Myc-driven tumorigenesis involves a non-transcriptional role for Myc in over-activating replication origins. We show here that the mechanism underlying this process involves a direct role for Myc in activation of Cdc45-MCM-GINS (CMG) helicases at Myc-targeted sites. Myc induces decondensation of higher-order chromatin at targeted sites and is required for chromatin access at a chromosomal origin. Myc-driven chromatin accessibility promotes Cdc45/GINS recruitment to resident MCMs, and activation of CMGs. Myc-Box II, which is necessary for Myc-driven transformation, is required for Myc-induced chromatin accessibility, Cdc45/GINS recruitment, and replication stimulation. Myc interactors GCN5, Tip60, and TRRAP are essential for chromatin unfolding and recruitment of Cdc45, and co-expression of GCN5 or Tip60 with MBII-deficient Myc rescues these events and promotes CMG activation. Finally, Myc and Cdc45 interact and physiologic conditions for CMG assembly require the functions of Myc, MBII, and GCN5 for Cdc45 recruitment and initiation of DNA replication.

B cell activation is dependent on a large increase in transcriptional output followed by focused expression on secreted immunoglobulin as the cell transitions to an antibody producing plasma cell. The rapid transcriptional induction is facilitated by the release of poised RNA pol II into productive elongation through assembly of the super elongation complex (SEC). We report that a SEC component, the Eleven nineteen Lysine-rich leukemia (ELL) family member 3 (ELL3) is dynamically up-regulated in mature and activated human B cells followed by suppression as B cells transition to plasma cells in part mediated by the transcription repressor PRDM1. Burkitt’s lymphoma and a sub-set of Diffuse Large B cell lymphoma cell lines abundantly express ELL3. Depletion of ELL3 in the germinal center derived lymphomas results in severe disruption of DNA replication and cell division along with increased DNA damage and cell death. This restricted utilization and survival dependence reveal a key step in B cell activation and indicate a potential therapeutic target against B cell lymphoma’s with a germinal center origin.

The N-terminal domain of the retinoblastoma (Rb) tumor suppressor protein (RbN) harbors in-frame exon deletions in partially penetrant hereditary retinoblastomas and is known to impair cell growth and tumorigenesis. However, how such RbN deletions contribute to Rb tumor-and growth-suppressive functions is unknown. Here we establish that RbN directly inhibits DNA replication initiation and elongation using a bipartite mechanism involving N-terminal exons lost in cancer. Specifically, Rb exon 7 is necessary and sufficient to target and inhibit the replicative CMG helicase, resulting in the accumulation of inactive CMGs on chromatin. An independent N-terminal loop domain, which forms a projection, specifically blocks DNA polymerase ␣ (Pol-␣) and Ctf4 recruitment without affecting DNA polymerases and ␦ or the CMG helicase. Individual disruption of exon 7 or the projection in RbN or Rb, as occurs in inherited cancers, partially impairs the ability of Rb/RbN to inhibit DNA replication and block G 1 -to-S cell cycle transit. However, their combined loss abolishes these functions of Rb. Thus, Rb growth-suppressive functions include its ability to block replicative complexes via bipartite, independent, and additive N-terminal domains. The partial loss of replication, CMG, or Pol-␣ control provides a potential molecular explanation for how N-terminal Rb loss-offunction deletions contribute to the etiology of partially penetrant retinoblastomas. Mutational inactivation or deletion of the retinoblastoma (Rb) tumor suppressor gene occurs in multiple cancer types, including retinoblastoma, osteosarcoma, and breast and small cell lung cancers, and deregulation or inactivation of regulatory components of the Rb pathway is a hallmark of human cancers (1). The Rb protein functions to harness a variety of cellular processes important in tumorigenesis, including regulation of the cell cycle, apoptosis, differentiation, stress responses, and DNA replication. The role of Rb in these processes derives to a large extent from interactions of proteins with the C terminus of Rb that contains a large pocket domain (1-5), and most Rb loss-of-function mutations compromise pocket structure and/or function and are highly penetrant alleles of inherited cancer in humans and mice (6).Multiple observations indicate that the N-terminal domain of Rb (RbN) (residues 1 to 400) also plays an important role in growth suppression and tumorigenesis. Indeed, nearly 20% of cancer-associated in-frame mutations in Rb are located in the Nterminal region (6). These lesions leave an intact C-terminal pocket and generate stable forms of Rb that bind E2F transcription factors and localize to the nucleus in a fashion similar to that of wild-type Rb (wt-Rb) (6-10). Several in-frame RbN exon deletions in familial retinoblastomas have been reported, including individual losses of exon 4 (Ex4), Ex5, Ex7,. Inframe deletions and mutations have also been found in Rb exons 6 and 8 in prostate cancers and astrocytomas, respectively (15, 16). Furthermore, in contrast to pock...