-dependent and time-dependent inhibition of cell growth that is specific for the BRAFV600E-mutant melanoma cell lines with inhibition of mRNA and protein expression. Xenograft mouse trials show increased tumor growth delay and necrosis with the BRAFV600E-complementary PNA-peptide conjugates as compared with the saline and scrambled PNA sequence controls. Similarly, quantitative measurement shows a 2.5-fold decrease in Ki67 and a 3-fold increase in terminal deoxynucleotidyl transferase dUTP nick end labeling expression with this approach. PNA-delivery peptide conjugates represent a novel way to target BRAFV600E and represent a new approach in targeting selective oncogenes that induce tumor growth.

Selection of homeotic proteins for binding to a human DNA replication origin.

We have previously shown that a cell cycle-dependent nucleoprotein complex assembles in vivo on a 74 bp sequence within the human DNA replication origin associated to the Lamin B2 gene. Here, we report the identification, using a one-hybrid screen in yeast, of three proteins interacting with the 74 bp sequence. All of them, namely HOXA13, HOXC10 and HOXC13, are orthologues of the Abdominal-B gene of Drosophila melanogaster and are members of the homeogene family of developmental regulators. We describe the complete open reading frame sequence of HOXC10 and HOXC13 along with the structure of the HoxC13 gene. The specificity of binding of these two proteins to the Lamin B2 origin is confirmed by both band-shift and in vitro footprinting assays. In addition, the ability of HOXC10 and HOXC13 to increase the activity of a promoter containing the 74 bp sequence, as assayed by CAT-assay experiments, demonstrates a direct interaction of these homeoproteins with the origin sequence in mammalian cells. We also show that HOXC10 expression is cell-type-dependent and positively correlates with cell proliferation.

DNA methylation in small cell lung cancer defines distinct disease subtypes and correlates with high expression of EZH2.

Small cell lung cancer (SCLC) is an aggressive malignancy characterized by early metastasis, rapid development of resistance to chemotherapy and genetic instability. This study profiles DNA methylation in SCLC, patient-derived xenografts (PDX) and cell lines at single-nucleotide resolution. DNA methylation patterns of primary samples are distinct from those of cell lines, whereas PDX maintain a pattern closely consistent with primary samples. Clustering of DNA methylation and gene expression of primary SCLC revealed distinct disease subtypes among histologically indistinguishable primary patient samples with similar genetic alterations. SCLC is notable for dense clustering of high-level methylation in discrete promoter CpG islands, in a pattern clearly distinct from other lung cancers and strongly correlated with high expression of the E2F target and histone methyltransferase gene EZH2. Pharmacologic inhibition of EZH2 in a SCLC PDX markedly inhibited tumor growth.

A repeated element in the human lamin B2 gene covers most of an intron and reiterates the exon/intron junction.

Nuclear lamins are intermediate filament-type proteins forming a fibrillar meshwork that underlies the inner nuclear membrane. We have previously reported the identification of the human lamin B2 gene that maps to the subtelomeric band p13.3 of chromosome 19 in close proximity of a human DNA replication origin. Here we report the identification within the human lamin B2 gene of a novel repeated element (variable number of tandem repeats: VNTR) that appears to have a very recent origin, being absent in the genome of mouse and primates such as cercopitheques, lemurs and macaques. The VNTR is adjacent to exon 8 of the lamin B2 gene which, albeit encoding the nuclear localization signal of the protein, is highly divergent both at amino acid and nucleotide level among species. Moreover the VNTR, characterized by a repeated unit of about 100 bp, covers most of intron 8 of the gene and reiterates both the last 7 bp of the upstream exon and the exon/intron junction. RT-PCR experiments carried out on HeLa cell RNA suggest that none of the downstream junctions is used during the processing of the lamin B2 pre-mRNA.

Concurrent loss of the PTEN and RB1 tumor suppressors attenuates RAF dependence in melanomas harboring (V600E)BRAF.

Identifying the spectrum of genetic alterations that cooperate with critical oncogenes to promote transformation provides a foundation for understanding the diversity of clinical phenotypes observed in human cancers. Here, we performed integrated analyses to identify genomic alterations that co-occur with oncogenic BRAF in melanoma and abrogate cellular dependence upon this oncogene. We identified concurrent mutational inactivation of the PTEN and RB1 tumor suppressors as a mechanism for loss of BRAF/MEK dependence in melanomas harboring (V600E)BRAF mutations. RB1 alterations were mutually exclusive with loss of p16(INK4A), suggesting that whereas p16(INK4A) and RB1 may have overlapping roles in preventing tumor formation, tumors with loss of RB1 exhibit diminished dependence upon BRAF signaling for cell proliferation. These findings provide a genetic basis for the heterogeneity of clinical outcomes in patients treated with targeted inhibitors of the mitogen-activated protein kinase pathway. Our results also suggest a need for comprehensive screening for RB1 and PTEN inactivation in patients treated with RAF and MEK-selective inhibitors to determine whether these alterations are associated with diminished clinical benefit in patients whose cancers harbor mutant BRAF.

PML is induced by oncogenic ras and promotes premature senescence.

Oncogenic ras provokes a senescent-like arrest in human diploid fibroblasts involving the Rb and p53 tumor suppressor pathways. To further characterize this response, we compared gene expression patterns between ras-arrested and quiescent IMR90 fibroblasts. One of the genes up-regulated during ras-induced arrest was promyelocytic leukemia (PML) protein, a potential tumor suppressor that encodes a component of nuclear structures known as promyelocytic oncogenic domains (PODs). PML levels increased during both ras-induced arrest and replicative senescence, leading to a dramatic increase in the size and number of PODs. Forced PML expression was sufficient to promote premature senescence. Like oncogenic ras, PML increased the levels of p16, hypophosphorylated Rb, phosphoserine-15 p53, and expression of p53 transcriptional targets. The fraction of Rb and p53 that colocalized with PML markedly increased during ras-induced arrest, and expression of PML alone forced p53 to the PODs. E1A abolished PML-induced arrest and prevented PML induction and p53 phosphorylation in response to oncogenic ras. These results imply that PML acts with Rb and p53 to promote ras-induced senescence and provide new insights into PML regulation and activity.

INK4a/ARF mutations accelerate lymphomagenesis and promote chemoresistance by disabling p53.

The INK4a/ARF locus encodes upstream regulators of the retinoblastoma and p53 tumor suppressor gene products. To compare the impact of these loci on tumor development and treatment response, the Emu-myc transgenic lymphoma model was used to generate genetically defined tumors with mutations in the INK4a/ARF, Rb, or p53 genes. Like p53 null lymphomas, INK4a/ARF null lymphomas formed rapidly, were highly invasive, displayed apoptotic defects, and were markedly resistant to chemotherapy in vitro and in vivo. Furthermore, INK4a/ARF(-/-) lymphomas displayed reduced p53 activity despite the presence of wild-type p53 genes. Consequently, INK4a/ARF and p53 mutations lead to aggressive tumors by disrupting overlapping tumor suppressor functions. These data have important implications for understanding the clinical behavior of human tumors.

E1A signaling to p53 involves the p19(ARF) tumor suppressor.

The adenovirus E1A oncogene activates p53 through a signaling pathway involving the retinoblastoma protein and the tumor suppressor p19(ARF). The ability of E1A to induce p53 and its transcriptional targets is severely compromised in ARF-null cells, which remain resistant to apoptosis following serum depletion or adriamycin treatment. Reintroduction of p19(ARF) restores p53 accumulation and resensitizes ARF-null cells to apoptotic signals. Therefore, p19(ARF) functions as part of a p53-dependent failsafe mechanism to counter uncontrolled proliferation. Synergistic effects between the p19(ARF) and DNA damage pathways in inducing p53 may contribute to E1A's ability to enhance radio- and chemosensitivity.