Fragile X Mutations Affect Boys and Their Grandfathers
18.4 Cancer Genes and MicroRNAs
Most mutations that cause cancer are in oncogenes or tumor sup- pressor genes . A third category includes mismatch mutations in DNA repair genes (see section 12.6) that allow other mutations to persist. When such mutations activate oncogenes or inactivate tumor suppressor genes, cancer results. DNA repair disorders are often inherited in a single-gene fashion, and are quite rare.
They tend to cause diverse and widespread tumors.
Oncogenes
Genes that normally trigger cell division when it is appropri- ate are called proto-oncogenes. They are active where and when high rates of cell division are necessary, such as in a
Figure 18.10 Shifting the balance in a tissue toward cells that divide. If a mutation renders a differentiated cell able to divide to yield other cells that frequently divide, then over time these cells may take over, forming an abnormal growth.
= Stem or progenitor cell
2/30 = 6.6%
Cell divisions
Abnormal growth
Mutation
Cell divisions
%
5/30 = 16.6%
8/30 = 26.6%
Figure 18.11 Too much repair may trigger tumor formation. If epithelium is occasionally damaged, resting stem cells can become activated and divide to fill in the tissue. If injury is chronic, the persistent activation of stem cells to renew the tissue can veer out of control, fueling an abnormal growth.
o N
rmal
repair
Resting epithelium
Acute injury Chronic injury
Persistent activation of stem cells
Cancer
Resting stem cell Activated stem cell
Self-renewal Cancer cells Repair
Injury and activation of tissue
Repair
antibody gene. Recall from chapter 17 that antibody genes nor- mally move into novel combinations when a B cell is stimulated and they are very actively transcribed. Cancers associated with viral infections, such as cervical cancer following HPV infec- tion, may be caused when proto-oncogenes are mistakenly acti- vated with antibody genes. Similarly, in Burkitt lymphoma, a cancer common in Africa, a large tumor develops from lymph glands near the jaw. People with Burkitt lymphoma are infected with the Epstein-Barr virus, which stimulates specific chromo- some movements in maturing B cells to assemble antibodies against the virus. A translocation places a proto-oncogene on chromosome 8 next to an antibody gene on chromosome 14.
The oncogene is overexpressed, and the cell division rate increases. Tumor cells of Burkitt lymphoma patients have the translocation ( figure 18.12 ).
We can use the information of changes in gene expres- sion that promote cancer to diagnose, treat, or track response to treatment, even without knowing what the expression pat- terns mean. For example, ocular melanoma affects pigment cells in the eye—it is much more deadly than common skin melanoma. Many cases of ocular melanoma spread, 95 percent to the liver. Researchers extracted mRNAs from affected eyes and measured the expression of ten genes. They derived two
“molecular signatures”—patterns of mRNAs that are more or less abundant than normal—one predicting a low risk of spread to the liver, the other a high risk. This information is used to guide treatment choices.
Fusion Proteins with New Functions
An oncogene is also activated when a proto-oncogene moves next to another gene, and the gene pair is transcribed and translated together, as if they are one gene. The double gene product, called a fusion protein, activates or lifts control of cell division.
A fusion oncoprotein causes acute promyelocytic leu- kemia. (Leukemias differ by the type of white blood cell affected.) A translocation between chromosomes 15 and 17 of growth factors that cause mitosis to fill in the damaged
area with new cells. When that proto-oncogene is activated at a site other than a wound—as an oncogene—it still hikes growth factor production and stimulates mitosis. However, because the site of the action is not damaged tissue, the new cells form a tumor.
Some proto-oncogenes encode transcription factors that, as oncogenes, are too highly expressed. (Recall from chapter 10 that transcription factors bind to specific genes and acti- vate transcription.) The products of these activated genes then contribute to the cancer cell’s characteristics. Oncogenes may also block apoptosis. As a result, damaged cells do not die, but divide.
Increased Expression in a New Location
A proto-oncogene can become an oncogene when it is placed next to a gene that boosts its expression. A virus infecting a cell, for example, may insert DNA next to a proto-oncogene.
When the viral DNA is rapidly transcribed, the adjacent proto- on cogene (now an oncogene) is also rapidly transcribed.
Increased production of the oncogene’s encoded protein then switches on genes that promote mitosis, triggering the cascade of changes that leads to cancer. Viruses cause cervical cancer, Kaposi sarcoma, and acute T cell leukemia.
A proto-oncogene can also be activated when it is moved next to a gene that is normally very actively transcribed. This can happen when a chromosome is inverted or translocated, placing a gene in a new chromosomal environment. For exam- ple, a cancer of the parathyroid glands in the neck is associated with an inversion on chromosome 11, which places a proto- oncogene next to a DNA sequence that controls transcription of the parathyroid hormone gene. When the gland synthesizes the hormone, the oncogene is expressed, too. Cells in the gland divide, forming a tumor.
Ironically, the immune system contributes to cancer when a translocation or inversion places a proto-oncogene next to an
Figure 18.12 A translocation that causes cancer. (a) The cause of Burkitt lymphoma is translocation of a proto-oncogene on chromosome 8 to chromosome 14, next to a highly expressed antibody gene. Overexpression of the translocated proto-oncogene, now an oncogene, triggers the molecular and cellular changes of cancer. (b) Burkitt lymphoma often affects the jaw.
antibody gene
Translocation
Chromosomal site of oncogene Chromosomal
site of
antibody gene Chromosomal site of
Chromosomal site of proto-oncogene
a. b.
many times—turns off transcription. Such hypermethyla- tion is an epigenetic change, because the mRNA sequence is unaffected.
Wilms’ tumor is an example of a cancer that develops from loss of tumor suppression. A gene that normally halts mitosis in the rapidly developing kidney tubules in the fetus is absent. As a result, an affected child’s kidney retains pockets of cells dividing as frequently as if they were still in the fetus, forming a tumor. Following are descriptions of specific tumor suppressor genes.
Retinoblastoma (RB)
RB (MIM 180200) is a rare childhood eye tumor . In 1597, a Dutch anatomist described the eye cancer as a growth “the size of two fists.” In 1886, researchers identified inherited cases. At that time, the only treatment was removal of the affected eye.
Today, children with an affected parent or sibling, who have a 50 percent chance of having inherited the mutant RB gene, can be monitored from birth so that noninvasive treatment can begin early. Full recovery is common. Often the first abnor- mal sign is an unusual gray area that appears in an eye in a photograph—the tumor reflects light differently than unaf- fected parts of the eye.
About half of the 1 in 20,000 infants who develop RB inherit susceptibility to the disorder: They harbor one germline mutant allele for the RB gene in each of their cells. Cancer develops in any somatic cell where the second copy of the RB gene mutates. Therefore, inherited retinoblastoma requires two point mutations or deletions, one germline and one somatic.
In sporadic (noninherited) cases, two somatic mutations occur in the RB gene. Either way, RB usually starts in a cone cell of the retina, which provides color vision. Study of RB was the origin of the “two-hit” hypothesis of cancer causation—
that two mutations (germline and somatic or two somatic) are required to cause a cancer related to tumor suppressor deletion or malfunction.
Many children with RB have deletions in the same region of the long arm of chromosome 13, which led researchers to the cancer-causing gene. In 1987, they found the RB gene and iden- tified its protein product, which linked the cancer to control of the cell cycle. The RB protein normally binds transcription fac- tors so that they cannot activate genes that carry out mitosis. It normally halts the cell cycle at G 1 . When the RB gene is mutant or missing, the hold on the transcription factor is released, and cell division ensues.
Mutations in the RB gene cause other cancers. Chil- dren successfully treated for retinoblastoma often develop bone cancer as teens or bladder cancer as adults. Mutant RB genes have been found in the cells of patients with breast, lung, or prostate cancers, or acute myeloid leukemia, who never had the eye tumors. These other cancers may be caused by expression of the same genetic defect in different tissues.
brings together a gene coding for the retinoic acid cell sur- face receptor and an oncogene called myl. The fusion protein functions as a transcription factor, which, when overex- pressed, causes cancer. The nature of this fusion protein explains why some patients who receive retinoid (vitamin A-based) drugs recover. Their immature, dedifferentiated cancer cells, apparently stuck in an early stage of develop- ment where they divide frequently, suddenly differentiate, mature, and then die. Perhaps the cancer-causing fusion pro- tein prevents affected white blood cells from getting enough retinoids to specialize, locking them in an embryonic like, rapidly dividing state. Supplying extra retinoids allows the cells to resume their normal developmental pathway.
Reading 18.1 tells the story of a young magazine editor who recovered from a different type of leukemia thanks to a drug developed from understanding how a fusion protein causes cancer.
Receiving a Too-Strong Division Signal
In about 25 percent of women with breast cancer, affected cells have 1 to 2 million copies of a cell surface protein called Her-2/
neu that is the product of an oncogene. The normal number of these proteins is only 20,000 to 100,000.
The Her-2/neu proteins are receptors for epidermal growth factor. The receptors traverse the plasma membrane, extending outside the cell into the extracellular matrix and also dipping into the cytoplasm. They function as a tyrosine kinase, as is the case for the leukemia described in Reading 18.1 . When the growth factor binds to the tyrosine of the receptor, the tyrosine picks up a phosphate group, which signals the cell to activate transcription of genes that stimulate cell division. In Her-2/
neu breast cancer, too many tyrosine kinase receptors send too many signals to divide.
Her-2/neu breast cancer usually strikes early in adult- hood and spreads quickly. However, a monoclonal antibody- based drug called Herceptin binds to the receptors, blocking the signal to divide (see figure 17.16). Interestingly, Herceptin works when the extra receptors arise from multiple copies of the gene, rather than from extra transcription of a single Her-2/
neu gene.
Tumor Suppressors
Some cancers result from loss or silencing of a gene that normally suppresses tumor formation by blocking the activi- ties of other genes. Such a tumor suppressor gene normally inhibits expression of genes involved in all of the activities that turn a cell cancerous, listed in table 18.2 . Cancer can result when a tumor suppressor’s control is lifted. This can happen if the gene has a deletion, or if the promoter region binds too many methyl (CH 3 ) groups, which blocks tran- scription. Binding of CH 3 groups to “CpG islands”—regions in the starts of genes where the sequence “CG” repeats
When 23-year-old Glamour magazine editor Erin Zammett Ruddy went for a routine physical in November 2001, she expected reassurance that her healthy lifestyle had indeed been keeping her healthy (figure 1).
After all, she felt great. What she got, a few days later, was a shock.
Instead of having 4,000 to 10,000 white blood cells per milliliter of blood, she had more than 10 times that number—and many of the cells were cancerous.
“I had just returned from a nice, long lunch to find a message from my doctor. Could I call back? Something had come up in my blood work,” recalled Erin. “I was diagnosed with chronic myelogenous leukemia. CML is cancer, and until very recently, it proved fatal in the vast majority of cases.”
Although there is hardly a “good” time to find out that you have cancer, Erin’s diagnosis came just a few months after a landmark report of a new drug—and, ironically, an article in Glamour about three CML survivors. A successful cancer drug typically helps about 20 percent of the patients who take it, often just extending life a few months. But cancer in the blood had vanished in 53 of 54 initial patients, usually quickly. So Erin contacted the lead researcher, Brian Druker, and joined the group. Her cancer was reversed—with just a pill a day, and no side effects.
The drug, Gleevec, is now the standard treatment for CML and a few other cancers. The story of its development illustrates how understanding the genetic events that start and propel a cancer can guide development of an effective weapon.
The tale of Gleevec began on August 13, 1958, when two men entered hospitals in Philadelphia and reported weeks of fatigue.
Each had very high white blood cell counts and were diagnosed with CML. Too many immature white blood cells were crowding the healthy cells. The men’s blood samples eventually fell into the hands of pathologist Peter Nowell and cytogeneticist David Hungerford.
They had developed ways to stimulate white blood cells to divide in culture, and they probed the chromosomes of both leukemic and normal-appearing white blood cells in the two tired men and five others with CML.
Nowell and Hungerford discovered a small, unusual chromosome that was only in the leukemic cells. This was the first chromosome abnormality to be linked to cancer. Later, it would be dubbed “the Philadelphia chromosome” (Ph1). The link between the cancer and the chromosome anomaly held up in other patients.
With refinements in chromosome banding, important details emerged. In 1972, Janet Rowley at the University of Chicago used new stains that distinguished AT-rich from GC-rich chromosome regions to tell that Ph1 is the result of a translocation (see figure 13.19). By 1984, researchers had homed in on the two genes juxtaposed in the translocation between chromosomes 9 and 22. Therein lay the clues that would lead to Erin’s treatment.
One gene from chromosome 9 is called the Abelson oncogene (abl), and the other gene, from chromosome 22, is called the breakpoint cluster region (bcr). Two different fusion genes form.
The bcr-abl fusion gene is part of the Philadelphia chromosome, and it causes CML. The encoded fusion protein, called the BCR- ABL oncoprotein, is a form of the enzyme tyrosine kinase, which is the normal product of the abl gene. The cancer-causing form of tyrosine kinase is active for too long, which sends signals into the cell, stimulating it to divide too many times. (The other fusion gene does not affect health.)
The discovery that a fusion oncoprotein started the cellular changes that cause CML gave drug researchers a target. Through the 1980s, they tested more than 400 small molecules in search of one that would block the activity of the errant tyrosine kinase, without derailing other important enzymes. When they found a candidate in 1992, Druker joined the effort and led the way in developing it into Gleevec. Figure 2 shows how the drug works—it nestles into the pocket on the tyrosine kinase that must bind ATP to stimulate cell division. With ATP binding blocked, cancer cells do not receive the message to divide, and they cease doing so. After passing safety tests, the drug worked so dramatically that it set a new speed record for drug approval—10 weeks.
Erin and the other patients were able to track their progress in several ways:
■ “Hematological remission” meant that the percentage of leukemia cells in the blood fell.
■ “Cytogenetic remission” meant that the percentage of cells with the Ph1 chromosome fell.
■ “Molecular remission” meant that the level of mRNA representing the fusion gene fell.
Although molecular remission is the goal of CML treatment, in actuality, fusion gene mRNA rarely reaches undetectable levels. As a result, patients can become resistant to Gleevec—relapse occurs in 3 to 16 percent of patients, depending on how sick they were when diagnosed. Resistance is a result of natural selection. Those few cancer cells able to divide in the presence of the drug eventually take over. Again, genetic research came to the rescue. By discovering how
Reading 18.1
Erin’s Story: How Gleevec Treats Leukemia
Figure 1 “My third bone marrow biopsy—you never get used to the pain,” said Erin. Gleevec has treated her leukemia.
(Continued)
Figure 2 How Gleevec treats chronic myelogenous leukemia. In CML, a translocation forms the fusion oncoprotein BCR-ABL, which functions as a tyrosine kinase. A tyrosine (an amino acid) of a substrate molecule picks up a phosphate from the ATP nestled in the oncoprotein, making the substrate able to bind to another protein, called an effector, that triggers runaway cell division (a). Gleevec replaces the ATP (b). Without phosphorylation of the tyrosine on the substrate, division stops. As cancer progresses, mutations in the DNA of some cells make the shape of their pockets unable to bind the drug. Newer drugs can replace Gleevec once the cancer becomes resistant.
Source: Adapted from “Drug therapy: Imantinib mesylate—A new oral targeted therapy” by Savage & Antman: New England Journal of Medicine 346: 683–693. Copyright © 2002 Massachusetts Medical Society. All rights reserved. Reprinted by permission.
P P P ATP BCR–ABL
oncoprotein Substrate
Gleevec Tyrosine
P P P ADP
Substrate
Tyrosine
Chronic myelogenous
leukemia Effector
BCR–ABL
oncoprotein Substrate Tyrosine
Substrate
Tyrosine
Chronic myelogenous
leukemia
Effector
a. b.
resistant cells evade the drug, researchers tweaked Gleevec, making it bind more strongly, and developed new drugs that fit the slightly altered active site in resistant cancer cells.
As for Erin, she decided to go off the drug while pregnant.
Although she risked relapse, she did not want to expose a fetus to the powerful drug. Her son Alex was born in 2008, healthy. Follow Erin’s progress on her blog: http:/www.glamour.com/lifestyle/blogs/editor).
(Concluded)
p53 Normally Prevents Many Cancers
Another single gene that causes a variety of cancers when mutant is p53. Recall from chapter 12 that the p53 protein tran- scription factor “decides” whether a cell repairs DNA replica- tion errors or dies by apoptosis. If a cell loses a p53 gene, or if the gene mutates and malfunctions, a cell with damaged DNA is permitted to divide, and cancer may be the result.
More than half of human cancers involve a point muta- tion or deletion in the p53 gene. This may be because p53 pro- tein is a genetic mediator between environmental insults and development of cancer ( figure 18.13 ). A type of skin cancer, for example, is caused by a p53 mutation in skin cells damaged by an excessive inflammatory response that can result from repeated sunburns. That is, p53 may be the link between sun exposure and skin cancer.
In most p53 -related cancers, mutations occur only in somatic cells. However, in the germline condition Li-Fraumeni syndrome (MIM 151623), family members who inherit a mutation in the p53 gene have a very high risk of developing cancer—50 percent do
so by age 30, and 90 percent by age 70. A somatic p53 mutation in the affected tissue results in cancer because a germline mutation in the gene is already present.
Stomach Cancer
Golda Bradfield died of stomach cancer in 1960. By the time some of her grown children developed the cancer too, the grandchildren began to realize that their family had a terrible legacy. Genetic testing revealed that the disease was familial diffuse gastric cancer (MIM 192090). An “exon skipping”
mutation deleted DNA from the tumor suppressor gene that encodes E-cadherin, which is essential for cell adhesion in lin- ing cells.
Golda’s grandchildren had genetic tests. Eleven of them had inherited the mutant gene, but scans of their stomachs did not show any tumors. Still, they all had their stomachs removed.
It was good that they did, because most of them already had hun- dreds of tumors, too tiny to have been seen on medical scans.
The cousins without stomachs are doing well. Like people who