title: The N Terminus of the Retinoblastoma Protein Inhibits DNA Replication via a Bipartite Mechanism Disrupted in Partially Penetrant Retinoblastomas.
abstract: 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 α (Pol-α) and Ctf4 recruitment without affecting DNA polymerases ε and δ 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 G1-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-α control provides a potential molecular explanation for how N-terminal Rb loss-of-function deletions contribute to the etiology of partially penetrant retinoblastomas.
authors.affiliation: Department of Molecular Oncology, Moffitt Cancer Center and Research Institute, Tampa, Florida, USA.
authors.forename: Sergiy I
authors.initials: SI
authors.lastname: Borysov
authors.affiliation: Department of Molecular Oncology, Moffitt Cancer Center and Research Institute, Tampa, Florida, USA University of South Florida Cancer Biology Ph.D. Program, Tampa, Florida, USA.
authors.forename: Brook S
authors.initials: BS
authors.lastname: Nepon-Sixt
authors.affiliation: Department of Molecular Oncology, Moffitt Cancer Center and Research Institute, Tampa, Florida, USA University of South Florida Cancer Biology Ph.D. Program, Tampa, Florida, USA Mark.Alexandrow@moffitt.org.
authors.forename: Mark G
authors.initials: MG
authors.lastname: Alexandrow
mesh_terms: D000818:Animals; D002453:Cell Cycle; D045744:Cell Line, Tumor; D004256:DNA Polymerase I / Q000378:metabolism; D004261:DNA Replication; D017353:Gene Deletion; D006801:Humans; D064168:Minichromosome Maintenance Complex Component 7 / Q000378:metabolism; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D012175:Retinoblastoma / Q000235:genetics* / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014981:Xenopus
chemical_list: D016160:Retinoblastoma Protein; D004256:DNA Polymerase I; C101624:MCM7 protein, human; D064168:Minichromosome Maintenance Complex Component 7