Detailed Notes on Colon Cancer: HNPCC and FAP

Colon Cancers: HNPCC and FAP

Colorectal Cancer

• Third most common cancer, second most frequent cause of cancer-related death.
• Normal cells in the lining of the colon or rectum change, grow uncontrollably, and don't die.
• Usually starts as a noncancerous polyp that can become a cancerous tumor over time.

Forms of Colorectal Cancer

• Hereditary: Family history, younger age of onset, specific gene defects.
  • Examples: Familial adenomatous polyposis (FAP), hereditary nonpolyposis colorectal cancer (HNPCC or Lynch syndrome).
• Sporadic: Absence of family history, older population, isolated lesion.
• Familial: Family history, higher risk if the index case is young (<50 years) and the relative is a first-degree relative.
• Histopathology: Generally adenocarcinoma.

Function of the Colon and Rectum

• The colon and rectum comprise the large intestine (large bowel).
• The primary function is to turn liquid stool into formed fecal matter.

Risk Factors for Colorectal Cancer

• Polyps (noncancerous or precancerous growth associated with aging).
• Age.
• Inflammatory bowel disease (IBD).
• Diet high in saturated fats, such as red meat.
• Personal or family history of cancer.
• Obesity.
• Smoking.
• Other factors.

Inherited Colorectal Cancer Syndromes

• Lynch syndrome (HNPCC):
  • Also known as Hereditary Non Polyposis Colorectal Cancer.
  • Approximately 2-5% of colorectal cancer cases.
  • Prevalence of 1 in 200-2,000 (depends on population).
• Familial Adenomatous Polyposis (FAP):
  • Less than 1% of colorectal cancer cases.
  • Prevalence of 1 in 8,000-14,000 (depends on population).
  • Autosomal dominant inheritance.

Colorectal Cancer Genes

• Lynch syndrome (HNPCC): Mutations in DNA repair genes lead to an accumulation of mutations which may result in malignancy.
• FAP: Mutations in a tumor suppressor gene cause an increase in cell proliferation and a decrease in cell death when mutated.

Lynch Syndrome (HNPCC)

• Genetically heterogeneous.
• Clinical testing is available for 4 genes: MLH1 & MSH2 (most common), MSH6 & PMS2.
• Research testing may be available for other genes.
• High penetrance.
• Characterized by:
  • Earlier onset than sporadic cancer.
  • More aggressive, proximal, right-sided tumors.
  • Risk for extra-colonic tumors.
  • Distinct tumor pathology.

Mutations Linked to HNPCC

• Caused by a germline mutation in a gene responsible for DNA mismatch repair (MMR).
• Over 90% of the mutations occur in MSH2 or MLH1, which are MMR genes: MLH1 (50%) and MSH2 (40%).
• Present in Incomplete Penetrance.

Tasks of MMR (Mismatch Repair)

• Identify the old and new DNA strands (old strand assumed to be correct).
• Make a single-strand cut in the new strand on one side of the mistake.
• Make a second cut on the other side of the mistake.
• Remove the DNA that contains the error.
• Replace the DNA, using the old strand as a template to define the sequence.

Human Mismatch Repair (MMR)

• Recruitment of PCNA, RFC, DNA Pold, Helicase, Exonuclease.
• Incision of the new strand.
• Excision of the new strand.
• Repair synthesis (IDL).
• hMutSa + hMutLa recruitment.
• hMutSa + hMutSb recruitment.

Comparison: FAP, HNPCC, Sporadic Adenomas, Sporadic Cancers

Defects in MMR Function

• Absent or non-functional proteins.
• Loss of MMR capacity.
• Dominant-negative effect: abnormal proteins bind with normal ones.
• Inability to separate, destroy, and resynthesize mutated DNA.

Human MMR Genes

• MLH1 (3p21) (Mut L homolog 1).
• MSH2 (2p16) (Mut S homolog 2).
• PMS1 (2q31-33) (postmeiotic segregation increased 1).
• PMS2 (7p22).
• MSH3 (5q3).
• MSH6 (2p16) (= GT Binding Protein).

Colorectal Cancer and Early Detection

• Can be prevented through regular screening and removal of polyps.
• Early diagnosis means a better chance of successful treatment.
• Screening should begin at age 50 for "average risk" individuals or sooner if there's a family history, symptoms, or personal history of inflammatory bowel disease.

Investigations

• Faecal occult blood:
  • Guaiac test (Hemoccult) - based on pseudoperoxidase activity of haematin.
    • Sensitivity of 40-80%; Specificity of 98%.
    • Dietary restrictions - avoid red meat, melons, horseradish, vitamin C, and NSAIDs for 3 days before the test.
  • Immunochemical test (HemeSelect, Hemolex) - based on antibodies to human haemoglobins.
    • Used for screening, not diagnosis.

Investigations (Continued)

• Colonoscopy:
  • Can visualize lesions < 5mm.
  • Small polyps can be removed or at a later stage by endoscopic mucosal resection.
  • Performed under sedation.
  • Consent: bleeding, infection, perforation (1 in 3000), missed diagnosis, failed procedure, anaesthetic/medical risks.
  • Warn: bowel prep, abdominal bloating/discomfort afterwards, no driving for 24 hours.
• Double contrast barium enema:
  • Doesn't require sedation.
  • Avoids risk of perforation.
  • More limited in detecting small lesions.
  • All lesions need to be confirmed by colonoscopy and biopsy.
  • Performed with sigmoidoscopy.
  • Second line in patients who failed/cannot undergo colonoscopy.

Colorectal Cancer Staging

• Staging describes the depth of the tumor and where it has spread.
• It's the most important tool doctors have to determine a patient’s prognosis.
• Staging is described by the TNM system: Tumor size, spread to lymph Nodes, and whether the cancer has Metastasized.
• Treatment depends on the stage of the cancer.

Stages of Colorectal Cancer

• Stage 0: Cancer in situ, located in the mucosa.
  • Treatment: Removal of the polyp (polypectomy).
• Stage I: Cancer has grown through the mucosa and invaded the muscularis.
  • Treatment: Surgery to remove the tumor and some surrounding lymph nodes.
• Stage II: Cancer has grown beyond the muscularis but hasn't spread to the lymph nodes.
  • Colon cancer: Surgery and sometimes chemotherapy after surgery.
  • Rectal cancer: Surgery, radiation therapy, and chemotherapy.
• Stage III: Cancer has spread to the regional lymph nodes.
  • Colon cancer: Surgery and chemotherapy.
  • Rectal cancer: Surgery, radiation therapy, and chemotherapy.
• Stage IV: Cancer has spread outside of the colon or rectum to other areas of the body.
  • Treatment: Chemotherapy. Surgery to remove the colon or rectal tumor may or may not be done.
  • Additional surgery to remove metastases may also be done in carefully selected patients.

Summary: Staging of Colorectal Cancer

5-Year Survival for CRC by Stage

• All Stages: 65%
• Localized (Stage I and II): 70-90%
• Regional Stage III: 25-70%
• Distant (Stage IV): 5-10%

Clinical Trials for Colorectal Cancer Treatment

• Research studies involving people; test new treatment and prevention methods.
• Determine whether methods are safe, effective, and better than the best known treatment.
• Purpose: answer a specific medical question in a highly structured, controlled process.
• Evaluate methods of cancer prevention, screening, diagnosis, treatment, and/or quality of life.

Chemotherapy Agents for Colorectal Cancer

Targeted Therapy of Colorectal Cancer

Surgical Approach to Managing CRC

• Resect colon (or its parts) without disrupting normal arterial supply.
• Important arteries include: Middle colic artery, Superior mesenteric artery, Right colic artery, Ileocolic artery, Inferior mesenteric artery, Left colic artery, Sigmoid arteries.
• Surgical procedures include: right hemicolectomy, sigmoidectomy, abdominoperineal resection of the rectum.

Hereditary Colorectal Cancer Syndromes: FAP

• Familial adenomatous polyposis (FAP) accounts for about 1% of colorectal cancer cases.
• People with FAP typically develop hundreds to thousands of colon polyps.
  • The polyps are initially benign but have nearly a 100% chance of developing into cancer if left untreated.
• Mutations in the APC gene cause FAP; genetic testing is available.
• Yearly screening for polyps is recommended.
• Attenuated familial adenomatous polyposis (AFAP) is related to FAP; people have fewer polyps.

Familial Adenomatous Polyposis Details

• APC gene: High penetrance.
• Characterized by:
  • Early onset.
  • >100 adenomatous polyps.
  • Variant form:
    • Attenuated FAP may occur with >10 but <100 polyps.

Consequences of FAP

• Colorectal adenomatous polyps begin to appear at an average age of 16 years (range 7-36 years).
• Average age at diagnosis: 34-43 years, when >95% have polyps.
• Age-related incidence of colon cancer:
  • 21 years: 7%
  • 45 years: 87%
  • 50 years: 93%

Genetics of FAP

• Autosomal dominant inheritance.
• Caused by mutations in the APC tumor suppressor gene on chromosome 5q.
• Up to 30% of patients have de novo germline mutations.
• Most families have unique mutations.
• Most mutations are protein-truncating.
• Genotype/phenotype relationships are emerging.

APC Mutations and Tumor Development

• APC mutations are the earliest event in tumor development.
• Steps:
  1. Loss of APC tumor-suppressor gene (chromosome 5).
  2. Activation of K-ras oncogene (chromosome 12).
  3. Loss of DCC tumor-suppressor gene (chromosome 18).
  4. Loss of p53 tumor-suppressor gene (chromosome 17).
  5. Other changes.

APC Function Discovery

• Researchers used cell extracts from FAP tumors/normal cells to "fish" for APC partners using Immuno-Precipitation (IP) and co-immuno Precipitation (Co-IP) experiments.
• They identified Armadillo (β-catenin) as an APC binding partner.

Armadillo and Body Plan Development in Drosophila

• Armadillo was earlier identified in Drosophila for body plan development.
• Experiments by Eric Wieschaus and Christiane Nüsslein-Volhard involved removing single genes and looking for effects on the body plan.

Signal Transduction Pathways

• Five signal transduction pathways shape virtually all cell fates choices in flies and vertebrates (including humans):
  • RTK.
  • TGF-β.
  • Wnt.
  • Hedgehog.
  • Notch.
• All play key roles in many cancers.

Wingless Signaling

• Wingless signaling specifies cell fates in the ventral epidermis.
• The Wingless signal influences the fates of neighboring cells.
• Cell-cell signaling can be influenced by the distance between cells.
• Signal transduction moves information from the cell surface to the nucleus and other cellular targets.
• Signal transduction can regulate cell proliferation.

Current Model for Wnt Signaling

• No Wg/Wnt Signal: β-catenin is destroyed.
• With Wg/Wnt Signal: β-catenin accumulates.
• Wnt target genes are turned OFF or ON accordingly.

Mouse Models of Colon Cancer

• Apc (Min).
• Smad.
• DNA mismatch repair.
• Ras.

β-catenin (Armadillo) Function

• Found in two places:
  1. In the adherens junctions.
  2. In the nucleus, as a transcriptional co-activator of TCF to form a bipartite transcription factor.

Armadillo and TCF Activate Wingless Target Genes

• Without Wingless signal: Repression of Wingless-responsive genes.
• With Wingless signal: Activation of Wingless-responsive genes.

APC and Arm Repeats

• APC is a complex protein.
• Contains Arm repeats (protein-protein interaction motif), SAMP repeats (bind Axin), and a mutation cluster region (MCR).
• All tumors carry one allele that makes a truncated APC protein.
• Truncations almost always occur in the MCR.

Colon Architecture

• Consists of crypts (proliferating cells) and villi (differentiated cells).
• Genes turned ON by Wnt signaling are expressed in crypts, while those turned OFF are expressed in villi.

Wnt Signaling and Cell Cycle Regulation

• Wnt signaling turns OFF the transcription of a key cell cycle regulator: the CDK inhibitor p21.
• The effect on p21 expression is indirectly mediated by the transcription factor c-myc.
• Transcription of the myc gene is directly regulated by TCF/beta-catenin.

Molecular Mechanisms Underlying APC

• Stromal cells send Wnt signals.
• Cells that receive them become stem cells.
• As cells migrate away from the crypt, they differentiate.

Current Model of Molecular Mechanisms Underlying APC

• Normal epithelium progresses to hyper-proliferative epithelium, early adenoma, intermediate adenoma, late adenoma, carcinoma, and metastasis.
• This progression involves loss of APC, activation of K-ras, deletion of 18q, loss of TP53, and other alterations.

Activated Wnt Signaling Triggers Colon Cancer

• Cancer pathways co-opt developmental pathways during tumor initiation and progression.
• Normal colon epithelial cells and normal embryonic cells are compared to show the role of Wnt signaling in cell migration and proliferation.
• Mutations can lead to colon polyps or early melanoma due to proliferation or blocked apoptosis.