Cancer Genetics - Colorectal Cancers

Colorectal cancer

• Fourth most common cancer

• 4% of men and women will be diagnosed during their lifetime

• 5 - 10% of CRC have strong hereditary components

Colorectal cancer screening options

• Stool Based test

  • Check stool for signs of colon cancer; less invasive

    • Highly sensitive Fecal Immunochemical test (FIT)

    • Highly sensitive guaiac-based fecal occult blood test (gFOBT)

    • Multi-targeted stool DNA test

• Visual Examinations

  • Colonoscopy

    • Removes polyps, can both find and prevent colon cancer

  • CT Colonoscopy (Virtual)

  • Sigmoidoscopy

Adenomatous polyps (Adenomas)

• Precancerous

• Polpys have the potential to develop into colorectal cancer over time, most common precancerous polyps

  • Tubular adenomas

    • Most common; lower risk of malignancy

  • Villous adenomas

    • Higher risk of turning cancerous due to greater dysplasia

  • Tubulovillous adenomas

    • A mix of tubular and villous structures, with an intermediate cancer risk

Serrated polyps

• Precancerous

• Can develop into colorectal cancer through a different molecular pathway than adenomas

• Hyperplastic polpys

  • Generally small and low risk but may be a concern if large or numerous

• Sessile serrated lesions / Sessile serrated adenomas

  • Higher risk of progression to colorectal cancer, especially if dysplasia is present

• Traditional serrated adenomas

  • Rare but have a high risk of malignancy

Hamartomatous Polyps - Associated with Genetic Syndromes

• Typically benign but can be associated with inherited cancer syndromes

  • Juvenile polyps

    • Can occur sporadically or as part of Juvenile Polyposis Syndrome (Refers to type of polyp not age)

  • Peutz-Jeghers polyps

    • Associated with Peutz-Jeghers Syndrome which increases CRC and other cancer risks

Inflammatory polyps

• Non-Cancerous

• Found in individuals with inflammatory bowel disease (IBD) (eg. ulcerative colitis, Crohn’s disease)

• Typically benign but indicated colorectal cancer risk

Hyperplastic polyps

• Usually benign

• Small, commonly found in rectum or left colon

• Considered low risk, except when part of the serrated polyp pathway

Risk factors for colorectal cancer

Non-Modifiable

• Aging

• Family history of cancer

• Heredity

• Inflammatory bowel disease (IBD)

• Personal history of colorectal cancer or colorectal polyps

• Personal history of lower abdominal radiation

Modifiable

• Diet high in red, processed or charred meats

• Smoking

• Lack of exercise

• Obesity

• Moderate to high levels of alcohol use

• Failure to undergo screening

Warning Signs of Colorectal Cancer

• Abdominal discomfort (frequent gas, pain, bloating, fullness, cramps)

• Nausea / vomiting

• Marked changes in bowel habits

• Diarrhea or constipation

• Blood in stool

• Stool has become more narrow

• Unexplained weight loss

• Fatigue

• Jaundice

• Internal bleeding

• Anemia

Common sites of metastasis for colorectal cancer

• Lungs

• Liver

• Tissue lining the abdomen (peritoneum)

• Cancer

• Large bowel

Treatment for colorectal cancer

• Surgery

• Chemotherapy

• Radiation

• Immunotherapy

Lynch Syndrome

• Inherited autosomal dominant

• Genes

  • MLH1

  • MSH2

  • PMS2

  • EPCAM deletion

  • Constitutional MLH1 hypermethylation

• 1 in 279

  • Estimated 2 - 4% of individuals with colorectal cancer have Lynch Syndrome

    • ~2.5% of endometrial cancer cases

• Most commonly inherited form of colorectal cancer

  • Increased risk of endometrial, ovarian, gastric, urinary tract, brain, pancreatic and other cancers

Lynch Syndrome Diagnosis

• A pathogenic germline variant in mismatch repair (MMR) gene

  • MLH1

  • MSH2

  • MSH6

  • PMS2

• A pathogenic germline deletion in EPCAM

  • EPCAM is a gene located upstream from MSH2

  • Deletion in EPCAM silences the expression of MSH2 gene, by causing hypermethylation of the MSH2 promoter

Colorectal cancer in Lynch Syndrome

• Elevated lifetime risk for colorectal cancer

• Colorectal cancer evolve from polyps called adenomas

  • Tumor site mostly in proximal colon (right-sided colorectal cancer)

  • Larger

  • Flatter

  • More high-grade dysplasia and villous histology

• Progression from adenoma => carcinoma

  • More rapid compared to non-syndromic colorectal cancer

  • New cancers can occur within 2- 3 years following a negative colonoscopy

Testing criteria for Lynch Syndrome

• Amsterdam II Criteria

• Revised Bethesda Guidelines

• Testing should be considered in patients with at least 5% risk of Lynch Syndrome

  • MMRpro

  • PREMM

  • MMRpredict prediction models

Amsterdam II Criteria

• At least three relatives diagnosed with a cancer associated with LS (colorectal, endometrial, small bowel, ureteral, or renal pelvis); all of the following must be met

  • One must be a first-degree relative of the other two

  • At least two successive generations must be affected

  • At least one diagnosed before 50 years

  • FAP should be excluded in the colorectal cancer cases if any

  • Tumor should be verified whenever possible

Bethesda Guidelines

• Colorectal cancer diagnosed in patient who is younger than age 50

• Presence of synchronous or metachronous LS-associated tumors

• Colorectal cancer with MSI-H histology (Crohn’s-like lymphocytic reaction, mucinous / signet differentiation, or medullary growth pattern) diagnosed under age 50

• Colorectal cancer diagnosed in a patient with two or more first- or second- degree relatives with LS- related cancers regardless of age

Hallmark features of Lynch Syndrome tumors - deficient MMR activity

• Either MSI-H or MMR IHC can be used to screen tumors for deficient MMR activity

• For families that are highly suspicious for Lynch Syndrome, testing for both MSI-H and MMR IHC should be considered

• MSI-H and MMR IHC can be performed on any cancer type

  • The sensitivity and specificity in diagnosing Lynch Syndrome, is dependent on the cancer type

• Patients with MMR deficient tumors of any kind and no MLH1 promoter hypermethylation should be offered germline testing

IHC Staining

• IHC staining detects the presence or absence of mismatch repair (MMR) proteins

  • MLH1

  • MSH2

  • MSH6

  • PMS2

• Loss of staining (absence of protein expression) suggests a deficiency in the corresponding MMR gene, which may indicate Lynch Syndrome

• IHC is fast and cost-effective

MMR proteins

• MSH6 protein joins with MSH2 protein to form a two-protein called a dimer

  • Identifies locations on the DNA where errors have been made during DNA replications

• MLH1 + PMS2 protein to form a two-protein complex called a dimer'

  • Coordinates the activities of other proteins that repair errors made during DNA replication

• MLH1 dominant to PMS2

• MSH2 dominant to MSH6

Interpreting IHC Results

• Normal (Intact Staining)

  • All hour MMR proteins are present = suggests tumor is not associated with Lynch syndrome

• Loss of MLH1 and PMS2

  • Likely due to MLH1 promoter methylation (sporadic cancer)

  • Further testing for BRAF mutation or MLH1 methylation to rule out sporadic

• Loss of MSH2 and MSH6

  • Suggests germline mutation in MSH2

  • Strong Lynch syndrome indicator

• Loss of MSH6 alone

  • Suggests MSH6 mutation

  • Linked to Lynch syndrome

• Loss of PMS2 alone

  • Suggests PMS2 mutation

  • Also linked to Lynch syndrome

• If any MMR is absent and no methylation is found = genetic testing for Lynch is recommended

Microsatellite Instability

• Microsatellite instability (MSI), are short, repetitive DNA sequences scattered throughout the genome

• MSI segments are prone to replication errors, such as insertions or deletions

• MSI occurs due to defects in MMR repair system

  • In normal cells, the MMR system repairs errors

• Germline mutations in an MMR gene can lead to defective repair

• Lynch Syndrome tumors exhibit high MSI instability (MSI-H)

  • MSI-H tumors have better overall survival and respond well to immunotherapy

MSI Testing in Lynch Syndrome

• Polymerase chain reaction (PCR)-Based MSI Testing

  • Compares the length of microsatellite regions in tumor DNA to normal tissue DNA

  • Uses a standard panel of 5 microsatellite markers (Bethesda panel)

    • Two mononucleotide repeats: BAT-25, BAT-26

    • Three dinucleotide repeats D2s123, D5S346, D17S250

• MSI Interpretation

  • MSI-H (High)

    • >= 2 - 5 markers show instability → suggestive of Lynch syndrome

  • MSI-L (Low)

    • 1 marker shows instability → unlikely to be Lynch-related

  • MSS (Microsatellite Stable)

    • No instability detected → suggests intact MMR function

CA-125

• Not routinely recommended as a screening test for ovarian cancer

• Lacks specificity and sensitivity

• Elevated levels can occur due to benign conditions leading to false positives

• Some ovarian cancers do not produce CA-125 leading to false negatives

• CA-125 is used for

  • Monitoring ovarian cancer in patients with a known diagnosis

  • Assessing treatment response and detecting recurrence

  • Supporting diagnosis

Constitutional Mismatch Repair Deficiency Syndrome: Reproductive Implication

• Genes

  • MLH1

  • MSH2

  • MSH6

  • PMS2

• Autosomal recessive

• Early onset and often childhood cancers

  • Brain

  • Gastrointestinal

  • Hematological

  • Phenotype mimics NF1

Lynch vs CMMRD

• Pathogenic variant

  • Lynch = 1 copy, CMMRD = 2 copies

  • Lynch = MMR deficiency in pre-cancerous / cancer cells, CMMRD = complete MMR in all cells

  • Lynch = asymptomatic until cancer, CMMRD = cafe au lait macules, skin hypopigmentation, Lisch nodules

  • Lynch = colorectal, endometrial, urothelial, ovarina, gastric, biliary tract, CMMRD = brain, haematological, LS associated

  • Lynch = 44 - 61 years onset, CMMRD = 12 - 30 years onset

Muir-Torre Syndrome

• Autosomal dominant, variant of Lynch

• Characterized by:

  • Sebaceous skin tumors

  • Internal cancers

• Management:

  • Lynch syndrome guidelines

  • Local excision

  • Radiation

Turcot Syndrome

• Variant of Lynch Syndrome

• Rare

• Characterized by:

  • Colorectal cancer and brain tumors, specifically glioblastomas

• Age of onset 20 - 40 years compared to 50 - 80 years

• Treatment

  • Surgical resection

  • Radiation therapy

  • Chemotherapy

Hereditary Diffuse Gastric Cancer

• Genes

  • CDH1 (E-cadherin) at 16q22.1

  • CTNNA1 at 5q31.2

• Autosomal dominant

• Risk for diffuse gastric cancer aka signet ring carcinoma is 70% for men and 56% for women

  • Women have 42% risk for invasive lobular breast cancer

• Management

  • Gastric - prophylactic gastrectomy, upper endoscopy

  • Breast - CBE, mammogram, MRI, prophylactic b/l mastectomies

• Other

  • Cleft lip with/without clef palate

Pancreatitis

• Inflammation of pancreas

• May be acute (sudden onset), recurrent acute, or chronic

• Hereditary pancreatitis

  • Diagnosed in individuals and families with germline highly penetrant heterozygous gain of function variants in PRSS1 gene

  • Genes

    • CFTR - AD (pancreatitis) and AR (Cystic fibrosis)

    • CTRC - AD

    • PRSS1 - AD

    • SPINK1 - AD/AR

    • CASR - AD

• Familial pancreatitis

  • Describe families with two or more closely related individuals with pancreatitis

  • Other causes of pancreatitis excluded, including PRSS1-related hereditary pancreatitis, CFTR-related pancreatitis, gallstones, trauma and other etiologies