DNA Replication, Telomere Dynamics, and the Biology of Aging

Upcoming Laboratory and Course Activities

• Lab Gels and Activities: The session begins with the conclusion of running gels in the lab. Students are expected to complete several activities during the laboratory period.
• Creative Learning Exercise: Tomorrow, the instructor will facilitate a creative session where students utilize construction paper, markers, and colored pencils. The objective is for students to draw or create (e.g., origami) a representation of what they have learned so far, focusing on creativity and artistry rather than simple regurgitation of facts.
• Current Session: The class will use a "cookie" activity during the current day's lecture.

Overview of DNA Replication and Genomic Integrity

• DNA as a Template: DNA is critical as it holds the template for the genetic code.
• The Replication Process: Before replication occurs, the parent strand must unwind and be properly prepared. This process involves specific enzymes to ensure the integrity of the genetic copy.
• Impact of Disruptions: Disruptions or mutations in the enzymes responsible for unwinding DNA (specifically helicases) lead to premature aging diseases.
  • Examples: Werner syndrome and Progeria are caused by helicase mutations.
• The Lagging Strand Problem: During every round of replication, a portion of the lagging strand shortens. If the machinery protecting the ends of chromosomes fails, it results in a condition described as a telomeropathy.
• Replication Fork Components:
  • Leading Strand: Synthesized continuously.
  • Lagging Strand: Synthesized discontinuously.
  • Primers: DNA polymerase requires primers to bind and start copying. Tomorrow's PCR (Polymerase Chain Reaction) activity will focus on priming the ends of a gene or sequence of interest.
  • RNA Polymerase: Responsible for creating the actual RNA primer.
  • DNA Polymerase: Extends the matching nucleotides on the template strand.

Specific Enzymes of DNA Replication and Repair

• Enzymatic Diversity: There are many types ("flavors") of enzymes involved in nucleotide catalysis.
• DNA Polymerases in Prokaryotes/Humans:
  • DNA Polymerase I: Specifically used on the lagging strand.
  • DNA Polymerase II: Specifically dedicated to DNA repair. It stops the process to fix mutations or aberrations.
  • DNA Polymerase III: The main, most common enzyme used for the main strand synthesis.
• Nucleases: These enzymes remove nucleotides to facilitate repair.
  • Endonucleases: Cleave or remove nucleotides from internal positions within the DNA sequence.
  • Exonucleases: Remove nucleotides from either end ($exo$ meaning outside) of the DNA.
• DNA Ligase: Closes the gaps or nicks in the DNA, particularly on the lagging strand, to ensure a continuous strand.
• Unwinding Enzymes:
  • DNA Helicase: Breaks the hydrogen bonds between parent strands to "unzip" the DNA. These are weak bonds, allowing for separation.
  • Topoisomerase (DNA Gyrase): Relieves "supercoiling" or tension in the DNA as it unwinds. The instructor compares supercoiling to the tangled, curly cord of an old landline telephone.
  • Single-Stranded DNA Binding Proteins (SSBs): These proteins keep the separated strands apart and prevent them from immediately re-forming hydrogen bonds (winding back up).

The RecQ Helicase Family and Progeroid Syndromes

• RecQ Helicases: A specific family of helicases that utilize $ATP$ for energy to split DNA strands.
• Isoforms: There are five specific isoforms found in humans ($Homo\,sapiens$), compared to simpler organisms like yeast or $E.\,coli$:
  1. $RecQ1$
  2. $BLM$
  3. $WRN$ (the Werner gene)
  4. $RecQ4$
  5. $RecQ5b$
• Disease-Linked Mutations: Mutations in three specific isoforms cause clinical syndromes:
  • $WRN$ (Werner Syndrome): Mimics the normal aging process but occurs prematurely. It is categorized as a segmental progeroid syndrome. It is an autosomal recessive disorder located on chromosome 8. It results in a loss of function of the DNA helicase.
  • $BLM$ (Bloom Syndrome).
  • $RecQ4$ (Rothmund-Thomson Syndrome).
• Protein Domains: All isoforms share a common helicase domain, but they differ in auxiliary domains (e.g., $WRN$ has an exonuclease domain which others lack), which dictates their specific independent functions.
• Case Study Context: A 48-year-old woman with Werner syndrome can present a phenotype appearing to be in her 70s due to accelerated cellular aging.
• Hutchinson-Guilford Progeria Syndrome: A childhood disorder where children die from aging-related diseases typically not seen in youth.

Telomeres and the "End-Replication" Problem

• Lagging Strand Shortening: Every time a cell divides ($40$ to $60$ doublets), the lagging strand shortens. When the final RNA primer is removed from the end of the lagging strand, there is no upstream $3'$ end for DNA polymerase to extend from, leaving a gap.
• Telomeric Ends: The repetitive sequences at the ends of chromosomes. The common sequence mentioned is often highly conserved.
• The Hayflick Limit: Cells have a finite doubling capacity ($40$ to $60$ times). Once reached, the cell faces three potential fates:
  1. Senescence: The cell stops dividing but remains metabolically active.
  2. Apoptosis: Programmed cell death.
  3. Immortalization: The cell continues to divide indefinitely (characteristic of cancer).
• Immortalization and Tumor Suppressors: For a cell to become immortalized, it usually must inhibit tumor suppressor genes like $p53$ and Retinoblastoma ($Rb$), which normally arrest the cell cycle at the $G_1$ phase.

Telomerase and the Shelterin Complex

• Telomerase (TERT): An enzyme (telomerase reverse transcriptase) that adds nucleotides to the ends of chromosomes to repair shortening.
  • Structure: It is shaped like a "hand" that cups the chromosome end.
  • Mechanism: It uses its own internal RNA template to synthesize DNA repeats.
• The Shelterin Complex: A complex of six different proteins that protects the telomeric ends.
  • Function: It prevents enzymes from "chewing up" the ends of the gene and prevents double-stranded breaks.
  • Regulation: It regulates telomerase activity and prevents aneuploidy (an abnormal number of chromosomes, departing from the standard $46$).
• Alternative Lengthening of Telomeres (ALT): Approximately $10\%$ of cancers use this alternative pathway. Instead of using telomerase, they use homologous recombination to extend chromosome ends. This pathway is also typically expressed in stem cells for self-renewal.

Clinical and Biological Implications of Telomeres

• High Telomerase Expression: Found in cells that are highly replicative, such as:
  • Germ cells (ovaries and testes).
  • Stem cells.
  • Digestive system (gut) cells.
  • Cancer cells ($95\%$ of tumors overexpress telomerase to achieve immortalization).
• Oncogenes and Hormones: Certain factors can induce telomerase expression:
  • $c-MYC$: A known oncogene.
  • Estrogen and Testosterone: Hormones that promote division in reproductive tissues.
• Aplastic Anemia: A condition where patients have telomeres that are $4$ to $5$ times shorter than normal, often serving as a precursor to leukemia.
• Mouse Models: Mice are useful but have telomeres $5$ to $10$ times longer than humans despite a shorter lifespan. Mice lacking telomerase can actually remain healthy for several generations, making it difficult to directly equate mouse telomere biology to human disease.

Advanced Telomereopathies and DNA Repair

• Primary Telomereopathies: Caused by mutations in the core telomere maintenance machinery or the shelterin complex (e.g., $DKC1$ gene mutations).
  • Dyskeratosis Congenita (DC/DKC): Presents with leukoplakia, nail dystrophy, hyperpigmentation, aplastic anemia, and cirrhosis of the liver.
• Secondary Telomereopathies: Mutations in DNA repair proteins that affect telomere stability.
  • Ataxia Telangiectasia (AT): An autosomal recessive neurodegenerative disorder caused by mutations in the $ATM$ (Ataxia Telangiectasia Mutated) kinase.
  • ATM/ATR Kinase Function: These kinases detect DNA damage and signal the DNA Damage Response ($DDR$). They upregulate $p53$, which induces $p21$ (a $CKI$) to inhibit cyclin/$CDK$ complexes and arrest the cell cycle in $G_1$. Over $400$ unique mutations exist in the $ATM$ gene.

The Role of NAD+ in DNA Repair and Aging

• NAD+ (Nicotinamide Adenine Dinucleotide): A critical molecule for redox reactions (oxidation and reduction). It declines significantly with age.
• PARP and DBC1:
  • $PARP$ (Polyadenosine diphosphate ribose polymerase): An essential enzyme for DNA repair.
  • $DBC1$ (Deleted in Breast Cancer 1): Normally inhibits $PARP$.
  • $NAD^{+}$ Role: $NAD^{+}$ binds to the domain of $DBC1$, preventing it from inhibiting $PARP$. Therefore, high $NAD^{+}$ levels allow $PARP$ to remain active and repair DNA.
• NAD+ Boosters: Companies like Elysium produce $NAD^{+}$ precursors. Studies in mice show that replenishing $NAD^{+}$ can improve cognition (linked to Alzheimer's research) and longevity by overturning $DBC1$ inhibition of $PARP$.

Questions & Discussion

• Student Question on School Size: Discussion regarding the size of high schools in Texas (graduating classes of $3,000$ to $5,000$) compared to Miami (private school class of $60$). Mention of a Texas football stadium costing up to $500,000,000$.
• Discussion on TeloYears: A DNA test company (founded by a Nobel laureate) that measures cellular age via a blood drop for $89$.
  • Student/Instructor Critique: The test is not FDA-approved and doesn't provide a full genomic picture. Many people question the value of knowing their "internal age" if they cannot change the outcome.
• Question on Heredity: A student asked if mothers who have children later in life pass on shorter telomeres. The instructor noted it is a "conundrum" and depends on various factors, highlighting the difficulty of studying these associations in humans.
• Question on Progenitor Cells: The instructor defined endothelial progenitor cells as "supercells" trapped between a stem cell and a mature cell state, capable of traveling to tissues like the lung or heart to differentiate and repair damage. Depletion of these cells leads to declined tissue function and aging diseases.