Clinical Oncology SC017: Many Of My Family Members Have Cancers? Cancer Genetics And Cytogenetics Flashcards
Cancer: A genetic disease
- Overactivity of Oncogenes
- Gene amplification
- Point mutation
- Chromosomal translocation
- Viral promoter - Inactivation of Tumour suppressor genes
- Deletion
- Mutation
- Epigenetic inactivation (e.g. Methylation)
- Above changes can be Acquired (Somatic) / Inherited (Germline)
- **Inherited mutation in **Tumour suppressor genes —> most common causes of familial cancer
Kundson’s 2-hit hypothesis
Tumour suppressor gene (TSG)
- Protect cells from neoplastic transformation
- 2 copies are present in each cell (Paternal + Maternal alleles)
- Expression of 1 allele enough to suppress tumour formation
- 2-hit hypothesis: Both alleles need to be inactivated in the same cell
Sporadic vs Familial cancers:
- Sporadic: both copies of TSG have to be inactivated in the same cell —> chance is low + takes a long time for mutations to accumulate so 2nd hit can happen
- Familial: 1 copy of TSH already defective in all cells at birth (Germline mutation) —> normal development still possible ∵ a functioning copy remain in all cells —> when remaining copy inactivated in any cell —> cancer —> much higher observed cancer risk + occur at an **early age + may develop **multiple cancers (synchronous / metachronous, ∵ other cells also loss 1 TSG —> once loss the other TSG —> cancer again (vs sporadic: need to lose both TSG))
Familial cancer
- Previously thought to be rare disease
- 2 common forms of familial cancers characterised in past 20 years
- Seen in daily clinical practice
- Substantial difference can be made to functioning of the whole family if they can be recognised + institute appropriate cancer preventive measures
- No specific clinical phenotype —> ∴ diagnosis rely on attention to family history of cancer + referral for genetic diagnoses
Microsatellite instability
Expansion + Contraction of small repeat sequences during DNA replication
- Related to defect in the DNA mismatch repair (MMR) system
- Mismatch repair activity deficient —> DNA length differ
Colorectal cancer with microsatellite instability
2 groups:
1. Germline mutation in the DNA mismatch repair genes (Lynch Syndrome / HNPCC) (4% of all CRC)
- Tendency for early-onset / with family history
- High risk of cancer for individual and family members
- The need for prophylactic screening
- Promoter methylation of the MLH1 gene which is somatically acquired / somatic mutations (11% of all CRC)
- Tendency for late-onset
- No risk of inheritance
Both groups may response to immune checkpoint inhibitor treatment
***Lynch syndrome (previously called HNPCC)
- 4% of total CRC
- Familial occurrence of CRC with early onset age
- **Autosomal dominant inheritance with **incomplete penetrance —> One functioning TSG already enough —> Need to lose both to loss TS function
- Some famiilies have Extra-colonic cancers (Endometrial, Ovarian, Gastric, Hepatobiliary, Small bowel, Transitional cell carcinoma of renal pelvis / ureter)
- No known premonitory phenotypic stigmata —> diagnosis require careful analysis of family pedigree
- > 90% of HNPCC family satisfying Amsterdam criteria are due to germline DNA MMR gene mutation (MSH2, MLH1)
- Younger the age of colon cancer onset, the risk of LS is higher: <=35: 60%, 36-45: 20%
Genetic basis:
- Microsatellite instability
- Germline mutation in one of the DNA mismatch repair genes
—> **MSH2
—> **MLH1
—> MSH6
—> PMS2
—> EPCAM deletion (leading to inactivation of MSH2)
Amsterdam criteria (Henry Lynch):
1. >=3 relatives with histologically verified CRC; one is a 1st degree relative of the other 2
2. >=2 successive generations affected
3. 1 of CRC diagnosed <50 yo
Revised Bethesda criteria (criteria to select patients at risk of HNPCC for MSI analysis):
1. Colorectal cancer diagnosed in a patient ***<50 yrs
2. Synchronous / Metachronous CRC or other HNPCC-associated tumours regardless of age
3. CRC diagnosed in >=2 1st or 2nd degree relatives with HNPCC-related tumours, regardless of age
Histological spectrum of LS-related extra-colonic cancers in Revised Bethesda guideline:
1. ***Endometrial carcinoma (risk: 60% (SpC OG))
2. Ovarian carcinoma (risk: 10-15%)
3. Gastric adenocarcinoma
4. Cholangiocarcinoma
5. Pancreas adenocarcinoma
6. Small bowel adenocarcinoma
7. Transitional cell carcinoma of renal pelvis / bladder
8. Brain (Glioma / Glioblastoma)
9. Sebaceous gland adenomas
10. Keratoacanthoma
Screening + Surveillance for LS families
- Family members are at risk of inheriting the disease / defective genes (50% chance)
- Enough evidence to support that screening and surveillance of these high risk individuals can :
1. Detect premalignant lesions
2. Detect cancer at early stages
3. Prevent cancer development and overall decrease CRC morbidity and mortality
Recommended screening protocol for LS gene carrier:
1. Colonoscopy
- 20-25 every 2 years, >40 every year
- Reduce CRC incidence by 60%, mortality by 100%
- Gynaecological examination
- Endometrial aspirate
- Trans-abdominal / trans-vaginal USG
- Serum ovarian tumour marker (CA-125)
- 30-35 every 1-2 years - Upper endoscopy (only for families with history of gastric cancer)
- 30-35 every 1-2 years - Ultrasound kidney and bladder, Urine cytology
- 30-35 every 1-2 years
Genetic Diagnosis for HNPCC
- Analysis of MSI in tumour tissue
- can be performed in paraffin blocks
- paired tumour / normal tissue
- ***IHC staining for MMR protein -
**Genetic diagnosis of MMR genes in blood samples
- combination of methods including **direct DNA sequencing + ***MLPA analysis (or NGS analysis)
- once the germline mutation is defined for an index patient, simpler molecular diagnostic test can be devised for the specific family
Genetic diagnosis protocol for Lynch syndrome
**HK Hereditary Gastrointestinal Cancer Genetic Diagnosis Laboratory:
Microsatellite instability (MSI) detected in tumour (Positive: can be Germline / Somatic)
—> test for **MSH2 loss / MSH6 loss / PMS2 loss / ***MLH1 loss
—> Germline testing by MLPA for genomic deletions
HK Hereditary Gastrointestinal Cancer Genetic Diagnosis Laboratory
- based in Department of Pathology, HKU
- charitable genetic diagnosis service
- receiving a territory-wide referral to perform genetic testing for CRC patients who satisfy the Bethesda guideline
- genetic testing, genetic counseling and referral for prophylactic screening for confirmed gene carrier
- HA started providing the service in QMH since 2021
NB:
- Majority of MSI is acquired / sporadic (NOT familial) —> most common promoter methylation of MLH1 gene —> silence the gene
Presymptomatic Genetic Diagnosis
Advantages:
1. Identify defective gene carriers from non-defective gene carriers within these families
2. Vigilant clinical screening, early intervention, and possibly chemopreventive measures can be targeted to high-risk members
3. Prophylactic surgery to prevent gynaecological cancers after completion of family in female gene carrier
4. Low-risk members are spared from repeated colonoscopy
Genetic Counseling:
1. Before decision to undertake genetic testing
2. Given again as guided by the results
3. Low-risk individuals are relieved from the unnecessary psychological burden
4. For high-risk individuals, detail explanations concerning the **nature of the disease and discussions on the possible **options and **alternatives on **prevention
Barrier:
For taking genetic test:
1. Want to know
2. Care for well-being of their family (children)
3. Prevention
4. Enable them to be accessible to screening in which they cannot afford
Against taking genetic test:
1. Do not want to know
2. Genetic stigma
3. Psychological burden
4. Worry about insurance
Familial Adenomatous Polyposis (FAP)
- A clinical phenotype characterized by development of adenomas in the colon starting in early teenage (>100 in number) (∵ 2nd hit is very easy to get vs Lynch (not common to get 2nd hit))
- **Autosomal dominant inheritance pattern + **complete penetrance
- Majority is caused by germline mutation in the ***APC gene
- A small number of patients may be caused by mutation in the ***MUTYH gene which is inherited in autosomal recessive manner
- Adenomas, if left untreated, will ***definitely progress to cancer
- A subset of patients may have fewer number of adenomas (average of 30) and known as attenuated FAP
- Genetic diagnosis performed in ***blood to identify the mutated APC gene
- ***Prophylactic surgery to resect the whole colon in APC gene carrier
Hereditary breast / ovarian cancer
**BRCA gene mutations (Deleterious mutation):
- 30-70% of patients with **Hereditary breast / ovarian cancer
—> Breast and ovarian cancer syndrome (individuals have both breast + ovarian cancer)
—> Site specific breast cancer syndrome (only breast cancer)
—> Site specific ovarian cancer syndrome (only ovarian cancer)
- 5-10% of ***ALL breast and ovarian cancers
- Autosomal dominant hereditary syndromes
BRCA1 and BRCA2 gene:
1. BRCA1 mutation carriers
- 85% of developing breast cancer
- 35-60% of developing ovarian, fallopian tube, primary peritoneal cancer
- BRCA2 mutation carriers
- 85% of developing breast cancer
- 10-27% of developing ovarian, fallopian tube, primary peritoneal cancer (∵ same cell type: Mullerian cell type)
- 6% of developing male breast cancer
Characteristics of BRCA gene mutation carriers:
- Breast cancer diagnosed at an **early age
- **Bilateral breast cancer
- History of both breast and ovarian cancer
- Presence of breast cancer in >=1 **male family members
- **Multiple cases of breast cancer in family
- ***Both breast and ovarian cancer in family
- >=1 members with two primary cancers
Criteria for assessment of high risk:
1. Number of affected relatives
2. Relationship of affected relatives
3. Age at diagnosis of breast cancer
4. Presence of ovarian cancer in family
5. Presence of associated malignancies
6. >=3 members of direct lineage with breast / ovarian cancer
Risk for other cancers in BRCA mutation carriers
- Prostate
- BRCA1: Possible
- BRCA2: Definite - Pancreas
- BRCA1: Possible
- BRCA2: Definite - Fallopian tube
- BRCA1: Definite
- BRCA2: Definite - Endometrium
- BRCA1: Possible
- BRCA2: No evidence - Cervix
- BRCA1: No evidence
- BRCA2: Possible - Hepatobiliary
- BRCA1: Possible
- BRCA2: Possible - Stomach
- BRCA1: Possible
- BRCA2: Possible - Colorectal
- BRCA1: Possible
- BRCA2: No evidence - H+N
- BRCA1: No evidence
- BRCA2: Possible - Melanoma
- BRCA1: No evidence
- BRCA2: Possible - Risk to male cancers
- BRCA1: Little / None
- BRCA2: Definite
Mutation vs Polymorphism
Mutation:
- change
- disease-causing-change
Polymorphism:
- non-disease-causing change
- change found at frequency ≥1%
Use term “sequence variation” to prevent confusion
Testing for BRCA gene mutation
- Both are large genes with many exons
- No mutational hot-spots for both genes
- Frameshift, nonsense, or splice mutations resulting in truncated protein contribute to 85% of detected mutations in BRCA1
- Direct DNA sequencing used for mutation detection
- MLPA analysis for detection of large genomic deletions and duplications
- Next-generation Sequencing
Who:
Families identified to have high risk
- Should always start with “Index” testing —> Family member with diagnosis of breast / ovarian cancer
- Once the germline mutation is identified for index patient, then “carrier testing” can be offered to unaffected family members
Interpretation:
- Test outcome may be
—> Positive
—> Negative
—> Uncertain
- To distinguish a disabling mutation from a benign genetic variant
- Need to first to identify the germline mutation in “Index” patient
- If **NO mutation can be found in any of the index patients, then BRCA testing should **NOT
be offered to unaffected relatives
- **Negative result useful only if a BRCA mutation has been identified in an **affected 1st degree
relative
Positive result:
- A clinically significant mutation is identified and is associated with specific increased cancer risk
- Offspring + siblings have ***50% risk of carrying this mutation
- Testing for this mutation becomes available for blood relatives of the index case
Negative result:
For index case:
- The tested individual does not carry an inherited mutation
- A mutation does exist but current testing methods have not been able to identify it
- A mutation does exist but in a gene that has not yet been identified
For carrier testing:
- No mutation is identified
- Cancer risks of this individual are the same as the general population
- Offspring of this individual will not inherit this mutation
Uncertain result:
- A genetic change is identified but it is not currently known if this change is linked with cancer risk (i.e. variants of unknown significance)
- Further testing, including that of relatives, and/or development of future tests may help better predict the nature of this genetic change
Pathology of BRCA-related breast cancer
- Histologic subtypes
- BRCA1: Invasive carcinoma, NST (75%), Medullary (~5%), Atypical Medullary (10-30%)
- BRCA2: Invasive carcinoma, NST (75%), Atypical Medullary (<5%), Lobular (~10%), Presence of DCIS + well formed tubules - Grade
- BRCA1: High (grade III, 75%)
- BRCA2: Medium/high (grade II, 45%; grade III, 45%) - ER
- BRCA1: Negative (75%)
- BRCA2: Positive (75%) - HER2
- BRCA1: Negative (95%)
- BRCA2: Negative (95%) - Triple negative
- BRCA1: Half
- BRCA2: Nil - CK5
- BRCA1: Positive (50%)
- BRCA2: Negative (90%) - p53
- BRCA1: Positive (50%)
- BRCA2: Positive (40%)
Basal-like subtype of invasive breast carcinoma
- Grade 3 invasive carcinoma / metaplastic cancer
- Geographic necrosis
- Pushing borders of invasion
- Stromal lymphocytic response
- High mitotic count
- IHC: CK5/6(+), EGFR(+)
- ER(-), HER2(-)
- Actin(-), p63(-)
Basal-like cancers:
- Distinct genomic, expression and protein profile
- Distinct morphology
- Often but not invariably ***Triple Negative
- Found in 80-90% BRCA1 mutation breast cancers and 15% breast cancers with no family history
Clinical behaviour:
- Younger patients
- Frequently “interval cancers” with rapid progression
- Tend to have poor prognosis
- Visceral metastases to brain and lung
- Not associated with locoregional relapse after conservative treatment
- Pathological complete response to neoadjuvant chemotherapy
