Epigenetic Reprogramming Flashcards
Why is epigenetic reprogramming necessary?
- If an individual is producing gametes of the opposite sex, the imprints must be reset to match the sex of the embryo.
**Example: If I inherit genes from my father and later have a child, those genes must acquire a maternal imprint since I am the mother.
How many times does epigenetic reprogramming occur and when?
Twice:
1. During gamete formation (germline reprogramming) → Ensures the correct epigenetic marks for sperm or egg.
- After fertilization (pre-implantation reprogramming) → Prepares the zygote for embryonic development.
What percentage of the genome undergoes epigenetic reprogramming?
96%
What must be erased before the next generation?
- Epigenetic marks, such as DNA methylation, must be erased before the next generation.
- Environmental factors (e.g., stress, diet) can influence these marks.
How do imprinted genes tackle epigenetic reprogramming?
retain their parent-of-origin methylation patterns in gametes and bypass epigenetic reprogramming after fertilization.
Outline the mammalian life cycle and when epigenetic programming occurs
- Primordial germ cells (PGCs) are specified early in embryonic development.
- PGCs migrate to the developing gonads.
- Meiosis occurs.
- Gametes differentiate into sperm or eggs.
- Fertilization of the oocyte by sperm creates a zygote.
- Zygote undergoes pre-implantation reprogramming.
- New PGCs form, and the cycle repeats.
Why does epigenetic reprogramming occur during gamete formation?
- Ensures imprinted genes are correctly reset for the sex of the embryo.
- Erases parental epigenetic memories.
- Facilitates gametogenesis.
- Maintains transposable element silencing.
- Reduces mutation rates in the germline.
Why does epigenetic reprogramming occur during pre-implantation development?
- Resets the zygotic epigenome for naïve pluripotency.
- There is evidence of evolutionary conflicts between maternal and paternal genomes.
What occurs during the gamete formation (germline) epigenetic resetting?
- PGCs undergo genome-wide epigenetic reprogramming to erase somatic epigenetic marks.
- DNA methylation is erased as PGCs migrate along the genital ridge.
- Gonadal germ cells enter meiosis and differentiate into gametes.
- The genome is remethylated in a sex-specific manner for fertilization.
What is a major source of confusion in the time-sequence of epigenetic reprogramming?
- Gametogenesis starts early in embryonic development (week 5-6 in humans), so it only erases the epigenetic marks formed during the first few weeks of pregnancy.
Sequence of events:
Zygote forms → Pre-implantation reprogramming occurs → Gamete formation begins → Individual grows to adulthood → Fertilization occurs → Cycle repeats.
How is epigenetic memory erased?
- Global DNA demethylation
- Genomic imprint erasure
- X-chromosome reactivation (females only)
- Chromosome restructuring (to prepare for meiosis and gametogenesis)
What are the sex specific differences between germ cells?
- XX germ cells enter meiosis.
- Enter meiosis during fetal development → This means they start dividing before birth.
- They pause at a specific stage (prophase I of meiosis) and remain there until ovulation later in life.
- XY germ cells enter mitotic arrest
- This means they stop dividing and remain in a paused state until after birth.
- Later in life (puberty), they will restart dividing and begin spermatogenesis (sperm production).
What is the “same process” for XY and XX embryos?
Both male (XY) and female (XX) PGCs undergo epigenetic reprogramming:
- Global DNA demethylation (to erase previous epigenetic marks).
- Reorganization of chromatin (preparing for gametogenesis).
- Imprint erasure (resetting parental epigenetic marks).
How do the DNA methylation levels fluctuate in germline compared to early development?
High (~70%) in germline.
Low (~4%) in early development due to demethylation enzymes.
How is DNA methylation % measured?
If all of the Cs in a CpG have a methyl group then it is 100% methylated
If none of the Cs in a CpG have a methyl group then it is 0% methylated
When does remethylation occur in males and females?
- Males → Before birth in spermatogonial stem cells.
- Females → After birth in growing oocytes.
**Mature oocytes return to ~70% DNA methylation.
What is the methylation pattern insight from mice studies?
- PGCs start with ~70% methylation.
- Genome-wide demethylation occurs during PGC migration.
- Sex-specific remethylation occurs:
Males → Starts at embryonic day 13.5 (E13.5).
Females → Occurs after birth.
What are the exceptions to global demethylation?
- 4% of the genome remains methylated, including retrotransposable elements.
- Hypothesis: Incomplete repression of DNA methylation targets these regions.
What is the key thing to remember with imprinted genes and methylation?
Imprinted genes do not lose their methylation during pre-implantation reprogramming.
What evidence was there that the IGf2r gene follows the general rule of demethylation?
- Southern blot analysis with restriction enzymes showed that male embryonic stem cells have unmethylated copies of IGF2R.
- The IGF2R locus is not an imprinted gene in mice (though it is imprinted in some other species, including humans).
- Imprinted genes (such as IGF2 itself, but not IGF2R in mice) do escape the pre-implantation demethylation process and retain their methylation.
What are the differences between demethylation in the maternal and paternal genome in pre-implantation reprogramming?
-Paternal genome undergoes active demethylation.
- Maternal genome undergoes passive demethylation during DNA replication.
*resetting the genome for embryonic development
What is thought to be responsible for imprinting memory?
Non-coding RNAs
