By Sue Rochman

What is Cancer?

Cancer is many diseases, but all with common denominators

By Sue Rochman

A GOOD CELL GONE BAD
Unlike diseases that are contagious and that are acquired from someone else, cancer begins inside our own bodies. "Our bodies are made of tissue, and tissues are made of cells," explains Robert Weinberg, a professor of biology at the Massachusetts Institute of Technology. "Cancer is simply an abnormal tissue made of abnormal cells. It is not an implant from outside the body; it is some of our own native cells that have gone awry and started growing abnormally."

In 1980, Weinberg and his colleagues discovered the first human oncogene—a mutated gene that causes normal cells to form tumors—in a human bladder cancer. Finding that gene, called ras, helped to prove what researchers had speculated: that genetic mutations cause human cancers. A mutation occurs when a gene is damaged or changed in a way that alters the message that gene carries.

Genetic messages tell cells to create proteins—the ubiquitous biological molecules that make up and run the body, including the processes of cell growth and death—and a mutation can result in the wrong protein or no protein at all. Over the past two decades, more than 50 human oncogenes have been discovered.

Genetic mutations can occur quite easily. There are somewhere between 10 and 100 trillion cells in the adult human body, and more than one trillion new cells are created every day. Each time a cell divides to create two new daughter cells, there is the potential for an error in the replication of the genome. Typically, an error doesn't cause any problems. But if a descendant of this cell acquires another error, and so does one of its descendants, and so does one of its descendants, and so on, a cell can be transformed from normal to cancerous.

Cancer cells might do three problematic things in the body, says Larry Norton, deputy physician-in-chief for Breast Cancer Programs at Memorial Sloan-Kettering Cancer Center in New York City. One, the cells multiply continually, forming a tumor or a mass. Two, the cells often invade into the surrounding tissue so that they start to destroy the function of a normal organ—this is called invasion. Three, the cells sometimes spread to other parts of the body—a process called metastasis. The development of new therapies is based on interfering with these three abnormal properties, says Norton, "and we have to pay attention to all of them."

SIX TRAITS OF CANCER
For a cell to take part in these three activities, it has to acquire behaviors a normal cell doesn't have. Six years ago, Weinberg and his colleague Douglas Hanahan, a professor of biochemistry at the University of California, San Francisco, argued in a seminal article in the journal Cell that there are "six essential alterations" resulting from genetic mutations that are shared by most, if not all, cancer cells. They referred to these six traits as the hallmarks of cancer.

These traits "don't all occur within one cell," explains Weinberg. "They occur within a lineage of cells and a cell's descendants, and its descendant's descendants and so forth." And they don't occur within a week, or a month, or a year, but over an extended period. "The time course for most cancers is 30, 40 or 50 years, which explains why cancer is largely a disease of old people. You have to live long enough for these things to happen, and each of these steps takes on average 10 or 15 years."