Cell Cycle and Cancer Genetics

Germline (inherited) mechanisms — non-modifiable

• **Two-hit tumor suppressor loss (Knudson hypothesis)**: one defective allele is inherited, so a single somatic "second hit" unmasks loss of function. Explains bilateral, multifocal, early-onset retinoblastoma (RB1), Li-Fraumeni syndrome (TP53), familial adenomatous polyposis (APC), and hereditary breast/ovarian cancer (BRCA1/2).
• DNA repair deficiency: mismatch repair loss (Lynch syndrome → microsatellite instability), homologous recombination loss (BRCA1/2), nucleotide excision repair loss (xeroderma pigmentosum), ATM (ataxia-telangiectasia). Repair failure raises the mutation rate, accelerating accumulation of oncogenic hits.
• Ancestry and founder mutations: Ashkenazi Jewish ancestry enriches specific BRCA1/2 founder alleles; NCCN criteria use ancestry plus pedigree to trigger germline testing.
• Age and constitutional syndromes: cumulative somatic mutation burden makes age the dominant risk factor for most epithelial cancers; Down syndrome carries increased risk of acute leukemia.

Somatic/environmental mechanisms — largely modifiable

• Tobacco smoke: polycyclic aromatic hydrocarbons form bulky DNA adducts → KRAS activation and TP53 mutation; the single most preventable cause of cancer death, and the reason USPSTF recommends low-dose CT lung screening in eligible current/former smokers.
• Ultraviolet radiation: pyrimidine dimers requiring nucleotide excision repair → basal/squamous cell carcinoma and melanoma.
• Ionizing radiation: double-strand breaks and translocations (thyroid, leukemia, sarcoma).
• Oncogenic viruses: high-risk HPV E6 degrades p53 and E7 inactivates Rb (cervical, oropharyngeal); EBV (Burkitt, nasopharyngeal); HBV/HCV and chronic inflammation (hepatocellular); HTLV-1 (adult T-cell leukemia); HHV-8 (Kaposi sarcoma). HPV vaccination per ACIP and cervical screening per USPSTF are the modifiable levers.
• Chronic inflammation/irritation: reactive oxygen species and proliferative drive in Barrett esophagus, ulcerative colitis, chronic pancreatitis, H. pylori gastritis.
• Chemical carcinogens: aflatoxin B1 (TP53 codon 249), aniline dyes (bladder), vinyl chloride (angiosarcoma), asbestos (mesothelioma), benzene (leukemia), alcohol (acetaldehyde).
• Obesity, physical inactivity, and unopposed estrogen: adipokine/insulin–IGF-1 signaling and estrogen-driven proliferation raise endometrial, postmenopausal breast, and colorectal risk.
• Immunosuppression: transplant or HIV impairs immune surveillance, unmasking virally driven tumors.

Cell Cycle Phases and Checkpoints

• G1 Phase: Cell growth and preparation for DNA synthesis; G1/S checkpoint ensures DNA integrity and nutrient availability before replication begins
• S Phase: DNA replication; replication fork checkpoints detect stalled forks and DNA damage
• G2 Phase: Continued growth and preparation for mitosis; G2/M checkpoint (spindle checkpoint) prevents mitosis until DNA is properly replicated and damage is repaired
• M Phase: Mitosis and cytokinesis; spindle checkpoint ensures all chromosomes are properly attached to microtubules before anaphase

Cyclin-CDK Complexes Drive Cell Cycle Progression

• G1/S transition: Cyclin E-CDK2 and Cyclin A-CDK2 phosphorylate Rb protein, releasing E2F transcription factors that drive S phase gene expression
• G2/M transition: Cyclin B-CDK1 (also called MPF—maturation-promoting factor) triggers mitotic events including nuclear envelope breakdown, chromosome condensation, and spindle formation
• CDK inhibitors (p21, p27) bind and inactivate cyclin-CDK complexes; increased during cellular stress to halt progression

Tumor Suppressor Genes—"Brakes" on Cell Cycle

• RB protein (Rb): Hypophosphorylated Rb binds E2F, repressing S phase genes; loss of RB (common in retinoblastoma, lung, and breast cancers) causes uncontrolled entry into S phase
• p53 ("Guardian of the Genome"): Activated by DNA damage, hypoxia, or oncogene stress; induces p21 (CDK inhibitor) to arrest cell cycle OR activates pro-apoptotic genes (BAX, PUMA) if damage is irreparable; mutated in >50% of human cancers
• PTEN: Phosphatase antagonizing PI3K/AKT signaling; loss promotes survival signaling
• BRCA1/BRCA2: DNA repair proteins; germline mutations predispose to early-onset breast and ovarian cancers
• APC: Adenomatous polyposis coli; loss causes uncontrolled Wnt signaling in colorectal cancer; APC mutations are the initiating event in familial adenomatous polyposis

Proto-Oncogenes—"Accelerators" Promoting Growth

• RAS family: Activate MAPK/ERK signaling; ~30% of cancers harbor RAS mutations (particularly KRAS in pancreatic, colorectal, and lung cancers)
• MYC: Transcription factor driving proliferation and metabolic reprogramming; dysregulated in lymphomas (t(8;14) translocation), neuroblastoma, and breast cancer
• HER2 (ErbB2): Growth factor receptor; overexpressed in ~20% of breast cancers; targeted by trastuzumab (Herceptin)
• BCL-2: Anti-apoptotic protein; overexpressed in follicular lymphoma (t(14;18) translocation)

Multi-Hit Hypothesis

• Cancer typically requires 3-7 mutations in key genes (combination of oncogenic activation and tumor suppressor loss)
• Early lesions (e.g., adenoma in colorectal cancer) may progress through intermediate adenomas to carcinoma
• Clonal evolution with sequential mutations increases aggressiveness

Chromosomal Instability Pathways

• Mismatch repair (MMR) deficiency: Lynch syndrome (hereditary nonpolyposis colorectal cancer—HNPCC); microsatellite instability (MSI); misfidelity in repetitive sequences
• Base excision repair (BER) and nucleotide excision repair (NER) defects: Xeroderma pigmentosum increases UV-induced skin cancer risk
• Homologous recombination defects: BRCA1/2 mutations impair repair of double-strand breaks; synthetic lethality with PARP inhibitors

• Uncontrolled proliferation and tissue invasion: Enlarging mass, often painless initially; location-dependent symptoms (dysphagia with esophageal cancer, hematochezia with colorectal cancer)
• Constitutional B-symptoms (especially lymphomas): Fever, night sweats, unintentional weight loss reflecting increased metabolic demands and cytokine release
• Paraneoplastic syndromes: Hypercalcemia (PTHrP from squamous cell carcinoma), SIADH (small cell lung cancer causing hyponatremia), dermatomyositis (associated with internal malignancy)
• Symptoms from metastatic disease: Bone pain (osteolytic or osteoblastic lesions), neurological symptoms (brain mets), dyspnea (pulmonary mets), abdominal pain (hepatic mets)
• Genetic cancer syndrome presentations: Familial clustering (BRCA mutations—breast/ovarian at age <50), early-onset cancers, Lynch syndrome with colorectal and endometrial cancers by age 40-50, familial adenomatous polyposis with hundreds of polyps and colorectal cancer by age 30-40
• Complications of rapidly dividing cells: Tumor lysis syndrome (massive cell death releasing K+, phosphate, uric acid—especially in hematologic malignancies), secondary infections (immunosuppression from chemotherapy), bleeding (thrombocytopenia, DIC)

• Histopathology: Gold standard showing dysplastic/neoplastic cells with high nuclear-to-cytoplasmic ratio, abnormal mitotic figures, loss of differentiation; grading (Gleason for prostate, Fuhrman for renal cell carcinoma) and staging (TNM) guide prognosis
• Molecular testing for oncogenic mutations and alterations:
• PCR/sequencing for point mutations (e.g., BRAF V600E in melanoma, EGFR mutations in lung adenocarcinoma predicting tyrosine kinase inhibitor response)
• Fluorescence in situ hybridization (FISH) for gene amplifications (HER2 in breast cancer) and translocations (BCR-ABL in CML, t(8;14) in Burkitt lymphoma)
• Karyotyping for chromosomal abnormalities
• Microsatellite instability (MSI) testing for Lynch syndrome and immunotherapy eligibility
• Next-generation sequencing (NGS) for comprehensive mutation profiling
• Tumor markers:
• PSA (prostate cancer), α-fetoprotein/β-hCG (testicular cancer and hepatocellular carcinoma), CEA (colorectal cancer), CA-125 (ovarian cancer)—supportive but not diagnostic; used for surveillance
• Imaging: CT, MRI, PET-CT for staging and assessing metastatic disease
• Important diagnostic considerations:
• Age of onset is critical—early-onset breast cancer (<40 years) or colorectal cancer should raise suspicion for hereditary syndromes requiring genetic counseling
• Family history of specific cancers (breast/ovarian in premenopausal women suggests BRCA; colorectal and endometrial cancers in multiple relatives suggests Lynch syndrome)
• Some cancers are screened in asymptomatic populations (colorectal cancer, cervical cancer, breast cancer) to detect early-stage, more treatable disease

• Surgery: Gold standard for localized disease and early-stage tumors; achieves cure in many solid malignancies (breast cancer, colorectal cancer, etc.); palliative in advanced disease
• Chemotherapy:
• **Traditional cytotoxic

Oncologic emergencies (recognize and treat immediately)

• Tumor lysis syndrome: massive release of intracellular contents from rapidly cycling tumors (Burkitt lymphoma, ALL) → hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia with AKI from urate/calcium-phosphate deposition. NCCN and ASCO advise risk-stratified prophylaxis with aggressive IV hydration plus allopurinol (xanthine oxidase inhibitor) or rasburicase (recombinant urate oxidase) for high risk; peaked T waves or rising creatinine after cytoreduction is the signal.
• Febrile neutropenia: chemotherapy-induced marrow suppression with loss of the inflammatory response — fever may be the only finding. IDSA guidance is immediate blood cultures and empiric antipseudomonal beta-lactam monotherapy (cefepime or piperacillin-tazobactam) without waiting for a source.
• Malignant spinal cord compression: epidural metastasis → progressive back pain worse at night, then weakness and sphincter loss. Best next step is urgent whole-spine MRI plus corticosteroids (dexamethasone) before radiation or surgery.
• Hypercalcemia of malignancy: PTHrP or osteolysis → confusion, polyuria, short QT; treat with IV isotonic saline plus an antiresorptive (bisphosphonate such as zoledronic acid, or denosumab).
• SVC syndrome and immune checkpoint inhibitor myocarditis (troponin rise with new conduction disease; high-dose corticosteroids per ASCO immune-related adverse event guidance) are likewise emergencies.

Treatment-related complications

• Anthracycline cardiomyopathy: topoisomerase IIβ inhibition and iron-mediated reactive oxygen species cause dose-dependent, largely irreversible myocyte loss; falling LVEF on serial echo signals it. Resulting HFrEF is treated per ACC/AHA/HFSA heart failure guideline with all four pillars — ARNI (or ACEI/ARB), beta blocker, MRA, and SGLT2 inhibitor.
• Trastuzumab cardiotoxicity: HER2 blockade in cardiomyocytes; typically reversible and not dose-dependent, requiring LVEF surveillance.
• Therapy-related myeloid neoplasms: alkylators cause MDS/AML with chromosome 5q/7 loss after a latency of years; topoisomerase II inhibitors cause AML with 11q23 (KMT2A/MLL) rearrangement earlier.
• Organ-specific toxicities: bleomycin pulmonary fibrosis, cisplatin nephro- and ototoxicity, vincristine peripheral neuropathy, cyclophosphamide hemorrhagic cystitis (prevented with mesna), and tamoxifen-associated endometrial carcinoma presenting as postmenopausal bleeding.

• Rb phosphorylation state is the whole G1/S story: *hypo*phosphorylated Rb holds E2F hostage (brake ON); cyclin D-CDK4/6 → cyclin E-CDK2 phosphorylation releases E2F (brake OFF). A stem describing constitutive E2F activity wants Rb loss or CDK4/6 hyperactivity — the rationale for CDK4/6 inhibitors (palbociclib) in hormone receptor–positive breast cancer.
• p53 does not inhibit CDKs directly — it transactivates p21, which inhibits cyclin-CDK complexes, and BAX/PUMA for apoptosis. Choosing "p53 binds and inactivates cyclin-CDK" is the classic distractor.
• HPV is the two-hit virus: E6 degrades p53, E7 inactivates Rb. If a question pairs koilocytes with loss of two checkpoint proteins, this is it.
• Knudson two-hit: bilateral or multifocal, early-onset tumors (retinoblastoma, with osteosarcoma as the classic second malignancy) mean an inherited germline first hit; unilateral late tumors are sporadic.
• Translocation-to-tumor pairs examiners love: t(8;14) MYC → Burkitt (starry-sky); t(14;18) BCL-2 → follicular lymphoma; t(9;22) BCR-ABL → CML (imatinib); t(15;17) PML-RARA → APL, where ATRA is given and DIC/differentiation syndrome is the danger.
• Synthetic lethality: BRCA1/2-mutant tumors cannot repair double-strand breaks by homologous recombination, so PARP inhibition (olaparib) is lethal to the tumor but spared in normal cells.
• Single best next step for suspected Lynch syndrome: tumor MMR immunohistochemistry or MSI testing first, then confirmatory germline sequencing with genetic counseling — NCCN endorses universal MMR/MSI screening of colorectal and endometrial cancers. MSI-high/dMMR status also predicts response to PD-1 blockade (pembrolizumab).
• **Cyclin B-CDK1 (MPF) governs G2/M, not G1/S** — a frequent swap in answer choices. Remember that microtubule agents (taxanes stabilize, vinca alkaloids destabilize) act at M phase, while antimetabolites are S-phase specific.