Cycle 8: Control of Gene Expression

• Types of stem cells


  Embryonic, adult/ somatic, iPSC, umbilical cord stem cells
• What do stem cells express?

  Telomerase
• Totipotent


  Can form an entire viable organism (the zygote is the only truly totipotent cell)
• Pluripotent


  Can form nearly all cells
• Multipotent


  Can differentiate into a family of cells
• Unipotent


  Can only form one cell type (epidermal stem cell)
• Difference between a somatic cell and a unipotent stem cell


  Unipotent cells don't reach cell senescence, they self-renew. Somatic cells eventually reach their Hayflick limit and must enter irreversible cell cycle arrest
• All stem cells have the exact same

  Genome (excluding spontaneous mutations)
• Spatial and temporal regulation of gene expression determine

  What cell type a stem cell can give rise to
• Spatial regulation


  Where (in what cell/ part of the body)
• Temporal regulation


  When (at what time/ under what conditions)
• Steps of stem cell division


  Hibernating stem cell (in the niche), symmetric self-renewal, asymmetric self-renewal, symmetric differentiation
• Symmetric self-renewal and symmetric differentiation are coupled to

  Maintain count
• Progenitor cells are committed to


  Differentiation
• 2 types of progenitor cells

  Multipotent progenitor, committed progenitor
• Role of stem cells in the body

  Regenerate adult tissues (ex: epidermal stem cells, intestinal villi stem cells)
• Transcription factors are key to

  Determining differentiation (Wnt, EGF, Notch. Stem cells require the factors to enter G1)
• Induced Pluripotent Stem Cells (iPSCs) uses

  Macular degeneration (risk of cancer), burn victims (iPSC skin not fully identical)
• Lac operon

  Form of transcriptional regulation in E. coli. Only want to express Lac operon when lactose is in the environment. Otherwise, glucose is preferentially metabolized
• LacZ function

  Digests lactose into galactose and glucose
• LacY function

  Transports lactose into cell
• When lactose is present


  Allolactose isomer binds to Lac repressor and inactivates it. RNA polymerase binds to promoter, transcription is initiated
• Spliceosomes are


  Complexes made of snRNPs (protein + RNA) and the pre-mRNA
• Pre-mRNA is cut at very specific places


  Called 5' and 3' splice site recognition sequences (3' splice site is NCAG), intron is removed and exons are joined together
• Normal mRNA splicing


  snRNPs that recognize the 5' splice site look downstream for the next 3' splice site that has a snRNP. snRNPs that recognize the 3' splice site look upstream for the closest 5' splice site that has a snRNP.
• Alternative splicing is normal too

  A snRNP at a 5’ splice site never looks for a second snRNP at the next 5’ splice site. It only cares about the next 3’ splice site. May produce a functional protein
• Aberrant mRNA splicing


  For example a 3' is mutated beyond recognition. This may produces a non-functional protein, if exon 3 is important to function
• B-thalassemia (when splicing goes wrong)


  Blood disorder caused by abnormal hemoglobin, 2 key points (G-A mutation in intron creates a new 3' splice site, expression of an early stop codon causes premature termination of translation
• What drug is used for treatment of sickle cell anemia and b-thalassemia?

  CTX001 drug
• Fetal hemoglobin has a higher affinity for oxygen than


  Adult hemoglobin (gene is present in adults, just turned off)
• Drop in fetal HbF is due to

  BCL11A (a transcription factor)
• Steps of treatment for sickle cell anemia and b-thalassemia

  Hematopoietic stem cells and progenitor cells from healthy donors (don't express HbF - they are from adults). Edit using CRISPR -Cas9 (mutate the erythroid enhancer region so that BCL11A is not produced, production of HbF). Grow and put in patient bone marrow. Patient stem cells can send out progenitors, differentiate into RBC with HbF. Stem cells self renew (always have supply of stem cells with HbF)