Cellular Biology and Cell Cycle
Understanding cellular biology and the cell cycle is fundamental to comprehending cancer biology, pharmacology of cytotoxic drugs, and mechanisms of disease at the molecular level.
Key Facts
The cell cycle consists of G1 (growth), S (DNA synthesis), G2 (preparation for mitosis), and M (mitosis) phases Cyclins and CDKs (cyclin-dependent kinases) regulate cell cycle progression — key targets for CDK4/6 inhibitors (palbociclib) The G1/S checkpoint (restriction point) is controlled by Rb protein and p53 — mutated in >50% of cancers p53 (TP53) is the 'guardian of the genome' — triggers cell cycle arrest, DNA repair, or apoptosis Apoptosis (programmed cell death) is mediated by caspases; dysregulation leads to cancer and autoimmune disease Telomeres shorten with each division; telomerase reactivation enables unlimited replication in cancer cells Mismatch repair (MMR) deficiency leads to microsatellite instability (MSI) — seen in Lynch syndrome and some sporadic cancers Cell cycle-specific drugs: Methotrexate (S phase), vincristine (M phase), bleomycin (G2/M)
Overview
Key Facts
The cell cycle is the ordered series of events leading to cell division. Dysregulation of cell cycle control is a hallmark of cancer, and understanding these mechanisms underpins modern oncological therapeutics.
Epidemiology
Cell cycle dysregulation is relevant to all cancers. In the UK, approximately 375,000 new cancer diagnoses occur annually, with aberrant cell cycle control contributing to tumourigenesis in virtually all cases.
Aetiology
Normal cell cycle regulation involves:
- Growth factor signalling: Mitogenic signals via RTKs (receptor tyrosine kinases) → RAS-MAPK and PI3K-AKT pathways
- Cyclin-CDK complexes: Sequential activation drives progression through cell cycle phases
- Tumour suppressors: Rb, p53, p21, p27 act as brakes
- Checkpoint controls: DNA damage checkpoints prevent propagation of errors
Pathophysiology
Hallmarks of cancer (Hanahan and Weinberg):
- Sustaining proliferative signalling
- Evading growth suppressors
- Resisting cell death (apoptosis)
- Enabling replicative immortality (telomerase)
- Inducing angiogenesis
- Activating invasion and metastasis
- Genome instability and mutation
- Tumour-promoting inflammation
- Deregulating cellular energetics
- Avoiding immune destruction
Clinical Presentation
Clinical Relevance
- Cancer: Uncontrolled cell proliferation due to oncogene activation and/or tumour suppressor loss
- Chemotherapy side effects: Cytotoxic drugs target rapidly dividing cells — hence toxicity to bone marrow, GI epithelium, hair follicles
- Genetic syndromes: Li-Fraumeni (TP53 mutation), retinoblastoma (Rb mutation), Lynch syndrome (MMR deficiency)
Red Flags Suggesting Malignancy
- Unexplained weight loss
- Persistent unexplained symptoms >3 weeks
- Unexplained lymphadenopathy
- New mass or lump
- Unexplained anaemia
- Night sweats
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| Benign neoplasm | Encapsulated, slow-growing, no invasion | Biopsy, imaging |
| Malignant neoplasm | Invasion, metastasis, rapid growth | Biopsy, staging CT, tumour markers |
| Hyperplasia | Increased cell number, normal cells | Biopsy (endometrial, prostatic) |
| Dysplasia | Abnormal cells, pre-malignant | Biopsy (cervical, Barrett's) |
| Metaplasia | Replacement of one cell type by another | Biopsy (Barrett's, squamous metaplasia) |
| Hypertrophy | Increased cell size (not number) | Clinical/imaging assessment |
Diagnosis / Investigation
Bedside
- Clinical examination for masses, lymphadenopathy
- Dermoscopy for skin lesions
Bloods
- Tumour markers: CEA, CA-125, AFP, PSA, HCG, LDH (limited diagnostic value, useful for monitoring)
- FBC: Cytopenia suggesting marrow involvement
- LDH: Elevated in high cell turnover (lymphoma)
Imaging
- CT/MRI: Staging and characterisation
- PET-CT: Metabolic activity assessment
Special Tests
- Histopathology: Gold standard for diagnosis — grade and type
- Immunohistochemistry: Cell markers (Ki-67 proliferation index, ER/PR, HER2)
- Cytogenetics/FISH: Chromosomal abnormalities (Philadelphia chromosome, HER2 amplification)
- Molecular profiling: NGS panels, MSI testing, BRCA status
- Flow cytometry: Leukaemia/lymphoma immunophenotyping
Management
Non-pharmacological
- Cancer screening programmes (cervical, breast, bowel, lung in selected populations)
- Genetic counselling for hereditary cancer syndromes
- Lifestyle modification (smoking cessation, diet, exercise)
Pharmacological
Cell cycle-targeted therapies:
- CDK4/6 inhibitors (palbociclib 125mg OD days 1-21 of 28-day cycle): G1 arrest in HR+/HER2- breast cancer
- Cell cycle-specific chemotherapy: Methotrexate (S), vincristine (M), etoposide (S/G2)
- Cell cycle-non-specific: Alkylating agents, anthracyclines (effective in all phases)
- p53-restoring therapies: Under investigation
- Telomerase inhibitors: Imetelstat — clinical trials ongoing
Referral Criteria
- Suspected cancer → 2-week-wait referral pathway
- Family history suggestive of hereditary syndrome → clinical genetics
- Abnormal screening result → specialist follow-up
Prognosis
- Ki-67 index >30% indicates high proliferation rate and generally worse prognosis
- TP53 mutations associated with poor prognosis in most cancer types
- MSI-high tumours have better prognosis and respond well to immune checkpoint inhibitors
- BRCA-mutated cancers respond to PARP inhibitors (olaparib, niraparib)
- Understanding cell cycle biology has enabled development of targeted therapies improving survival across multiple cancer types
Other Relevant Information
Cell Cycle Phase Summary
| Phase | Duration | Key Events | Drugs Active in Phase |
|---|---|---|---|
| G1 | Variable (hours-days) | Cell growth, RNA/protein synthesis | CDK4/6 inhibitors |
| S | 6-8 hours | DNA replication | Methotrexate, 5-FU, cytarabine |
| G2 | 2-4 hours | Preparation for mitosis, error checking | Bleomycin |
| M | 1-2 hours | Mitosis and cytokinesis | Vincristine, taxanes |
| G0 | Variable | Quiescent/resting state | Most drugs inactive |
Key Tumour Suppressor Genes
| Gene | Syndrome | Cancer Type |
|---|---|---|
| TP53 | Li-Fraumeni | Multiple (breast, sarcoma, brain, adrenal) |
| Rb | Retinoblastoma | Retinoblastoma, osteosarcoma |
| APC | Familial adenomatous polyposis | Colorectal |
| BRCA1/2 | Hereditary breast/ovarian cancer | Breast, ovarian, prostate, pancreatic |
| MLH1/MSH2 | Lynch syndrome | Colorectal, endometrial |