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Functions of Tumor-Suppressor Proteins

Research Summary

William Kaelin is trying to understand why mutations affecting tumor-suppressor genes cause cancer. His long-term goal is to lay the foundation for the development of new anticancer therapies that are based on the biochemical functions of specific tumor-suppressor proteins.

Over time, cells can accumulate changes (mutations) in their DNA. If the DNA is a blueprint for making a protein (that is, a gene), the behavior of the cell might be altered. If the right combination of genes becomes altered, the cell will become cancerous. Some genes (tumor-suppressor genes) prevent malignant behavior. These genes contribute to cancer when they are inactivated as a result of mutations. Other genes (proto-oncogenes) can promote cancer if they acquire new properties (for example, become overactive) as a result of mutations (at which point they are called oncogenes). Most common cancers involve both inactivation of specific tumor-suppressor genes and activation of certain proto-oncogenes.

Our laboratory studies the functions of the proteins encoded by specific tumor-suppressor genes. Our long-term goal is to help develop new anticancer therapies. For example, it might be possible to develop a drug that would mimic the behavior of a particular tumor-suppressor protein (for example, by inactivating a specific enzyme involved in cell growth). It might also be possible to design strategies for killing only those cells in which a particular tumor-suppressor protein has been inactivated (thus sparing normal cells).

Figure 1: Structure of HIF1α bound to pVHL...

The Retinoblastoma Protein
The retinoblastoma tumor-suppressor protein (pRB) binds to a family of proteins collectively referred to as E2F. When not bound to pRB, E2F can turn on specific genes (that is, increase the production of the proteins encoded by those genes) that stimulate cell growth. In contrast, these same genes are turned off once pRB binds to E2F. Hence, E2F means "go" and pRB means "stop."

DNA is normally associated with proteins called histones, which modulate how easy it is for the cell to "read" the information contained within the DNA and use it to make proteins. The functions of histones, in turn, are regulated by the addition or removal of small chemical entities such as methyl groups (a methyl group consists of one carbon atom and three hydrogen atoms). In addition to E2F, pRB interacts with a protein called RBP2 (also called JARID1A or KDM5A). We recently discovered that RBP2 catalyzes (accelerates) the removal of methyl groups from histones under certain circumstances. Our earlier work suggested that loss of pRB deregulated RBP2. We have now made mice in which we can turn RBP2 on or off at will. Using these mice, we found that loss of RBP2 inhibits the development of specific types of tumors, including tumors linked to loss of pRB. This work has motivated the development of drugs that inhibit RBP2.

The von Hippel–Lindau Tumor Protein
People with von Hippel–Lindau (VHL) disease develop blood vessel tumors (hemangioblastomas) of the brain and eye, as well as tumors of the kidneys and adrenal glands. Such individuals have inherited a mutated, or altered, VHL tumor-suppressor gene from either parent. Tumors develop when the remaining normal copy of the VHL gene (from the other parent) is inactivated, rendering the cell incapable of making the normal VHL protein (pVHL). VHL mutations are also very common in kidney cancers arising in the general population, but these mutations are acquired spontaneously, not inherited from a parent.

We and others previously showed that pVHL attaches a chemical "flag" (called polyubiquitin) to certain proteins in the cell. This flag is recognized by a cellular machine, called the proteasome, that degrades proteins. The best-understood pVHL target is the protein HIF (hypoxia-inducible factor), which controls genes that are turned on by low oxygen. The presence of oxygen is required for pVHL to target HIF for destruction. In the absence of oxygen (or in tumor cells lacking normal pVHL), HIF accumulates and activates genes that promote survival in a low-oxygen environment (such as occurs when the blood supply to an organ is compromised). Some of these genes direct the synthesis of proteins that induce blood vessel formation, such as VEGF (vascular endothelial growth factor). We used laboratory models to establish that HIF is a driving force in the development of kidney cancer. This work then motivated the successful clinical testing, and eventual Food and Drug Administration approval, of drugs that directly or indirectly inhibit HIF or VEGF for the treatment of kidney cancer.

We and others discovered that pVHL binds to a short, discrete region of HIF. At the heart of this region is a proline residue (1 of the 20 amino acid building blocks used to make proteins) that, in the presence of oxygen, becomes chemically modified by the addition of another chemical flag (a hydroxyl group, consisting of an oxygen atom and a hydrogen atom), leading to its recognition by pVHL. Proteins that accelerate specific chemical reactions are called enzymes. We purified the enzyme EglN1 (also called PHD2) that hydroxylates HIF in the presence of oxygen. Laboratory experiments by us and others suggest that drugs that inhibit EglN1 would be useful for the treatment of anemia, heart attacks, and strokes. To facilitate the development of such drugs, we made a genetically engineered mouse in which tissues lacking adequate oxygen emit light, which can then be monitored using a sensitive camera, as well as mice in which EglN1 can be inactivated at different time points after birth.

EglN1 and RBP2 both belong to a large family enzymes that require a specific chemical, called 2-oxoglutarate, in order to function. A number of cancers have recently been shown to have mutations that cause them to accumulate chemicals, such as fumarate, succinate, or 2-hydroxyglutarate, which compete with 2-oxoglutarate and thereby potentially interfere with enzymes such as EglN1 and RBP2. We are using a variety of approaches to identify which enzymes, when altered by fumarate, succinate, or 2-hydroxyglutarate, can cause cancer. We are also trying to find specific vunerabilities created by high levels of these chemicals in the hope that this information can be used to design new therapeutics.

Grants from the National Institutes of Health and the Breast Cancer Research Foundation provided partial support for some of these projects.

As of March 08, 2013

Scientist Profile

Dana-Farber Cancer Institute
Cancer Biology, Medicine and Translational Research