4. Colorectal Cancer - HNPCC, FAP, MUTYH Flashcards
(25 cards)
What causes Lynch syndrome?
Germline mutations in mismatch repair genes - MLH1, MSH2, MSH6, PMS2
Followed by secondary somatic loss of remaining copy
What cancers are caused by HNPCC?
CRC, endometrial carcinoma, small intestine, pancreatic, gastric, ovariant
What’s the contribution of each gene to HNPCC?
40% MLH1
40% MSH2
10% MSH6
7% PMS2
3% deletion of 3’ of EPCAM - creates EPCAM-MSH2 fusion transcript with hypermethylated MSH2 promoter –> reduced expression
What is the purpose of IHC?
Test for presence of MLH1, MSH2, MSH6, PMS2 with antibodies
Often shows concurrent loss of MSH2 & MSH6 or MLH1 & PMS2 - occur as heterodimers in MMR pathway
What is the purpose of MSI testing? How is it tested?
Most Lynch syndrome cases show length alterations of microsatellites due to MMR
Test 5 mononucleotide markers - 2 or more altered markers = MSI-H
MSI-H = increased risk tumour due to MMR gene defect
What tests are carried out to identify sporadic cases of CRC?
- MLH1 promoter methylation - in high proportion of sporadic cases, rarely in germline. Results in loss of MLH1 on IHC
- BRAF mutations associated with MLH1 hypermethylation, indicates sporadic
V600E commonly seen in non-MSI-H tumours
Why is it necessary to determine the size of EPCAM deletions?
Higher risk of extracolonic cancers if MSH2 coding sequence included (similar to MSH2 mutations)
Lower risk if deletion causes promoter hypermethylation
Why do mismatches occur?
Incorporation of mismatched bases and DNA slippage during replication causes insertion/deletion loops
Failure to repair –> missense and frameshift
What is the role of the different MMR complexes?
MSH2-MSH6 (MutS-alpha) - repairs single base mismatches and mononucleotide repeats
MSH2-MSH3 (MutS-beta) - repairs dinucleotide repeat errors
MLH1-PMS2 - coordinates the other DNA-repair protein effectors to repair mismatches arising during DNA replication
What is the treatment for HNPCC?
Full colectomy, routine colonoscopy as preventative measure, removal of pre-cancerous polyps, chemoprevention
What are the considerations for predictive testing in HNPCC?
Risk depends on gene and gender
Highest risk in males with MLH1 and MSH2 variants
What are the first tests in the Lynch syndrome pathway?
What if the result if negative/positive?
MSI or IHC
Normal IHC or MSI - no further testing for Lynch, suggests it’s sporadic - do BRAF, RAS
Positive IHC for MSH2, MSH6, PMS2 - do germline panel
Positive IHC for MLH1 - do BRAF V600E
Positive MSI - do BRAF V600E
What testing is indicated following BRAF V600E?
Why?
BRAF V600E positive –> no further testing as V600E associated with sporadic CRC
Negative –> MLH1 promoter hypermethylation
What testing is indicated following MLH1 promoter hypermethylation?
Positive –> no further testing as MLH1 hypermeth associated with sporadic CRC
Negative –> germline Lynch syndrome testing
What is the clinical phenotype of FAP?
Familial adenomatus polyposis
Development of 1000s of adenomatous colonic polyps in 2nd decade
100% risk of CRC if untreated, ~10 years after polyps
What gene causes FAP? What is its role?
APC
TSG involved in Wnt signalling pathway
Regulates B-canetin degradation - accumulates in absence of APC –> activates TFs –> constitutive activation of the canonical WNT signaling pathway
Increased proliferation, decreased apoptosis and differentiation
What is attenuated FAP and what causes it?
Fewer polyps, due to APC mutations in exon 9 and 5’/3’ UTRs
What is MUTYH-associated polyposis?
AR, similar presentation to AFAP
What is the role of MUTYH?
Base excision repair gene
When is MUTYH testing indicated?
Test alongside APC
Usually patients with 10-100 polyps, no APC mutation + AR family history
What is MSI?
Microsatellite instability
Change in length of of microsatellite due to errors in DNA replication that aren’t corrected due to deficient MMR
What does MSI testing inform, other than MMR deficiency?
Guides therapy: MSI-H = better response to immunotherapy rather than 5-FU chemo
How is MLH1 promoter methylation tested for?
Bisulphate treatment of DNA - converts unmethylated cytosines at CpG islands to uracil by deamination
PCR of promoter region then analyse of degree of methylation at five CpG sites using pyrosequencing
When is somatic Lynch syndrome testing indicated?
Tumours with MMR loss on IHC
BRAF codon 600 normal
MLH1 promoter normal
No germline MMR mutations
