Tumor Suppressor Genes

• Their protein product inhibits mitosis.
• When mutated, the mutant allele behaves as a recessive; that is, as long as the cell contains one normal allele, tumor suppression continues. (Oncogenes, by contrast, behave as dominants; one mutant, or overly-active, allele can predispose the cell to tumor formation).

Example 1: RB - the retinoblastoma gene

Familial retinoblastoma

Multiple tumors in the retinas of both eyes occurring in the first weeks of infancy.

Sporadic retinoblastoma

A single tumor appears in one eye sometime in early childhood before the retina is fully developed and mitosis in it ceases.

Familial retinoblastoma

Familial retinoblastoma occurs when the fetus inherits from one of its parents a chromosome (number 13) that has its RB locus deleted (or otherwise mutated). The normal Rb protein prevents mitosis.

A random mutation of the remaining RB locus in any retinal cell completely removes the inhibition provided by the Rb protein, and the affected cell grows into a tumor. So, in this form of the disease, a germline mutation plus a somatic mutation of the second allele leads to the disease.

Sporadic retinoblastoma

In this disease, both inherited RB genes are normal and a single cell must be so unlucky as to suffer a somatic mutation (often a deletion) in both in order to develop into a tumor. Such a double hit is an exceedingly improbable event, and so only rarely will such a tumor occur. (In both forms of the disease, the patient's life can be saved if the tumor(s) is detected soon enough and the affected eye(s) removed.)

Example 2: p53

The product of the tumor suppressor gene p53 is a protein of 53 kilodaltons (hence the name).

The p53 protein prevents a cell from completing the cell cycle if

• its DNA is damaged or
• the cell has suffered other types of damage.

• the damage is minor, p53 halts the cell cycle — hence cell division — until the damage is repaired.
• the damage is major and cannot be repaired, p53 triggers the cell to commit suicide by apoptosis.

These functions make p53 a key player in protecting us against cancer; that is, an important tumor suppressor gene. More than half of all human cancers do, in fact, harbor p53 mutations and have no functioning p53 protein.

Mice have been cured of cancer by treating them with a peptide that turns on production of the p53 protein in the tumor cells. However, there may be a tradeoff involved: excess production of the p53 protein leads to accelerated aging in mice [Link]. The converse appears also to be true: mice expressing high levels of the anti-aging protein Sirt1 [Link] have their production of p53 depressed and are more susceptible to cancer. However, under more physiological conditions, p53 seems to protect against both cancer and aging — perhaps by protecting cells from oxidative damage.

Example 3: p16^INK4a

The product of the tumor suppressor gene INK4a is a protein of 16 kilodaltons (hence the name).

Like p53, it blocks progression through the cell cycle — in this case by inhibiting the action of the cyclin-dependent kinase Cdk4.

As an animal ages, its cells produce increasing amounts of p16^INK4a. This is probably a good thing in that it reduces the risk of the cell entering uncontrolled mitosis, i.e., becoming a cancer. However, again like p53, there is a tradeoff. As levels of p16^INK4a rise in adult stem cells and progenitor cells, their ability to reproduce and thus replace lost or damaged tissue diminishes.

• Mice expressing higher-than-normal levels of p16^INK4a show earlier cell senescence while
• mice in which p16^INK4a activity is blocked continue to repair damaged tissue efficiently but run a higher risk of getting cancer.

In humans, deletions and other mutations of p16^INK4a are found in a variety of cancers.

Loss Of Heterozygosity (LOH)

Because tumor suppressor genes are recessive, cells that contain one normal and one mutated gene — that is, are heterozygous — still behave normally. (Exception: one X-linked tumor suppressor gene [WTX] has been found. In males, having only one X chromosome, a damaging point-mutation in WTX or its deletion is all that is needed to eliminate tumor-suppression. Females are also at risk if the mutation or deletion occurs on the X chromosome that is not inactivated.)

However, there are several mechanisms which can cause a cell to lose its normal gene and thus be predisposed to develop into a tumor. These may result in a "loss of heterozygosity" or "LOH".

Mechanisms of LOH:

1. Deletion of
   • the normal allele;
   • the chromosome arm containing the normal allele;
   • the entire chromosome containing the normal allele (resulting in aneuploidy).
2. In females, X-inactivation of the X chromosome carrying the normal allele.
3. Loss of the chromosome containing the normal allele followed by duplication of the chromosome containing the mutated allele.
4. Mitotic recombination. The study of tumor suppressor genes revealed (for the first time) that crossing over — with genetic recombination — occasionally occurs in mitosis (as it always does in meiosis).

In #3 and #4, the resulting cell now carries two copies of the "bad" gene. This is called "reduction to homozygosity".

Mutation is not the only way to inactivate tumor suppressor genes.

• a similarly blocked promoter on the other allele (producing the same effect as #2 above);
• a loss of that locus on the other chromosome (like the LOH in #1 above);
• an inactivating mutation in the other allele.

Tumor suppressor genes = anti-oncogenes

Genes like RB and p53 are also called anti-oncogenes. They were first given this name because they reverse, at least in cell culture, the action of known oncogenes.

This image (courtesy of Moshe Oren, from Cell 62:671, 1990) shows petri dishes which were seeded with the same number of mouse cells that had been transformed by two oncogenes: myc and ras. Many of those on the left have grown into colonies of cells. However, the cells plated on the right also contained the tumor suppressor p53 gene. Only a few have been able to grow into colonies.

Human Papilloma Viruses (HPV)

The name anti-oncogene may be even more appropriate than originally thought. Both the Rb protein and the p53 protein turn out to complex directly in the cell with an oncogene product of some human papilloma viruses.

Once inside the cells of their host, human papilloma viruses synthesize

• a protein designated E7 and
• another designated E6.

Result: E2F is now free to bind to the promoters of genes (like c-myc) that cause the cell to enter the cell cycle (right). Thus this version of E7 is an oncogene product.

The E6 protein of a second human papilloma virus implicated in cervical cancer binds the p53 protein targeting it for destruction by proteasomes and thus removing the block on the host cell's entering the cell cycle.

• the absence of activators of transcription and
• the presence of repressors of transcription.