Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

TUMOUR SUPPRESSOR GENES I

1
...

2
...
e
...

3
...



4
...

5
...

• Certain p53 mutations provide an exception as they can confer a dominant
negative effect on the tetramer
...
A loss of both alleles is usually required to disrupt tumour suppressor function
...
Sinn et al
...

2
...
The same result was seen with transgenic mice bearing ras
...
When the authors then crossed the ras and myc transgenic animals, the progeny has
a greatly enhanced rate of tumour formation
...
This provided in vivo evidence for oncogene co-operation
...
However, these tumours were focal in nature, in that, they were limited to one
specific area
...

6
...

7
...






Cell Fusion Studies:
• Normal cells are fused with tumour cells
...

• This results in loss of malignancy, but with time, some of the hybrids reacquire the
malignant phenotype
...

• These genes are called tumour suppressor genes
...
In rare cases, susceptibility to cancer can be inherited, for example retinoblastoma,
familial breast cancer etc
...
These conditions are associated with the inheritance of one defective allele of a
susceptibility gene, with cancer usually arising following loss of the remaining
‘good’ allele
...
Mutations that disable gene function are therefore recessive (as loss of both copies
if required for disease development)
...
Inactivating, recessive mutations are a defining feature of tumour suppressor genes
and are in contrast to the activating, dominant mutations associated with
oncogenes
...
Retinoblastoma is a rare tumour of the eye affecting children generally between 2-6
years of age
...

6
...

7
...













8
...

9
...




Two-Hit Hypothesis and Retinoblastoma:
1
...

• In the sporadic form, both mutations are somatic events
...

The second mutation event is a somatic event and occurs with high
probability
...
Sporadic RB causes single, unilateral tumours because the acquisition of more
tumours is less likely, in comparison with familial RB, as it requires only one somatic
event instead of two
...
Question: if mutations in the Rb gene are recessive, why is the disease inherited
dominantly?
• Because there is a high probability that a second mutation will occur: all
retinoblasts have only one functional Rb gene and are therefore vulnerable
to the effects of a second mutation
...

4
...

5
...

6
...





Loss of the remaining WT allele in Sporadic RB:
1
...
The probability of a second point mutation event occurring is very low – therefore,
the loss of the second WT allele can occur in many ways leading to a loss of
heterozygosity:
• Nondisjunction
• Nondisjunction & duplication
• Deletion
• Recombination
• Gene conversion

3
...

• Mitotic recombination – recombination between homologous chromosomal
arms occurring during somatic cell proliferation, often during the G2 phase of
the cell cycle (rather than during mitosis)
...
(2) State created by a failure of this separation
...
The first Rb gene is inactivated by a point mutation, thus leaving the cell in a
heterozygous configuration: having one wild-type and one defective gene copy –
Rb+/-
...
Since the mutant Rb allele was recessive at the cellular level, this heterozygous cell
would continue to exhibit a wild-type phenotype
...
In mitotic recombination, the chromosomal arm carrying the WT Rb allele might be
replaced with a chromosomal arm carrying the mutant allele derived from the paired
homologous chromosome
...
Due to reciprocal exchange of information, the chromosomes would remain the
same full-length under the microscope
...
However, at the genetic and molecular level, one of the cells emerging from this
recombination event would have shed its remaining WT Rb allele and would
therefore become Rb-/-
...
In many tumours, LOH seems to be achieved through the loss of an entire
chromosome due to inappropriate chromosomal segregation at mitosis –
nondisjunction
...
In a descendent cell, one of the three chromosomes (an aneuploidy cell) resulting
from the initial nondisjunction event may be shed, leaving two identical copies of
one of the homologous chromosomes behind
...
Let’s consider if the same happened in a monosomic daughter cell, in that it
contained only one copy of the sister chromatid with the mutant Rb allele
...
Nondisjunction with duplication – duplication of the chromosome harbouring the
mutant allele
...

5
...





Function of Tumour Suppressors:
1
...

2
...

3
...

4
...


5
...

• Nucleotide excision repair: XPA gene (Xeroderma pigmentosum – caused by
DNA damage by UV light)
...

• Homologous recombination: BRAC1/2 (hereditary breast cancer)
...
A loss of caretaker gene function leads to mutations in other genes, including
oncogenes or gatekeeper tumour suppressor genes
...
In contrast to gatekeeper genes, restoration of caretaker gene function would not
reverse tumourigenicity
...


8
...
e
...





Rb protein:
1
...
e
...

2
...

3
...

4
...
E2F1, E2F2, and
E2F3), which regulate the expression of the cell cycle genes
...
The hypophosphorylated form of Rb prevents entry into the S phase
...





6
...

7
...

8
...

9
...

10
...

11
...

12
...






13
...

14
...



15
...

16
...

17
...

18
...

19
...




Note:
• Besides RB1, there are two other closely-related “pocket” proteins:
- P107 form complexes with E2F4 and E2F5 à repressors
...


• However, these two do not seem to be tumour suppressors
...
The central role of Rb in the regulation of cell proliferation is highlighted by the fact
that a number of unrelated DNA tumour viruses have evolved mechanisms for
disrupting Rb signalling
...
These viruses hijack the host cell’s DNA replication machinery in order to replicate
their own genome
...
They induce host cells to proliferate so that new viral DNA can be synthesised
alongside replication of the host genome, and in order to achieve this, the viruses
must overcome Rb-dependent inhibition of E2F
...
The small tumour viruses are:
• SV40 T Ag
• Adenovirus E1A
• Papilloma virus E7

5
...

6
...

7
...




Simian Virus 40 (SV40):
1
...

2
...

3
...

4
...

5
...



Uncontrolled proliferation à tumour formation

• Rb loss alone is not sufficient enough to result in cellular transformation as
additional gene mutations are also required, i
...
disruption of the p53 pathway and
possible oncogene activation
...


6
...

Because the viral life cycle ultimately results in cell lysis to enable the release
of new virus particles, infected rodent cells do not die but are forced to
replicate due to Rb inhibition by the viral protein T-Ag
...

In primates, the viral life cycle should be allowed to progress to completion
because T-Ag can recruit DNA polymerase to viral template DNA and so
enable viral DNA replication and new virion production

---