24 Colorectal cancer Flashcards
The most common diagnosed malignancies worldwide is which one?
colorectal cancer-CRC
Three distinct molecular pathways lead to the development of CRC
1) the conventional suppressor pathway or chromosomal instability (CIN) pathway;
2) the serrated pathway or CpG island methylator phenotype (CIMP);
3) the microsatellite instability (MSI) pathway
What does it mean if polyp is serrated?
Serrated polyps are a type of growth that stick out from the surface of the colon or rectum. The polyps are defined by their saw-toothed appearance under the microscope. They can often be hard to find. The only way to determine the types of polyps is by removing them and examining them under a microscope.
Comprehensive exome sequencing studies have demonstrated that individual CRCs harbor an average of 76 gene mutations.
Describe Conventional suppressor pathway or CIN pathway of CRC
1) CIN pathway demonstrates accelerated gains or losses of large portions or whole chromosomes and commonly results in gross chromosomal or karyotypic abnormalities.
2) The consequences of CIN is a high frequency of aneuploidy, genomic amplifications and loss of heterozygosity.
3) About 60% of all CRCs including FAP, follow this CIN pathway;
4) Poor prognosis;
5) Accumulations of mutations is specific oncogenes and tumor suppressor genes plus chromosomal abnormalities in CIN pathways, lead to initiation and progression of CRC over a period of years to decades.
Genes and signal pathways involved in conventional suppressor or CIN pathway of CRC and steps lead to CRC
1) CIN pathway is initiated by inactivation of the APC/β-catenin/Wnt signaling pathway;
2) typically by mutation of one copy of APC gene;
3) Then a second event leads to inactivation of the second APC allele through deletion or additional mutations;
4) APC genes alterations lead to the development of dysplasia in aberrant cryptic foci and early adenomas.
5) Sequential accumulations of other gene mutations: KRAS, DCC, SMAD4, TP53.
What is APC stands for?
1) adenomatous polyposis coli;
2) located on chr. 5q21;
3) a tumor suppressor gene
Role of APC in CRC
1) By negatively regulating canonical WNT signaling, APC counteracts proliferation, promotes differentiation, facilitates apoptosis and suppresses invasion and tumor progression.
2) APC further antagonizes canonical WNT signaling by interacting with and counteracting β-catenin in the nucleus.
% of all CRC have MSIs?
MSIs found in 15% of all CRCs
Why are MSIs prone to accumulation of mutations?
MSIs are prone to accumulation of mutations due to inefficient binding of DNA polymerases to these sequence motifs
What is MSI?
1) MSIs are stretches of short-tandem DNA sequence repeats, about 1- 6bp in length.
2) MSIs are found throughout the human genome.
Genes involved in MSI pathway of CRC?
1) The mismatch repair (MMR) system: MSH2, MLH1, MSH3, MSH6, PSM2;
2) As MSIs are present in the coding regions of key genes for regulation of cell growth and apoptosis, loss of MMR function may result in frameshift mutations due to expansion or contraction of these regions and create an environment of uncontrolled cell survival and carcinogenesis.
Functions of the mismatch repair (MMR) genes
1) MMR genes encode proteins for proper repair of DNA sequence mismatch errors missed by DNA polymerase;
2) Function to preserve the genomic integrity.
Effects of loss of MMR function
As MSIs are present in the coding regions of key genes for regulation of cell growth and apoptosis, loss of MMR function may result in frameshift mutations due to expansion or contraction of these regions and create an environment of uncontrolled cell survival and carcinogenesis.
1) frameshift mutations;
2) uncontrolled cell survival and carcinogenesis
TGF-βRII frameshift mutations found in how many % of CRC with MSIs?
TGF-βRII frameshift mutations reported in 90% of CRCs with MSI.
How does inactivation of MMR happen in Lynch Syndrome?
In Lynch Syndrome, MMR inactivation may be caused by a germline mutation in one of the four MMR genes:
MSH2,
MLH1,
MSH6,
PSM2
How does inactivation of MMR happen in sporadic CRC with MSIs?
In sporadic CRC with MSIs, inactivation of MMR caused by aberrant epigenetic methylation of MLH1.
Which MMR genes have MSIs themselves?
1) MSH3 and MSH6.
2) MSH3 and MSH6 may be mutated in MSI-high (MSI-H) CRCs.
When compared to MSI-stable (MSS) CRCs, MSI-associated CRCs demonstrate a better prognosis at all stages, despite their known resistance to some chemotherapy regimens.
Function of DNA methylation
1) an epigenetic modification that regulates gene expression;
2) is essential for normal embryonic development and functions in X-chromosome inactivation and genomic imprinting;
3) The normal mammalian genome contains methylated CpG islands in non-promoter regions and promoter regions.
4) About half of CpG islands are located in promoter regions around the transcriptional start site, and are unmethlyated in normal cells.
5) Genes with these unmethlyated CpG islands will undergo normal transcription in the presence of transcriptional activators.
CIMP pathway in CRC
1) The CIMP pathway refers to widespread CpG island methylation within promoter regions of tumor suppressor genes.
2) In caner cells, hypermethylation of CpG islands within promoter regions leads to transcriptional silencing of tumor suppressor genes and loss of gene function, contributing to the tumorigenic process.
3) The CIMP positive phenotype accounts for about 35% of all CRCs.
4) Hypermethylation CpG islands–> gene silencing of tumor suppressor–> loss of gene function–>CRC
The most common CRC arising through CIMP pathway begin with sessile serrated adenomas (SSAs)
sessile serrated adenomas
1) SSAs;
2) prone to hypermethylation of a number of genes rich in CpG islands within the promoter regions.
Frequent activating mutation causes SSAs
BRAF mutations
% of CRC are MSI-stable and CIMP+
60%
% of CRC are MSI-High and CIMP+
40%
Most of sporadic MSI-H CRCs result from epigenetic silencing of which gene?
hMLH1 hypermethylation of CpG islands of promoter regions
Role of loss hMLH1 protein function in SSAs
1) leads to rapid accumulation of additional mutations in other genes: TGF-β, BAX;
2) then leads to cancer progression
Clinical features of SSAs with hMLH1 hypermethylation
1) cytologic dysplasia;
2) then rapid malignant transformation
CpG island hypermethylation may also occur in tumor suppressor genes other than hMLH1, resulting in CIMP+ MSS CRCs.
Hereditary colon cancer and polyposis syndromes are associated with a high risk of developing CRC. These account for how many % of all CRC?
<10%
Lynch Syndrome
1) The most common causes for developing hereditary CRC;
2) 2-7% of all CRCs;
3) Lynch syndrome is an autosomal dominant disorder that carries an increased risk for the development of CRC, endometrial cancer, and other cancers.
Lynch Syndrome is typically caused by a germline mutation in one of the MMR genes, what are these genes?
1) MLH1, MSH2, MSH6 and PSM2;
2) Loss of MMR function leads to development of CRC through the MSI pathway.
Which MMR genes mutations are involved in 90% of Lynch Syndrome?
MLH1, MSH2
What is the two-hit hypothesis of tumorigenesis of Lynch Syndrome?
Germline mutation in one copy of one MMR gene leads to somatic inactivation of the remaining wild-type allele.
Novel mechanisms identified in a subset of Lynch Syndrome
1) Hypermethylation of the MSH2 promoter without MMR gene mutations;
2) germline deletions in the 3’-region of the EPCAM (epithelial cell adhesion molecule)
What are the clinical guidelines used to evaluate the risk for Lynch Syndrome?
1) Amsterdam criteria;
2) Revised Bethesda guidelines - recommend MSI testing in CRCs
Tests used to diagnosis of Lynch Syndrome
1) Molecular testing for MSI or IHC;
2) Further differentiation of sporadic CRC from hereditary MSI-H CRC.
NCCN guidelines now recommend evaluation of MSI status by molecular or IHC on all resected CRCs, or CRCs diagnosed in patients <70-yrs old and in patient >70 meet the Bethesda.
Familial Adenomatous Polyposis (FAP)
1) FAP is the most common polyposis syndrome;
2) FAP is associated with about 0.5% of all CRC;
3) FAP is autosomal dominant but 25% of FAP are caused by de novo germline mutations
Clinical feature of FAP
1) characterized by the presence of hundreds to thousands of adenomas;
2) 100% of risk of development of CRC;
3) FAP median age for CRC development is 36-yrs old;
4) Classic FAP typically shows >100 colonic polyps and associated with other cancers
What is attenuated FAP (AFAP)?
1) Less severe form;
2) less than 100 adenomas, thus flat morphology;
3) 69% of risk of development of CRC
Gene mutations that causes FAP and AFAP
APC gene on chr. 5q21
The development of CRC in FAP/AFAP follows which pathway?
the conventional suppressor CIN pathway: mainly APC gene mutations
List the inherited adenomatous polyposis syndromes
1) FAP;
2) AFAP;
3) MAP (MUTYH-associated polyposis);
4) PPAP (polymerase proofreading associated polyposis);
5) HMPS (hereditary mixed polyposis syndrome)
MAP: MUTYH-associated polyposis
1) Autosomal recessive;
2) 0.5-1% of all CRCs;
3) MAP has a cumulative risk for CRC development of 80% by 70 years of age;
4) Caused by biallelic germline mutation in MUTYH
What is MUTYH gene?
1) a base-excision repair gene for oxidative DNA damage;
2) encodes MYH glycosylase
Who is recommended to test for MAP?
1) patients with more than 10 adenomatous polyps;
2) and with a family history of CRC consistent with recessive inheritance;
3) patient is negative for APC mutations
PPAP: polymerase proofreading associated polyposis
1) autosomal dominant;
2) characterized by the presence of multiple colorectal adenomas and early onset CRC
Germline mutated genes associated with PPAP
POLE and POLD1:
1) two DNA polymerases with exonuclease activity
2) germline mutations in the proofreading domains of POLE and POLD1
HMPS: hereditary mixed polyposis syndrome
1) autosomal dominant;
2) presents with polyps that display multiple and mixed morphologies: serrated polyps, Peutz-Jeghers polyps, juvenile polyps, and conventional adenomas
Gene mutations associated with HMPS
HMPS is associated with a 40 kb duplication present at the 3’ end of SCG5 gene and upstream of the GREM1 locus, leading to increased GREM1 expression.
