Tumor Genetics: Oncogenes, Tumor Suppressor Genes, DNA Repair Defects, Microsatellite Instability

Foundations and mechanisms

Cancer is a clonal genetic disease in which a single cell and its progeny acquire heritable alterations that confer a selective growth advantage. These alterations may be point mutations, small insertions/deletions, copy-number gains or losses, chromosomal translocations, epigenetic silencing, or whole-chromosome aneuploidy. Normal human cells replicate a diploid genome of approximately 3.2 × 10⁹ base pairs; high-fidelity DNA polymerases plus repair pathways reduce the final mutation rate to roughly 10⁻⁹ to 10⁻¹⁰ mutations/base/cell division. Cancer develops when “driver” mutations accumulate faster than they are eliminated by DNA repair, apoptosis, senescence, and immune surveillance.

Core classes of cancer genes

Class	Normal function	Oncogenic mechanism	Inheritance pattern at cellular level	High-yield examples
Oncogenes	Promote growth, survival, or proliferation when appropriately activated	Gain-of-function mutation, amplification, translocation, promoter/enhancer activation	Dominant: one activated allele can drive phenotype	RAS, MYC, ABL, HER2/ERBB2, BCL2
Tumor suppressor genes	Inhibit cell-cycle progression, repair damage, or promote apoptosis	Loss-of-function mutation, deletion, methylation, loss of heterozygosity	Recessive: usually both alleles lost; exceptions include haploinsufficiency/dominant-negative effects	TP53, RB1, APC, PTEN, CDKN2A
DNA repair genes	Maintain genomic integrity	Loss causes hypermutation and genomic instability	Usually recessive at cellular level	MLH1, MSH2, BRCA1/2, ATM, XPA-G

Oncogenes are altered versions of proto-oncogenes. Mechanisms include: activating point mutations, such as KRAS locked in a GTP-bound state; gene amplification, such as HER2 amplification in breast/gastric carcinoma or N-MYC amplification in neuroblastoma; and translocations, such as t(9;22) creating BCR-ABL in chronic myeloid leukemia. Tumor suppressor genes restrain proliferation. Knudson’s two-hit hypothesis, classically described for retinoblastoma, states that both alleles must be inactivated before loss of function becomes clinically relevant. A patient with a germline RB1 mutation inherits the first hit in every cell and requires only one somatic second hit, causing earlier, often bilateral tumors.

Gatekeepers, caretakers, and cell-cycle checkpoints

Gatekeeper genes directly control cell growth and death. RB1 restrains the G1-to-S transition by binding E2F transcription factors; phosphorylation of RB by cyclin D-CDK4/6 releases E2F, allowing DNA synthesis. TP53, the “guardian of the genome,” is stabilized by DNA damage and induces CDKN1A encoding p21, which inhibits CDKs and arrests the cell cycle. If damage is severe, p53 promotes apoptosis through genes such as BAX and PUMA. TP53 is the most commonly mutated gene in human cancers, present in approximately 50% of malignancies.

Caretaker genes do not primarily regulate proliferation; instead, they preserve genomic fidelity. Loss creates a mutator phenotype, increasing the probability of oncogene activation and tumor suppressor loss. Cancer cells also bypass replicative senescence: normal somatic cells undergo approximately 40–60 population doublings before telomere shortening triggers senescence, whereas many cancers reactivate telomerase via TERT promoter mutations or amplification.

DNA repair pathways and characteristic defects

• Mismatch repair corrects base-base mismatches and small insertion/deletion loops after replication. Defects in MLH1, MSH2, MSH6, or PMS2 cause microsatellite instability.
• Nucleotide excision repair removes bulky helix-distorting lesions such as UV-induced pyrimidine dimers. Defects cause xeroderma pigmentosum with extreme UV sensitivity and increased skin cancers.
• Base excision repair removes small non-bulky lesions, such as deaminated or oxidized bases.
• Homologous recombination repairs double-strand breaks using a sister chromatid as template; BRCA1 and BRCA2 are high-yield examples.
• Nonhomologous end joining ligates double-strand breaks without a template and is more error-prone.

Microsatellite instability

Microsatellites are short tandem repeat sequences, commonly 1–6 base pairs repeated many times. Because DNA polymerase can slip during replication, these regions are especially vulnerable to insertion/deletion errors. Normal mismatch repair corrects these errors; when mismatch repair is defective, repeat lengths become variable, producing microsatellite instability (MSI). MSI is a molecular signature of deficient mismatch repair and is central to Lynch syndrome, an autosomal dominant cancer predisposition syndrome most often involving germline MLH1 or MSH2 mutations. Sporadic colorectal cancers can also show MSI, commonly from MLH1 promoter hypermethylation rather than germline mutation.

Conceptually, tumors evolve through selection. A typical solid tumor often contains multiple driver alterations, commonly estimated around 4–7 major driver events, plus numerous passenger mutations. Genetic instability may be chromosomal instability, producing aneuploidy and loss of heterozygosity, or microsatellite instability, producing small repeat-length errors. For Step 1, the key distinction is that oncogenes are activated by gain of function, tumor suppressors are inactivated by loss of function, and DNA repair defects accelerate the acquisition of both.

Clinical assessment and investigations

Clinical presentation: when to suspect a tumor genetic mechanism

Most cancers arise from acquired somatic mutations accumulated over years, but Step 1 questions often signal a germline cancer predisposition syndrome by emphasizing age, family pattern, multiplicity, or characteristic tumor type. A germline mutation is present in every cell and provides the “first hit”; a second somatic hit inactivates the remaining allele for many tumor suppressor genes, consistent with the Knudson two-hit hypothesis.

• Young age at diagnosis: colorectal cancer before age 50, breast cancer before age 45, sarcoma in childhood, or retinoblastoma in infancy suggests inherited predisposition.
• Multiple primary tumors: bilateral breast cancer, bilateral retinoblastoma, multiple colon polyps, or synchronous/metachronous colorectal cancers.
• Characteristic family history: autosomal dominant vertical transmission, multiple affected relatives, and cancers across generations.
• Syndrome-defining pathology: medullary thyroid carcinoma in MEN2, hemangioblastomas in von Hippel-Lindau disease, osteosarcoma/adrenocortical carcinoma in Li-Fraumeni syndrome, or sebaceous tumors in Muir-Torre variant of Lynch syndrome.

Differential diagnosis by clinical pattern

Presentation clue	High-yield diagnosis	Gene / mechanism	Typical tumors
Hundreds to thousands of colon polyps in adolescence	Familial adenomatous polyposis	APC tumor suppressor; Wnt/β-catenin activation	Colorectal adenocarcinoma; desmoid tumors; osteomas in Gardner variant
Colon cancer with few polyps, endometrial cancer, family clustering	Lynch syndrome	Mismatch repair genes: MLH1, MSH2, MSH6, PMS2; microsatellite instability	Colorectal, endometrial, ovarian, gastric, small bowel, urinary tract
Breast and ovarian cancer; early-onset or male breast cancer	Hereditary breast/ovarian cancer	BRCA1/BRCA2; homologous recombination repair defect	Breast, ovarian, prostate, pancreatic cancers
Sarcoma, breast cancer, brain tumor, leukemia, adrenocortical carcinoma	Li-Fraumeni syndrome	TP53; defective G1/S checkpoint and apoptosis	Multiple early-onset cancers
Retinal/cerebellar hemangioblastomas, renal cell carcinoma, pheochromocytoma	Von Hippel-Lindau disease	VHL; increased HIF-1α and VEGF	Clear cell renal carcinoma, CNS hemangioblastoma
Medullary thyroid carcinoma, pheochromocytoma, mucosal neuromas	MEN2	RET proto-oncogene gain-of-function	Medullary thyroid carcinoma, pheochromocytoma, parathyroid hyperplasia

Investigations: tumor testing versus germline testing

A key Step 1 distinction is whether a test examines the tumor only or the patient’s inherited genome. Somatic tumor testing identifies mutations driving a particular cancer and can guide classification or targeted therapy. Germline testing uses normal tissue, typically blood or saliva, to determine inherited cancer risk. Tumor sequencing alone cannot prove heredity because the variant may be acquired in the tumor.

• Immunohistochemistry: detects presence or absence of proteins in tumor tissue. Loss of MLH1, MSH2, MSH6, or PMS2 suggests mismatch repair deficiency. Loss of RB protein supports RB1 pathway disruption.
• PCR-based microsatellite instability testing: compares repeated DNA sequences in tumor versus normal tissue. Instability reflects polymerase slippage that is not corrected because mismatch repair is defective.
• FISH: detects gene amplification, deletion, or translocation using fluorescent probes. Classic examples include HER2 amplification in breast/gastric cancer and MYCN amplification in neuroblastoma.
• Karyotyping: detects large chromosomal abnormalities, such as t(9;22) Philadelphia chromosome in CML, but has lower resolution than FISH or sequencing.
• Next-generation sequencing: identifies point mutations, small insertions/deletions, copy-number changes, and mutational signatures; analytic sensitivity often detects variants at approximately 2–5% variant allele frequency, depending on platform and depth.

Interpretation and thresholds

Test / criterion	Important threshold	Interpretation
Amsterdam II criteria for Lynch syndrome	At least 3 relatives with Lynch-associated cancer, across 2 generations, with 1 diagnosed before age 50; one must be a first-degree relative of the other two	Clinical screening rule; highly specific but not sensitive
Microsatellite instability PCR	MSI-high: instability in ≥30–40% of tested markers or ≥2 of 5 classic Bethesda markers	Suggests mismatch repair deficiency; common in Lynch syndrome and sporadic MLH1-silenced tumors
MLH1 loss on IHC	Loss of MLH1 and PMS2 together	May reflect germline MLH1 mutation or sporadic MLH1 promoter hypermethylation; BRAF V600E favors sporadic colorectal cancer
HER2 testing	IHC 3+ or FISH HER2/CEP17 ratio ≥2.0	HER2 amplification/overexpression; reflects oncogene activation via receptor tyrosine kinase signaling
Ki-67 proliferation index	No universal normal cutoff; higher percentage indicates greater proliferative fraction	Marker of cycling cells; useful conceptually for tumor aggressiveness but context-dependent

Current practice guidelines, including major oncology and pathology society recommendations, support universal mismatch repair or MSI testing for newly diagnosed colorectal cancers, and increasingly for endometrial cancers, because family history alone misses many Lynch syndrome cases. On Step 1, the most important interpretation is mechanistic: loss of mismatch repair causes microsatellite instability, whereas defects in nucleotide excision repair, such as xeroderma pigmentosum, cause UV-induced pyrimidine dimer persistence and markedly increased skin cancer risk.

Common diagnostic pitfalls

• Oncogene activation requires one allele: examples include RET, RAS, MYC, BCR-ABL, and HER2. Germline gain-of-function RET causes MEN2.
• Tumor suppressor loss usually requires both alleles: examples include RB1, TP53, APC, VHL, PTEN, and BRCA1/2.
• BRCA1/2 are tumor suppressors, not oncogenes: they repair double-strand breaks by homologous recombination; loss causes chromosomal instability.
• MSI is not the same as chromosomal instability: MSI involves short repeat-length errors; chromosomal instability involves aneuploidy, deletions, amplifications, and translocations.

Management, pharmacology and procedures

Principles of genetically guided cancer management

In tumor genetics, management begins by distinguishing germline pathogenic variants, present in every cell and heritable, from somatic tumor mutations, acquired only in the neoplasm. Germline testing affects the patient and relatives; somatic testing guides tumor-specific therapy. Testing usually requires informed consent, pretest probability assessment, and post-test counseling because results may be pathogenic, likely pathogenic, benign, or a variant of uncertain significance, which should not drive irreversible procedures.

• Germline testing: blood or saliva DNA; used for BRCA1/2, APC, MLH1/MSH2/MSH6/PMS2, RET, RB1, TP53.
• Somatic testing: tumor tissue or circulating tumor DNA; detects actionable mutations such as EGFR, BRAF V600E, ALK fusions, HER2 amplification, NTRK fusions, and MSI-high status.
• Microsatellite instability testing: performed by PCR/next-generation sequencing or mismatch repair immunohistochemistry. Loss of MLH1, MSH2, MSH6, or PMS2 protein suggests mismatch repair deficiency; MSI-high tumors often respond to immune checkpoint blockade.

Targeted pharmacology: high-yield gene-drug relationships

Genetic lesion	Prototype therapy	Mechanism	Representative dose and major toxicities
BCR-ABL fusion, t(9;22)	Imatinib	ATP-competitive tyrosine kinase inhibitor; blocks constitutively active ABL kinase in CML and some ALL	400 mg orally daily in chronic-phase CML; edema, nausea, myelosuppression, hepatotoxicity
HER2/ERBB2 amplification	Trastuzumab	Monoclonal antibody against HER2 receptor; inhibits signaling and promotes antibody-dependent cellular cytotoxicity	8 mg/kg IV loading, then 6 mg/kg every 3 weeks; cardiomyopathy, especially with anthracyclines
EGFR activating mutation	Osimertinib	EGFR tyrosine kinase inhibitor active against common sensitizing mutations and T790M resistance mutation	80 mg orally daily; rash, diarrhea, QT prolongation, interstitial lung disease
BRAF V600E	Vemurafenib or dabrafenib plus trametinib	Blocks mutant BRAF; MEK inhibition reduces paradoxical MAPK activation	Dabrafenib 150 mg orally twice daily plus trametinib 2 mg orally daily; fever, rash, cardiomyopathy, secondary skin tumors
BRCA1/2 loss or homologous recombination deficiency	Olaparib	PARP inhibitor causing synthetic lethality: impaired single-strand break repair becomes lethal when double-strand break repair is defective	300 mg orally twice daily; anemia, nausea, fatigue, rare myelodysplastic syndrome/AML
MSI-high or mismatch repair deficient tumor	Pembrolizumab	Anti-PD-1 antibody restores T-cell activity against mutation-rich tumors with many neoantigens	200 mg IV every 3 weeks or 400 mg every 6 weeks; autoimmune colitis, pneumonitis, hepatitis, endocrinopathies

A landmark concept is synthetic lethality: loss of either pathway alone is tolerated, but loss of both causes cell death. PARP inhibitors exploit this in BRCA-deficient tumors. Another high-yield example is acute promyelocytic leukemia with PML-RARA t(15;17): all-trans retinoic acid, commonly 45 mg/m²/day orally in divided doses, induces differentiation of malignant promyelocytes and is combined with arsenic trioxide or chemotherapy. It rapidly improves disseminated intravascular coagulation risk but can cause differentiation syndrome with fever, pulmonary infiltrates, hypotension, and edema.

Procedures, surveillance, and prevention in hereditary cancer syndromes

Management of inherited tumor suppressor or DNA repair defects emphasizes surveillance and risk-reducing procedures because the mutation is present before cancer develops.

• Familial adenomatous polyposis, APC mutation: hundreds to thousands of adenomas; colorectal cancer risk approaches 100% without colectomy. Colonoscopic surveillance begins around age 10-12 years, with prophylactic colectomy when polyp burden becomes unmanageable.
• Lynch syndrome, mismatch repair defects: colonoscopy generally begins at age 20-25 years or 2-5 years before the earliest family cancer, repeated every 1-2 years. Endometrial and ovarian cancer risk counseling is important.
• BRCA1/2 pathogenic variants: enhanced breast screening often includes annual MRI beginning around age 25 years and mammography around age 30 years. Risk-reducing bilateral salpingo-oophorectomy is commonly considered after childbearing, around age 35-40 for BRCA1 and 40-45 for BRCA2. Prophylactic mastectomy may reduce breast cancer risk by more than 90%.
• MEN2, RET activating mutation: prophylactic thyroidectomy prevents medullary thyroid carcinoma; timing depends on RET mutation risk category, with highest-risk variants requiring surgery in infancy.
• Hereditary retinoblastoma, RB1 mutation: requires ophthalmologic surveillance and treatment such as laser, cryotherapy, chemotherapy, or enucleation; survivors have increased osteosarcoma risk, especially after radiation.

Complications and follow-up

Follow-up integrates genetic risk, treatment toxicity, and recurrence assessment. Targeted therapies require mechanism-based monitoring: left ventricular ejection fraction for trastuzumab, liver enzymes for many kinase inhibitors, complete blood counts for PARP inhibitors, and thyroid, adrenal, hepatic, pulmonary, and gastrointestinal assessment for immune checkpoint inhibitors. Tumor response in solid cancers is often measured radiographically using RECIST: partial response requires at least a 30% decrease in the sum of target lesion diameters, while progressive disease requires at least a 20% increase plus an absolute increase of at least 5 mm or new lesions.

Acute complications after highly effective therapy include tumor lysis syndrome, especially in bulky, rapidly proliferating malignancies. It causes hyperkalemia, hyperphosphatemia, secondary hypocalcemia, hyperuricemia, acute kidney injury, and arrhythmias. Prevention includes aggressive IV hydration and urate-lowering therapy: allopurinol 300 mg/day orally inhibits xanthine oxidase, whereas rasburicase 0.2 mg/kg IV enzymatically degrades uric acid and is avoided in G6PD deficiency due to hemolysis risk.

Exam controversies and advanced synthesis

From “gene list memorization” to biologic classification

USMLE Step 1 questions increasingly test whether you can classify a mutated gene by mechanism, not merely by name. A useful synthesis is: oncogenes are accelerators, tumor suppressor genes are brakes, and DNA repair genes are genome-maintenance systems. However, exam pitfalls arise because some genes have context-dependent behavior.

Category	Classical rule	Mechanistic nuance	High-yield examples
Oncogene	One activating hit is sufficient at the cellular level	Gain-of-function by point mutation, amplification, or translocation	RAS, MYC, BCR-ABL, HER2/ERBB2, ALK
Tumor suppressor gene	Two-hit hypothesis: both alleles inactivated	Loss of heterozygosity, deletion, methylation, nonsense mutation; inherited cases carry the first hit in the germline	RB, APC, BRCA1/2, NF1, PTEN
Dominant-negative tumor suppressor	Exception-like behavior	One mutant protein interferes with normal protein complexes	TP53, because p53 functions as a tetramer
DNA repair gene	Often behaves like a tumor suppressor	Loss increases mutation rate rather than directly stimulating proliferation	MLH1, MSH2, MSH6, PMS2, BRCA1/2, XPA-G

Microsatellite instability: what is tested, and what is controversial

Microsatellites are short tandem repeats, commonly 1–6 base pairs long. During DNA replication, polymerase slippage creates insertion/deletion loops. Normally, mismatch repair proteins correct these errors: MSH2/MSH6 recognize mismatches, MLH1/PMS2 coordinate repair. Loss produces microsatellite instability-high tumors, classically in Lynch syndrome and sporadic MLH1-promoter-methylated colorectal cancer.

Current pathology practice commonly uses either immunohistochemistry for MMR proteins or PCR/next-generation sequencing for MSI. A classic National Cancer Institute 5-marker PCR panel defines MSI-high as instability in ≥2 of 5 markers, approximately ≥30%–40%; MSI-low is instability in 1 marker; microsatellite stable shows no instability. Modern mononucleotide panels and NGS-based assays are more commonly used clinically. Approximate sensitivity for detecting Lynch-associated MMR deficiency is 85%–95% for MSI testing and 83%–100% for MMR IHC, with lower sensitivity for some MSH6/PMS2-associated tumors.

• Universal testing: major guidelines, including NCCN-style practice, support universal MMR/MSI screening in newly diagnosed colorectal cancer and endometrial cancer because family-history criteria miss cases.
• Amsterdam II criteria: ≥3 relatives with Lynch-associated cancer, across ≥2 generations, with ≥1 diagnosis before age 50, and one must be a first-degree relative of the other two.
• Pitfall: loss of MLH1 and PMS2 on IHC in colorectal cancer is often sporadic. Testing for BRAF V600E mutation or MLH1 promoter methylation helps distinguish sporadic MLH1 silencing from Lynch syndrome; BRAF V600E is generally not a Lynch feature.

Targeted therapy as proof of mechanism, not management minutiae

Step 1 rarely asks treatment algorithms, but drugs illustrate molecular logic. Know the principle: identify a driver lesion, then exploit dependence on that pathway or a synthetic-lethal vulnerability.

Molecular lesion	Therapeutic concept	Representative agent	Mechanism and notable data
BCR-ABL fusion, t(9;22)	Constitutive tyrosine kinase inhibition	Imatinib, typically 400 mg orally daily; half-life ~18 h	Blocks ATP-binding site of ABL kinase; landmark IRIS trial established durable superiority over interferon-based therapy in chronic myeloid leukemia.
HER2/ERBB2 amplification	Receptor blockade	Trastuzumab, commonly 8 mg/kg IV loading then 6 mg/kg every 3 weeks; half-life ~28 days	Monoclonal antibody against HER2; adverse association: reversible cardiomyopathy, especially with anthracyclines.
BRCA1/2 loss	Synthetic lethality	Olaparib, 300 mg orally twice daily; half-life ~12 h	PARP inhibition prevents single-strand break repair; BRCA-deficient cells cannot repair resulting double-strand breaks by homologous recombination.
MSI-high or dMMR tumor	Immune checkpoint sensitivity due to high neoantigen load	Pembrolizumab, 200 mg IV every 3 weeks or 400 mg every 6 weeks; half-life ~22 days	Anti-PD-1 antibody. KEYNOTE-016/164/158 supported tissue-agnostic approval; pooled objective response rates were approximately 40% in MSI-H/dMMR cancers.

Common viva-level pitfalls

• Inherited cancer syndromes are usually inherited as autosomal dominant predispositions, even when the affected gene is a tumor suppressor requiring two cellular hits. The inherited allele is the first hit; the tumor acquires the second.
• A mutation in a tumor is not automatically germline. Tumor-only sequencing can detect somatic variants, germline variants, or artifacts. Germline testing requires normal tissue, usually blood or saliva, and variants are classified using ACMG categories: pathogenic, likely pathogenic, variant of uncertain significance, likely benign, benign.
• High tumor mutation burden is not identical to MSI. MSI is a specific signature of mismatch repair failure. Tumor mutational burden is often reported as mutations per megabase; a commonly used immunotherapy threshold is ≥10 mutations/Mb, but this is assay- and cancer-dependent.
• Not every driver is actionable. KRAS activation predicts lack of response to anti-EGFR therapy in colorectal cancer, but mutant RAS itself has historically been difficult to inhibit, although KRAS G12C inhibitors now exist for selected tumors.
• Promoter methylation can mimic mutation. Epigenetic silencing of MLH1 or CDKN2A can phenocopy loss-of-function mutation without changing the coding sequence.

The highest-yield synthesis is that cancer genetics links cell-cycle control, DNA repair fidelity, apoptosis, telomere maintenance, angiogenesis, and immune evasion. Step 1 questions often disguise the diagnosis by describing the mechanism: loss of G1/S checkpoint control suggests RB; failure to arrest after DNA damage suggests p53; chromosomal instability with hundreds of colonic adenomas suggests APC; insertion/deletion errors in repetitive DNA suggest mismatch repair deficiency.