Familial cancer syndromes Flashcards
What percentage of CRC is familial and what are the associated genetic syndromes?
15% is familial
3-5% is due to Lynch syndrome
1% due to FAP (APC), Juvenille polyposis syndrome (SMAD4 BMPR1A), Peutz-jeghers (STK11) and PTEN harmatoma syndrome
BMPR1A and PTEN may be deleted in 10q23 contiguous del syndrome with syndromic features dev delay and polyps
What genes are mutated in Lynch syndrome
MLH1, MSH2 (~70%)
PMS2- 1&
MSH6- 6-10%
EPCAM <1%
Lynch syndrome results in an increased risk of which cancers?
CRC, endometrial, small intestine, hepato-billiary, pancreatic, gastric, renal pelvis, bladder, prostrate, ovary, breast
what is the mean age of onset of Lynch?
45 yrs. it is older for MSH6 and PMS2 as these are the non-obligate partners onf the MutSa and MutLa heterodimers so are associated with later onset and reduced penetrance
what is the mutations spectrum
- LOF mutations
- MLH1 and MSH2 are most commonly mutated and have highest penetrance
- 3% are due to deletions of the 3’ edn of the EPCAM gene, this results in loss of the stop codon and poly-A signal resulting in polymerase readthrough into downstream MSH2 and promoter hypermethylation. the EPCAM deletion can also include the MSH2 promoter or coding sequence. The result for all is LOF of MSH2
- 10Mb iinversion of chromosome 2 can disrupt MSH2 and is reported in cases of unexplained lynch syndrome
what is the clinical criteria for testing for lynch syndrome?
2 different criteria:
Amsterdam- developed to identify LS patients for research studies
Bethesda- designed to identify CRC patients who should be tested for LS. Lower specificity but higher sensitivity than Amsterdam criteria
describe IHC testing in LS
95% sensitivity
standard to test FFPE tumour tissues for the expression of LS genes by IHC
- If the obligate partner is lost e.g. MLH1 or MSH2 you will also see concurrent loss of the partner. But if the non-obligate partner is lost (MSH6 or OMS2 there is not concurrent loss of the obligate partner)
- If a patient shows loss of staining by IHC there is no need to also test for MSI as this is assumed
- IHC has lower sensitvity than MSI as it may miss LOF mutations that do not result in NMD e.g. missense mutations
- therefore if a patient is -ve for IHC should also perform MSI testing
- techinically difficult to distinguish +ve staining from artefact
if a patient shows loss of staining for MLH1/PMS2 they should have MLH1 methylation studies before proceeding to germline mutation testing
describe MSI testing in LS
MMR proteins repair mismatches and a defect in the pathway results in an increased mutation rate.
micro satellites are highly mutable making them a good target to screen for MMR deficiency
- generally screen tumour vs germline (blood samples) or regions of tumour and non-tumour on an FFPE slide.
-2/5 markers should show instability to confirm MSI
- if only 1 marker shows instability it is not enough to confirm MSI but may indicate the need for further studies
- can also be used for ovarian and endometrial cancer, but has been designed for CRC so is most snsitive for this
-if MSI is not detected an MMR defect is ruled out.
what are the most common mutations in sporadic CRC?
MLH1 promoter methylation is present in 15% and results in an absence of MLH1 by IHC. it can be detected by MS-MLPA which can methylation sites in the promoter region (can also detect MSH2 methylation due to an EPCAM deletion).
- rarely seen as a germline chage and is acquuired somatically.
V600E BRAF mutation is associated with MLH1 and indicates sporadic CRC- can be used to screen for sporadic CRC and avoid unnecessary germline mutation testing
What techniques are used for germline mutation testing in LS?
- sequencing and MLPA (although more likely to now all be performed as part of an NGS panel
- PMS2 is complicated by the presence of a highly homologous pseudogene so need to use long range PCR with primers anchored in region of divergence between the native and pseuogene to select for the native gene. nester PCR with preimers for the exons of interest for sanger seq.
what is the testing strategy?
1) test tumour for IHC or MSI- cant diagnose LS but can confirm a defect in MMR and IHC may indicate the likely affected gene
2) MLH1 promoter methylation and BRAF V600E testing to rule out spradic cases
3) test for germline MMR proteins
Describe the role of the MMR genes?
MSH2-MSH6 (MutSa) recognises single base mismatches and monoculoetide repts. MutSb recognises larger looped out errors e.g. dint rpts
MutSa inititiate repair by binding to the mismatch. Other molecules are recruited- PCNA, RCF, MutLa(MLH1-PMS2). this assembly activates the exonuclease activity of PPMS2 which makes ss breaks near the mismatch and opens EXO1 nucelase entry site. this mismatch is then excised, gap filled by a pol and nick ligated
What is associated with homozygous/compound het LS mutation?
associated with constituional mimsatch repair cancer syndrome- rare childhood cancer syndrome resulting in hematological, brain/CNS, CRC, intestinal polyps
what are the treatment and surveillance options for CRC?
full coloectomy and ilorectal anastomosis recommended
- prohylactic colectomy is not recommended as colonoscopy is effective
- removal of ovary and uterus may be considered after completion of childbearing
surveillance: colonoscopy and removal of precancerous lesions every 1-2 years from 25 years
aspirin is recommended for chemoprevention in at risk individuals
Predictive testing
Available to all at risk adults. CRC not associated with childhood cancer so testing before then is not required.
requires referral from a GC
-ve result reduces risk to that of the general population
+ve result increases risk of developing cancer- level of risk is dependent on gene, age, mutation, lifestyle, sex,
What is FAP
Familial Adenomatous polyposis
most common polyps syndrome due to AD APC mutations
- characterized by the development of 100s of colonic polyps in the seconsd decade and 95% have polyps by 35
- almost 100% penetrance if left untreated- treatment is by colctomy at an early age (when 20-30 poyps are present)
- screening starts at 10-12 years and detects most CRC before onset
PST in FAP
PST is offered from 10yrs so that colonoscopy can be offered due to early age of onset
- VHL and NF1 are other cancer syndromes with early onset where early PST is appropriate
APC gene
Loss of APC is an early event in colorectal adenoma.
The APC gene function in the wnt pathway by regulating phosphorylation is B-catenin and marking it for degradation, in the absence of APC B-catenin is not degraded resulting in uncontrolled cell proliferation.
APC mutation spectrum
- there is an alternatively spliced transcript of exon 9 lacking codons 312-412. This isoform is present in normal tissues and if the mutated codon is in this region it can be removed by normal splicing in the intestinal mucosa resulting in a milder phenotype.
- large final exona and nonsense muts in thus region do no always undergo NMD resulting in the presence of a partially functional protein.
- 5’ mutation. There is an internal ribosomal entry site early in the transcript. therefore truncating mutation in the 1st exon can escape NMDF by initiating translation at a downstream sites- results in attenuated FAP
- Missense mutations do no confirm a diagnosis. No statistically significant increased cancer risk and PST is not offered
How is FAP diagnosed?
based on family history and CRC phnotype >100 polyps.
Genetic testing by sequencing and MLPA. Now commonly included in a CC panel
15% of APC -ve cases have a MUTYH mutation so this testing should be considered.
What is the treatment of FAP?
safest preventative strategy is surgical removal of the colon when polyps start to develop
chemotherapy includes NSAIDs, COX-2 inhibitors
What is attenuated FAP?
milder phnotype with fewer polyps (<100), likely to be underdiganosed and associated with specific mutations.
What is MAP?
MUTYH associated polyposis
AR
Similar to FAP (10-100 polyps) family history can help distinguish the likely diagnosis.
FAP and MAP are example of simillar phenotype different mechanism
what is the age of onset of MAP
mean age of onset is 48yrs with 40-100% lifetime risk
there is a small increased risk of adenoma in heterozygotes but screening is not warranted.
What is the role of the MUTYH gene?
Involved in BER- repair of the most common form of oxidative damage 8-oxoguanine which pairs with A instead of C resulting in a G:C to T:A transition. The glycolase excises the 8)G from the sugar phosphate backbone so the base can be repaired.
MAP mutation spectrum
almost all mutations are missense and their affect on the glycolase range from a completer to partial loss of activity
Y179C and G396D account for 90% of mutation in N EU mutations
60% of tumours also have a KRAS mutation
Testing strategy for MAP
testing is offered to patients with 10-100 polyps and no APC mutation (in reality both are likely to be tested together as part of a panel)
- can pre-screen for common N EU mutations
- failure to detect a second mut could indicate the presence of a large del, rearrangement, or another gene is responsible
what screening is offered for MAP?
colonoscopy from 18yrs and upper GI tract investigations from 25-30 therefore PST is not offered to minors
What is the incidence and risk factors of breast cancer?
Most common female cancer in the UK with 1 in 8 lifetime risk. Incidence is increasing in western countries with increased life expectancy and move to a more wetern lifetsyle
risk factors: age, genetics, HRT, smoking, obesity, alcohol. Strongest risk factor is a strong family history of BC- 15-20% of BC has a family but gene found in known familial BC gene in ~10% of cases
what factors support a suspicion of a BRCA1/2 mutation?
- BC < 50yrs
- 2x primary BC
- breast and ovarian cancer in a single individual
- breast and ovarian cancer in close relative from the same side of the family
- male BC
ovarian cancer at any age
BRCA1/2 mutation in a family member
algorithms have been developed to calculate the liklihood that an individual have a BRCA1/2 mutation (BODICEA, Myriad, BRCAPRO)
