The impact of HTLV-1 expression on the 3D structure and expression of host chromatin.

A typical HTLV-1-infected individual carries >104 different HTLV-1-infected T cell clones, each with a single-copy provirus integrated in a unique genomic site. We previously showed that the HTLV-1 provirus causes aberrant transcription in the flanking host genome and, by binding the chromatin architectural protein CTCF, forms abnormal chromatin loops with the host genome. However, it remained unknown whether these effects were exerted simply by the presence of the provirus or were induced by its transcription. To answer this question, we sorted HTLV-1-infected T-cell clones into cells positive or negative for proviral plus-strand expression, and then quantified host and provirus transcription using RNA-seq, and chromatin looping using quantitative chromosome conformation capture (q4C), in each cell population. We found that proviral plus-strand transcription induces aberrant transcription and splicing in the flanking genome but suppresses aberrant chromatin loop formation with the nearby host chromatin. Reducing provirus-induced host transcription with an inhibitor of transcriptional elongation allows recovery of chromatin loops in the plus-strand-expressing population. We conclude that aberrant host transcription induced by proviral expression causes temporary, reversible disruption of chromatin looping in the vicinity of the provirus.

A frequent PLCγ1 mutation in adult T-cell leukemia/lymphoma determines functional properties of the malignant cells.

BACKGROUND: Development of adult T-cell leukemia/lymphoma (ATL) involves human T-cell leukemia virus type 1 (HTLV-1) infection and accumulation of somatic mutations. The most frequently mutated gene in ATL (36 % of cases) is phospholipase C gamma1 (PLCG1). PLCG1 is also frequently mutated in other T-cell lymphomas. However, the functional consequences of the PLCG1 mutations in cancer cells have not been characterized. METHODS: We compared the activity of the wild-type PLCγ1 with that of a mutant carrying a hot-spot mutation of PLCγ1 (S345F) observed in ATL, both in cells and in cell-free assays. To analyse the impact of the mutation on cellular properties, we quantified cellular proliferation, aggregation, chemotaxis and apoptosis by live cell-imaging in an S345F+ ATL-derived cell line (KK1) and a KK1 cell line in which we reverted the mutation to the wild-type sequence using CRISPR/Cas9 and homology-directed repair. FINDINGS: The PLCγ1 S345F mutation results in an increase of basal PLC activity in vitro and in different cell types. This higher basal activity is further enhanced by upstream signalling. Reversion of the S345F mutation in the KK1 cell line resulted in reduction of the PLC activity, lower rates of proliferation and aggregation, and a marked reduction in chemotaxis towards CCL22. The PLCγ1-pathway inhibitors ibrutinib and ritonavir reduced both the PLC activity and the tested functions of KK1 cells. INTERPRETATION: Consistent with observations from clinical studies, our data provide direct evidence that activated variants of the PLCγ1 enzyme contribute to the properties of the malignant T-cell clone in ATL. FUNDING: MRC (UK) Project Grant (P028160).

The human leukemia virus HTLV-1 alters the structure and transcription of host chromatin in cis.

Chromatin looping controls gene expression by regulating promoter-enhancer contacts, the spread of epigenetic modifications, and the segregation of the genome into transcriptionally active and inactive compartments. We studied the impact on the structure and expression of host chromatin by the human retrovirus HTLV-1. We show that HTLV-1 disrupts host chromatin structure by forming loops between the provirus and the host genome; certain loops depend on the critical chromatin architectural protein CTCF, which we recently discovered binds to the HTLV-1 provirus. We show that the provirus causes two distinct patterns of abnormal transcription of the host genome in cis: bidirectional transcription in the host genome immediately flanking the provirus, and clone-specific transcription in cis at non-contiguous loci up to >300 kb from the integration site. We conclude that HTLV-1 causes insertional mutagenesis up to the megabase range in the host genome in >104 persistently-maintained HTLV-1+ T-cell clones in vivo.

The impact of HTLV-1 on the cellular genome.

Human T-lymphotropic virus type-1 (HTLV-1) is the causative agent of adult T-cell leukaemia/lymphoma (ATL), an aggressive CD4+ T-cell malignancy. The mechanisms of leukaemogenesis in ATL are incompletely understood. Insertional mutagenesis has not previously been thought to contribute to the pathogenesis of ATL. However, the recent discovery that HTLV-1 binds the key chromatin architectural protein CTCF raises the hypothesis that HTLV-1 deregulates host gene expression by causing abnormal chromatin looping, bringing the strong HTLV-1 promoter-enhancer near to host genes that lie up to 2Mb from the integrated provirus. Here we review current opinion on the mechanisms of oncogenesis in ATL, with particular emphasis on the local and distant impact of HTLV-1 on the structure and expression of the host genome.

The retrovirus HTLV-1 inserts an ectopic CTCF-binding site into the human genome.

Human T-lymphotropic virus type 1 (HTLV-1) is a retrovirus that causes malignant and inflammatory diseases in ∼10% of infected people. A typical host has between 10(4) and 10(5) clones of HTLV-1-infected T lymphocytes, each clone distinguished by the genomic integration site of the single-copy HTLV-1 provirus. The HTLV-1 bZIP (HBZ) factor gene is constitutively expressed from the minus strand of the provirus, whereas plus-strand expression, required for viral propagation to uninfected cells, is suppressed or intermittent in vivo, allowing escape from host immune surveillance. It remains unknown what regulates this pattern of proviral transcription and latency. Here, we show that CTCF, a key regulator of chromatin structure and function, binds to the provirus at a sharp border in epigenetic modifications in the pX region of the HTLV-1 provirus in T cells naturally infected with HTLV-1. CTCF is a zinc-finger protein that binds to an insulator region in genomic DNA and plays a fundamental role in controlling higher order chromatin structure and gene expression in vertebrate cells. We show that CTCF bound to HTLV-1 acts as an enhancer blocker, regulates HTLV-1 mRNA splicing, and forms long-distance interactions with flanking host chromatin. CTCF-binding sites (CTCF-BSs) have been propagated throughout the genome by transposons in certain primate lineages, but CTCF binding has not previously been described in present-day exogenous retroviruses. The presence of an ectopic CTCF-BS introduced by the retrovirus in tens of thousands of genomic locations has the potential to cause widespread abnormalities in host cell chromatin structure and gene expression.

A synthetic lethal interaction between APC/C and topoisomerase poisons uncovered by proteomic screens.

The Anaphase-promoting complex/cyclosome (APC/C) cofactor Cdh1 modulates cell proliferation by targeting multiple cell-cycle regulators for ubiquitin-dependent degradation. Lack of Cdh1 results in structural and numerical chromosome aberrations, a hallmark of genomic instability. By using a proteomic approach in Cdh1-null cells and mouse tissues, we have identified kinesin Eg5 and topoisomerase 2α as Cdh1 targets involved in the maintenance of genomic stability. These proteins are ubiquitinated and degraded through specific KEN and D boxes in a Cdh1-dependent manner. Whereas Cdh1-null cells display partial resistance to Eg5 inhibitors such as monastrol, lack of Cdh1 results in a dramatic sensitivity to Top2α poisons as a consequence of increased levels of trapped Top2α-DNA complexes. Chemical inhibition of the APC/C in cancer cells results in increased sensitivity to Top2α poisons. This work identifies in vivo targets of the mammalian APC/C-Cdh1 complex and reveals synthetic lethal interactions of relevance in anticancer treatments.

Correlation of mutant menin stability with clinical expression of multiple endocrine neoplasia type 1 and its incomplete forms.

Germline mutations of the tumor suppressor gene MEN1 are found not only in typical multiple endocrine neoplasia type 1 (MEN1) but also in its incomplete forms such as familial isolated hyperparathyroidism (FIHP) and apparently sporadic parathyroid tumor (ASPT). No definitive genotype-phenotype correlation has been established between these clinical forms and MEN1 gene mutations. We previously demonstrated that mutant menin proteins associated with MEN1 are rapidly degraded by the ubiquitin-proteasome pathway. To examine whether the intracellular stability of mutant menin is correlated with clinical phenotypes, we developed a method of evaluating menin stability and examined 20 mutants associated with typical MEN1 (17 missense, two in-frame deletion, one nonsense) and 21 mutants associated with FIHP or ASPT (19 missense, two in-frame deletion). All tested mutants associated with typical MEN1 showed reduced stability. Some missense and in-frame deletion mutants (G28A, R171W, T197I, E255K, E274A, Y353del and E366D) associated with FIHP or ASPT were almost as stable as or only slightly less stable than wild-type menin, while others were as unstable as those associated with typical MEN1. Some stable mutants exhibited substantial biological activities when tested by JunD-dependent transactivation assay. These findings suggest that certain missense and in-frame mutations are fairly stable and retain intrinsic biological activity, and might be specifically associated with incomplete clinical phenotypes. The menin stability test will provide useful information for the management of patients carrying germline MEN1 mutations especially when they have missense or in-frame variants of ambiguous clinical significance.

Indispensable role of the Runx1-Cbfbeta transcription complex for in vivo-suppressive function of FoxP3+ regulatory T cells.

Naturally arising regulatory T (Treg) cells express the transcription factor FoxP3, which critically controls the development and function of Treg cells. FoxP3 interacts with another transcription factor Runx1 (also known as AML1). Here, we showed that Treg cell-specific deficiency of Cbfbeta, a cofactor for all Runx proteins, or that of Runx1, but not Runx3, induced lymphoproliferation, autoimmune disease, and hyperproduction of IgE. Cbfb-deleted Treg cells exhibited impaired suppressive function in vitro and in vivo, with altered gene expression profiles including attenuated expression of FoxP3 and high expression of interleukin-4. The Runx complex bound to more than 3000 gene loci in Treg cells, including the Foxp3 regulatory regions and the Il4 silencer. In addition, knockdown of RUNX1 showed that RUNX1 is required for the optimal regulation of FoxP3 expression in human T cells. Taken together, our results indicate that the Runx1-Cbfbeta heterodimer is indispensable for in vivo Treg cell function, in particular, suppressive activity and optimal expression of FoxP3.

Foxp3 controls regulatory T-cell function by interacting with AML1/Runx1.

Naturally arising CD25+CD4+ regulatory T cells (T(R) cells) are engaged in the maintenance of immunological self-tolerance and immune homeostasis by suppressing aberrant or excessive immune responses, such as autoimmune disease and allergy. T(R) cells specifically express the transcription factor Foxp3, a key regulator of T(R)-cell development and function. Ectopic expression of Foxp3 in conventional T cells is indeed sufficient to confer suppressive activity, repress the production of cytokines such as interleukin-2 (IL-2) and interferon-gamma (IFN-gamma), and upregulate T(R)-cell-associated molecules such as CD25, cytotoxic T-lymphocyte-associated antigen-4, and glucocorticoid-induced TNF-receptor-family-related protein. However, the method by which Foxp3 controls these molecular events has yet to be explained. Here we show that the transcription factor AML1 (acute myeloid leukaemia 1)/Runx1 (Runt-related transcription factor 1), which is crucially required for normal haematopoiesis including thymic T-cell development, activates IL-2 and IFN-gamma gene expression in conventional CD4+ T cells through binding to their respective promoters. In natural T(R) cells, Foxp3 interacts physically with AML1. Several lines of evidence support a model in which the interaction suppresses IL-2 and IFN-gamma production, upregulates T(R)-cell-associated molecules, and exerts suppressive activity. This transcriptional control of T(R)-cell function by an interaction between Foxp3 and AML1 can be exploited to control physiological and pathological T-cell-mediated immune responses.

A whole MEN1 gene deletion flanked by Alu repeats in a family with multiple endocrine neoplasia type 1.

Multiple endocrine neoplasia type 1 is an autosomal dominant cancer syndrome characterized by pituitary, parathyroid and enteropancreatic endocrine tumors, which is caused by germline mutations of the tumor suppressor gene MEN1. In the case reported here, the patient had family with this disease whose germline MEN1 mutation was undetectable by conventional sequencing analysis. Further investigations involving polymorphism analyses, gene dose assay and nucleotide sequencing identified a large germline deletion of approximately 29 kilobase pairs spanning the whole MEN1 gene. The deletion was flanked by Alu repetitive sequences, suggesting unequal homologous recombination as the deletion mechanism. The polymorphism linkage data suggested that an asymptomatic son of the proband did not carry the family mutation. More direct evidence was obtained by gene dose assay and deletion-specific polymerase chain reaction, which demonstrated the normal MEN1 gene dosage and the absence of the deletion breakpoints in this asymptomatic subject and thus definitely excluded the possibility of disease predisposition.

Detection of APC gene deletion by double competitive polymerase chain reaction in patients with familial adenomatous polyposis.

Familial adenomatous polyposis (FAP) is an autosomal dominant familial cancer syndrome caused by germline mutations of the tumor suppressor adenomatous polyposis coli (APC) gene. Heterozygous apc mutations have been identified in the majority of classical FAP patients who develop more than 100 colorectal adenomas. However, classical FAP patients often fail to display germline APC mutations detectable by routine mutation analysis. These apparently mutation-negative cases may be caused by heterozygous large genomic deletions. In the present study, FAP patients who showed no APC germline mutation detectable by the protein truncation assay and direct sequencing of protein coding exons were screened for APC gene deletion by a gene dose assay based on double competitive polymerase chain reaction. Gene dosage measurements within exon 15 of the APC gene identified two patients with gene deletion and one with possible gene duplication among 41 apparently mutation-negative patients. The deleted sequences in the two patients were determined by fine gene dose mapping around the APC gene and nucleotide sequencing of the deletion breakpoints. They were approximately 435-kilobase pair (kb) and 737-kb regions including the whole APC gene and flanked by a 4-base pair repeat and LINE-1 repetitive sequences, respectively. The chimeric LINE-1 element created at the breakpoint in the latter case also contained a short sequence derived from another LINE-1 element, suggesting a complex unequal homologous recombination event. These findings indicate that this gene dose assay is a useful technique to detect large gene deletions of the APC gene and to determine their genomic breakpoints.

Coactivator-associated arginine methyltransferase 1, CARM1, affects pre-mRNA splicing in an isoform-specific manner.

Molecular diversity through alternative splicing is important for cellular function and development. However, little is known about the factors that regulate alternative splicing. Here we demonstrate that one isoform of coactivator-associated arginine methyltransferase 1 (named CARM1-v3) associates with the U1 small nuclear RNP-specific protein U1C and affects 5' splice site selection of the pre-mRNA splicing. CARM1-v3 was generated by the retention of introns 15 and 16 of the primary transcript of CARM1. Its deduced protein lacks the C-terminal domain of the major isoform of CARM1 and instead has v3-specific sequences at the C terminus. CARM1-v3, but not the other isoforms, strongly stimulates a shift to the distal 5' splice site of the pre-mRNA when the adenoviral E1A minigene is used as a reporter and enhances the exon skips in the CD44 reporter. A CARM1-v3 mutant lacking the v3-specific sequences completely lost the ability to regulate the alternative splicing patterns. In addition, CARM1-v3 shows tissue-specific expression patterns distinct from those of the other isoforms. These results suggest that the transcriptional coactivator can affect the splice site decision in an isoform-specific manner.

Menin missense mutants associated with multiple endocrine neoplasia type 1 are rapidly degraded via the ubiquitin-proteasome pathway.

MEN1 is a tumor suppressor gene that is responsible for multiple endocrine neoplasia type 1 (MEN1) and that encodes a 610-amino-acid protein, called menin. While the majority of germ line mutations identified in MEN1 patients are frameshift and nonsense mutations resulting in truncation of the menin protein, various missense mutations have been identified whose effects on menin activity are unclear. For this study, we analyzed a series of menin proteins with single amino acid alterations and found that all of the MEN1-causing missense mutations tested led to greatly diminished levels of the affected proteins in comparison with wild-type and benign polymorphic menin protein levels. We demonstrate here that the reduced levels of the mutant proteins are due to rapid degradation via the ubiquitin-proteasome pathway. Furthermore, the mutants, but not wild-type menin, interact both with the molecular chaperone Hsp70 and with the Hsp70-associated ubiquitin ligase CHIP, and the overexpression of CHIP promotes the ubiquitination of the menin mutants in vivo. These findings reveal that MEN1-causing missense mutations lead to a loss of function of menin due to enhanced proteolytic degradation, which may be a common mechanism for inactivating tumor suppressor gene products in familial cancer.

Effects of exposure of CHO-K1 cells to a 10-T static magnetic field.

PURPOSE: To evaluate whether exposure to strong static magnetic fields (SMFs), of up to 10 T, affects the growth and cycle distribution of and the micronucleus formation in monolayered Chinese hamster ovary CHO-K1 cells. MATERIALS AND METHODS: The authors developed a system to expose cultured cells to strong SMFs immediately after the cells are seeded. Cell growth rate was evaluated according to cell number count. Cell cycle distribution experiments were performed by using flow cytometric analysis. In these experiments, the cells were exposed to SMFs for up to 4 days. The frequency of micronucleus formation with only SMF exposure at x-ray irradiation was analyzed at microscopic observation. RESULTS: Long-term exposure to a 10-T SMF for up to 4 days did not affect cell growth rate or cell cycle distribution. Exposure to SMFs alone did not affect micronucleus frequency. In x-ray-irradiated cells, exposure to a 1-T SMF did not affect micronucleus frequency, but exposure to a 10-T SMF resulted in a significant (P <.05) increase in micronucleus frequency. CONCLUSION: Strong (10-T) SMFs have no effect on cell growth, cell cycle distribution, or micronucleus frequency, but they may cause an increase in the micronucleus formation induced by 4-Gy x rays.

Menin, the multiple endocrine neoplasia type 1 gene product, exhibits GTP-hydrolyzing activity in the presence of the tumor metastasis suppressor nm23.

MEN1, the gene responsible for multiple endocrine neoplasia type 1, is a tumor suppressor gene that encodes a protein called menin, of unknown function with no homology to any known protein. Here we demonstrate that menin interacts with a putative tumor metastasis suppressor nm23H1/nucleoside diphosphate (NDP) kinase A in mammalian cells. Given the roles of nm23 as a multi-functional protein, we searched for the possible function of menin. Menin has no effect on the known activities of nm23; that is, nucleoside diphosphate kinase, protein kinase, or GTPase-activating protein for Ras-related GTPase Rad. However, we found that menin hydrolyzes GTP to GDP efficiently in the presence of nm23, whereas nm23 or menin alone shows little or no detectable GTPase activity. Furthermore, menin contains sequence motifs similar to those found in all known GTPases or GTP-binding proteins and shows low affinity but specific binding to GTP/GDP. These results suggest that menin is an atypical GTPase stimulated by nm23.

The EWS/NOR1 fusion gene product gains a novel activity affecting pre-mRNA splicing.

In extraskeletal myxoid chondrosarcoma, chromosomal translocation creates a gene fusion between EWS and the orphan nuclear receptor NOR1. The resulting fusion gene product, EWS/NOR1, has been believed to lead to malignant transformation by functioning as a transcriptional activator, but an alternative mechanism may also be involved. Here, using a newly developed functional complementation screening in yeast, we found that EWS/NOR1, but not EWS or NOR1, complemented the loss of function of the small nuclear ribonucleoprotein Snu23p, an essential factor for pre-mRNA splicing in yeast. To verify the potential function of EWS/NOR1 in mammalian cells, we next showed that overexpression of EWS/NOR1 caused increased usage of the distal 5'-splice site of pre-mRNA splicing and that EWS/NOR1 interacted with the human splicing protein U1C; neither EWS nor NOR1 had the same activity or interaction as EWS/NOR1. Altogether, our findings reveal that EWS/NOR1 gains a novel activity affecting pre-mRNA splicing.