Why do we want to identify inherited cancer genes?
- Gsin an understanding of disease mechanism
- Help identify targets for therapy
- Identify risk populations
- Identify people that might respond best to particular drugs (pharmacogenetics)
- Identify why people might react badly to drugs
How are LOH detected
Microsatellite markers when blood and tumour DNA samples are compared
Why are familial cancer carriers more likely to develop cancer
Because the first "hit" in the tumour suppressor gene has been inherited and is present in all the cells in their body. Only requires the second hit (often loss of a larger chromosomal region including the WT allele on the other chromosome)
How have familial cancers been identified
Positional cloning - linkage analysis followed by cloning and sequencing
Identification of deletions and LOH in tumours
Testing tumours for mutations of genesknown to be involved in cell cycle regulation
Recently, sequencing of candidate genes in people with a family history of cancer.
What are examples of the rare cancer syndromes and give examples of germline mutations.
>1000 relative risk,
mutations in Rb1 and APC.
What are examples of familial forms of common cancers
10 - 100 relative risk
Breast and colon cancer.
Mutations in BRCA1/2, MSH2/1 etc
What is the process for positional cloning
Assemble some families with many affected individuals
Genotype their DNA using microsatellite markers that span the whole genome
Look to see if the disease co-segregates with markers on a particular chromosome (done by linkage analysis)
Identify likely candidate genes in the region of interest and look for mutations carried by people with the disease but not by healthy family members.
What are characteristics of familial breast cancer
Early age of onset
Breast and ovarian cancer in the same familiy
bilateral disease is frequent (both breasts effected)
Males in the family are also effected.
What is linkage analysis
Genotype markers and observe whether or not particular marker alleles segregate with disease in families in which there are multiple cases.
We can use these data to work out the likelihood our marker is linked to the disease.
The answer is in the form of lod score which is a measure of the likeliness that the disease and the marker are linked.
What did linkage analysis of familial breast cancer reveal.
lod score of 5.98 in early onset at marker D17S74 at chromosome 17q21
chromosome 17 is linked to breast cancer
What is genetic heterogeneity?
Multiple genes are responsible for causing the disease in question
What is the role of the BRCA2/1 genes in DNA repair
ATM phosphorylates MRE11 and NBS1 in response to DNA damage
The RAD50, NRE11, NBS1 complex binds at sites of DNA double strand breaks and initiates DNA repair
Mutations in these genes cause familial syndroms with genomic instability.
What are the common features of NBS and AT mutants
Cells show increased sensitivity to ionising radiation
Fail to induce p53 at the G1/S checkpoint.
Fail to stop DNA synthesis in response to ionising radiation.
Phenotype is one of failing to signal DNA damage,
What is the role of BRCA1 with the MRE11/RAD50/NBS1 complex pathway
BRCA1 also associates with the complex
ATM phosphorylates BRCA1
AT mutant cells are unable to phosphorylate BRCA1
Mutated BRCA1 lacking two phosphorylation sites is unable to rescue the radiation hypersensitivity of a BRCA1 deficient cell line.
This pathway is important to protect cells against DNA damage
Draw out the pathway of BRCA1 and its relationship with other complexes/ATM