Chapter 4: Growth factor signaling and oncogenes (Book 4.2)

Question 1

There are two major classifications of mutated genes that contribute to carcinogenesis. Those are.. (also think about their general description)?

Answer 1

Oncogenes (gene has mutated, what causes an increase in product produced and therefore acts in a dominant manner) and tumor suppressor genes (mutation has caused loss of function).

Question 2

In the book there’s this block of text about the discovery of oncogenes through the use of viruses in animals. I will not discuss this part here. Just know that the early observation that viruses could cuase cancer in animals, lead to the discovery of oncogenes.

Answer 2

Okay

Question 3

Describe the life cycle of retroviruses

Answer 3

They inject their infectious RNA into a host cell. Viral RNA is reverse-transcribed into DNA and is then called a provirus. The provirus is randomly integrated into the host genome. Translation of the provirus in the genome produces the viral proteins for the synthesis of new viral particles.

Question 4

How are retroviruses involved in the formation of oncogenes?

Answer 4

A virus can acquire fragments of genes from the host at integration sites and this process may result in the creation of oncogenes. (Viral DNA may also me translated as a fusion protein, in conjunction with cellular DNA, resulting in a novel fusion protein, or host genes may fall under the regulation of viral regulatory sequences).

Question 5

Just know that the name for a proto-oncogene as a normal cellular (c) gene is referred to as c-src. The name for an oncogene with an altered form transduced by retroviruses (v) is referred to as v-src.

Answer 5

Okay

Question 6

There’s a viral oncogene, v-sis. Its protein product was cytoplasmic and was found to be a truncated version of a growth factor normally secreted by platelets, called platelet-derived growth factor (PDGF). It’s normal role is in e.g. would healing. What’s significant about the oncogenic form?

Answer 6

-It’s aberrant location (cytoplasmic rather than secreted). - The subsequent activation of the PDGF signal pathway at inappropriate times, leading to unregulated growth.

Question 7

What oncogene was originally identified from the avian erythroblastosis leukemia virus?

Answer 7

The oncogene v-erbB, it is a truncated form of the epidermal growth factor receptor (EGFR) whereby the extracellular domain is deleted.

Question 8

What identity belongs to the proto-oncogene that is related to the oncogene v-erbB?

Answer 8

The cellular gene is c-erbB and the identity of the product of the proto-oncogene is EGFR.

Question 9

The mutated EGFR receptor triggers …. in the absence of EGF.

Answer 9

Cell division

Question 10

What is another way how normal c-erbB contributes to carcinogenesis?

Answer 10

Increasing the amount of normal c-erbB product by gene amplification (this particularly happens in breast cancer).

Question 11

What role does RET signaling have?

Answer 11

It plays an important role in kidney development and neuronal differentiation during embryogenesis.

Question 12

What the proto-oncogene ret code for? What is its function?

Answer 12

It codes for another growth factor tyrosine kinase receptor that heterodimerizes with cell-surface co-receptors GFR-α1-4. This, so that the signal for glial-derived neurotrophic factor (GDNF) family ligand is transduced.

Question 13

How is the signal for glial-derived neurotrophic factors (GDNF) family ligands transduced?

Answer 13

The ligands will not directly bind to RET, but first will form a complex with co-receptors. These recruit RET. RET then undergoes a conformational change, dimerizes and undergoes autophosphorylation.

Question 14

What type of carcinoma often carries somatic chromosomal rearrangements involving the amino-terminal parts of numerous genes and the sequences of ret that code for the tyrosine kinase domain?

Answer 14

Papillary thyroid carcinoma

Question 15

What kind of proteins results from this chromosomal rearrangements in papillary thyroid carcinoma?

Answer 15

A fusion protein that displays cytoplasmic kinase activity independent of GDNF signaling.

Question 16

Which three familial autosomal dominant tumor syndromes are associated with germline mutations?

Answer 16

Multiple endocrine neoplasia 2A (MEN2A), MEN2B and familial medullary thyroid carcinoma.

Question 17

What is mutated in multiple endocrine neoplasia 2A (MEN2A) and what results from this?

Answer 17

Mutations in conserved extracellular cysteines. This results in intermolecular disulfide bonds that cause constitutive RET dimerization and aberrant activation.

Question 18

What is mutated in multiple endocrine neoplasia 2B (MEN2B) and what results from this?

Answer 18

As substitution mutation whereby conserved Met is replaced by Thr (Met918Thr). Conserved Met is characteristic of the substrate binding domain of the receptor tyrosine kinase. The substitution results in altered substrate acces, leading to increased kinase activity and altered substrate specificity.

Question 19

Oncogenic activation of receptor tyrosine kinases occurs through specific mutations that lead to constitutive … (1), …(2), or … (3).

Answer 19

1. tyrosine activation 2. dimerization 3. altered substrate specificity

Question 20

What is the most commonly mutated oncogene in human cancers?

Answer 20

The ras gene.

Question 21

The majority of mutation are located in codons 12, 13 and 61. What is the consequence of these mutations?

Answer 21

Loss of GTPase activity of the RAS protein which puts RAS in a constitutive active state due to GTPase not being able to exchange Ras bound GTP for Ras bound GDP.

Question 22

Some specific mutations in the ras gene are characteristic for specific cancer. What mutation is characteristic for bladder carcinoma and what for lung cancer?

Answer 22

-Bladder carcinoma: G12V point mutation within codon 12 which results in the substitution of valine (GTC) for glycine (GGC). -Lung cancer: point mutation within codon 12 where cysteine gets substituted.

Question 23

What is an important part of a transformation assay to demonstrate oncogenes and their genetic alterations (or in other words: on what is a transformation assay based?

Answer 23

Cancer cells can be distinguished from normal cells in cell culture conditions. It is based on the characteristic that cancer cells grow as foci against a monolayer of normal cells.

Question 24

Name another transducer that can become oncogenic.

Answer 24

B-raf, oncogenic activation of B-raf is common in melanomas.

Question 25

What does the common mutated form of B-Raf (V600E) found in melanomas cause?

Answer 25

Constitutive kinase activity and insensitivity to feedback mechanisms. Which ultimately leads to abnormal cell growth.

Question 26

1. What are genes that code for cytoplasmic tyrosine kinases? 2. What are genes that code for serine/threonine kinases? 3. What are genes that code for nuclear kinases?

Answer 26

1. SRC 2. Raf and MAPK 3. ABL

Question 27

We’ve already seen that SRC can become inactive through its SH2 domain that binds to phosphorylated tyrosine residue (Tyr530) which results in a conformation that blocks the SRC kinase active site. In colon cancer, the protein product of oncogenic src is characterized by a truncation at Tyr530. What is the result from this?

Answer 27

This aberrant protein is unable to adopt the inactive conformation and therefore kinase activity is constitutive.

Question 28

What is the product and function of the c-abl gene?

Answer 28

The product is a nuclear tyrosine kinase that plays a role in DNA-damage induced apoptosis.

Question 29

Normally, the c-abl gene is activated by ionizing radiation and particular drugs via the serine/threonine kinase ATM. How does oncogenic activation occur?

Answer 29

Through chromosomal translocation t(9;22) whereby abl becomes juxtaposed to a breakpoint cluster region bcr. When transcribed, this gives rise to a fusion protein with novel features.

Question 30

How does the BCR-ABL gene look like and how does this result in oncogenic activation?

Answer 30

Translocated BCR retains domains I and II, while ABL retains the SH2 and -3 domains, the kinase domain, the DNA-binding domain and the actin-binding domain. BCR-ABL molecules form a complex (through BCR domain I). Autophosphorylation occurs within BCR domain II, which triggers activation of the ABL tyrosine kinase.

Question 31

How does the role of the SH2 domain of SRC compare with that of the SH2 domain of Grb2?

Answer 31

In both cases, the SH2 domain recognizes a phosphotyrosine residue. The SH2 domain of SRC regulates intramolecular interactions, but the SH2 domain of Grb2 regulates intermolecular interactions between itself and receptor tyrosine kinases.

Question 32

Components of AP-1, Jun and Fos, are encoded by proto-oncogenes c-jun and c-fos. Normally, c-fos mRNA is short-lived so that the response to a mitogen is transient. What kind of mutation causes a decrease in v-fos mRNA instability which leads to oncogenic activation?

Answer 32

Truncation of the 3’end of v-fos eliminates a motif involed in mRNA instability. Another way of oncogenic activation can be deletion of a regulatory promotor sequence, such that transcription of fos gene occurs even in absence of serum mitogens.

Question 33

How does oncogenic activation of c-Myc occur?

Answer 33

Through constitutive expression and overexpression of the c-Myc protein. This can be done throug chromosomal translocation of myc to a location that falls within the regulation of the strong promotor of immunoglobulin genes.

Question 34

In what type of cancer is oncogenic activation of c-myc through chromosomal translocation seen?

Answer 34

In Burkitt’s lymphoma

Question 35

There’s another oncogene, besides v-erbB, that was originally identified from the avian erythroblastosis leukemia virus. What is this gene and what is the proto-oncogenic form?

Answer 35

v-erbA. The identity of the product of the proto-oncogene or cellular gene c-erbA is the thyroid hormone receptor (T3).

Question 36

How is oncogenic activation of c-erbA or thyroid hormone receptor achieved?

Answer 36

By mutations that prevent thyroid hormone binding and inhibit transcriptional activation.

Question 37

Why is the v-erbA mutation a dominant negative mutation?

Answer 37

Because the product of the mutation codes for receptors that can bind to DNA and block acces of wild-type receptors, including other steroid hormone receptor family members that may form heterodimers with thyroid hormons.

Question 38

What do somatic mutations of thyroid hormone receptors give rise to?

Answer 38

They give rise to product that have lost their ability to bind thyroid hormone and regulate transcription.

Question 39

Describe briefly how the following mutations can lead to the formation of oncogenes: 1. point mutations/deletion in coding sequences 2. point mutations/deletions in regulatory sequences 3. chromosomal translocations 4. insertional mutagenesis caused by viral integration 5. gene amplification

Answer 39

1. point mutations/deletion in coding sequences –> structural and functional changes. 2. point mutations/deletions in regulatory sequences –> overexpression 3. chromosomal translocations –> fusion proteins with novel functions 4. insertional mutagenesis caused by viral integration –> aberrant expression 5. gene amplification –> increase in gene dose and protein production