When two different mammalian cell types are fused to generate a stable hybrid cell line, genes that are active in only one of the parents are frequently shut off, a phenomenon called extinction. In this study two distinct, complementary mechanisms for such extinction of growth hormone gene expression were identified. In hybrids formed by fusing fibroblasts to pituitary cells, pituitary-specific proteins that bind to the growth hormone promoter were absent. In addition, a negative regulatory element located near the rat growth hormone promoter was specifically activated.
A human c-erbA oncogene homologue is closely proximal to the chromosome 17 breakpoint in acute promyelocytic leukemia (somatic cell Contributed by Peter C. Nowell, April 5, 1984 ABSTRACT A human cDNA library was screened for sequences homologous to the erbA gene of avian erythroblastosis virus (AEV). One such clone, cHerbA-1, was used to map the chromosomal location of highly homologous human sequences that were found to be present on chromosome 17 as judged by Southern blot screening of a panel of mouse-human hybrid cell lines segregating human chromosomes. cHerbA-1 was hybridized in situ to metaphase chromosomes from a normal male subject and from a female patient with an acute promyelocytic leukemia (APL) having the typical t(15;17) translocation. The results localized the cellular c-erbA sequences on chromosome 17 to the q21-q24 region of normal chromosomes and indicated that the c-erbA sequences remained on the 17q-chromosome in the APL cells, suggesting that they could be assigned to the 17(q21-q22) region. For additional data, we hybridized human neoplastic cells derived from a poorly differentiated acute leukemia carrying a t(17;21) translocation with thymidine kinase (TK)-deficient LMTK-mouse cells. A resulting hybrid, containing only the 21q+ chromosome, did not have human c-erbA sequences. Since the breakpoint on 17q in this translocation was similar to that in the APL t(15;17) translocation, this supported the assignment of c-erbA to the q21-q22 region of chromosome 17. The apparent close proximity of the c-erbA sequences to the chromosomal breakpoints in these two leukemias suggests a possible role for this oncogene homologue in the development of these neoplasms.
Histone genes were knapped to at least three human chromosomes by Southern blot analysis of DNAs from a series of mouse-human somatic cell hybrids (using 32p-labeled cloned human histone DNA as probes). Chromosome assignment was confirmed by in situ hybridization of radiolabeled histone gene probes (3H-labeled) to metaphase chromosomes. One human histone gene cluster (AHHG41) containing an H3 and H4 gene resides only on chromosome 1, whereas other clusters containing core (H3, H4, M2A, and H2B) alone (XHHG17) or core together with HI histone genes (AHHG415) have been assigned to chromosomes 1, 6, and 12. These results suggest that the multigene family of histone coding' sequences that reside in a series of clusters may be derived from a single cluster containing one each of the genes for the five principal classes of histone proteins. During the course of evolution, a set of events, probably involving reduplication, sequence modifil cation, and recombination, resulted in the present pattern of human histone gene distribution among several chromosomes.Human histone genes are represented as a family of moderately repeated sequences with variations in the structure, organization, and regulation of the different copies (1)(2)(3)(4)(5). Functionally, there are three classes of histone genes, with the expression of most of the genes coupled with DNA replication (6)(7)(8), =10% expressed in a non-cell-cycle-dependent manner (9, 10), and several nonexpressed pseudogenes (3). Our present understanding of human histone gene organization is based on analysis of a series of cloned 15-to 20-kilobase (kb) human genomic DNA segments containing one to five core or core plus HI histone genes (1-5, 11). The human histone genes are represented by at least 11 different types of these cloned clusters, and there is no evidence of a simple tandem repeat as observed for the histone genes expressed early during development in several lower eukaryotes (12,13). Other mammalian histone genes appear to be organized in a manner similar to that in humans (14, 15). Yet, to date, the structural relationship between these histone gene clusters remains to be resolved. MATERIALS AND METHODSSomatic Cell Hybrids. The construction and characterization of the various somatic cell hybrids have been reported (16,17 [3H]dATP to a specific activity of 2.0 x 107 cpm/,ug. Hybridization was in 2x SSC/50% formamide/10% dextran sulfate, pH 7.0, in the presence of a 1000-fold excess of salmon sperm DNA. The radiolabeled histone gene probes were denatured for 5 min at 70°C and rapidly chilled prior to hybridization at a concentration of 50 ng/ml. Hybridization was for 18 hr at 37°C, followed by extensive washing in hybridization buffer at 39°C to remove nonspecifically bound probe. The preparations were dehydrated by a series of ethanol washes, dipped in Kodak NTP-2 liquid nuclear track emulsion, and stored in the dark at 4°C for 1-4 weeks. Chromosome banding was carried out with Wright's Giemsa stain, and the preparations were analyzed by light microsc...
A human histone gene cluster was assigned to chromosome 1 by Southern blot analysis of DNA's from a series of mouse-human somatic cell hybrids with 32P-labeled cloned human H4 and H3 histone DNA as probes. Localization of this histone gene cluster on the long arm of chromosome 1 was confirmed by in situ hybridization of this DNA probe to metaphase chromosomes.
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