A researcher is investigating compounds that modulate the cell cycle as possible chemotherapeutic agents against peripheral T-cell lymphoma. The researcher discovers a group of natural compounds with inhibitory activity against histone deacetylases, a class of enzymes that remove acetyl groups from the lysine residues of histones. A histone deacetylase inhibitor most likely causes which of the following?

Prevention of DNA strand reannealing

Increased heterochromatin formation

Suppression of gene transcription

A 62-year-old man seeks evaluation at a local walk-in clinic for mid-low back pain of several weeks. He has tried different rehabilitation therapies and medications with no improvement. He was prescribed some pain medications and sent home last week, but the patient presents today with difficulty walking and worsening of his back pain. He was referred to the ER, where he was examined and found to have hypoesthesia from T12 to S4–S5, significant muscle weakness in both lower limbs, and reduced knee and ankle deep tendon reflexes. A hypotonic anal sphincter with conserved deep anal pressure was demonstrated on digital rectal examination, as well as a multinodular, asymmetric prostate. Imaging studies showed multiple sclerotic bone lesions along the spine. Subsequently, a prostate core biopsy was obtained which confirmed the diagnosis of prostate cancer. Which of the following characteristics would you expect in the specimen?

Well-formed glands with an increase in interglandular stroma

Prostatic intraepithelial neoplasia

Small, closely-packed, well-formed glands

A 74-year-old retired female teacher is referred to the endocrinology clinic. She is very concerned about a large mass in her neck that has progressively enlarged over the past 2 weeks. She also reports a 15 pound weight loss over the last 3 months. She now has hoarseness and difficulty swallowing her food, giving her a sensation that food gets stuck in her windpipe when she swallows. There is no pain associated with swallowing. Her speech is monotonous. No other gait or language articulation problems are noted. Testing for cranial nerve lesions is unremarkable. On palpation, a large, fixed and non-tender mass in the thyroid is noted. Cervical lymph nodes are palpable bilaterally. The patient is urgently scheduled for an ultrasound-guided fine needle aspiration to guide management. Which of the following is the most likely gene mutation to be found in this mass?

Inactivating mutation of the p53 tumor suppressor gene

Activating mutation of the Ras protooncogene

Tumor suppressor genes — USMLE Lesson

Tumor Suppressor Genes - The Cellular Brakes

Function as cellular "brakes" to halt uncontrolled cell proliferation. Require a "two-hit" loss of function to trigger malignancy (Knudson's hypothesis).

p53 ("Guardian of the Genome"): Monitors DNA damage; initiates apoptosis. Mutated in >50% of human cancers.
Rb (Retinoblastoma): Controls the G1/S checkpoint.

⭐ Li-Fraumeni syndrome results from a germline p53 mutation, predisposing individuals to various cancers at a young age.

Two-hit hypothesis: hereditary vs. non-hereditary cancer

p53 - Guardian's Gambit

Function: "Guardian of the Genome"; activated by hypoxia, DNA damage, or oncogene expression.
Mechanism: Halts cell cycle at G1/S checkpoint via p21 induction to allow for DNA repair. If repair fails, it triggers apoptosis by upregulating BAX.
Regulation: Tightly controlled by MDM2, a ubiquitin ligase that promotes p53 degradation.

⭐ Inactivation of p53 allows cells with damaged DNA to proliferate, a hallmark of most human cancers. Li-Fraumeni syndrome results from a germline mutation in one p53 allele.

p53 pathway: wild-type vs. mutant effects

Rb & Friends - The Cycle Sentinels

Rb (Retinoblastoma protein): The "Governor" of the cell cycle; master regulator of the G1 → S checkpoint.
- Active (hypophosphorylated): Binds and inhibits E2F transcription factor, halting the cell cycle.
- Inactive (hyperphosphorylated): Releases E2F, allowing progression to S phase.
Regulation Pathway:
- Growth factors → ↑ Cyclin D/CDK4 → Hyperphosphorylates Rb.
- p16 (a CDK inhibitor) blocks the Cyclin/CDK complex, keeping Rb active.
Knudson's Two-Hit Hypothesis:
- Hereditary: Germline mutation (1st hit) + somatic (2nd hit) → Bilateral retinoblastoma, osteosarcoma.
- Sporadic: Two somatic hits → Unilateral retinoblastoma.

⭐ Patients with familial retinoblastoma (one germline Rb mutation) have a greatly increased risk of developing osteosarcoma later in life.

More Suppressors - The Watchful Guardians

Gene	Function / Pathway	Key Associated Syndromes & Cancers
APC	Negative regulator of β-catenin/WNT pathway.	FAP (Familial Adenomatous Polyposis) → near 100% risk of colon cancer. 📌 Adenomatous Polyposis Coli.
BRCA1/BRCA2	DNA repair via homologous recombination.	Hereditary Breast & Ovarian Cancer (HBOC). ↑ risk for prostate & pancreatic cancer. BRCA1 often linked to triple-negative breast cancer.
WT1	Transcription factor for renal & gonadal differentiation.	Wilms Tumor (nephroblastoma). Part of WAGR Syndrome (Wilms, Aniridia, Genitourinary anomalies, Retardation).
VHL	Ubiquitin ligase for HIF-1α (hypoxia-inducible factor).	von Hippel-Lindau disease. ↑ risk for clear cell Renal Cell Carcinoma, pheochromocytoma, hemangioblastomas.

Tumor suppressor genes are the "brakes" of cell growth, requiring a "two-hit" loss of function (Knudson hypothesis).

p53, the "guardian of the genome," is the most frequently mutated gene, halting proliferation of DNA-damaged cells.

Rb controls the G1/S checkpoint; inactivation is linked to retinoblastoma and osteosarcoma.

Inherited germline mutations (first hit) in APC or BRCA1/2 predispose to hereditary cancer syndromes.

Loss of heterozygosity (LOH) is the classic mechanism for the required second hit.

Unlock the full lesson and continue reading

Signup to continue reading this lesson and unlimited access questions, flashcards, AI notes, and more

Scan to download app

UNLOCK FREE ACCESS