Histone proteins and the nucleosomes they form with DNA are the fundamental building blocks of eukaryotic chromatin. A diverse array of post-translational modifications that often occur on tail domains of these proteins has been well documented. Although the function of these highly conserved modifications has remained elusive, converging biochemical and genetic evidence suggests functions in several chromatin-based processes. We propose that distinct histone modifications, on one or more tails, act sequentially or in combination to form a 'histone code' that is, read by other proteins to bring about distinct downstream events.

The language of covalent histone modifications.

Whether and how human tumours are genetically unstable has been debated for decades. There is now evidence that most cancers may indeed be genetically unstable, but that the instability exists at two distinct levels. In a small subset of tumours, the instability is observed at the nucleotide level and results in base substitutions or deletions or insertions of a few nucleotides. In most other cancers, the instability is observed at the chromosome level, resulting in losses and gains of whole chromosomes or large portions thereof. Recognition and comparison of these instabilities are leading to new insights into tumour pathogenesis.

Genetic instabilities in human cancers

The organization of chromatin into higher-order structures influences chromosome function and epigenetic gene regulation. Higher-order chromatin has been proposed to be nucleated by the covalent modification of histone tails and the subsequent establishment of chromosomal subdomains by non-histone modifier factors. Here we show that human SUV39H1 and murine Suv39h1—mammalian homologues of Drosophila Su(var)3-9 and of Schizosaccharomyces pombe clr4—encode histone H3-specific methyltransferases that selectively methylate lysine 9 of the amino terminus of histone H3 in vitro. We mapped the catalytic motif to the evolutionarily conserved SET domain, which requires adjacent cysteine-rich regions to confer histone methyltransferase activity. Methylation of lysine 9 interferes with phosphorylation of serine 10, but is also influenced by pre-existing modifications in the amino terminus of H3. In vivo, deregulated SUV39H1 or disrupted Suv39h activity modulate H3 serine 10 phosphorylation in native chromatin and induce aberrant mitotic divisions. Our data reveal a functional interdependence of site-specific H3 tail modifications and suggest a dynamic mechanism for the regulation of higher-order chromatin.

Regulation of chromatin structure by site-specific histone H3 methyltransferases

Tgfbeta signaling in growth control, cancer, and heritable disorders

We used cDNA microarrays to explore the variation in expression of approximately 8,000 unique genes among the 60 cell lines used in the National Cancer Institute's screen for anti-cancer drugs. Classification of the cell lines based solely on the observed patterns of gene expression revealed a correspondence to the ostensible origins of the tumours from which the cell lines were derived. The consistent relationship between the gene expression patterns and the tissue of origin allowed us to recognize outliers whose previous classification appeared incorrect. Specific features of the gene expression patterns appeared to be related to physiological properties of the cell lines, such as their doubling time in culture, drug metabolism or the interferon response. Comparison of gene expression patterns in the cell lines to those observed in normal breast tissue or in breast tumour specimens revealed features of the expression patterns in the tumours that had recognizable counterparts in specific cell lines, reflecting the tumour, stromal and inflammatory components of the tumour tissue. These results provided a novel molecular characterization of this important group of human cell lines and their relationships to tumours in vivo.

/pdf/systematic-variation-in-gene-expression-patterns-in-human-1qajp6duxj.pdf

Systematic variation in gene expression patterns in human cancer cell lines.

We have generated and characterized a novel site-specific antibody highly specific for the phosphorylated form of the amino-terminus of histone H3 (Ser10). In this study, we used this antibody to examine in detail the relationship between H3 phosphorylation and mitotic chromosome condensation in mammalian cells. Our results extend previous biochemical studies by demonstrating that mitotic phosphorylation of H3 initiates nonrandomly in pericentromeric heterochromatin in late G2 interphase cells. Following initiation, H3 phosphorylation appears to spread throughout the condensing chromatin and is complete in most cell lines just prior to the formation of prophase chromosomes, in which a phosphorylated, but nonmitotic, chromosomal organization is observed. In general, there is a precise spatial and temporal correlation between H3 phosphorylation and initial stages of chromatin condensation. Dephosphorylation of H3 begins in anaphase and is complete immediately prior to detectable chromosome decondensation in telophase cells. We propose that the singular phosphorylation of the amino-terminus of histone H3 may be involved in facilitating two key functions during mitosis: (1) regulate protein-protein interactions to promote binding of trans-acting factors that "drive" chromatin condensation as cells enter M-phase and (2) coordinate chromatin decondensation associated with M-phase.

Mitosis-specific phosphorylation of histone H3 initiates primarily within pericentromeric heterochromatin during G2 and spreads in an ordered fashion coincident with mitotic chromosome condensation.

The centrosomes are thought to maintain genomic stability through the establishment of bipolar spindles during cell division, ensuring equal segregation of replicated chromosomes to two daughter cells. Deregulated duplication and distribution of centrosomes have been implicated in chromosome segregation abnormalities, leading to aneuploidy seen in many cancer cell types. Here, we report that STK15
 (also known as BTAK
 and aurora2
 ), encoding a centrosome-associated kinase, is amplified and overexpressed in multiple human tumour cell types, and is involved in the induction of centrosome duplication-distribution abnormalities and aneuploidy in mammalian cells. STK15
 amplification has been previously detected in breast tumour cell lines
 1
 and in colon tumours
 2
 ; here, we report its amplification in approximately 12% of primary breast tumours, as well as in breast, ovarian, colon, prostate, neuroblastoma and cervical cancer cell lines. Additionally, high expression of STK15
 mRNA was detected in tumour cell lines without evidence of gene amplification. Ectopic expression of STK15
 in mouse NIH 3T3 cells led to the appearance of abnormal centrosome number (amplification) and transformation in vitro
 . Finally, overexpression of STK15
 in near diploid human breast epithelial cells revealed similar centrosome abnormality, as well as induction of aneuploidy. These findings suggest that STK15
 is a critical kinase-encoding gene, whose overexpression leads to centrosome amplification, chromosomal instability and transformation in mammalian cells.

Tumour amplified kinase STK15 / BTAK induces centrosome amplification, aneuploidy and transformation

Managing the centrosome numbers game: from chaos to stability in cancer cell division

Cell form and cytoskeletal organization were investigated in 13 human breast carcinoma cell lines in vitro. Using tubulin antibodies and indirect immunofluorescence to detect the arrangement of cytoplasmic microtubules, three distinct cell phenotypes were recognized: (a) cells with extensive arrays of microtubules (type 1); (b) cells which were diffusely stained with microtubules apparent only near the cell margins (type II intermediate); and (c) cells in which individual microtubules could not be detected and only diffuse fluorescence was apparent (type II diffuse). Type I cells were flattened epithelial-like cells, much like normal mammary epithelial cells, which when stained with actin antibody displayed many brightly fluorescent parallel cables or "stress fibers." Many microtubules and microfilament bundles were observed in type I cells when examined by transmission electron microscopy. Type II cells were more rounded, often grew in multilayered colonies, and displayed fewer microtubules and microfilament bundles when examined by either immunofluorescence or electron microscopy. Type II cells ranged from very small rounded cells with diffuse tubulin and actin immunofluorescence (type II diffuse) to more flattened cells in which microtubules and actin cables were observed near the flattened cell margins (type II intermediate). Since all of the cells were derived initially from malignant metastatic lesions and some were tumorigenic when injected into athymic nude mice, we assume that they remained malignant in vitro. Thus, in human breast carcinoma cells in vitro, it is not possible to associate any specific cell morphology or cytoskeletal phenotype with cancer or metastasis in vivo. Whether or not these same conclusions hold for breast tumor cells in situ remains to be determined.

/pdf/variations-in-cell-form-and-cytoskeleton-in-human-breast-2bh9ebw7lq.pdf

Bill R. Brinkley

Papers

Mitosis-specific phosphorylation of histone H3 initiates primarily within pericentromeric heterochromatin during G2 and spreads in an ordered fashion coincident with mitotic chromosome condensation.

Tumour amplified kinase STK15 / BTAK induces centrosome amplification, aneuploidy and transformation

Managing the centrosome numbers game: from chaos to stability in cancer cell division

Variations in Cell Form and Cytoskeleton in Human Breast Carcinoma Cells in Vitro

Supernumerary centrosomes and cancer: Boveri's hypothesis resurrected