APPENDIX 27
Memorandum from Newcastle Fertility Centre
SUMMARY
The House of Lords Select Committee Report and
the Donaldson Report made special mention of the application of
nuclear transfer techniques to prevent transmission of mitochondrial
disease from mother to child. Pronuclear transfer is likely to
be the safest and most effective approach. However, the lack of
clarity in the wording of the clause relating to nuclear transfer
techniques in the HFE Act imposes severe limitations on the development
of these techniques. We therefore urge the Select Committee to
ensure that the clear biological distinction between a nucleus
and a pronucleus is clearly reflected in the law.
1. Defects of the mitochondrial genome are
a very important inherited cause of disease. Recent studies from
the North East of England indicate that approximately 1 in 10,000
patients are clinically affected and one in 5,000 individual at
risk of developing mitochondrial disease. Patients with mitochondrial
defects often have progressive neurological disease and at present
there is no successful treatment. Mitochondrial DNA is the only
genetic material outside of the nucleus and is present in multiple
copies. Mitochondrial DNA is strictly maternally inherited. Whilst
increasing numbers of patients with mitochondrial DNA defects
are being recognised, sadly we have little to offer our patients.
Genetic advice is difficult and because of the frequency of mitochondrial
DNA disease, and the lack of effective treatment or genetic counseling.
Thus, there is a need for research into the development of assisted
conception techniques to prevent transmission.
2. The House of Lords Select Committee Report
on Stem Cell Research (2002) encouraged the application of nuclear
transfer techniques to prevent transmission mitochondrial mutations
from mother to child. Nuclear transfer is theoretically possible
at a number of developmental stages, but any experimental procedures
have to be guided by ethical, safety and practical considerations.
Evidence from our work and others[210]
demonstrates safety and efficacy of pronuclear transfer in preventing
transmission of mutated mitochondrial DNA in a mouse model. This
approach allows oocytes to mature under physiological conditions,
and to be fertilized using standard IVF techniques. Furthermore,
the pronuclei can be easily visualized by light microscopy, which
facilitates safe manipulation.
3. An alternative and possibly ethically
less contentions approach would be to transfer genetic material
before the eggs are fertilized. One option would be to transfer
germinal vesicles between immature donor and recipient oocytes.
However, this requires harvest of immature oocytes followed by
maturation in vitro. These are not established clinical procedures
and there is a large body of evidence to suggest that this approach
is significantly less efficient than the conventional technique
of harvesting mature oocytes.[211]
Furthermore, epigenetic programming in the female germline occurs
largely during oocyte maturation.[212]
Thus, disruptions to this process would increase the risk of aberrant
epigenetic programming, which may result in an increased incidence
of conditions such as Beckwith-Wiedermann and Angelman syndromes.
4. Another possibility would be to transfer
genetic material between mature unfertilized eggs. This has a
number of major disadvantages. Firstly, mature eggs are arrested
at metaphase of the second division. The chromosomes are therefore
not enclosed by a nuclear membrane and cannot be visualized by
light microscopy during micromanipulation. Thus, it would be necessary
to use a fluorescent label such as Hoeshst 33342 which would require
UV illumination. This is likely to induce DNA damage and may also
have other unknown developmental consequences. Secondly, removal
of chromosomes would necessitate disruption of the meiotic spindle,
which is likely to predispose to missegregation of chromosomes.
5. For the reasons set out above, we believe
that pronuclear transfer is the safest option for preventing transmission
of mitochondrial mutations. In support of this, studies in mice
through at least five generations have not shown any long term
harmful implications of pronuclear transfer (Lawrence Smith, Personal
communication). The next step in the development of this technique
is to perform in vitro studies of human embryos following
pronuclear transfer. We submitted an application for a research
licence to the HFEA in February 2004. Our proposal involved the
use of abnormally fertilized zygotes, which are normally discarded.
Unfortunately, there does not appear to have been much progress
in processing the application and the limited information we have
been given suggests that there is uncertainty about the interpretation
of HFE Act, 1990. Further progress in the development of a safe
and effective treatment for preventing transmission of mitochondrial
disease will depend on clarification of the legal position. We
therefore urge the Select Committee to ensure that the clear biological
distinction between a pronucleus and a nucleus in reflected in
the law.
6. What is the difference between a pronucleus
and a nucleus? The pronuclear stage represents a unique cell cycle
stage marking the transition from meiotic to mitotic cell division.
Following sperm entry the oocyte completes the second meiotic
division. The chromosomes retained in the oocyte become enclosed
within the newly formed female pronucleus. Around the same time
the sperm head decondenses to form the male pronucleus. Thus,
each pronucleus contains a haploid number of either maternal or
paternal chromosomes. The pronuclei generally persist as two separate
entities until their membranes breakdown at onset of the first
mitotic division. During the first mitotic division, replicated
pairs of maternal and paternal sister chromatids segregate to
two new daughter cells. Thus, each cell of the resulting two-cell
embryo receives a diploid number of chromosomes (one copy of each
maternal and paternal chromosome). Before exit from mitosis the
chromosomes of each daughter cell become enclosed in the newly
formed nuclear membrane. This marks the first appearance
of the embryonic nucleus. Thus, the clause in the HFE Act prohibiting
replacement of "a nucleus of a cell of an embryo with a nucleus
taken from a cell of any person, embryo or subsequent development
of an embryo" (HFE Act, 1990), is only relevant from the
two-cell stage onwards.
7. In conclusion, progress in the development
of techniques to prevent transmission of mitochondrial disease
from mother to child has been stymied by the misinterpretation
of current legislation. We wish to seek clarification of both
the potential importance and acceptance within the current HFE
Act for this procedure.
June 2004
210 Meirelles F, Smith LC. Mitochondrial genotype
segregation in a mouse heteroplasmic lineage produced by embyonic
karyoplast transplantation. Genetics. 1997; 145: 445-451. Back
211
Trounson, A., C. Anderiesz and G. Jones, Maturation of human
oocytes in vitro and their developmental competence. Reproduction,
2001. 121(1): p 51-75. Back
212
Lucifero D, Mertineit C, Clarke HJ et al. Methylatio
dynamics of imprinted genes in mouse germ cells. genomics 2002;
79: 530-538. Back
| 
