Screening for Mutations in the Germline Flashcards
Why do we want to find cancer mutations? Who may benefit?
1) . Implications for patient:
- May have risk of other cancers - surgery/screening
- Treatment options - e.g. PARP inhibitors
- BRCA1/2 - breast and ovarian cancer
- HNPCC - colorectal and endometrial cancer risk
2) . Risk to relatives:
- Can offer predictive testing
- Prophylactic surgery/screening
- Relief/less screening (morbidity/financial)
3) . Where does the process start?
- ‘worried-well’ with FH may go to GP and then get referred to clinical genetics dept. Could also get referred to a family history clinic for triage.
- Affected individual with FH and early onset cancer may get referred to clinical genetics.
How do we decide which families are going to benefit from mutation testing?
- Need a large enough family history of relevant tumours - e.g. breast/ovarian (BRCA1/2), colorectal/endometrial (HNPCC).
- Age of onset will be low if it is an inherited cancer.
- There are formulae to calculate the likelihood that an individual has a cancer mutation - funding cut offs.
- Phenotype may give indication e.g. FAP and multiple GI polyps.
- Biomarkers of tumours from individual/family that suggest that there is a predisposition. BRCA1 results in triple negative tumours, HNPCC gives microsatellite instability and loss of antibody staining in IHC.
Discuss Familial Adenomatous Polyposis (FAP).
- In FAP there are usually >100-1000 polyps in the colon and rectum by the second decade of life.
- May also be extracolonic features.
- APC gene - AD - genotype-phenotype correlation - mutations in the first 4 exons tend to give rise to attenuated polyposis where you have <100 polyps. Starts to give rise to difficulties in identifying them as individuals who may have mutations in the APC gene.
- Two common 5bp deletions account for about 15-20% of APC mutations but all the rest are scattered throughout the gene. You just need to sequence the whole gene. Usually truncating mutations. 80% are point mutations. 7-12% are large genomic deletions.
- MYH-associated polyposis - 15-200 colonic polyps by age 50 -Autosomal recessive - MYH gene - 2 mutations account for 82% mutant alleles in UK caucasian populations.
- There is an overlap between the MYH and APC associated polyposis phenotypes.
Are there any common mutations in the APC gene seen in FAP?
- Two common 5bp deletions account for about 15-20% of APC mutations but all the rest are scattered throughout the gene.
- You just need to sequence the whole gene.
- Usually truncating mutations.
- 80% are point mutations.
- 7-12% are large genomic deletions.
What locus heterogeneity do we get with HNPCC?
- 4 genes where mutations may occur. The 2 major genes are MLH1 and MSH2 which are responsible for about 90% of the mutations. MSH6 is responsible for about 10% and PMS2 a few %.
- If you need to screen all 4 genes you need to look through 60 exons.
- Locus heterogeneity is not so much a problem now that we can make NGS gene panels.
What locus heterogeneity do we get with familial breast and ovarian cancer?
- BRCA1 and BRCA2 genes - 63 exons to search through.
- Locus heterogeneity is not so much a problem now that we can make NGS gene panels.
How does allele heterogeneity affect our detection of cancer mutations?
- There is heterogeneity of mutations across different populations - some cancer mutations will be far more common in certain ethnic groups etc.
- There are no obvious major mutations for HNPCC, BRCA1/2 in caucasian UK populations so we need to sequence the whole genes.
- BRCA1/2 there are founder mutations in other populations - e.g. Iceland there are common founder mutations. In Ashkenazy Jewish populations there are 3 mutations which are high pop. frequency so BPGs advise testing in all predictive tests.
Ashkenazy BRCA mutations
- 2 in BRCA1, 1 in BRCA 2.
- BRCA1 185delAG (old nom) - 1% Ashk Jewish Women - 20% early onset breast cancer.
- BRCA2 6174delT (old nom) - 1-1.5% Ashk Jews - 8% early onset breast cancer.
What common BRCA mutations are seen?
Ashkenazy BRCA mutations
- 2 in BRCA1, 1 in BRCA 2.
- BRCA1 185delAG (old nom) - 1% Ashk Jewish Women - 20% early onset breast cancer.
- BRCA2 6174delT (old nom) - 1-1.5% Ashk Jews - 8% early onset breast cancer.
There are no obvious major mutations for BRCA1/2 in caucasian UK populations so we need to sequence the whole genes.
Describe the BRCA1/2 genes.
- Both large genes.
- Both involved in DNA repair by homologous recombination and integrity of the genome.
- More Alu repeats are seen in BRCA1 introns than seen in BRCA2 - therefore a larger proportion of mutations in BRCA1 will be by whole exon deletions and duplications than in BRCA2 - still MLPA both. Alu repeats can cause unequal crossover.
In which gene are more whole exon deletions likely to occur and why - BRCA1 or BRCA2?
- More Alu repeats are seen in BRCA1 introns than seen in BRCA2 - therefore a larger proportion of mutations in BRCA1 will be by whole exon deletions and duplications than in BRCA2 - still MLPA both.
- Alu repeats can cause unequal crossover.
- Do still see whole exon deletions in BRCA 2.
What is the risk of a female BRCA1 mutation carrier developing cancer?
- BRCA1 - 65% risk of breast cancer, 39% risk of ovarian cancer by age 70.
What is the risk of a female BRCA2 mutation carrier developing cancer?
- BRCA2 - 45% risk of breast cancer, 11% risk of ovarian cancer by age 70.
Other than breast and ovarian cancer what other cancer risks may be increased by BRCA mutations?
Increased risk of other cancers:
- Increased risk of prostate and pancreatic cancer in BRCA2.
- Increased risk of prostate, pancreatic, endometrial, cervical cancer in BRCA1.
What may we find when looking for mutations in BRCA1/2 genes? What do we need to do with this information?
- May find numerous different mutations.
- The problems come when interpreting mis-sense changes of unknown significance.
- Need to go through variant assessment process to classify previously unseen variants - conservation? amino-acid effect? is the change in a functionally important part of the protein? any functional tests? Segregation studies.
- There are some known pathogenic mis-sense mutations.
- Most BRCA2 truncating mutations are pathogenic but stop codon in last exon of BRCA2 is actually a benign polymorphism.
- Sometimes individuals may have both a BRCA1 and 2 mutation.
What can be done to confirm whether a variant is altering splicing?
Can check blood RNA.