Organizational changes in chromatin at different malignant stages of Friend erythroleukemia

Distribution of globin genes and histone variants in micrococcal nuclease-generated subfractions of chromatin from Friend erythroleukemia cells at different malignant states

The distribution of the alpha and beta-globin genes and histone variants was examined in micrococcal nuclease-generated chromatin fractions of three Friend murine erythroleukemia cell types differing in malignant potential and inducibility to erythroid differentiation. A preferential concentration of globin gene sequences, as compared to satellite DNA, was noted in a physiological salt-soluble, histone H1-depleted, mononucleosomal chromatin fraction (Sup 120) in all Friend cell types, even those in which the globin gene was not capable of transcriptional activation by chemical induction. The level of globin gene enrichment in the Sup 120 fraction was highest in the most malignant and inducible cell type. The chemical induction of erythroid differentiation in this cell line did not change the distribution of globin genes in the chromatin fractions. The Sup 120 chromatin fraction prepared from mouse brain nuclei was not enriched in globin genes. Besides the previously reported low H2A. 1/H2A.2 ratio [Blankstein and Levy: Nature 260:638-640, 1976], chromatin from the most tumorigenic cell type showed the lowest H2B.2 to H2B.1 ratio, highest levels of histone H4 acetylation, and the most pronounced change in relative amounts of two major electrophoretic bands of histone H1 variants as compared to the less malignant cell types. The histone variant content of the micrococcal nuclease-generated chromatin fractions from the three Friend cell types reflected the core histone variant differences for the respective intact nuclei. However, the electrophoretic separation of mononucleosomes by size revealed several classes with different H2A variant ratios. The results demonstrate the existence of structural differences in globin gene and histone variants in erythroleukemia cell chromatin associated with distinguishable phenotypes during malignant cell progression.

Chromatin reorganization during emergence of malignant Friend tumors: early changes in H2A and H2B variants and nucleosome repeat length

Serial examination of five newly derived Friend murine tumors during early subcutaneous passages showed continuing changes in chromatin composition and structure over the first 10 to 20 passages followed by a period of stabilization over the subsequent 20 passages. These changes were reflected in a decrease in two major histone variants, H2A.1 and H2B.2, with a coordinate increase in histone variants, H2A.2 and H2B.1, and a changing nucleosome repeat length (NRL). The absolute values differed among the five tumors, but all five showed the same general direction of change. There was no obvious relationship among the NRL, H2A, and H2B histone variant values. A low H2A.1/H2A.2 ratio was found in Friend tumors of high malignant potential. Cell lines derived in vitro also showed directional changes in the H2A and H2B variants similar to those of their tumor cell parents, but with different kinetics. Our findings suggest that Friend tumor establishment is accompanied by an early period of chromatin reorganization marked by changes in several parameters of chromatin structure.

Vitamin deficiency and cancer

Vitamins, either as coenzymes or hormone-like substances play an important role in the expression of genetic information and in the regulation of cellular metabolism, differentiation, and maturation. Recent epidemiologic evidence suggests that certain forms of cancer are more prevalent among populations with a limited intake of certain nutrients. Other evidence indicates that localized deficiency of a nutrient can occur in hormone-sensitive cells without overt systemic manifestations of deficiency disease. Folic acid, because of its key role in nucleic acid biosynthesis, and vitamin A, because of its role in epithelial maintenance are of particular interest. The recognition of folate-dependent fragile sites on chromosomes suggests that folate and related compounds may play a role in preventing tumor-specific chromosome translocations and the insertion of viral oncogenes. Improved understanding of the role of nutrients in cellular maturation could provide clues for primary cancer prevention and better methods of treatment.

A model chromatin assembly system. Factors affecting nucleosome spacing

Poly[d(A-T)].poly[d(A-T)], when reconstituted with chicken erythrocyte core histones and subsequently incubated with sufficient histone H5 in a solution containing polyglutamic acid, forms structures resembling chromatin. H5 induces nucleosome alignment in about two hours at physiological ionic strength and 37 degrees C. The nucleosome spacing and apparent linker heterogeneity in the assembled nucleoprotein are very similar to those in chicken erythrocyte chromatin. Also, condensed chromatin-like fibers on the polynucleotide can be visualized. The binding of one mole of H5 per mole of core octamer is necessary to generate the physiological nucleosome spacing, which remains constant with the addition of more H5. The nucleosome repeat length is not a function of the core histone to poly[d(A-T)] ratio for values lower than the physiological ratio. With increasing ratios, in excess of the physiological value, nucleosome spacing first becomes non-uniform, and then takes on the close packing limit of approximately 165 base-pairs. In addition to eliminating possible base sequence effects on nucleosome positioning, poly[d(A-T)] allows nucleosomes to slide more readily than does DNA, thereby facilitating alignment. Evidence is presented that polyglutamic acid facilitates the nucleosome spacing activity of histone H5, primarily by keeping the nucleoprotein soluble. This model system should be useful for understanding how different repeat lengths arise in chromatin.