Abstract 433: Triplication of HMGN1 promotes B cell acute lymphoblastic leukemia (B-ALL) through suppression of H3K27me3

Our goal is to identify oncogenic loci in regions of recurrent DNA copy number alterations in cancer. Constitutional trisomy 21 (Down syndrome) carries a 20-fold increased risk of B-ALL, and chr.21 gains are the most common acquired aneuploidy in B-ALL. Interstitial amplification in the chr.21q22 region (iAMP21) is also a recurrent finding in B-ALL and carries a poor prognosis. However, the gene(s) on chr.21 responsible for this association remain unclear. We studied the Ts1Rhr mouse, which carries germline triplication of 31 genes homologous to human chr.21q22. Chr.21q22 triplication was sufficient to promote B cell autonomous self-renewal and maturation defects, and cooperated with BCR-ABL or CRLF2 with JAK2 R683G to accelerate leukemogenesis. Chr.21q22 triplication also resulted in histone H3K27 hypomethylation at gene promoters, and the expression signature of triplicated B cells was enriched for genes targeted by polycomb repressor complex 2 (PRC2), which trimethylates H3K27. Thus, chr.21q22 triplication may deregulate B cell development by causing H3K27 hypomethylation at genes critical for progenitor cell growth. In support of this hypothesis, pharmacologic inhibition of PRC2 function was sufficient to confer self-renewal in wild-type B cells, while inhibition of H3K27 demethylases blocked self-renewal induced by chr.21q22 triplication. In three independent B-ALL cohorts, PRC2/H3K27 gene signatures distinguished leukemias with +21 from those without, validating the same biology in human disease. One of the 31 triplicated genes, HMGN1, encodes a nucleosome binding protein known to modulate chromatin structure and facilitate transcriptional activation. When we overexpressed HMGN1 in BaF3 proB cells, H3K27me3 decreased proportionally to the level of overexpression. We next knocked down each of the 31 triplicated genes with lentivirally-expressed shRNAs (5 per gene) and assessed the effects on growth of Ts1Rhr and wild-type primary B cells. Strikingly, Hmgn1 was the top scoring gene and all 5 hairpins targeting Hmgn1 were depleted in the assay. Finally, we studied transgenic mice (HMGN1_OE) that overexpress human HMGN1 (∼2-fold total overexpression). HMGN1_OE mice had a defect in B cell maturation, increased proB colony forming capacity, and a transcriptional signature overlapping with that of triplication of all 31 Ts1Rhr genes. In a bone marrow transplant model driven by BCR-ABL, recipients of HMGN1_OE bone marrow developed B-ALL with decreased latency (median 33 days vs not reached) and increased penetrance (17/18 vs 4/17 mice died by 80 days; leukemia-free survival difference P<0.001) compared to recipients of wild-type bone marrow. These data indicate that HMGN1 is a B-ALL oncogene, and therapies targeting HMGN1 or the downstream effects of HMGN1 overexpression on epigenetic histone modifications may be effective in B-ALL with copy number gains involving chr.21q22.

Citation Format: Andrew A. Lane, Bjoern Chapuy, Charles Y. Lin, Trevor Tivey, Hubo Li, Elizabeth Townsend, Diederik van Bodegom, Tovah A. Day, Shuo-Chieh Wu, Huiyun Liu, Akinori Yoda, Gabriela Alexe, Anna Schinzel, Timothy J. Sullivan, Sebastien Malinge, Jordan Taylor, Kimberly Stegmaier, Jacob Jaffe, Michael Bustin, Geertruy te Kronnie, Shai Izraeli, Marian Harris, Kristen Stevenson, Donna Neuberg, Lewis B. Silverman, Steven E. Sallan, James E. Bradner, William C. Hahn, John D. Crispino, David Pellman, David M. Weinstock. Triplication of HMGN1 promotes B cell acute lymphoblastic leukemia (B-ALL) through suppression of H3K27me3. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 433. doi:10.1158/1538-7445.AM2014-433

Triplication of a 21q22 region contributes to B cell transformation through HMGN1 overexpression and loss of histone H3 Lys27 trimethylation

Down syndrome confers a 20-fold increased risk of B cell acute lymphoblastic leukemia (B-ALL)¹ and polysomy 21 is the most frequent somatic aneuploidy amongst all B-ALLs². Yet, the mechanistic links between chr.21 triplication and B-ALL remain undefined. Here we show that germline triplication of only 31 genes orthologous to human chr.21q22 confers murine progenitor B cell self-renewal in vitro, maturation defects in vivo, and B-ALL with either BCR-ABL or CRLF2 with activated JAK2. Chr.21q22 triplication suppresses H3K27me3 in progenitor B cells and B-ALLs, and “bivalent” genes with both H3K27me3 and H3K4me3 at their promoters in wild-type progenitor B cells are preferentially overexpressed in triplicated cells. Strikingly, human B-ALLs with polysomy 21 are distinguished by their overexpression of genes marked with H3K27me3 in multiple cell types. Finally, overexpression of HMGN1, a nucleosome remodeling protein encoded on chr.21q22^3–5, suppresses H3K27me3 and promotes both B cell proliferation in vitro and B-ALL in vivo.

Metabolomics reveals a role for the chromatin-binding protein HMGN5 in glutathione metabolism

High mobility group nucleosome-binding protein 5 (HMGN5) is a chromatin architectural protein that binds specifically to nucleosomes and reduces the compaction of the chromatin fiber. The protein is present in most vertebrate tissues however the physiological function of this protein is unknown. To examine the function of HMGN5 in vivo, mice lacking the nucleosome-binding domain of HMGN5 were generated and characterized. Serological analysis revealed that compared to wild-type littermates (Hmgn5^+/Y), mice with a targeted mutation in the HMGN5 gene (Hmgn5^tm1/Y), had elevated serum albumin, non-HDL cholesterol, triglycerides, and alanine transaminase, suggesting mild hepatic abnormalities. Metabolomics analysis of liver extracts and urine revealed clear differences in metabolites between Hmgn5^tm1/Y and their Hmgn5^+/Y littermates. Hmgn5^tm1/Y mice had a significant increase in hepatic glutathione levels and decreased urinary concentrations of betaine, phenylacetylglycine, and creatine, all of which are metabolically related to the glutathione precursor glycine. Microarray and qPCR analysis revealed that expression of two genes affecting glutathione metabolism, glutathione peroxidase 6 (Gpx6) and hexokinase 1 (Hk1), was significantly decreased in Hmgn5^tm1/Y mouse liver tissue. Analysis of chromatin structure by DNase I digestion revealed alterations in the chromatin structure of these genes in the livers of Hmgn5^tm1/Y mice. Thus, functional loss of HMGN5 leads to changes in transcription of Gpx6 and Hk1 that alter glutathione metabolism.

Loss of the nucleosome-binding protein HMGN1 affects the rate of N-nitrosodiethylamine-induced hepatocarcinogenesis in mice

We report that HMGN1, a nucleosome-binding protein that affects chromatin structure and function, affects the growth of N-nitrosodiethylamine (DEN)-induced liver tumors. Following a single DEN injection at 2 weeks of age, Hmgn1(tm1/tm1) mice, lacking the nucleosome-binding domain of HMGN1, had earlier signs of liver tumorigenesis than their Hmgn1(+/+) littermates. Detailed gene expression profiling revealed significant differences between DEN-injected and control saline-injected mice, but only minor differences between the injected Hmgn1(tm1/tm1) mice and their Hmgn1(+/+) littermates. Pathway analysis revealed that the most significant process affected by loss of HMGN1 involves the lipid/sterol metabolic pathway. Our study indicates that in mice, loss of HMGN1 leads to transcription changes that accelerate the progression of DEN-induced hepatocarcinogenesis, without affecting the type of tumors or the final total tumor burden of these mice.

IMPLICATIONS

Loss of HMGN1 leads to accelerated progression of DEN-induced hepatocarcinogenesis in mice.

High mobility group protein N5 (HMGN5) and lamina-associated polypeptide 2α (LAP2α) interact and reciprocally affect their genome-wide chromatin organization

The interactions of nuclear lamins with the chromatin fiber play an important role in regulating nuclear architecture and chromatin function; however, the full spectrum of these interactions is not known. We report that the N-terminal domain of the nucleosome-binding protein HMGN5 interacts with the C-terminal domain of the lamin-binding protein LAP2α and that these proteins reciprocally alter their interaction with chromatin. Chromatin immunoprecipitation analysis of cells lacking either HMGN5 or LAP2α reveals that loss of either protein affects the genome-wide distribution of the remaining partner. Our study identifies a new functional link between chromatin-binding and lamin-binding proteins.

High mobility group N proteins modulate the fidelity of the cellular transcriptional profile in a tissue- and variant-specific manner

The nuclei of most vertebrate cells contain members of the high mobility group N (HMGN) protein family, which bind specifically to nucleosome core particles and affect chromatin structure and function, including transcription. Here, we study the biological role of this protein family by systematic analysis of phenotypes and tissue transcription profiles in mice lacking functional HMGN variants. Phenotypic analysis of Hmgn1(tm1/tm1), Hmgn3(tm1/tm1), and Hmgn5(tm1/tm1) mice and their wild type littermates with a battery of standardized tests uncovered variant-specific abnormalities. Gene expression analysis of four different tissues in each of the Hmgn(tm1/tm1) lines reveals very little overlap between genes affected by specific variants in different tissues. Pathway analysis reveals that loss of an HMGN variant subtly affects expression of numerous genes in specific biological processes. We conclude that within the biological framework of an entire organism, HMGNs modulate the fidelity of the cellular transcriptional profile in a tissue- and HMGN variant-specific manner.

Microtubule dynamics alter the interphase nucleus

Microtubules are known to drive chromosome movements and to induce nuclear envelope breakdown during mitosis and meiosis. Here we show that microtubules can enforce nuclear envelope folding and alter the levels of nuclear envelope-associated heterochromatin during interphase, when the nuclear envelope is intact. Microtubule reassembly, after chemically induced depolymerization led to folding of the nuclear envelope and to a transient accumulation of condensed chromatin at the site nearest the microtubule organizing center (MTOC). This microtubule-dependent chromatin accumulation next to the MTOC is dependent on the composition of the nuclear lamina and the activity of the dynein motor protein. We suggest that forces originating from simultaneous polymerization of microtubule fibers deform the nuclear membrane and the underlying lamina. Whereas dynein motor complexes localized to the nuclear envelope that slide along the microtubules transfer forces and/or signals into the nucleus to induce chromatin reorganization and accumulation at the nuclear membrane folds. Thus, our study identified a molecular mechanism by which mechanical forces generated in the cytoplasm reshape the nuclear envelope, alter the intranuclear organization of chromatin, and affect the architecture of the interphase nucleus.

Gene network reconstruction reveals cell cycle and antiviral genes as major drivers of cervical cancer

Although human papillomavirus was identified as an aetiological factor in cervical cancer, the key human gene drivers of this disease remain unknown. Here we apply an unbiased approach integrating gene expression and chromosomal aberration data. In an independent group of patients, we reconstruct and validate a gene regulatory meta-network, and identify cell cycle and antiviral genes that constitute two major subnetworks upregulated in tumour samples. These genes are located within the same regions as chromosomal amplifications, most frequently on 3q. We propose a model in which selected chromosomal gains drive activation of antiviral genes contributing to episomal virus elimination, which synergizes with cell cycle dysregulation. These findings may help to explain the paradox of episomal human papillomavirus decline in women with invasive cancer who were previously unable to clear the virus.

Concanavalin A binds to puffs in polytene chromosomes

CHANGES in transcriptional activity at defined loci are often correlated with significant local structural changes in the genome(1), and in polytene chromosomes, such changes are thought to be associated with compositional or conformational changes in the protein complement at these particular bands(2,3). Thus, various studies on Balfoiani rings and specific 'puffs' in such chromosomes are useful for elucidating the role of defined chromosomal components in both chromosome structure and gene activity. Such studies require specific probes which will allow in situ localisation of a chromosomal component during the various stages of puffing. Antibodies specific to purified histone fractions(4-7), HMG proteins(8), RNA polymerase(9) and non-histone protein subfractions(10) have been used in studies on chromatin and chromosome structure. We reported previously that concanavalin A (Con A) specifically binds to three types of non-histone proteins present in chromatin purified from rat liver nuclei and suggested that derivatives of Con A might serve as specific probes to study the in situ organisation of these non-histone proteins(11). We have now reacted fluorescein-labelled Con A with polytene chromosomes isolated from different developmental stages of Chironomus thummi and visualised the location of the bound Con A by fluorescence microscopy. We observed that the fluorescent lectin, which has an affinity for glucose- and mannose-containing molecules, specifically bound to the transcriptionally active regions of chromosome IV. The extent of binding of Con A to the Balbiani rings present in regions b and c of chromosome IV is proportional to the size of the respective ring. Our results indicate that glucose- or mannose-containing molecules are present in these Balbiani rings and that the availability of these sugars to interact with Con A can be correlated with the developmental stage of a puff. We suggest that lectins can be useful cytological tools with which to study the in situ organisation of defined chromosomal components during various functional states of the genome.

HMGN1 protein regulates poly(ADP-ribose) polymerase-1 (PARP-1) self-PARylation in mouse fibroblasts

In mammalian cells, the nucleosome-binding protein HMGN1 (high mobility group N1) affects the structure and function of chromatin and plays a role in repair of damaged DNA. HMGN1 affects the interaction of DNA repair factors with chromatin and their access to damaged DNA; however, not all of the repair factors affected have been identified. Here, we report that HMGN1 affects the self-poly(ADP-ribosyl)ation (i.e., PARylation) of poly(ADP-ribose) polymerase-1 (PARP-1), a multifunctional and abundant nuclear enzyme known to recognize DNA lesions and promote chromatin remodeling, DNA repair, and other nucleic acid transactions. The catalytic activity of PARP-1 is activated by DNA with a strand break, and this results in self-PARylation and PARylation of other chromatin proteins. Using cells obtained from Hmgn1(-/-) and Hmgn1(+/+) littermate mice, we find that in untreated cells, loss of HMGN1 protein reduces PARP-1 self-PARylation. A similar result was obtained after MMS treatment of these cells. In imaging experiments after low energy laser-induced DNA damage, less PARylation at lesion sites was observed in Hmgn1(-/-) than in Hmgn1(+/+) cells. The HMGN1 regulation of PARP-1 activity could be mediated by direct protein-protein interaction as HMGN1 and PARP-1 were found to interact in binding assays. Purified HMGN1 was able to stimulate self-PARylation of purified PARP-1, and in experiments with cell extracts, self-PARylation was greater in Hmgn1(+/+) than in Hmgn1(-/-) extract. The results suggest a regulatory role for HMGN1 in PARP-1 activation.

High-mobility group nucleosome-binding protein 1 acts as an alarmin and is critical for lipopolysaccharide-induced immune responses

Alarmins are endogenous mediators capable of promoting the recruitment and activation of antigen-presenting cells (APCs), including dendritic cells (DCs), that can potentially alert host defense against danger signals. However, the relevance of alarmins to the induction of adaptive immune responses remains to be demonstrated. In this study, we report the identification of HMGN1 (high-mobility group nucleosome-binding protein 1) as a novel alarmin and demonstrate that it contributes to the induction of antigen-specific immune responses. HMGN1 induced DC maturation via TLR4 (Toll-like receptor 4), recruitment of APCs at sites of injection, and activation of NF-κB and multiple mitogen-activated protein kinases in DCs. HMGN1 promoted antigen-specific immune response upon co-administration with antigens, and Hmgn1(-/-) mice developed greatly reduced antigen-specific antibody and T cell responses when immunized with antigens in the presence of lipopolysaccharide (LPS). The impaired ability of Hmgn1(-/-) mice to mount antigen-specific immune responses was accompanied by both deficient DC recruitment at sites of immunization and reduced production of inflammatory cytokines. Bone marrow chimera experiments revealed that HMGN1 derived from nonleukocytes was critical for the induction of antigen-specific antibody and T cell responses. Thus, extracellular HMGN1 acts as a novel alarmin critical for LPS-induced development of innate and adaptive immune responses.

The chromatin-binding protein HMGN1 regulates the expression of methyl CpG-binding protein 2 (MECP2) and affects the behavior of mice

High mobility group N1 protein (HMGN1), a nucleosomal-binding protein that affects the structure and function of chromatin, is encoded by a gene located on chromosome 21 and is overexpressed in Down syndrome, one of the most prevalent genomic disorders. Misexpression of HMGN1 affects the cellular transcription profile; however, the biological function of this protein is still not fully understood. We report that HMGN1 modulates the expression of methyl CpG-binding protein 2 (MeCP2), a DNA-binding protein known to affect neurological functions including autism spectrum disorders, and whose alterations in HMGN1 levels affect the behavior of mice. Quantitative PCR and Western analyses of cell lines and brain tissues from mice that either overexpress or lack HMGN1 indicate that HMGN1 is a negative regulator of MeCP2 expression. Alterations in HMGN1 levels lead to changes in chromatin structure and histone modifications in the MeCP2 promoter. Behavior analyses by open field test, elevated plus maze, Reciprocal Social Interaction, and automated sociability test link changes in HMGN1 levels to abnormalities in activity and anxiety and to social deficits in mice. Targeted analysis of the Autism Genetic Resource Exchange genotype collection reveals a non-random distribution of genotypes within 500 kbp of HMGN1 in a region affecting its expression in families predisposed to autism spectrum disorders. Our results reveal that HMGN1 affects the behavior of mice and suggest that epigenetic changes resulting from altered HMGN1 levels could play a role in the etiology of neurodevelopmental disorders.

High mobility group nucleosome-binding protein 1 acts as an alarmin critical for the induction of immune response (113.7)

Alarmins, defined as endogenous mediator(s) capable of promoting the recruitment and activation of antigen-presenting cells (APCs) including dendritic cells (DCs), can potentially promote immunity, however, their essential contribution to the induction immune responses remain to be demonstrated. Here we report the identification of HMGN1 as a novel alarmin that is critical to the induction of antigen-specific immune response. HMGN1 induced DC maturation and recruitment of APCs and activated NF-κB and multiple MAPKs, in a MyD88-dependent manner. HMGN1 promoted antigen-specific immune response upon co-administration with an antigen. Furthermore, knockout of HMGN1 in mice greatly reduced antigen-specific antibody and T cell responses upon intraperitoneal immunization with an antigen using LPS as an adjuvant. The impaired ability of HMGN1 KO mice to mount antigen-specific immune responses was accompanied by both deficient DC recruitment at sites of immunization and reduced production of inflammatory cytokines. Bone marrow chimera experiments revealed that HMGN1 derived from non-leukocytes played a more critical role in the induction of antigen-specific antibody and T cell responses. Thus, HMGN1 acts as a novel alarmin critical for the induction of adaptive immune response.

Effects of HMGN variants on the cellular transcription profile

High mobility group N (HMGN) is a family of intrinsically disordered nuclear proteins that bind to nucleosomes, alters the structure of chromatin and affects transcription. A major unresolved question is the extent of functional specificity, or redundancy, between the various members of the HMGN protein family. Here, we analyze the transcriptional profile of cells in which the expression of various HMGN proteins has been either deleted or doubled. We find that both up- and downregulation of HMGN expression altered the cellular transcription profile. Most, but not all of the changes were variant specific, suggesting limited redundancy in transcriptional regulation. Analysis of point and swap HMGN mutants revealed that the transcriptional specificity is determined by a unique combination of a functional nucleosome-binding domain and C-terminal domain. Doubling the amount of HMGN had a significantly larger effect on the transcription profile than total deletion, suggesting that the intrinsically disordered structure of HMGN proteins plays an important role in their function. The results reveal an HMGN-variant-specific effect on the fidelity of the cellular transcription profile, indicating that functionally the various HMGN subtypes are not fully redundant.

The role of chromatin structure in cell migration

Chromatin dynamics play a major role in regulating genetic processes. Now, accumulating data suggest that chromatin structure may also affect the mechanical properties of the nucleus and cell migration. Global chromatin organization appears to modulate the shape, the size and the stiffness of the nucleus. Directed-cell migration, which often requires nuclear reshaping to allow passage of cells through narrow openings, is dependent not only on changes in cytoskeletal elements but also on global chromatin condensation. Conceivably, during cell migration a physical link between the chromatin and the cytoskeleton facilitates coordinated structural changes in these two components. Thus, in addition to regulating genetic processes, we suggest that alterations in chromatin structure could facilitate cellular reorganizations necessary for efficient migration.

UV-induced histone H2AX phosphorylation and DNA damage related proteins accumulate and persist in nucleotide excision repair-deficient XP-B cells

DNA double strand breaks (DSB) may be caused by ionizing radiation. In contrast, UV exposure forms dipyrimidine photoproducts and is not considered an inducer of DSB. We found that uniform or localized UV treatment induced phosphorylation of the DNA damage related (DDR) proteins H2AX, ATM and NBS1 and co-localization of γ-H2AX with the DDR proteins p-ATM, p-NBS1, Rad51 and FANCD2 that persisted for about 6h in normal human fibroblasts. This post-UV phosphorylation was observed in the absence of nucleotide excision repair (NER), since NER deficient XP-B cells (lacking functional XPB DNA repair helicase) and global genome repair-deficient rodent cells also showed phosphorylation and localization of these DDR proteins. Resolution of the DDR proteins was dependent on NER, since they persisted for 24h in the XP-B cells. In the normal and XP-B cells p53 and p21 was detected at 6h and 24h but Mdm2 was not induced in the XP-B cells. Post-UV induction of Wip1 phosphatase was detected in the normal cells but not in the XP-B cells. DNA DSB were detected with a neutral comet assay at 6h and 24h post-UV in the normal and XP-B cells. These results indicate that UV damage can activate the DDR pathway in the absence of NER. However, a later step in DNA damage processing involving induction of Wip1 and resolution of DDR proteins was not observed in the absence of NER.

Efficient cell migration requires global chromatin condensation

Cell migration is a fundamental process that is necessary for the development and survival of multicellular organisms. Here, we show that cell migration is contingent on global condensation of the chromatin fiber. Induction of directed cell migration by the scratch-wound assay leads to decreased DNaseI sensitivity, alterations in the chromatin binding of architectural proteins and elevated levels of H4K20me1, H3K27me3 and methylated DNA. All these global changes are indicative of increased chromatin condensation in response to induction of directed cell migration. Conversely, chromatin decondensation inhibited the rate of cell migration, in a transcription-independent manner. We suggest that global chromatin condensation facilitates nuclear movement and reshaping, which are important for cell migration. Our results support a role for the chromatin fiber that is distinct from its known functions in genetic processes.

HMGN5/NSBP1: a new member of the HMGN protein family that affects chromatin structure and function

The dynamic nature of the chromatin fiber provides the structural and functional flexibility required for the accurate transcriptional responses to various stimuli. In living cells, structural proteins such as the linker histone H1 and the high mobility group (HMG) proteins continuously modulate the local and global architecture of the chromatin fiber and affect the binding of regulatory factors to their nucleosomal targets. HMGN proteins specifically bind to the nucleosome core particle through a highly conserved "nucleosomal binding domain" (NBD) and reduce chromatin compaction. HMGN5 (NSBP1), a new member of the HMGN protein family, is ubiquitously expressed in mouse and human tissues. Similar to other HMGNs, HMGN5 is a nuclear protein which binds to nucleosomes via NBD, unfolds chromatin, and affects transcription. This protein remains mainly uncharacterized and its biological function is unknown. In this review, we describe the structure of the HMGN5 gene and the known properties of the HMGN5 protein. We present recent findings related to the expression pattern of the protein during development, the mechanism of HMGN5 action on chromatin, and discuss the possible role of HMGN5 in pathological and physiological processes.

The nucleosome binding protein HMGN3 is expressed in pancreatic alpha-cells and affects plasma glucagon levels in mice

Glucose homeostasis requires the coordinated actions of various organs and is critically dependent on the proper functioning of the various cell types present in the pancreatic Langerhans islets. Here we report that chromatin architectural protein HMGN3 is highly expressed in all pancreatic endocrine islet cells, and that Hmgn3-/- mice which have a mild diabetic phenotype, have reduced glucagon levels in their blood. To elucidate the mechanism leading to altered glucagon secretion of Hmgn3-/- mice, we tested whether HMGN3 affect glucagon synthesis and secretion in alphaTC1-9 cells, a glucagon secreting cell line that is used to study pancreatic alpha-cell function. We find that in these cells deletion of either HMGN3 or other HMGN variants, does not significantly affect glucagon gene expression or glucagon secretion. Our studies demonstrate a link between HMGN3 and glucagon blood levels that is not directly dependent of the function of pancreatic alpha-cells.

The nucleosome-binding protein HMGN2 modulates global genome repair

The HMGN family comprises nuclear proteins that bind to nucleosomes and alter the structure of chromatin. Here, we report that DT40 chicken cells lacking either HMGN2 or HMGN1a, or lacking both HMGN1a and HMGN2, are hypersensitive to killing by UV irradiation. Loss of both HMGN1a and HMGN2 or only HMGN2 increases the extent of UV-induced G(2)-M checkpoint arrest and the rate of apoptosis. HMGN null mutant cells showed slower removal of UV-induced DNA lesions from native chromatin, but the nucleotide excision repair remained intact, as measured by host cell reactivation assays. These results identify HMGN2 as a component of the global genome repair subpathway of the nucleotide excision repair pathway, and may indicate that HMGN2 facilitates the ability of the DNA repair proteins to access and repair UV-induced DNA lesions in chromatin. Our finding that HMGNs play a role in global DNA repair expands the role of these proteins in the maintenance of genome integrity.

The interaction of NSBP1/HMGN5 with nucleosomes in euchromatin counteracts linker histone-mediated chromatin compaction and modulates transcription

Structural changes in specific chromatin domains are essential to the orderly progression of numerous nuclear processes, including transcription. We report that the nuclear protein NSBP1 (HMGN5), a recently discovered member of the HMGN nucleosome-binding protein family, is specifically targeted by its C-terminal domain to nucleosomes in euchromatin. We find that the interaction of NSBP1 with nucleosomes alters the compaction of cellular chromatin and that in living cells, NSBP1 interacts with linker histones. We demonstrate that the negatively charged C-terminal domain of NSBP1 interacts with the positively charged C-terminal domain of H5 and that NSBP1 counteracts the linker histone-mediated compaction of a nucleosomal array. Dysregulation of the cellular levels of NSBP1 alters the transcription level of numerous genes. We suggest that mouse NSBP1 is an architectural protein that binds preferentially to euchromatin and modulates the fidelity of the cellular transcription profile by counteracting the chromatin-condensing activity of linker histones.