Читать книгу The SAGE Encyclopedia of Stem Cell Research - Группа авторов - Страница 189

Progression through the cell cycle is necessary for the proliferation of both normal and cancer cells. The cell cycle consist of two functional phases (S phase and M phase) and two preparatory phases (G1 phase and G2 phase). These phases have multiple checkpoints to regulate, monitor, and prevent the cell cycle from progressing if certain requirements have not been met. Two main checkpoints are the G1/S checkpoint and the G2/M checkpoint. Tumor suppressor genes p53, p21, and p16 also regulate the cell cycle. Dysregulation of any part of the cycle results in uncontrolled growth. Other mechanisms involved in abnormal growth of cancer cells are the activation of cell growth by growth factors, inhibition of tumor suppressor genes, evasion of apoptosis, and upregulation of telomerase enzymes.

An image of a human brain using a magnetic resonance imaging (MRI) machine. In this patient, brain cancer has metastasized in the occipital lobe, shown in the darker gray mass at lower right. (Wikimedia Commons)

Brain cancer arises either from the transformation of progenitor cells or from the dedifferentiation of mature cells in response to genetic alteration. It has also been reported that some brain cancers possess a subpopulation of cancer stem-like cells having the capacity to initiate and sustain the tumor because of their ability to proliferate, self-renew, and be multi-potent. The self-renewal ability of these brain tumor stem cells correlates with increased malignancy (such as that seen in a medulloblastoma compared with a low-grade glioma). The origin of cancer stem cells is generally not known, but they may derive from a stem cell, a transient amplifying cell, or even a terminally differentiated cell that have acquired mutations that free them from the normal control on their proliferation and/or that give them stem cell characteristics, notably, by hijacking the capacity to self-renew and thereby creating a cancer cell that renews indefinitely.