RESEARCH ARTICLE The human origin recognition complex is essential for pre-RC assembly, mitosis, and maintenance of nuclear structure Hsiang-Chen Chou1,2†, Kuhulika Bhalla1†, Osama EL Demerdesh1, Olaf Klingbeil1, Kaarina Hanington1, Sergey Aganezov3, Peter Andrews1, Habeeb Alsudani1, Kenneth Chang1, Christopher R Vakoc1, Michael C Schatz3, W Richard McCombie1, Bruce Stillman1* 1Cold Spring Harbor Laboratory, Cold Spring Harbor, United States; 2Graduate Program in Molecular and Cellular Biology, Stony Brook University, Stony Brook, United States; 3Department of Computer Science, Whiting School of Engineering, Johns Hopkins University, Baltimore, United States Abstract The origin recognition complex (ORC) cooperates with CDC6, MCM2-7, and CDT1 to form pre-RC complexes at origins of DNA replication. Here, using tiling-sgRNA CRISPR screens, we report that each subunit of ORC and CDC6 is essential in human cells. Using an auxin-inducible degradation system, we created stable cell lines capable of ablating ORC2 rapidly, revealing multiple cell division cycle phenotypes. The primary defects in the absence of ORC2 were cells encountering difficulty in initiating DNA replication or progressing through the cell division cycle due to reduced MCM2-7 loading onto chromatin in G1 phase. The nuclei of ORC2-deficient cells were also large, with decompacted heterochromatin. Some ORC2-deficient cells that completed DNA replication entered into, but never exited mitosis. ORC1 knockout cells also demonstrated extremely slow cell proliferation and abnormal cell and nuclear morphology. Thus, ORC proteins and CDC6 are indispensable for normal cellular proliferation and contribute to nuclear organization. *For correspondence:
[email protected] †These authors contributed equally to this work Introduction Cell division requires the entire genome to be duplicated once and only once during S-phase of the Competing interest: See cell cycle, followed by segregation of the sister chromatids into two daughter cells.