Teodoro Lectures Flashcards

Question

Why do cells that do not have telomerase stop growing?

Answer 1

- Hayflick Limit – he first observed this in the 50s. - Reach senescence phase, the curve flattens out. Lead Hayflick to propose that human cells have built in mechanism where they have maximal number of cell divisions where they just stop dividing. - Human cells stop growing due to replicative senescence. *** tumour cells bypass rpelicative senescence, Cancer cells have infinite division potential within them, there is no limitation. Very different than most cells in body that have Hayflick Limit ***

Answer 2

- Most cells do not have telomerase activity, which means that they will experience the end replication problem as their DNA is replicated, it gets shorter during every round of replication. - As cells divide, see gradual decrease of telomere length through the end replication problem. Eventually reach certain length where they stop dividing (hit wall). Telomere gets short enough, the shelter complex is not there enough to protect the telomeres, and it is recognized for DNA damage response, triggering this response.

Answer 3

1) Telomere Shortening Triggers Damage Response: - Shortened telomeres activate damage-sensing proteins, ATM (recognizes double-stranded breaks) and ATR (recognizes single-stranded breaks). - ATM and ATR activate CHK1/CHK2 kinases and p53, which triggers the tumor-suppressor pathway. p53 turns on p21, a transcription factor that mediates senescence. 2) Rb Pathway and Cell Cycle Control: - Rb is activated by telomere shortening and regulates cell cycle arrest at the G1/S boundary. - Rb binds E2F, a transcription factor family that promotes S-phase entry by activating genes for DNA replication (e.g., DNA polymerase, nucleotide synthesis enzymes, telomerase). - When Rb binds E2F, it blocks S-phase entry, preventing cell division. Cancer Implications: - p53 and Rb are frequently mutated in cancers. - Mutations in upstream regulators (e.g., Ras) can inactivate Rb, disrupting its control over the cell cycle.

Answer 4

1) Function: p53 is a transcription factor and tumor suppressor protein, often referred to as a "universal cancer constant" because it is mutated in about half of all cancers. 2) Structure: p53 forms a tetramer (four copies) to effectively bind DNA. 3) Mutations: Most mutations are missense point mutations rather than complete deletions, making these mutations more oncogenic than simply losing p53. 4) Importance: Mice lacking p53 develop cancer rapidly, highlighting its critical role in tumor suppression. 5) Comparative Biology: Whales and elephants, which have many more cells than humans, possess multiple copies of p53, preventing widespread mutations and reducing cancer incidence. 6) Activation Triggers: p53 is activated by DNA breaks, telomere shortening (sheltin complex loss), UV light, cellular stress, and oncogenes, all of which promote senescence and help prevent cancer.

Answer 5

1) Overview: Loss of telomeres results in genomic instability through the Bridge-Fusion-Break (BFB) cycle, where the ends of chromosomes resemble DNA damage due to double-strand breaks. 2) DNA Repair Mechanism: When telomeres shorten, the cell's DNA repair mechanisms mistakenly try to fix the broken ends. This can lead to improper joining of sister chromosomes or fusion of different chromosomes, which is detrimental when they enter the cell cycle. 3) Centrosome Tug-of-War: When two chromosomes with centrosomes are present, they engage in a tug-of-war during cell division, causing random breaks in the genome. This leads to repeated cycles of breaking and fusion, perpetuating the BFB cycle until resolution or cell death occurs. 4) Consequences: BFB cycles contribute to aneuploidy and genomic instability, which are common in cancer cells. This instability is associated with: 5) Loss of tumor suppressor genes. Amplified growth pathways that accelerate cancer progression. Mutations: The most common mutation in this context is in the p53 gene. However, mutations in non-coding regions, particularly those affecting telomerase, occur more frequently than p53 mutations. 6) Implications: The end products of the BFB cycle can be particularly dangerous, leading to uncontrolled cell growth and cancer development.

Answer 6

1) Activation of Telomerase Expression 2) Alternative Lengthening of Telomere (ALT) pathway. -> uses homologous recombination; template is donated from long telomere to short telomere to lengthen it. -> TERRA is important for this whole process of the Alt pathway. TERRA is a scaffold and recruits the homologous recombinant machinery to the short telomere (first seen in cancer cells) - may also drive telomere maintenance in the immune system. Antigen presenting cells donate their telomeres to the T cells. Telomeric DNA is being transferred into the T cell, so it becomes longer in the T cell.

Answer 7

1) Telomerase Inhibitors - make active site inhibitors to block the enzyme - main drug: GRN36 2) TERT immunotherapy (getting immune system to target cancer cells) - take antigen presenting cells from a patient and programming them with peptides or mRNAs that code antigens to try and make the immune system mount an immune response to cells that present telomerase activity. - TERT peptides, mRNA and DNA vectors. 3) Telomere-disrupting agents - There are drugs that can stabilize these G form quadraplexes, when that happens telomerase does not extend, and the cell undergoes senescence. These are not in the clinic yet. These seem to work quite well in vitro. 4) Block Telomerase expression or biogenesis: - Dyskerin, part of telomerase holoenzyme complex - therapeutically inhibit telomerase.

Answer 8

- A knockout mouse has been made that completely lacks telomerase activity. - Phenotype of the Mouse… nothing! - Until after 6 generations of mating when the mice develop aging associated disorders: o Male and Female Infertility o Alopecia and Hair Graying o Heart failure o Immunosenescence. o Decreased regeneration of the digestive system, skin, and hematopoietic system. o Early death. o Less prone to cancer. - Cross section of Duodenum showing atrophy of villi.

Answer 9

- Common one is: Dyskeratosis congenita. (not a complete loss of telomerase activity, just a partial loss) o Skin pigmentation. o Nail dystrophy. o Mucosal Leukoplakia. o Bone Marrow Failure. o Pulmonary Disease. o Premature loss of teeth. o Premature hair loss/graying. o Early death. o Increased cancer. - Mutations in coding sequence for telomerase reverse transcriptase - Mutations in accessory proteins of telomerase o Dyskerin o NOP10 o NHP2 - Mutations will lead to disease. o DKC o Bone marrow failure syndromes o Pulmonary fibrosis

Answer 10

In Drosophila, telomeric retrotransposons, specifically HeT-A and TART, contribute to telomere maintenance by serving as a source of telomeric repeats. HeT-A is transcribed into RNA, which is then reverse transcribed into DNA and integrated into the chromosome ends, adding telomeric repeats that compensate for the DNA loss during replication. TART, on the other hand, can amplify HeT-A sequences, further reinforcing the telomeres. This retrotranspositional mechanism helps protect chromosome ends from degradation and prevents telomere shortening, maintaining chromosomal stability during cell division.

Answer 11

The T-loop, or telomere loop, protects chromosome ends by preventing degradation and masking them from being recognized as DNA breaks. It also regulates telomere length by influencing telomerase activity and prevents unwanted chromosome fusions, thus maintaining genomic stability.

Answer 12

- length of the band/fragment indicates the length of the telomere so it would be the longest band. - The intensity or darkness of the bands reflects the amount of DNA present. - generally, the longest telomere will be associated with the youngest age.

Answer 13

Dysfunctional telomeres activate two key proteins: ATM (Ataxia Telangiectasia Mutated) and ATR (ATM and Rad3-related). 1) ATM: This protein primarily responds to double-stranded DNA breaks and recruits the DNA damage repair machinery to address the damage caused by telomere dysfunction. 2) ATR: This protein is more specific to single-stranded DNA breaks and also helps in signaling the DNA damage response, but it functions similarly to ATM in activating downstream repair pathways.

Answer 14

The bridge-fusion-break (BFB) cycle occurs when dysfunctional telomeres cause chromosomes to fuse together during cell division, creating a chromosomal bridge. This tension can lead to breaks, resulting in random chromosomal fragments that may fuse again, perpetuating instability. This cycle contributes to aneuploidy and structural abnormalities in the karyotype, increasing the risk of cancerous transformations.