Histone variants are key players in shaping chromatin structure, and, thus, in regulating fundamental cellular processes such as chromosome segregation and gene expression. Emerging evidence points towards a role for histone variants in contributing to tumor progression, and, recently, the first cancer-associated mutation in a histone variant-encoding gene was reported. In addition, genetic alterations of the histone chaperones that specifically regulate chromatin incorporation of histone variants are rapidly being uncovered in numerous cancers. Collectively, these findings implicate histone variants as potential drivers of cancer initiation and/or progression, and, therefore, targeting histone deposition or the chromatin remodeling machinery may be of therapeutic value. Here, we review the mammalian histone variants of the H2A and H3 families in their respective cellular functions, and their involvement in tumor biology.

The histone variant macroH2A generally associates with transcriptionally inert chromatin; however, the factors that regulate its chromatin incorporation remain elusive. Here, we identify the SWI/SNF helicase ATRX (a-thalassemia/ MR, X-linked) as a novel macroH2A-interacting protein.Unlike its role in assisting H3.3 chromatin deposition, ATRX acts as a negative regulator of macroH2A's chromatin association. In human erythroleukemic cells deficient for ATRX, macroH2A accumulates at the HBA gene cluster on the subtelomere of chromosome 16, coinciding with the loss of a-globin expression. Collectively, our results implicate deregulation of macroH2A's distribution as a contributing factor to the a-thalassemia phenotype of ATRX syndrome. The replacement of canonical histones with histone variants contributes to the dynamic nature of chromatin.Due to amino acid differences and, in turn, unique posttranslational modifications, histone variants can alter nucleosome structure, stability, and binding of effector proteins. Histone variants have unique genomic localization patterns, and thus specialized roles such as regulating gene expression or chromosome segregation during cell division . Therefore, the differential genomic incorporation of histone variants directly impacts critical cellular functions.The histone variant macroH2A (mH2A) is a vertebratespecific member of the H2A family and is unusual due to the presence of a C-terminal macro domain (Pehrson and Fried 1992). Two different genes encode mH2A1 and mH2A2 (H2AFY and H2AFY2, respectively), and two splice forms of mH2A1 exist: mH2A1.1 and mH2A1.2 (Costanzi and Pehrson 2001). mH2A is abundant in heterochromatin, including senescence-associated heterochromatic foci (SAHF) and the inactivated X chromosome (Xi) (Costanzi and Pehrson 1998;Zhang et al. 2005). In vitro studies suggest that the macro domain sterically hinders access of transcription factors to DNA, while mH2A's L1 loop produces inflexible nucleosomes (Angelov et al. 2003;Chakravarthy et al. 2005).Our group has recently demonstrated a role for mH2A isoforms in suppressing melanoma progression, and others have linked mH2A expression or its splice patterns to breast and lung cancer (Sporn et al. 2009;Kapoor et al. 2010;Novikov et al. 2011). However, the factors that regulate the association of mH2A with chromatin remain obscure. Therefore, identifying regulators of the incorporation of histone variants at distinct genomic loci is key to understanding how chromatin domains are established and maintained and how these may go awry in disease.A second group of factors contributing to chromatin dynamics are ATP-dependent chromatin remodeling complexes that rearrange or mobilize nucleosomes. Deregulation of members of the SWI/SNF family is implicated in various cancers and mental retardation (MR) syndromes, including ATRX (a-thalassemia/MR, X-linked), (Wilson and Roberts 2011). Mutations in ATRX, predominantly found in the H3K9me3-binding ADD (ATRX-DNMT3-DNMT3L) and/or helicase domains, are associated with ATRX sy...

The process of cellular senescence generates a repressive chromatin environment, however, the role of histone variants and histone proteolytic cleavage in senescence remains unclear. Using models of oncogene-induced and replicative senescence, here we report novel histone H3 tail cleavage events mediated by the protease Cathepsin L. We find that cleaved forms of H3 are nucleosomal and the histone variant H3.3 is the preferred cleaved form of H3. Ectopic expression of H3.3 and its cleavage product (H3.3cs1), which lacks the first twenty-one amino acids of the H3 tail, is sufficient to induce senescence. Further, H3.3cs1 chromatin incorporation is mediated by the HUCA histone chaperone complex. Genome-wide transcriptional profiling revealed that H3.3cs1 facilitates transcriptional silencing of cell cycle regulators including RB/E2F target genes, likely via the permanent removal of H3K4me3. Collectively, our study identifies histone H3.3 and its proteolytically processed forms as key regulators of cellular senescence.

Human telomeric DNA consists of tandem repeats of the sequence 5'-d(TTAGGG)-3'. Guanine-rich DNA, such as that seen at telomeres, forms G-quadruplex secondary structures. Alternative forms of G-quadruplex structures can have differential effects on activities involved in telomere maintenance. With this in mind, we analyzed the effect of sequence and length of human telomeric DNA on G-quadruplex structures by native polyacrylamide gel electrophoresis and circular dichroism. Telomeric oligonucleotides shorter than four, 5'-d(TTAGGG)-3' repeats formed intermolecular G-quadruplexes. However, longer telomeric repeats formed intramolecular structures. Altering the 5'-d(TTAGGG)-3' to 5'-d(TTAGAG)-3' in any one of the repeats of 5'-d(TTAGGG)(4)-3' converted an intramolecular structure to intermolecular G-quadruplexes with varying degrees of parallel or anti-parallel-stranded character, depending on the length of incubation time and DNA sequence. These structures were most abundant in K(+)-containing buffers. Higher-order structures that exhibited ladders on polyacrylamide gels were observed only for oligonucleotides with the first telomeric repeat altered. Altering the sequence of 5'-d(TTAGGG)(8)-3' did not result in the substantial formation of intermolecular structures even when the oligonucleotide lacked four consecutive telomeric repeats. However, many of these intramolecular structures shared common features with intermolecular structures formed by the shorter oligonucleotides. The wide variability in structure formed by human telomeric sequence suggests that telomeric DNA structure can be easily modulated by proteins, oxidative damage, or point mutations resulting in conversion from one form of G-quadruplex to another.

Monomers of azobenzene were isolated in argon matrices at 15 K and characterized by infrared spectroscopy and theoretical calculations. When the equilibrium vapors existing over the azobenzene crystals at room temperature were trapped in the matrix, only the thermodynamically most stable E-azobenzene was detected. In an attempt to convert E-azobenzene into the Z isomer, the matrix-isolated E-monomers were irradiated either by broad-band or narrow-band UV-visible light of different wavelengths, in the 600-200 nm range. However, no E-to-Z transformation was observed under these conditions. In an alternative experiment, E-azobenzene was irradiated by UV-visible broad-band light in the gas phase prior to trapping in a matrix. In this case, the E-to-Z photoisomerization occurred, and both E- and Z-azobenzene monomers were detected in the matrix sample. Subsequent irradiation of the matrix with narrow-band tunable visible or UV light (λ < 550 nm) resulted in back conversion of Z-azobenzene into the E-form. The observed photoinduced E-to-Z isomerizations allowed for the reliable vibrational characterization of both azobenzene isomers. The two-dimensional potential energy surfaces of Z- and E-azobenzene were explored as functions of the torsional movement of the two phenyl rings. They exhibit large flat areas around the minima, for both isomers, allowing for large-amplitude zero-point torsional vibrations. For the Z-form, these vibrations were found to be responsible for significant changes in the equilibrium NN bond length (up to 0.3 pm). This also allowed to explain the experimentally observed frequency smearing of the N=N stretching vibration in this isomer.

Vacuum ultraviolet (VUV, 130-170 nm) photochemistry of the HOCO complex is studied by matrix-isolation infrared spectroscopy. The HOCO complexes in Ne, Ar, Kr, and Xe matrices are generated by ultraviolet (UV, 193 and 250 nm) photolysis of formic acid (HCOOH). VUV photolysis of the HOCO complexes is found to lead to the formation of the OHCO radical-molecule complexes and trans-HOCO radicals. It is shown that the matrix material, local matrix morphology, and possibly the HOCO complex geometry strongly affect the VUV photolysis pathways. The intrinsic reactivity of the matrix-isolated OHCO complex resulting in the formation of trans-HOCO is directly demonstrated for the first time. This reaction occurs in Ar, Kr, and Xe matrices upon annealing above 25 K and may proceed over the barrier. The case of a Ne matrix is very special because the formation of trans-HOCO from the OHCO complex is observed even at the lowest experimental temperature (4.5 K), which is in sharp contrast to the other matrices. It follows that quantum tunneling is probably involved in this process in the Ne matrix at such a low temperature. Infrared light also promotes this reaction in the Ne matrix at 4.5 K, which is not the case in the other matrices. The last findings show the effect of the environment on the tunneling and infrared-induced rates of this fundamental chemical reaction.

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Aromatic azo compounds have a wide range of industrial applications as dyes in optical and color-changing materials and can also be exploited in the design of new photodynamic molecular systems. The azo derivative 1-(cyclopropyl)diazo-2-naphthol was isolated in low-temperature cryogenic matrices, and its molecular structure, tautomeric equilibrium, and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. Only azo-enol forms having the OH group involved in a strong intramolecular hydrogen bond, forming a six-membered ring with the azo group, were found experimentally. Irradiation with a narrowband source in the near-UV range generates different rotameric and tautomeric azo-enol and keto-hydrazone forms that can be interconverted at different irradiation wavelengths.