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Scientists at Hollings Cancer Center at the Medical University of South Carolina have found that some cells can divide without a molecule that was previously thought necessary. This explains how liver cells can regenerate after injury and may leads to understanding of how cancer arises and how cancer cells evolve to have additional mutations, which accelerates growth and spread.
Cell division is necessary during periods of growth, such as embryonic development, and to replace dead or damaged cells. A key component of cell division is to accurately copy each chromosome providing identical DNA to each cell produced, termed DNA replication. Errors that occur during this process can result in cells with abnormal copies of chromosomes or deleterious mutations which can lead to cancer.
Human are multicellular organisms, to make multicellular organisms, it is important to copy cells, so DNA replication is very important. To divide properly and ensure accurate DNA replication, the cell must start this process at a specific location on each chromosome- the ‘origin of replication’. The entire process of firing these origins to start DNA replication, like the starting up of engines at the beginning of a car race, is highly regulated by a group of molecules called the ‘origin replication complex’ that binds to a certain place on the DNA within each chromosome and helps the cell recognize where to start DNA replication.
This complex helps each cell make a precise copy of its DNA before it divides, and thus ensures that all cells have a perfect genome. Some cells need a part of the origin recognition complex- ORC1, to copy their DNA, cells of the liver and placenta do not need ORC1. These cells are a rare type that routinely copy their DNA but do not divide, resulting in a larger cell with twice the normal amount of DNA. This process, termed an endocycle, can occur multiple times creating a cell with many times the normal amount of DNA.
The researchers found that in the liver, ORC1 is expressed at high levels in dividing cells. However, researchers found as the animal model ages and more liver cells start to endocycle, ORC1 levels drop indicating that ORC1 may not be needed to replicate DNA in endocycling cells. In order to test this idea, ORC1 was deleted in liver cells, and it was found that without OCR1, liver cells started to endocycle much sooner. ORC1 is essential for all forms of DNA replication. Cancer cells use processes similar to the endocycle to grow rapidly and become resistant to therapy, certain cancers have very low levels of ORC1 but are able to grow rapidly without it.