New insights into the pRB/ Condensin II interaction

KEYWORDS:

• cancer
• cell cycle
• Condensin II
• E2F
• pRB
• repetitive elements

This article refers to:

Disruption of CDK-resistant chromatin association by pRB causes DNA damage, mitotic errors, and reduces Condensin II recruitment

The retinoblastoma protein (pRB) is one of the most well-known, multifunctional tumor suppressor proteins whose deregulation has been implicated in a variety of human cancers.¹ As a key cell cycle regulator, pRB represses the transition from G1 to S phase through its interaction with E2F1.¹ CDK-dependent phosphorylation of pRB inhibits most interactions with E2Fs, allowing for the induction of genes essential for the subsequent cell cycle phases.¹ pRB also interacts with several chromatin modifiers, and its depletion leads to a more relaxed chromatin state, replication defects, condensation and segregation defects in mitosis, and increased levels of DNA damage and aneuploidy.¹ The tight linkage between pRB and the cell cycle, however, make it extremely difficult to assess whether the defects associated with genomic instability that are exhibited in a pRB-depleted state are a result of, or occur independently of pRB's role in regulating E2F-dependent genes necessary for cell cycle progression.

To shed light on these important questions, Ishak et. al. used mouse embryonic fibroblasts (MEFs) expressing a mutant pRb1^S/S allele which retains E2F binding necessary for maintaining transcriptional control of essential cell cycle genes, while disrupting residues that appear to be more important in the maintenance of genome stability.^2,3 Previous findings from this group showed that pRb1^S/S MEFs fail to recruit pRB to repetitive elements, resulting in their increased expression which was correlative with lymphoma development.²

In this study, ChIP-seq experiments further confirmed that pRb1^S/S MEFs lack pRB binding at repetitive elements including major satellites throughout the genome. The authors also showed that pRb1^S/S MEFs exhibit wild type levels of pRB binding at E2F target genes, and the expression of these genes was not altered. pRb1^S/S MEFs display increased levels of aneuploidy, DNA damage, and mitotic defects, similar to what was previously shown for pRb^−/− MEFs. Live cell imaging analyses also revealed increases in condensation defects and increases in anaphase bridge formation in pRb1^S/S MEFs during mitosis. In most cases, the anaphase bridges were left unresolved and correlated with elevated levels of partial segregation, missegregation, and binucleation events. Finally, pRb1^S/S MEFs exhibited increases in the frequency of micronuclei formation. All of these events have been associated with the development of aneuploidy.

The Condensin II complex is a chromatin organizing complex previously shown to bind to pRB, which also associates with loci containing repetitive elements and regulates their expression.^1,4 Condensin II consists of 2 structural maintenance of chromosome (SMC2 and SMC4) subunits. These subunits heterodimerize to form an active ATPase that constrains positive supercoils to condense DNA.⁵ The mammalian Condensin II complex also contains accessory chromosome associated proteins (CAP-G2, CAP-H2, and CAP-D3) that strongly influence complex formation and function.⁵ Mutation to the Condensin II subunit, CAP-H2 also results in genomic instability and murine lymphomas, although the tissue distribution differs from that seen in pRb1^S/S mice.⁶

As the authors were thorough in demonstrating that mitotic defects were not likely due to deregulation of cell cycle related E2F target genes, they next tested whether the important interactions between pRB and Condensin II were maintained in pRb1^S/S cells. Excitingly, they observed decreases in Condensin II loading at major satellite regions in pRb1^S/S mutant MEFs, where an increase in DNA damage was also observed. This suggests that pRB is needed to ensure Condensin II binding to major satellites, potentially as a means of preventing DNA damage.

Since genomic instability is a hallmark of cancer, investigating the mechanism by which mutations to pRB induce DNA damage and aneuploidy could provide insight into treating cancers where pRB malfunction is implicated. As both pRb1^S/S and Cap-H2 mutant mice develop lymphomas, further studies to investigate the deregulation of Condensin II in pRB-driven malignancies is warranted. Studies in Drosophila have shown that Condensin II affects the pairing of chromosomes at satellite repeats⁷ and it would be interesting to test whether this is also affected in pRb1^S/S MEFs. Finally, since Condensins have the propensity to influence global chromatin architecture throughout the cell cycle, future studies aimed at better understanding the effects of the pRb1^S/S mutation on the maintenance of topologically associated domain (TAD) boundaries and overall genome architecture could shed more light on the ability of pRB to promote genome stability.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.