Transcription activator BRG1 also known as ATP-dependent chromatin remodeler SMARCA4 is a protein that in humans is encoded by the SMARCA4 gene. [5]
The protein encoded by this gene is a member of the SWI/SNF family of proteins and is similar to the brahma protein of Drosophila. Members of this family have helicase and ATPase activities and are thought to regulate transcription of certain genes by altering the chromatin structure around those genes. The encoded protein is part of the large ATP-dependent chromatin remodeling complex SWI/SNF, which is required for transcriptional activation of genes normally repressed by chromatin. In addition, this protein can bind BRCA1, as well as regulate the expression of the tumorigenic protein CD44. [6]
BRG1 works to activate or repress transcription. Having functional BRG1 is important for development past the pre-implantation stage. Without having a functional BRG1, exhibited with knockout research, the embryo will not hatch out of the zona pellucida, which will inhibit implantation from occurring on the endometrium (uterine wall). BRG1 is also crucial to the development of sperm. During the first stages of meiosis in spermatogenesis there are high levels of BRG1. When BRG1 is genetically damaged, meiosis is stopped in prophase 1, hindering the development of sperm and would result in infertility. More knockout research has concluded BRG1’s aid in the development of smooth muscle. In a BRG1 knockout, smooth muscle in the gastrointestinal tract lacks contractility, and intestines are incomplete in some cases. Another defect occurring in knocking out BRG1 in smooth muscle development is heart complications such as an open ductus arteriosus after birth. [7] [8]
BRG1 (or SMARCA4) is the most frequently mutated chromatin remodeling ATPase in cancer. [9] Mutations in this gene were first recognized in human cancer cell lines derived from adrenal gland [10] and lung. [11] Later it was recognized that mutations exist in a significant frequency of medulloblastoma and pancreatic cancers, and in many other tumor subtypes. [12] [13] [14]
In cancer, mutations in BRG1 show an unusually high preference for missense mutations that are frequently heterozygous and target the ATPase domain. [15] [9] Mutations are enriched at highly conserved ATPase sequences, [16] which lie on important functional surfaces such as the ATP pocket or DNA-binding surface. [15] These mutations act in a genetically dominant manner to alter chromatin regulatory function at enhancers [15] and promoters. [16]
Mutations of BRG1 are associated with context-dependent expression changes at MYC-genes, which indicates that the BRG1 and MYC proteins are functionally related. [15] [11] [17] Another study demonstrated a causal role of BRG1 in the control of retinoic acid and glucocorticoid-induced cell differentiation in lung cancer and in other tumor types. This enables the cancer cell to sustain undifferentiated gene expression programs that affect the control of key cellular processes. Furthermore, it explains why lung cancer and other solid tumors are completely refractory to treatments based on these compounds that are effective therapies for some types of leukemia. [18]
The role of BRG1 in sensitivity or resistance to anti-cancer drugs had been recently highlighted by the elucidation of the mechanisms of action of darinaparsin, an arsenic-based anti-cancer drugs. Darinaparsin has been shown to induce phosphorylation of BRG1, which leads to its exclusion from chromatin. When excluded from the chromatin, BRG1 can no longer act as a transcriptional co-regulator. This leads to the inability of cells to express HO-1, a cytoprotective enzyme. [19]
SMARCA4 has been shown to interact with:
RSC is a member of the ATP-dependent chromatin remodeler family. The activity of the RSC complex allows for chromatin to be remodeled by altering the structure of the nucleosome.
In molecular biology, SWI/SNF, is a subfamily of ATP-dependent chromatin remodeling complexes, which is found in eukaryotes. In other words, it is a group of proteins that associate to remodel the way DNA is packaged. This complex is composed of several proteins – products of the SWI and SNF genes, as well as other polypeptides. It possesses a DNA-stimulated ATPase activity that can destabilize histone-DNA interactions in reconstituted nucleosomes in an ATP-dependent manner, though the exact nature of this structural change is unknown. The SWI/SNF subfamily provides crucial nucleosome rearrangement, which is seen as ejection and/or sliding. The movement of nucleosomes provides easier access to the chromatin, allowing genes to be activated or repressed.
Chromatin remodeling is the dynamic modification of chromatin architecture to allow access of condensed genomic DNA to the regulatory transcription machinery proteins, and thereby control gene expression. Such remodeling is principally carried out by 1) covalent histone modifications by specific enzymes, e.g., histone acetyltransferases (HATs), deacetylases, methyltransferases, and kinases, and 2) ATP-dependent chromatin remodeling complexes which either move, eject or restructure nucleosomes. Besides actively regulating gene expression, dynamic remodeling of chromatin imparts an epigenetic regulatory role in several key biological processes, egg cells DNA replication and repair; apoptosis; chromosome segregation as well as development and pluripotency. Aberrations in chromatin remodeling proteins are found to be associated with human diseases, including cancer. Targeting chromatin remodeling pathways is currently evolving as a major therapeutic strategy in the treatment of several cancers.
Tripartite motif-containing 28 (TRIM28), also known as transcriptional intermediary factor 1β (TIF1β) and KAP1, is a protein that in humans is encoded by the TRIM28 gene.
SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily B member 1 is a protein that in humans is encoded by the SMARCB1 gene.
Probable global transcription activator SNF2L2 is a protein that in humans is encoded by the SMARCA2 gene.
SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily A member 5 is a protein that in humans is encoded by the SMARCA5 gene.
Actin-like protein 6A is a protein that in humans is encoded by the ACTL6A gene.
AT-rich interactive domain-containing protein 1A is a protein that in humans is encoded by the ARID1A gene.
SWI/SNF complex subunit SMARCC1 is a protein that in humans is encoded by the SMARCC1 gene.
SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily E member 1 is a protein that in humans is encoded by the SMARCE1 gene.
SWI/SNF complex subunit SMARCC2 is a protein that in humans is encoded by the SMARCC2 gene.
AT-rich interactive domain-containing protein 1B is a protein that in humans is encoded by the ARID1B gene. ARID1B is a component of the human SWI/SNF chromatin remodeling complex.
SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily D member 1 is a protein that in humans is encoded by the SMARCD1 gene.
Protein polybromo-1 (PB1) also known as BRG1-associated factor 180 (BAF180) is a protein that in humans is encoded by the PBRM1 gene.
AT-rich interactive domain-containing protein 2 (ARID2) is a protein that in humans is encoded by the ARID2 gene.
SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily D member 3 is a protein that in humans is encoded by the SMARCD3 gene.
The Epstein–Barr virus nuclear antigen 2 (EBNA-2) is one of the six EBV viral nuclear proteins expressed in latently infected B lymphocytes is a transactivator protein. EBNA2 is involved in the regulation of latent viral transcription and contributes to the immortalization of EBV infected cells. EBNA2 acts as an adapter molecule that binds to cellular sequence-specific DNA-binding proteins, JK recombination signal-binding protein (RBP-JK), and PU.1 as well as working with multiple members of the RNA polymerase II transcription complex.
Robert E. Kingston is an American biochemist who studies the functional and regulatory role nucleosomes play in gene expression, specifically during early development. After receiving his PhD (1981) and completing post-doctoral research, Kingston became an assistant professor at Massachusetts General Hospital (1985), where he started a research laboratory focused on understanding chromatin's structure with regards to transcriptional regulation. As a Harvard graduate himself, Kingston has served his alma mater through his leadership.
Trevor K. Archer is a Bahamian research scientist who is a National Institutes of Health Distinguished Investigator and deputy director at the National Institute of Environmental Health Sciences. He leads the Chromatin and Gene Expression Group, who investigate chromatin, epigenetics and embryonic stem cells pluripotency.
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