Proper regulation of cell division is required for normal growth, development and genomic integrity;aspects of this control are lost or disrupted in human cancers. The cyclin-dependent kinases (CDKs) drive all major transitions in the cell division cycle. Mammalian cells have multiple downstream effector CDKs but, as we show, a single upstream CDK-activating kinase (CAK), Cdk7. Gene knockout and silencing studies suggested redundancy in the CDK network, by showing that Cdk2-long thought to be the principal driver of DNA synthesis (S) phase-was dispensable for viability. However, our work-using a chemical-genetic approach that selectively inactivates catalytic function of the targeted CDK while sparing non-catalytic, "scaffold" functions-uncovered strict requirements for Cdk2 activity in proliferation of both transformed and non- transformed human cells. An important function of Cdk2 is to prevent premature activation of Cdk1, and thereby ensure coordinated progression through S phase. That function depends on a kinetically more favorable pathway for activation of Cdk2, compared to Cdk1, due in part to different mechanisms of recognition by Cdk7. I hypothesize that specialized functions of Cdk2-in regulating gene expression, coordinating S phase and responding to DNA damage or replication stress-emerge from its unique mode of activation. I propose, moreover, that another distinct pathway by which Cdk7 activates Cdk4 and Cdk6-which function prior to cell-cycle commitment-is directly or indirectly coupled to mitogen-sensing pathways.
The specific aims are: 1. To dissect the G1/S regulatory network comprising Cdk7, Cdk2, Cdk4 and Cdk6 2. To probe specific functions of Cdk7 and Cdk2 in the DNA damage response 3. To target the Cdk2 activation pathway with small molecule inhibitors Our preliminary studies reveal a unitary CAK-CDK network that achieves regulatory flexibility through kinetically distinct modes of kinase activation and inactivation. I ow propose to probe specific functions of CAK and the downstream CDKs in coordinating constitutive cell-cycle events and ensuring effective responses to genotoxic insults;and to test a new paradigm for inhibiting a specific CDK by selective targeting of its activation pathway. These studies promise to advance basic understanding of cell-cycle control, and to reveal novel strategies for anti-CDK therapy of human cancer.

Public Health Relevance

Control of cell proliferation is essential to normal growth and development and becomes deranged in cancer cells. All aspects of the cell division cycle are regulated by a network of cyclin-dependent kinases (CDKs);CDK functions become disturbed in cancer cells, and some tumors appear to be addicted to elevated activity of specific CDKs, making CDKs logical targets for chemotherapy. Chemical genetics allows selective inhibition of one CDK at a time in engineered human cells-derived from cancers or normal tissue- to dissect their critical biological functions and to evaluate different CDKs, with distinct roles in cell division, as potential targets for inhibition by anti-cancer drugs.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
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Cellular Signaling and Regulatory Systems Study Section (CSRS)
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Hamlet, Michelle R
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Icahn School of Medicine at Mount Sinai
Schools of Medicine
New York
United States
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