Deregulation and Cancer

Deregulation of cell cycle control proteins plays a key role in the development of cancer. Overactivation of proteins that favor cell cycle progression, namely cyclins and CDKs, and the inactivation of proteins that impede cell cycle progression, such as CKIs, can result in uncontrolled cell proliferation.

In human tumors, it is genes encoding the proteins that control the transition from the G1 to the S phase that are most commonly altered. These genes include those for cyclins, CKIs, and pRb. Such mutations overcome the inhibitory effects of pRb on the cell cycle, causing cells to have a growth advantage. In some cancers, this occurs after the direct mutation of the pRb gene, resulting in the protein's loss of function. In a larger set of cancers, pRb is indirectly inactivated by the hyper-activation of CDKs. This may result from over expression of cyclins, from an activating mutation in CDK4, or from inactivation of CKIs.

There is much evidence to suggest that cyclins can act as oncogenes to induce cells to become cancerous. In particular the G1 cyclins, cyclin D1, and cyclin E have been implicated in the development of cancer. Overexpression of the cyclin D1 protein is frequently detected in human breast cancer, and increasing evidence suggests that cyclin E overexpression plays an important role in the pathogenesis of breast cancer.

CKIs antagonize the function of cyclins, and considerable evidence suggests that these proteins function as tumor suppressors. CKI function is often altered in cancer cells. The gene encoding p16, a protein that belongs to the INK family of CKIs, is mutated, deleted, or inactivated in a large number of human malignancies and tumors. Such alterations

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