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channel that promotes cytochrome c release rather than inhibiting it (see Fig. 18.16). The pro-death BH3-

only proteins (e.g., Bim and Bid) contain only the structural domain that allows them to bind to other Bcl-

2 family members (the BH3 domain), and not the domains for binding to the membrane, forming ion

channels, or binding to Apaf. Their binding activates the pro-death family members and inactivates the

antiapoptotic members. When the cell receives a signal from a pro-death agonist, a BH3 protein like Bid

is activated (see Fig. 18.16). The BH3 protein activates Bax (an ion-channel–forming proapoptotic

channel member), which stimulates release of cytochrome c. Normally, Bcl-2 acts as a death antagonist by

binding Apaf and keeping it in an inactive state. However, at the same time that Bid is activating Bax, Bid

also binds to Bcl-2, thereby disrupting the Bcl-2–Apaf complex and freeing Apaf to bind to released

cytochrome c to form the apoptosome. B. Cancer Cells Bypass Apoptosis

Apoptosis should be triggered by several stimuli such as withdrawal of growth factors, elevation of p53

in response to DNA damage, monitoring of DNA damage by repair enzymes, or release of TNF or other

immune factors. However, mutations in oncogenes can create apoptosis-resistant cells.

One of the ways this occurs is through activation of growth-factor–dependent signaling pathways that

inhibit apoptosis, such as the PDGF/Akt/BAD pathway. Nonphosphorylated BAD acts like Bid in

promoting apoptosis (see Fig. 18.16). Binding of the platelet-derived growth factor to its receptor

activates PI-3 kinase, which phosphorylates and activates the serine–threonine kinase Akt (protein kinase

B; see Chapter 11, Section III.B.3). Activation of Akt results in the phosphorylation of the proapoptoticBH3-only protein BAD, which inactivates it. The PDGF/Akt/BAD pathway illustrates the requirement of

normal cells for growth-factor stimulation to prevent cell death. One of the features of neoplastic

transformation is the loss of growth-factor dependence for survival. The MAP kinase pathway is also

involved in regulating apoptosis and sends cell-survival signals. MAP kinase kinase phosphorylates and

activates another protein kinase known as RSK. Like Akt, RSK phosphorylates BAD and inhibits its

activity. Thus, BAD acts as a site of convergence for the PI-3 kinase/Akt and MAP kinase pathways in

signaling cell survival. Gain-of-function mutations in the genes that control these pathways, such as ras,

create apoptosis-resistant cells.

When Bcl-2 is mutated and oncogenic, it is usually overexpressed; for example, in follicular

lymphoma and CML. Overexpression of Bcl-2 disrupts the normal regulation of proand antiapoptotic factors and tips the balance to an antiapoptotic stand. This leads to an inability to

destroy cells with damaged DNA, such that mutations can accumulate within the cell. Bcl-2 is also

a multidrug-resistant transport protein, and if it is overexpressed, it will block the induction of

apoptosis by antitumor agents by rapidly removing them from the cell. Thus, strategies are being

developed to reduce Bcl-2 levels in tumors that overexpress it before initiating drug or radiation

treatment.

C. MicroRNAs and Apoptosis

Recent work has identified a number of miRNAs which regulate apoptotic factors. Bcl-2, for example, is

regulated by at least two miRNAs, designated as miR-15 and miR-16. Expression of these miRNAs will

control Bcl-2 (an antiapoptotic factor) levels in the cell. If, for any reason, the expression of these

miRNAs is altered, Bcl-2 levels will also be altered, promoting either apoptosis (if Bcl-2 levels

decrease) or cell proliferation (if Bcl-2 levels increase). Loss of both of these miRNAs is found in 68%

of chronic lymphocytic leukemia (CLL) cells, most often caused by a deletion on chromosome 13q14.

Loss of miR-15 and -16 expression would lead to an increase in Bcl-2 levels, favoring increased cell

proliferation.

Other miRNA species have been identified which regulate factors involved in apoptosis. miR-21

regulates the expression of the programmed cell death 4 gene (PDCD4). PDCD4 is upregulated during

apoptosis and functions to block translation. Loss of miR-21 activity would lead to cell death because

PDCD4 would be overexpressed. However, overexpression of miR-21 would be antiapoptotic because

PDCD4 expression would be ablated.

The miR-17 cluster regulates the protein kinase B/Akt pathway by modulating the levels of PTEN (the

enzyme that converts phosphatidylinositol trisphosphate [PIP3] to phosphatidylinositol bisphosphate

[PIP2]), as well as the levels of the E2F family of transcription factors. An upregulation of miR-17, acting

as an oncogene, would decrease PTEN levels such that cellular proliferation is favored over apoptosis,

resulting from the constant activation of the Akt pathway.

VI. Cancer Requires Multiple MutationsCancer takes a long time to develop in humans because multiple genetic alterations are required to

transform normal cells into malignant cells (see Fig. 18.1). A single change in one oncogene or tumorsuppressor gene in an individual cell is not adequate for transformation. For example, if cells derived

from biopsy specimens of normal cells are not already “immortalized”—that is, able to grow in culture

indefinitely—addition of the ras oncogene to the cells is not sufficient for transformation. However,

additional mutations in a combination of oncogenes—for example, ras and myc—can result in

transformation (Fig. 18.17). Epidemiologists have estimated that four to seven mutations are required for

normal cells to be transformed.

Cells accumulate multiple mutations through clonal expansion. When DNA damage occurs in a

normally proliferative cell, a population of cells with that mutation is produced. Expansion of the mutated

population enormously increases the probability of a second mutation in a cell containing the first

mutation. After one or more mutations in proto-oncogenes or tumor-suppressor genes, a cell may

proliferate more rapidly in the presence of growth stimuli and with further mutations grow autonomously;

that is, independent of normal growth controls. Enhanced growth increases the probability of further

mutations. Some families have a strong predisposition to cancer. Individuals in these families have

inherited a mutation or deletion of one allele of a tumor-suppressor gene, and as progeny of that cell

proliferate, mutations can occur in the second allele, leading to a loss of control of cellular proliferation.

These familial cancers include familial retinoblastoma, FAPs of the colon, and multiple endocrine

neoplasia (MEN), one form of which involves tumors of the thyroid, parathyroid, and adrenal medulla

(MEN type II).

Studies of benign and malignant polyps of the colon show that these tumors have a number of different

genetic abnormalities. The incidence of these mutations increases with the level of malignancy. In theearly stages, normal cells of the intestinal epithelium proliferate, develop mutations in the APC gene, and

polyps develop (see Fig. 18.17). This change is associated with a mutation in the ras proto-oncogene that

converts it to an active oncogene. Progression to the next stage is associated with a deletion or alteration

of a tumor-suppressor gene on chromosome 5. Subsequently, mutations occur in chromosome 18,

inactivating a gene that may be involved in cell adhesion, and in chromosome 17, inactivating the p53

tumor-suppressor gene. The cells become malignant, and further mutations result in growth that is more

aggressive and metastatic. This sequence of mutations is not always followed precisely, but an

accumulation of mutations in these genes is found in a large percentage of colon carcinomas.

Michael T. had been smoking for 40 years before he developed lung cancer. The fact that

cancer takes so long to develop has made it difficult to prove that the carcinogens in

cigarette smoke cause lung cancer. Studies in England and Wales show that cigarette consumption

by men began to increase in the early 1900s. After a 20-year lag, the incidence in lung cancer in

men also began to rise. Women began smoking later, in the 1920s. Again, the incidence of lung

cancer began to increase after a 20-year lag.

VII. At the Molecular Level, Cancer Is Many Different Diseases

More than 20% of the deaths in the United States each year are caused by cancer, with tumors of the lung,

large intestine, and breast being the most common (Fig. 18.18). Different cell types typically use different

mechanisms through which they lose the ability to control their own growth. An examination of the genes

involved in the development of cancer shows that a particular type of cancer can arise in multiple ways.

For example, Patched and Smoothened are the receptor and coreceptor for the signaling peptide sonic

hedgehog. Either mutation of smoothened, an oncogene, or inactivation of patched, a tumor-suppressor

gene, can give rise to basal cell carcinoma. Similarly, TGF-β and its signal transduction proteins

Smad4/DPC are part of the same growth-inhibiting pathway, and either may be absent in colon cancer.

Thus, treatments that are successful for one patient with colon cancer may not be successful in a second

patient with colon cancer because of the differences in the molecular basis of each individual’s disease

(this now appears to be the case with breast cancer as well). Medical practice in the future will require

identifying the molecular lesions involved in a particular disease and developing appropriate treatments

accordingly. The use of proteomics, RNA-SEQ, and gene chip technology (see Chapter 17) to genotypetumor tissues, and to understand which proteins they express, will aid greatly in allowing patient-specific

treatments to be developed.

A treatment for CMLbased on rational drug design has been developed. The fusion

protein

Bcr-Abl is found only in transformed cells that express the Philadelphia chromosome and

not in normal cells. Once the structure of Bcr-Abl was determined, the drug imatinib (Gleevec)

was designed to specifically bind to and inhibit only the active site of the fusion protein and not

the normal protein. Imatinib was successful in blocking Bcr-Abl function, thereby stopping cell

proliferation, and in some cells inducing apoptosis, so the cells would die. Because normal cells

do not express the hybrid protein, they were not affected by the drug. The problem with this

treatment is that some patients suffered relapses, and when their Bcr-Abl proteins were studied it

was found that in some patients, the fusion protein had a single amino acid substitution near the

active site that prevented imatinib from binding to the protein. Other patients had an amplification

of the Bcr-Abl gene product. Other TKIs (such as dasatinib and nilotnib) can also be used in

treating CMLif a resistance to imatinib (Gleevec) is encountered. VIII. Viruses and Human Cancer

Three RNA retroviruses are associated with the development of cancer in humans: human Tlymphotrophic virus type 1 (HTLV-1), HIV, and hepatitis C. There are also DNA viruses associated with

cancer, such as hepatitis B, EBV, human papillomavirus (HPV), and herpesvirus (HHV-8).

HTLV-1 causes adult T-cell leukemia. The HTLV-1 genome encodes a protein, Tax, which is a

transcriptional coactivator. The cellular proto-oncogenes c-sis and c-fos are activated by Tax, thereby

altering the normal controls on cellular proliferation and leading to malignancy. Thus, tax is a viral

oncogene without a counterpart in the host-cell genome.

Infection with HIV, the virus that causes AIDS, leads to the development of neoplastic disease through

several mechanisms. HIV infection leads to immunosuppression and, consequently, loss of immunemediated tumor surveillance. HIV-infected individuals are predisposed to non-Hodgkin lymphoma, which

results from an overproduction of T-cell lymphocytes. The HIV genome encodes a protein, Tat, a

transcription factor that activates transcription of the interleukin-6 (IL-6) and interleukin-10 (IL-10)

genes in infected T-cells. IL-6 and IL-10 are growth factors that promote proliferation of T-cells, and thus,

their increased production may contribute to the development of non-Hodgkin lymphoma. Tat can also be

released from infected cells and act as an angiogenic (blood vessel–forming) growth factor. This property

is thought to contribute to the development of Kaposi sarcoma.

DNA viruses also cause human cancer, but by different mechanisms. Chronic hepatitis B infections

will lead to hepatocellular carcinoma. A vaccine currently is available to prevent hepatitis B infections.

EBV is associated with B- and T-cell lymphomas, Hodgkin disease, and other tumors. The EBV encodes

a Bcl-2 protein that restricts apoptosis of the infected cell. HHV-8 has been associated with Kaposi

sarcoma. Certain strains of papillomavirus are the major cause of cervical cancer, and a vaccine has been

developed against those specific papillomavirus strains. CLINICAL COM M ENTS

Mannie W. The treatment of a symptomatic patient with CMLwhose white blood cell count is in

excess of 50,000 cells/mLis initiated with a tyrosine kinase inhibitor (TKI). If one type is not

tolerated or successful, another one is tried. In addition, both γ- and β-interferon have shown promise in

increasing survival in these patients if they are intolerant to the TKI. Interestingly, the interferons have

been associated with the disappearance of the Philadelphia chromosome in dividing marrow cells of

some patients treated in this way. Hematopoietic stem cell transplantation can also be an option for

treatment.

Michael T. Surgical resection of the lung nodule with an attempt at cure was justified in Michael

T., who had a good prognosis with a T1,N0,M0 staging classification preoperatively. Because of his

history and the characteristics of the nodule, there was a high probability it was a malignant nodule and a

preoperative positron emission tomography (PET) scan did not show obvious signs of metastasis.

Unfortunately, Michael developed a metastatic lesion in the right temporal cortex of his brain 6

months later. Because metastases were almost certainly present in other organs, Michael’s brain tumor

was not treated surgically. In spite of palliative radiation therapy to the brain, Michael T. succumbed to

his disease just 9 months after its discovery, an unusually virulent course for this malignancy. On

postmortem examination, it was found that his body was riddled with metastatic disease.

The TNM system is a cancer staging system that standardizes the classification of tumors.

The T stands for the size and extent of the primary tumor (the higher number stands for a

larger and more extensive tumor), the N stands for the amount of regional lymph nodes that are

affected by the tumor (again, the higher the number, the worse the prognosis), and M stands for the

presence of metastasis (0 for none, 1 for the presence of metastatic cells).Clark T. Clark T. requires regular colonoscopies to check for new polyps in his intestinal tract. It

is recommended he initially has a colonoscopy 1 year after the surgery, and if no polyps are found,

the next colonoscopy can be at 3 years. Because the development of a metastatic adenoma requires a

number of years (because of the large numbers of mutations that must occur), frequent checks will enable

new polyps to be identified and removed before malignant tumors develop.

Calvin A. The biopsy of Calvin A.’s excised mole showed that it was not malignant. The most

important clinical sign of a malignant melanoma are changes in color or variation in color and

irregular borders. Unlike benign (nondysplastic) nevi, melanomas exhibit striking variations in

pigmentation, appearing in shades of black, brown, red, dark blue, and gray. Additional clinical warning

signs of a melanoma are enlargement of a preexisting mole, itching or pain in a preexisting mole, or

development of a new pigmented lesion during adult life. Calvin A. was advised to conduct a monthly

self-examination, to have a clinical skin examination once or twice yearly, to avoid sunlight, and to use

appropriate sunscreens.

Mutations associated with malignant melanomas include ras (gain of function in growth

signal-transduction oncogene), p53 (loss of function of tumor-suppressor gene), p16

(loss of

function in cdk inhibitor tumor-suppressor gene), cdk4 (gain of function in a cell-cycle

progression oncogene), and cadherin/β-catenin regulation (loss of regulation that requires

attachment).

BIOCHEM ICAL COM M ENTS

HNPCC and hereditary breast cancer both result from inherited mutations in genes involved in

DNA repair.

HNPCC is estimated to account for between 2% and 3% of all colon cancer cases. These syndromes

are heterogeneous, most likely owing to the finding that mutations in any of five genes could lead to colon

cancer. The disease genes include hMSH2, hMLH1, hPMS1, hPMS2, and hMSH6. These genes all play a

role in DNA mismatch repair, and all act as tumor suppressors (a loss of function is required for the tumor

to develop).

It is important to understand that the lack of a DNA mismatch repair enzyme actually does not directly

lead to cancer (such as an activating mutation in myc would do, for example). However, the lack of a

functional mismatch repair system increases the frequency at which new mutations are introduced into

somatic cells (particularly rapidly proliferating cells such as the colonic epithelium), such that eventually

a mutation will result in a gene necessary for proper growth control. Once that mutation occurs, tumors

can begin to develop.

Five percent to 10% of all breast cancer cases have been traced to inherited mutations in either one of

two genes, BRCA1 and BRCA2. BRCA1 maps to chromosome 17 and acts as a tumor suppressor. The

biochemical function of BRCA1 is to participate in the response to DNA damage. BRCA1 is

phosphorylated by a variety of kinases, each of which is activated by a different form of DNA damage.

BRCA1 is primarily involved in repairing double-strand breaks in DNA, and in transcription-coupledrepair. Once BRCA1 is phosphorylated, it will signal for cell cycle arrest in order to allow the DNA

damage to be repaired.

Women who carry a BRCA1 mutation have a 55% to 65% risk of developing breast cancer, and about

a 40% risk of ovarian cancer, by the age of 70 years. Men who carry BRCA1 mutations have been shown

to have a slight increase in breast cancer and prostate cancer.

The other gene involved in hereditary breast cancer is BRCA2, located on chromosome 13. BRCA2 is

required for DNA double-strand break repair, which is usually caused by ionizing radiation. As such, loss

of BRCA2 activity is required for cancer to develop, classifying BRCA2 as a tumor suppressor. BRCA2 is

also required for homologous recombination between sister chromatids during meiosis and mitosis.

BRCA2 mutations have also been liked to increased incidence of breast and ovarian cancer in women, and

breast cancer and prostate cancer in men. BRCA1 and BRCA2 mutations account for about 20% to 25% of

all hereditary breast cancer cases.

An understanding of the roles of BRCA1 and BRCA2 in repairing double-strand breaks in DNA has led

to the development of poly(ADP-ribose) polymerase 1 (PARP-1) inhibitors for the treatment of BRCA1-

or BRCA2-induced breast cancers. Double-strand break repair occurs either by

homologous

recombination (requiring the activities of BRCA1 and BRCA2 proteins) or through nonhomologous end

joining (NHEJ), which is an error-prone process, owing to trimming of the DNA ends before ligation.

Single-strand breaks in DNA are more common than double-strand breaks. The cellular mechanism

for repairing single-strand breaks is dependent on PARP-1. PARP-1 produces large branched chains of

poly(ADP-ribose), derived from NAD+, at the site of damage, which acts as a docking station for proteins

involved in repairing the single-strand break. Inhibiting PARP-1 would lead to an accumulation of singlestrand breaks in the DNA.

PARP-1 inhibitors are effective in killing BRCA1 or BRCA2 mutated cells in that when single-strand

breaks are not repaired, they often are converted to double-strand breaks when the replisome tries to

replicate through the break. In a cell lacking BRCA1 or BRCA2 activity, the only way the DNA can be

repaired is by NHEJ, which is an error-prone process. This leads to the cells accumulating a large

number of mutations, eventually leading to cell death. Cells with functional BRCA1 or BRCA2 activity

will not undergo that fate. Drugs which inhibit PARP-1 activity in cell culture are now in clinical trials,

with very promising results. KEY CONCEPTS

Cancer is the term applied to a group of diseases in which cells no longer respond to normal

constraints on growth.

Cancer arises owing to mutations in the genome (either inherited or formed in somatic cells).

The mutations that lead to cancer occur in certain classes of genes, including: Those that regulate cellular proliferation and differentiation

Those that suppress growth

Those that target cells for apoptosis Those that repair damaged DNA

Mutations that lead to cancer can be either gain-of-function mutations or loss of activity within a

protein.Gain-of-function mutations occur in proto-oncogenes, resulting in oncogenes. Loss-of-function mutations occur in tumor-suppressor genes.

Examples of proto-oncogenes are those involved in signal transduction and cell cycle progression:

Growth factors and growth-factor receptors Ras (a GTP-binding protein)

Transcription factors

Cyclins and proteins that regulate them

microRNAs which regulate growth-inhibitory proteins Examples of tumor-suppressor genes include the following:

Retinoblastoma (Rb) gene product, which regulates the G1-to-S phase of the cell cycle

p53, which monitors DNA damage and arrests cell-cycle progression until the damage has been

repaired Regulators of ras

microRNAs that regulate growth-promoting signals

Apoptosis (programmed cell death) leads to the destruction of damaged cells that cannot be

repaired, and it consists of three phases:

Initiation phase (external signals or mitochondrial release of cytochrome c) Signal integration phase

Execution phase

Apoptosis is regulated by a group of proteins of the Bcl-2 family, which consists of both proand

antiapoptotic factors.

Cancer cells have developed mechanisms to avoid apoptosis.

Multiple mutations are required for a tumor to develop in a patient, acquired over a number of

years.

Both RNA and DNA viruses can cause a normal cell to become transformed. Exploitation of DNA repair mechanisms may provide a novel means for regulating tumor cell

growth.

The diseases discussed in this chapter are summarized in Table 18.4.REVIEW QUESTIONS—CHAPTER 18

1.The ras oncogene in Clark T.’s malignant polyp differs from the c-ras proto-oncogene only in the

region that encodes the N terminus of the protein. This portion of the normal and mutant sequences is

as follows:

This mutation is similar to the mutation found in the ras oncogene in various tumors. What type of

mutation converts the ras proto-oncogene to an oncogene?

A. An insertion that disrupts the reading frame of the protein

B. A deletion that disrupts the reading frame of the proteinC. A missense mutation that changes one amino acid within the protein

D. A silent mutation that produces no change in the amino acid sequence of the protein

E. An early termination that creates a stop codon in the reading frame of the protein

2.The mechanism through which Ras becomes an oncogenic protein is which one of the following?

A. Ras remains bound to GAP. B. Ras can no longer bind cAMP.

C. Ras has lost its GTPase activity. D. Ras can no longer bind GTP.

E. Ras can no longer be phosphorylated by MAP kinase.

3.The ability of a normal cell to become a cancer cell can occur via a variety of mechanisms. Which

one of the following best describes such a mechanism?

A. Tumors arise via the acquisition of the ability to metabolize glucose at a faster rate than

noncancer cells.

B. Clonal expansion allows for a cell with a single mutation to become a cancer cell.

C. Mutations in proto-oncogenes can lead to uncontrolled cell growth.

D. Virtually all tumors arise via recombination events, leading to the formation of unusual and novel

genes.

E. Normal cellular oncogenes are mutated to proto-oncogenes, which leads to uncontrolled cellular

proliferation.

4.Loss of both p53 protein alleles is found in >50% of human tumors. Which one of the following is a

function of the p53 protein?

A. Halting replication in cells that have suffered DNA damage B. Targeting repaired cells to undergo apoptosis

C. Stimulating cyclin production D. Stimulating CDK production

E. Stimulating phosphorylation of Rb

5.A tumor-suppressor gene is best described by which one of the following?

A.A gain-of-function mutation leads to uncontrolled proliferation.

B.A loss-of-function mutation leads to uncontrolled proliferation.

C.When it is expressed, the gene suppresses viral genes from being expressed.

D.When it is expressed, the gene specifically blocks the G1/S checkpoint.

E.When it is expressed, the gene induces tumor formation.

B. Phosphorylation of BH3-only domain-containing proteins

C.Phosphorylation of Bcl-2

D.Increased expression of Apaf

E.Decreased expression of caspases

Phosphorylation of caspases

7. Inheriting a mutation in an enzyme necessary for DNA mismatch repair requires which one of thefollowing to occur before a cell loses its ability to regulate its own proliferation?

A.A mutation in BRCA1 or BRCA2

B.A mutation in the PDGF receptor

C.A mutation on one p53 gene

D.A mutation in the ras gene

E.A mutation in the corresponding normal allele

8. A tumor was found in which altered expression of a miRNA led to uncontrolled cellular

proliferation. If the target of this miRNA was the Myc protein, how best would this miRNA be

characterized?

A.As an oncogene

B.As a dominant-negative effector

C.As a tumor suppressor

D.As a factor that upregulates its target

E.As a regulatory factor for an enzyme important in DNA repair

9. A 3-year-old girl is seen by a pediatric ophthalmologist because of reduced vision within her left

eye. The doctor soon detects a mass growing within the eye, which is blocking her vision. Analysis

of DNA from the girl’s blood cells indicates a mutation in a tumor-suppressor gene which, when

mutated, most often leads to tumor formation within the eyes. Which one of the following is a

description of how this tumor-suppressor gene regulates the cell cycle?

A.It encodes for a cyclin.

B.In encodes for a CDK.

C.It encodes for a CKI.

D.It encodes a protein that regulates the transition from G0 to G1 phase in the cell cycle.

E.It encodes a protein that regulates the transition from G1 to S phase in the cell cycle.

10. An individual has been diagnosed with hereditary diffuse type stomach cancer, in which rather than

being located in one area of the stomach, the tumor is located in many areas of the stomach. A

mutation in which one of the following proteins would lead to such a disorder?

A.p53 protein

B.NF-1

C.Rb gene

D.Cadherins

E.Caspases

ANSWERS TO REVIEW QUESTIONS

1. The answer is C. The ras oncogene has a point mutation in codon 12 (position 35 of the DNA

chain) in which T replaces G. This changes the codon from one that specifies glycine to one that

specifies valine. Thus, there is a single amino acid change in the proto-oncogene (a valine for a

glycine) that changes ras to an oncogene.

2. The answer is C. Ras, when it is oncogenic, has lost its GTPase activity and thus remains active

for a longer time. Answer A is incorrect because GAP proteins activate the GTPase activity ofRas and this mutation would make Ras less active. cAMP does not interact directly with Ras

(thus, B is incorrect), and if Ras could no longer bind GTP, it would not be active (hence, D is

also incorrect). Ras is not phosphorylated by the MAP kinase (thus, E is incorrect).

3.The answer is C. Cancers develop from mutations in normal cellular genes (the proto-oncogenes)

that convert the genes to oncogenes. The oncogenes then alter cellular proliferation because they

are not regulated in the same way as their corresponding proto-oncogene. Whereas a small number

of tumors use recombination events to create a novel gene, this is not the usual mechanism for

most tumors to develop. The proto-oncogenes need multiple mutations (4 to 7) for full

transformation into oncogenes (clonal expansion of just a single mutation is usually not sufficient

to form a tumor in vivo).

4.The answer is A. The p53 protein is a transcription factor that regulates the cell cycle and

apoptosis. It has been named the “guardian of the genome” because it halts DNA replication in

cells that have suffered DNA damage and, if the damage is too difficult to repair, targets such cells

for apoptosis. Repaired cells are not targeted for apoptosis by p53. The p53 protein stimulates

production of proteins that inhibit cyclin–CDK complexes, which prevents the phosphorylation of

Rb and stops cells from further replicating their DNA.

5.The answer is B. Tumor-suppressor genes balance cell growth and quiescence. When they are not

expressed (via loss-of-function mutations), the balance shifts to cell proliferation and

tumorigenesis (thus, A is incorrect). Answer C is incorrect because tumor-suppressor genes do not

act on viral genes, answer D is incorrect because tumor-suppressor genes are not specifically

targeted to just one aspect of the cell cycle, and answer E is incorrect because a loss of

expression of tumor-suppressor genes leads to tumor formation, not expression of these genes.

6.The answer is B. Growth factor deprivation can lead to apoptosis owing to the action of BH3-

only domain members of the Bcl-2 family of proteins. The BH3-only domain proteins dimerize

with channel-forming proteins in the outer mitochondrial membrane, allowing cytochrome c to

leave the mitochondria and bind to Apaf, to form the apoptosome to initiate the apoptotic pathway.

Growth factors, via activation of the Akt/protein kinase B pathway, lead to the phosphorylation of

the BH3-only domain proteins and inactivate them, such that apoptosis is blocked. Because the

cell in question is always expressing active Akt, the cell will be in a constant antiapoptotic state.

Increased expression of cytochrome c will not lead to apoptosis or cell growth; it will not affect

cellular proliferation. Bcl-2 is an antiapoptotic factor, but it is not phosphorylated by the Akt

pathway. Increased expression of Apaf might lead to apoptosis if cytochrome c were released; its

increased expression would not lead to uncontrolled cellular proliferation. Decreased expression

of caspases would hinder apoptosis, but the Akt pathway does not alter the expression level of

these proteases. Caspases are also not phosphorylated by Akt.

7.The answer is E. In order for a mutation in a DNA mismatch repair enzyme to be fully manifest,

both alleles that code for this protein must be mutated; otherwise, 50% of the