23.1: Tumor Cells and the Onset of Cancer
Cancer is involved in many aspects of molecular cell biology and is an abbreviation in cellular behavior
Stem cells and other types of proliferating cells can aid in the formation of cancer cells
They multiply when there is an absence of any growth-promoting factors needed for the proliferation of cells
Transformation of tumor cell DNA transfected with cultured cells can happen
The invasion of surrounding tissues can be caused by cancer cells
Angiogenesis is needed to grow into a large mass by both primary and secondary tumors
23.2: The Genetic Basis of Cancer
Oncogenic mutations:
Dominant gain of function mutations in proto-oncogenes
Recessive loss of function mutations in tumor suppressor genes
Oncogenes are converted by one of the two alleles in proto-oncogenes when a mutation is activated
Cancer can be caused by slow-acting retroviruses by integrating near a proto-oncogene
RB is the first tumor suppressor gene to be recognized
23.3: Oncogenic Mutations in Growth Promoting Proteins
Constitutive receptor activity is caused by mutations or chromosomal translocations that let RTKs grow
Virus encoded proteins can bind and activate host cell receptors which cause proliferation
Transcription factors such as Fos, Jun, and Myc being produced unneeded can cause the transformation to occur sooner than needed
23.4 - Mutations Causing Loss of Growth-Inhibiting and Cell-Cycle Control
Normal growth and development depend on a finely tuned and highly regulated balance between growth-promoting and growth-inhibiting pathways
The complex mechanisms for regulating the eukaryotic cell cycle are prime targets for oncogenic mutations
Positive- and negative-acting proteins precisely control the entry of cells into and their progression through the cell cycle, which consists of four main phases: G1, S, G2, and mitosis
Mutations That Promote Unregulated Passage from G1 to S Phase Are Oncogenic
Once a cell progresses past a certain point in late G1, called the restriction point, it becomes irreversibly committed to entering the S phase and replicating its DNA
The expression of D-type cyclin genes is induced by many extracellular growth factors or mitogens
These cyclins assemble with their partners CDK4 and CDK6 to generate catalytically active cyclin-CDK complexes, whose kinase activity promotes progression past the restriction point
Mitogen withdrawal prior to passage through the restriction point leads to the accumulation of p16
The complete phosphorylation of Rb irreversibly commits the cell to DNA synthesis
A lot of tumours contain an oncogenic mutation that causes overproduction or loss of one of the components
The proteins that function as cyclin-CDK inhibitors play an important role in regulating the cell cycle
The loss of RB gene function also is found in more common cancers that arise later in life (e.g., carcinomas of lung, breast, and bladder)
These tissues, unlike retinal tissue, most likely produce other proteins whose function is redundant with that of Rb, and so the loss of Rb is not so critical
Tumours with inactivating mutations in Rb generally produce normal levels of cyclin D1 and functional p16 protein
Loss-of-Function Mutations Affecting Chromatin-Remodeling Proteins Contribute to Tumors
Mutations can undermine growth control by inactivating tumor-suppressor genes, but these genes can also be silenced by repressive chromatin structures
Recently the importance of chromatin-remodeling machines (like; the Swi/Snf complex) in transcriptional control has become increasingly clear
Human knowledge of the target genes regulated by Swi/Snf and other such complexes is incomplete
However, the targets evidently include some growth-regulating genes
With chromatin-remodeling complexes involved in so many cases of transcriptional control, it is expected that Swi/Snf and similar complexes will be linked to many cancers
Loss of p53 Abolishes the DNA-Damage Checkpoint
A critical feature of cell-cycle control is the G1 checkpoint, which prevents cells with damaged DNA from entering the S phase
The p53 protein is a sensor essential for the checkpoint control that arrests cells with damaged DNA in G1
Although p53 has several functions, its ability to activate transcription of certain genes is most relevant to its tumour-suppressing function
Cells with functional p53 become arrested in G1 when exposed to DNA-damaging irradiation, whereas cells lacking functional p53 don’t
Unlike other cell-cycle proteins, p53 is present at very low levels in normal cells because it is extremely unstable and rapidly degraded
The active form of p53 is a tetramer of four identical subunits
A missense point mutation in one of the two p53 alleles in a cell can abrogate almost all p53 activity because virtually all the oligomers will contain at least one defective subunit, and such oligomers cannot function as a transcription factor
Under stressful conditions, the ATM kinase also phosphorylates and thus activates Chk2, a protein kinase that phosphorylates the protein phosphatase Cdc25A, marking it for ubiquitin-mediated destruction
The activity of p53 normally is kept low by a protein called Mdm2
When Mdm2 is bound to p53, it inhibits the transcription-activating ability of p53 and catalyzes the addition of ubiquitin molecules, thus targeting p53 for proteasomal degradation
Phosphorylation of p53 by ATM displaces bound Mdm2 from p53, thereby stabilizing it
The Mdm2 gene is amplified in many sarcomas and other human tumours that contain a normal p53 gene
The activity of p53 also is inhibited by a human papillomavirus (HPV) protein called E6
The activity is not limited to inducing cell-cycle arrest
Apoptotic Genes Can Function as Proto-Oncogenes or Tumor-Suppressor Genes
During normal development, many cells are designated for programmed cell death, also known as apoptosis
Many abnormalities, including errors in mitosis, DNA damage, and an abnormal excess of cells not needed for the development of a working organ, also can trigger apoptosis
If cells do not die when they should and instead keep proliferating, a tumour may form
The resultant inappropriate overproduction of Bcl-2 protein prevents normal apoptosis and allows survival of these tumour cells
CLL tumours are therefore attributable to a failure of cell death
Another dozen or so proto-oncogenes that are normally involved in negatively regulating apoptosis have been mutated to become oncogenes
Conversely, genes whose protein products stimulate apoptosis to behave as tumour suppressors
The most common pro-apoptotic tumour-suppressor gene implicated in human cancers is p53
Among the genes activated by p53 are several encoding pro-apoptotic proteins such as Bax
Failure of Cell-Cycle Checkpoints Can Also Lead to Aneuploidy in Tumor Cells
It has long been known that chromosomal abnormalities abound in tumour cells. We have already encountered several examples of oncogenes that are formed by translocation, amplification, or both (e.g., c-myc, bcr-abl, bcl-2, and cyclin D1)
Another chromosomal abnormality characteristic of nearly all tumour cells is aneuploidy, the presence of an aberrant number of chromosomes—generally too many
Cells with abnormal numbers of chromosomes form when certain cell-cycle checkpoints are nonfunctional
23.5 - The Role of Carcinogens and DNA Repair in Cancer
DNA damage is unavoidable and arises by spontaneous cleavage of chemical bonds in DNA
By reaction with genotoxic chemicals in the environment or with certain chemical by-products of normal cellular metabolism
DNA Polymerases Introduce Copying Errors and Also Correct Them
The first line of defense in preventing mutations is DNA polymerase itself. Occasionally, when replicative DNA polymerases progress along with the template DNA, an incorrect nucleotide is added to the growing 3’ end of the daughter strand
E. coli DNA polymerases, for instance, introduce about 1 incorrect nucleotide per 104 polymerized nucleotides
Proofreading depends on the 3n5 exonuclease activity of some DNA polymerases
When an incorrect base is incorporated during DNA synthesis, the polymerase pauses
Then transfers the 3’ end of the growing chain to the exonuclease site, where the incorrect mispaired base is removed
Chemical Damage to DNA Can Lead to Mutations
DNA is continually subjected to a barrage of damaging chemical reactions
Even if DNA were not exposed to damaging chemicals, DNA is inherently unstable
Many spontaneous mutations are point mutations, which involve a change in a single base pair in the DNA sequence
One of the most frequent point mutations comes from the deamination of a cytosine (C) base, which converts it into a uracil (U) base
The commonly modified base 5-methyl cytosine forms thymine when it is deaminated
Some Carcinogens Have Been Linked to Specific Cancers
Environmental chemicals were originally associated with cancer through experimental studies in animals
The classic experiment is to repeatedly paint a test substance on the back of a mouse and look for the development of both local and systemic tumours in the animal
The ability of chemical and physical carcinogens to induce cancer can be accounted for by the DNA damage that they cause and by the errors introduced into DNA during the cells’ efforts to repair this damage
Although substances identified as chemical carcinogens have a broad range of structures with no obvious unifying features, they can be classified into two general categories
Chemical carcinogens are believed to be risk factors for many human cancers, a direct linkage to specific cancers has been established only in a few cases (the most important being lung cancer)
Lung cancer is not the only major human cancer for which a clear-cut risk factor has been identified
Aflatoxin, a fungal metabolite found in moldy grains, induces liver cancer
After chemical modification by liver enzymes, aflatoxin becomes linked to G residues in DNA and induces G-to-T transversions
Loss of High-Fidelity DNA Excision-Repair Systems Can Lead to Cancer
Cells have other repair systems for preventing mutations due to copying errors and exposure to carcinogens
Several DNA excision-repair systems that normally operate with a high degree of accuracy have been well studied
Loss of these systems correlates with increased risk for cancer
Without proper DNA repair, people with xeroderma pigmentosum or hereditary nonpolyposis colorectal cancer have a propensity to accumulate mutations in many other genes
Including those that are critical in controlling cell growth and proliferation
Base Excision Is Used to Repair Damaged Bases and Single-Base Mispairs
In humans, the most common type of point mutation is a C to T, which is caused by the deamination of 5-methyl C to T
The segment of the damaged strand containing the baseless deoxyribose is excised by an AP endonuclease that cuts the DNA strand near the abasic site
Human cells contain a battery of glycosylases, each of which is specific for a different set of chemically modified DNA bases
Depurination occurs spontaneously and is fairly common in mammals
The resulting abasic sites, if left unrepaired, generate mutations during DNA replication because they cannot specify the appropriate paired base
Loss of Mismatch Excision Repair Leads to Colon and Other Cancers
Another process, also conserved from bacteria to man, principally eliminates base-pair mismatches, deletions, and insertions that are accidentally introduced by polymerases during replication
Hereditary nonpolyposis colorectal cancer, arising from a common inherited predisposition to cancer
Results from an inherited loss-of-function mutation in one allele of either the MLH1 or the MSH2 gene; the MSH2 and MLH1 proteins are essential for DNA mismatch repair
Cells with at least one functional copy of each of these genes exhibit normal mismatch repair
One gene frequently mutated in colon cancers because of the absence of mismatch repair encodes the type II receptor for TGFB
Nucleotide Excision Repair Was Elucidated Through Study of Xeroderma Pigmentosum, a Hereditary Predisposition to Skin Cancers
Cells use nucleotide excision repair to fix DNA regions containing chemically modified bases, often called chemical adducts, that distort the normal shape of DNA locally
A key to this type of repair is the ability of certain proteins to slide along the surface of a double-stranded DNA molecule looking for bulges or other irregularities in the shape of the double helix
Remarkably, five polypeptide subunits of TFIIH, a general transcription factor, is required for nucleotide excision repair in eukaryotic cells, including two with homology to helicases
The use of shared subunits in transcription and DNA repair may help explain the observation that DNA damage in higher eukaryotes is repaired at a much faster rate in regions of the genome being actively transcribed than in non-transcribed regions—so-called transcription-coupled repair
Two systems have evolved to repair double-strand breaks—homologous recombination and DNA end-joining
The former is used during and after DNA replication when the sister chromatid is available for use as a template to repair the damaged DNA strand; homologous recombination is error-free
Error-Free Repair by Homologous Recombination: Yeasts can repair double-strand breaks induced by -irradiation
Isolation and analysis of radiation-sensitive (RAD) mutants that are deficient in this homologous recombination repair system facilitated the study of the process
Virtually all the yeast Rad proteins have homologs in the human genome and the human and yeast proteins function in an essentially identical fashion
At one time homologous recombination was thought to be a minor repair process in human cells
Repair of a double-strand break by homologous recombination involves reactions between three DNA molecules—the two DNA ends and the intact DNA strands from the sister chromatid
Error-Prone Repair by End-Joining: In multicellular organisms, the predominant mechanism for repairing double-strand breaks involves rejoining the nonhomologous ends of two DNA molecules
Even if the joined DNA fragments come from the same chromosome, the repair process results in the loss of several base pairs at the joining point
Occasionally broken ends from different chromosomes are joined together, leading to the translocation of pieces of DNA from one chromosome to another
Telomerase Expression Contributes to Immortalization of Cancer Cells
Telomeres, the physical ends of linear chromosomes, consist of tandem arrays of a short DNA sequence, TTAGGG in vertebrates
Telomeres provide the solution to the end-replication problem—the inability of DNA polymerases to completely replicate the end of a double-stranded DNA molecule
Telomerase, a reverse transcriptase that contains an RNA template, adds TTAGGG repeats to chromosome ends to lengthen or maintain the 5- to 20-kb regions of repeats that decorate the ends of human chromosomes
Most tumour cells, despite their rapid proliferation rate, overcome this fate by expressing telomerase
When treated with carcinogens, telomerase-null mice develop tumours less readily than normal mice do