According to a study conducted by Hollings Cancer Center scientists at the Medical University of South Carolina, DNA replication in murine cells is possible even in the absence of origin recognition complex 1 (ORC1), a protein encoded by the homonymous gene that was previously thought to be indispensable .
The largest subunit of the origin recognition complex (ORC1) is essential for assembly of the prereplicative complex, firing of DNA replication origins, and faithful duplication of the genome. Here, we generated knock-in mice with LoxP sites flanking exons encoding the critical ATPase domain of ORC1. Global or tissue-specific ablation of ORC1 function in mouse embryo fibroblasts and fetal and adult diploid tissues blocked DNA replication, cell lineage expansion, and organ development. Remarkably, ORC1 ablation in extraembryonic trophoblasts and hepatocytes, two polyploid cell types in mice, failed to impede genome endoreduplication and organ development and function. Thus, ORC1 in mice is essential for mitotic cell divisions but dispensable for endoreduplication. We propose that DNA replication of mammalian polyploid genomes uses a distinct ORC1-independent mechanism.
DNA replication and ORC1
Multicellular life depends on the ability of individual cells to divide; new cells must be created from existing ones when an organism is developing or every time older cells die or are damaged. Each of an organism’s cells should contain an exact copy of its parent cell’s DNA, as mutated DNA might make newborn cells dysfunctional or, worse, cancerous. For this reason, multicellular organisms have evolved quality-control mechanisms that make DNA replication as perfect as possible as well as repair mechanisms to fix imperfect DNA.
Cells use the protein encoded by the ORC1 gene as a tool to make sure that their offspring cells have their same DNA. The protein binds to the DNA location where the process of replication is supposed to start on each chromosome, which is something that the DNA replication machinery needs to ‘know’ in order to accurately replicate the cell’s genome.
Until now, it was generally accepted that ORC1 was essential for DNA replication in all cells; however, the study authored by Dr. Takayuki Okano-Uchida and Dr. Gustavo Leone, among others, showed that, while ORC1 is indeed essential for murine development, liver and placenta cells appear to be able to replicate their DNA even in the absence of this protein.
The scientists found out that DNA replication in two types of murine polyploid cells appears to be unaffected by the lack of ORC1. Polyploid cells, unlike most other (diploid) cells, have more than two copies of each chromosome. The cells in question were from the livers and placentae of the mice, and while they express high levels of ORC1 during division, ORC1 levels seem to fall as the animals age. At this point, the cells cease dividing but keep making copies of their own DNA (that is, they become polyploids)—a process known as endoreduplication or endocycling. This tipped off the scientists, who started to suspect that liver and placenta cells in mice might not need ORC1 to replicate their DNA. Indeed, the authors of the study tested their hypothesis by deleting the ORC1 in cells of the two polyploid tissues, and found out that in the absence of the protein, the cells would simply start endocycling sooner than normal. However, the same deletion in other tissues, such as embryo fibroblasts and adult diploid cells, ended up blocking DNA replication and cell division.
While this study offers insight on how ORC1 relates to murine development, it is unclear whether its results are also true in humans; it also remains to be seen whether what has been observed in murine liver cells will happen in human liver cells as well, which is what the researchers seek to test next. ORC1 might also be important in cancer cell replication; processes similar to endocycling have been observed in some cancers, whose cells don’t express much ORC1 at all and yet manage to spread like wildfire. According to Okano-Uchida, it is possible that DNA replication in the absence of ORC1 may lead to an accumulation of mutations as the cell replicates, which, in turn, may well drive cancer.
 Okano-Uchida, T., Kent, L. N., Ouseph, M. M., McCarty, B., Frank, J. J., Kladney, R., … & Leone, G. (2018). Endoreduplication of the mouse genome in the absence of ORC1. Genes & development, 32(13-14), 978-990.