BRCA1-induced large-scale chromatin unfolding and allele-specific effects of cancer-predisposing mutations

Large-scale chromatin organization and the localization of proteins involved in gene expression in human cells

Compartmentalization of the interphase nucleus is an important element in the regulation of gene expression. Here we investigated the functional organization of the interphase nucleus of HeLa cells and primary human fibroblasts. The spatial distribution of proteins involved in transcription (TFIIH and RNA polymerase II) and RNA processing and packaging (hnRNP-U) were analyzed in relation to chromosome territories and large-scale chromatin organization. We present evidence that these proteins are present predominantly in the interchromatin space, inside and between chromosome territories, and are largely excluded by domains of condensed chromatin. We show that they are present throughout the active and inactive X-chromosome territories in primary female fibroblasts, indicating that these proteins can freely diffuse throughout the interchromatin compartment in the interphase nucleus. Furthermore, we established that the in vivo spatial distribution of condensed chromatin in the interphase nucleus does not depend on ongoing transcription. Our data support a conceptually simple model for the functional organization of interphase nuclei.

Roles of BRCA1 in centrosome duplication

Centrosome duplication is under precise control and occurs only once in a normal mammalian cell cycle. Disruption of this process causes centrosome amplification, unequal segregation of chromosomes and, ultimately, tumorigenesis. Recent studies indicate that breast cancer suppressor gene 1 (BRCA1) plays an important role in regulating centrosome duplication. BRCA1 is located in the centrosome and binds to gamma-tubulin. It interacts with a variety of proteins that regulate centrosome duplication, including BRCA2, CDK2-Cyclin A, CDK2-Cyclin E, Gadd45, p21, p53 and Rb. Furthermore, targeted disruption of murine BRCA1 results in centrosome amplification, suggesting that BRCA1 serves as a negative regulator for centrosome duplication. This review will examine these data and discuss possible relationships between BRCA1 and its interacting proteins in centrosome duplication.

Alteration of large-scale chromatin structure by estrogen receptor

The estrogen receptor (ER), a member of the nuclear hormone receptor superfamily important in human physiology and disease, recruits coactivators which modify local chromatin structure. Here we describe effects of ER on large-scale chromatin structure as visualized in live cells. We targeted ER to gene-amplified chromosome arms containing large numbers of lac operator sites either directly, through a lac repressor-ER fusion protein (lac rep-ER), or indirectly, by fusing lac repressor with the ER interaction domain of the coactivator steroid receptor coactivator 1. Significant decondensation of large-scale chromatin structure, comparable to that produced by the approximately 150-fold-stronger viral protein 16 (VP16) transcriptional activator, was produced by ER in the absence of estradiol using both approaches. Addition of estradiol induced a partial reversal of this unfolding by green fluorescent protein-lac rep-ER but not by wild-type ER recruited by a lac repressor-SRC570-780 fusion protein. The chromatin decondensation activity did not require transcriptional activation by ER nor did it require ligand-induced coactivator interactions, and unfolding did not correlate with histone hyperacetylation. Ligand-induced coactivator interactions with helix 12 of ER were necessary for the partial refolding of chromatin in response to estradiol using the lac rep-ER tethering system. This work demonstrates that when tethered or recruited to DNA, ER possesses a novel large-scale chromatin unfolding activity.

Cancer susceptibility and the functions of BRCA1 and BRCA2

Inherited mutations in BRCA1 or BRCA2 predispose to breast, ovarian, and other cancers. Their ubiquitously expressed protein products are implicated in processes fundamental to all cells, including DNA repair and recombination, checkpoint control of cell cycle, and transcription. Here, I examine what is known about the biological functions of the BRCA proteins and ask how their disruption can induce susceptibility to specific types of cancer.