Three parent babies - Week 6

Question 1

Three parent babies

Answer 1

controversial - patient to model transfer

Question 2

Mitochondria

Answer 2

17,000 bases compared to nuclear genome 3 billion bases

Question 3

Lots of concerns in certain

Answer 3

groups about this technology

Question 4

Different techniques for

Answer 4

transferring mitochondria

Question 5

Lots of different additions and variants that cause disease in mitochondria so difficult to predict how many diseased mitochondria will end up in the egg
• Options available?

Answer 5

o Take donor eggs with typed mitochondria and then use those to form the artificial genetic material that way the baby is genetically related to the parents but has the mitochondria from a donor.

Question 6

Maternal Spindle Transfer MST -

Answer 6

1. Donor egg taken from woman with healthy mitochondria
2. The spindle of chromosomes is removed from the donor egg and discarded
3. The spindle of chromosomes is removed from the intending mother’s egg and transferred to the ‘enucleated’ donor egg; the intending mother’s egg is discarded.
4. The reconstructed oocyte contains the intending mother’s nuclear DNA and donor’s mitochondrial DNA.
5. The egg is then fertilized with the intending father’s sperm.
6. The embryo is cultured in vitro and develops in vitro and is then transferred to the womb of the woman who will carry the child.
   All of the techniques are based on modified IVF

Question 7

Meternal Spindle Transfer Potential risks

Answer 7

• Level of risk involved which each of the techniques – much controversy
• Around 1% of diseased mitochondria can be carried across with this technique

Question 8

MST mtDNA carryover:

Answer 8

PBT < MST < PNT (estimated <1%)

Question 9

MST - Technicality of procedure:

Answer 9

• Spindle-chromosome complex sensitive to manipulation; higher risk of chromosomal abnormalities than in PNT
• Visualization of spindle
• Operator dependent

Question 10

MST - Reagents

Answer 10

treatment of oocytes with cytoskeletal inhibitors for karyoplast removal; Sendai virus for fusion

Question 11

MST Ethical considerations

Answer 11

Manipulation and destruction of oocytes

nb: Embryos deemed not suitable for transfer may be discarded.

Question 12

Pronuclear Transfer PNT

Answer 12

1. The intending mother’s egg is fertilized by the intending father’s sperm.
2. The donor egg is also fertilized by the intending father’s sperm.
3. The pronuclei are removed from the single-celled zygote of the donor egg and discarded.
4. The pronuclei are removed from the intending mother’s fertilized egg and transferred to the enucleated fertilized donor egg. The enucleated fertilized egg of the intending mother is discarded.
5. The reconstructed embryo contains pronuclear DNA from the intending parents and healthy mitochondria from the donor.
6. The embryo develops in vitro and is transferred to the womb of the woman who will carry the child.

Question 13

Pronculear TRansfer potential risks

Answer 13

• mtDNA carryover: PBT < MST < PNT (estimated <2%)

Question 14

PNT Technicality of procedure:

Answer 14

Easier visualization than MST (pronuclei enclosed in karyoplast)
• Need to ensure inclusion of centrioles and other spindle assembly components
• Operator dependent

Question 15

PNT Reagents:

Answer 15

treatment of zygotes with cytoskeletal inhibitors for karyoplast removal; Sendai virus for fusion

Question 16

PNT Ethical considerations

Answer 16

Manipulation and destruction of fertilized eggs

Question 17

Oogenesis and Formation of Polar Bodies

Answer 17

§ The primordial germ cell (oogonium) undergoes mitosis in the fetus; at birth, the primary oocyte arrests in prophase of meiosis I (prophase I).
§ Beginning at puberty, once per month, a primary oocyte completes meiosis I and begins meiosis II, before arresting at metaphase II. At this time the first polar body is produced. The resultant secondary oocyte and first polar body are haploid.
§ The secondary oocyte is ovulated. If fertilized by a sperm, the secondary oocyte completes meiosis II and the second polar body (haploid) is formed. (polar bodies = genetic material and very little cytoplasm)

Question 18

Polar Body 1 Transfer (PB1T)

Answer 18

1. The chromosome spindle is removed from the donor egg and discarded.
2. The 1st polar body is removed from the intending mother’s egg and transferred to the enucleated donor egg; the intending mother’s egg is discarded.
3. The reconstructed oocyte contains the intending mother’s nuclear DNA and donor’s mitochondrial DNA.
4. The reconstructed egg is fertilized with the intending father’s sperm.
5. The embryo develops in vitro (PB2 extruded) and is transferred to the womb of the woman who will carry the child.

Question 19

Potential risks of PB1T

Answer 19

mtDNA carryover: PB1T < PB2T < MST < PNT (shouldn’t have many abnormal mitochondria – low risk transfer)

Question 20

Technicality of procedure of PB1T

Answer 20

potentially easier to obtain polar bodies, as they are already enclosed in their own cell membrane; can be removed with only micropipette

Question 21

Ethical considerations of PB1T

Answer 21

Manipulation and destruction of oocytes

nb: embryos deemed not suitable for transplant may be discarded.

Question 22

Polar Body 2 Transfer (PB2T)

Answer 22

1. The intending mother’s egg is fertilized by the intending father’s sperm. (not shown)
2. The donor egg is fertilized by the intending father’s sperm. (not shown)
3. The maternal pronuclei from the donor zygote is removed and discarded, leaving a half-enucleated egg.
4. The 2nd polar body from the intending mother’s zygote is transferred to the half-enucleated donor egg, which contains the paternal pronuclei and donor mtDNA.
5. The embryo develops in vitro and is transferred to the womb of the woman who will carry the child.

Question 23

Potential risks of PB2T

Answer 23

mtDNA carryover: PB1T < PB2T < MST < PNT

Question 24

Technicality of procedure of PB2T

Answer 24

• Identification of female pronuclei
• Potentially easier to obtain polar bodies, as they are already enclosed in their own cell membrane; can be removed with only micropipette

Question 25

Ethical considerations of PB2T

Answer 25

Manipulation and destruction of fertilized eggs

nb: embryos deemed not suitable for transplant may be discarded.

Question 26

what are the ethical considerations?

Answer 26

• Manipulation and destruction of fertilised eggs
• Mutant mitochondria may accidentally get carried over – unsure of future implications for the child then? (will they develop a mitochondrial disorder or will their future children inherit a mitochondrial disorder?)
• Side effects of having DNA from 3 parents in 1 person?
• The technology is largely unregulated in some countries i.e. Ukraine
• Potential misuse of mitochondrial transfer – should parents of mitochondrial transfer select only male embryos to prevent potentially changing the course of their blood line, or do parents have the right to have children of either sex despite the effect potential future generations