Mutation, DNA Repair, and Cancer

Chapter 15: Mutation, DNA Repair, and Cancer

Key Concepts

• Consequences of Mutations

• Causes of Mutations

• DNA Repair

• Cancer

Definition and Importance of Mutation

• Mutation: A heritable change in the genetic material essential for the continuity of life, providing a source of variation for natural selection.

  • New mutations tend to be more harmful than beneficial.

  • DNA repair systems can reverse DNA damage.

  • Cancer is fundamentally linked to gene mutations.

Effects of Mutations

• Most knowledge regarding mutations comes from experimental studies on various organisms.

• Mutations can alter the structure and amount of genetic material through various means including:

  • Changes in the structure and number of chromosomes.

  • Gene mutations are relatively minor shifts in the base sequence of specific genes.

Point Mutation Examples

• Base Substitution:

  • Example:

  • Original DNA Sequence: 5′ – CCCGCTA GATA – 3′

  • Mutated Sequence: 5′ – CCCGCGA GATA – 3′

• Addition or Deletion of a Single Base Pair:

  • Example:

  • Original Sequence: 5′ – GGC GCT AGA TC – 3′

  • Mutated Sequence: 5′ – GGC AGC T AGA TC – 3′

Consequences of Point Mutations

Types of Gene Mutations

• Silent Mutation:

  • Does not change the amino acid sequence due to the degeneracy of the genetic code.

• Missense Mutation:

  • Alters one amino acid in a polypeptide which might not affect function if the new amino acid is chemically similar.

  • Some can significantly impact protein functionality.

• Nonsense Mutation:

  • Converts a codon from normal to stop, resulting in a truncated peptide chain.

• Frameshift Mutation:

  • Addition/deletion of nucleotides (not in multiples of three) alters the downstream amino acid sequence.

Natural Selection and Mutations

• Natural selection impacts the prevalence of traits through generations.

  • Neutral mutations are not influenced by natural selection.

  • Beneficial or detrimental mutations may be acted upon by natural selection.

  • New mutations usually lead to less effective polypeptides.

Mutations and Human Disease

• Thousands of genetic diseases exist, often linked to mutations in single genes.

  • Sickle Cell Disease: A missense mutation in the HBB gene alters glutamic acid to valine.

  • Result: Hemoglobin units aggregate, altering red blood cell shape.

Examples of Genetic Diseases

Mutations Outside Coding Sequences

• Mutations can impact promoter sequences affecting transcription rates positively or negatively.

• Regulatory mutations in operator sites may prevent gene expression by inhibiting repressor binding.

Effects of Non-Coding Sequence Mutations

Germ-line vs. Somatic Mutations

• Germ-line Cells: Mutations in these cells (sperm/egg) can be transmitted to offspring.

• Somatic Cells: Non-heritable mutations; can cause genetic mosaics with different cells having varied genetic makeups.

Studying Mutations: The Lederbergs' Experiment

• Investigated the randomness of mutations using replica plating on bacteria.

  • Observed that mutations occurred prior to exposure to selective environments, suggesting randomness in mutation events.

Types of Mutations

• Spontaneous Mutations: Arising from natural biological processes, with an approximate background rate of 1 mutation per million genes per cell division.

• Induced Mutations: Resulting from environmental factors with higher rates than spontaneous mutations.

  • Mutagens: Environmental agents that alter DNA, causing mutations.

Common Causes of Gene Mutations

Examples of Mutagens

Examining Mutation Rate Using Ames Test

• Utilizes Salmonella typhimurium to test mutagenicity by determining the rate of secondary mutations that allow bacteria to synthesize histidine without its addition to media.

• Compares colony growth numbers between control and suspected mutagens to estimate mutation rates.

DNA Repair Mechanisms

• All organisms require repair systems to minimize mutations through:

  • Detection of DNA damage

  • Repair of damage

Types of DNA Repair Systems

• Direct Repair: A repair enzyme directly converts DNA back to correct structure.

• Nucleotide Excision Repair: Removes stretched segments of damaged DNA for resynthesis using complementary strand as a template.

• Methyl-Directed Mismatch Repair: Recognizes base pair mismatches, excising around the defect for repair.

Key Repair Mechanisms

Nucleotide Excision Repair (NER)

• Effectively corrects UV-induced damage and is present in all eukaryotes and prokaryotes.

• In E. coli, NER is facilitated by proteins (UvrA, UvrB, UvrC, UvrD) in addition to DNA polymerase and ligase for completion.

NER in Human Disease

• Genetic disorders like Xeroderma pigmentosum and Cockayne’s syndrome result from defective NER leading to photosensitivity due to UV-induced lesion repair failure.

Overview of Cancer

• Characterized by uncontrolled cell division, with significant prevalence in America: 1.5 million diagnoses annually and over 0.5 million fatalities.

  • Heritability: 10% of cancers show higher predispositions due to inherited traits; the remaining 90% involve non-heritable alterations.

Carcinogens

• Approximately 80% of cancers relate to carcinogens, agents promoting malignancy through genetic mutations affecting cell division.

Cancer Progression

• Cancers arise from a single mutated cell whose descendants continually mutate, resulting in tumors that can become benign (non-threatening) or malignant (cancerous).

  • Tumors: An mass of atypical cells that may progress to invade surrounding tissues (metastasis), ultimately threatening organism survival if untreated.

Cancer Promoting Genes

• Oncogenes: Mutated forms of genes that enhance uncontrolled cell growth.

  • Mutations can lead to increased gene activity.

• Tumor Suppressor Genes: Proteins encoded by these genes prevent cancer; mutations can result in loss of function, elevating cancer risk.

Oncogenes

• Critical role in cellular signaling governed by hormones known as growth factors influencing cell division and regulated by proteins like receptor kinases.

Examples of Oncogenes

Proto-oncogenes and Genetic Changes

• Proto-oncogene: Normal gene that can turn into an oncogene through mutations.

  • Common genetic alterations: Missense mutations, gene amplifications, chromosomal translocations, and retroviral insertions.

Mechanisms of Cancer Induction

• Missense Mutations: Change one amino acid; linked to malignancy through changes in signaling proteins.

• Gene Amplifications: Results in overproduction of proteins significant in cancer.

• Chromosomal Translocations: Results in abnormal fusion of genes leading to oncogene formation (e.g. bcr-abl in chronic myelogenous leukemia).

• Retroviral Insertions: Viral DNA integrations can lead to overexpression of proto-oncogenes, promoting cancer.

Viruses Associated with Cancer

Tumor-Suppressor Genes

• Functions include maintaining genome integrity, activating DNA repair mechanisms, and inhibiting cell growth.

Example Tumor-Suppressor Genes

Checkpoint Proteins

• Cyclins and cyclin-dependent kinases (cdks) control cell cycle progression.

• Checkpoint activators like p53 prevent further cell cycle progression under DNA damage conditions.

p53 Protein Role

• A checkpoint protein that halts progression from G1 to S phase upon DNA damage.

  • If damage is irreparable, p53 facilitates apoptosis through the activation of caspases, which digest specific proteins to enable cellular breakdown.

Negative Regulators of Cell Division

• Tumor-suppressor genes, including the Rb gene, inhibit cell division by regulating transcription factors involved in the cell cycle.

Rb Protein Function

• The Rb protein inhibits transcription factor E2F, repressing genes necessary for progression from G1 to S phase; if defective, E2F remains active perpetuating uncontrolled division.

Sequence of Cancer Development

• Cancer is typically not the result of a single mutation; it requires progressive genetic changes culminating in malignancy.

• Continued mutations enhance cancer's complexity making treatment progressively more challenging.

Metastasis

• The process by which cancer cells spread from the primary tumor to distant anatomical sites, often leading to patient mortality within a year if left untreated.