Download Ooplasmic donation in humans The potential for epigenic

Human Reproduction Vol.17, No.4 pp. 850–852, 2002
DEBATE
Ooplasmic donation in humans
The potential for epigenic modifications
Susan M.Hawes1,5, Carmen Sapienza2,3 and Keith E.Latham2,4
1Centre
for Early Human Development, Institute of Reproduction and Development, Monash University, Melbourne, Victoria 3168,
Australia, 2The Fels Institute for Cancer Research and Molecular Biology, 3Department of Pathology and 4Department of Biochemistry,
Temple University School of Medicine, 3307 North Broad Street, Philadelphia, PA 19140, USA
5To
whom correspondence should be addressed. E-mail: [email protected]
Ooplasm donation, wherein ooplasm is transferred from a donor oocyte to a recipient oocyte in an effort to increase
embryo viability, has been applied in the human, with resulting pregnancies and births. Neither the safety nor
efficacy of this method has been adequately investigated. Mitochondrial heteroplasmy in the blood of
children conceived using ooplasm donation has recently been described. A follow-up study of children born following
the use of this technique primarily focused on the presence of mitochondria from the donor oocyte highlighting
possible problems due to mitochondrial heteroplasmy. Other effects related to epigenetic events may also arise, but
have not been addressed. Studies using inbred mouse strains reveal that genetically diverse ooplasms can impose
diverse epigenetic modifications on parental genomes. Incompatibilities produced by combining maternal genome
and ooplasm from different genotypes leads to defects in gene expression and development. Such defects can be
heritable and observed in the next generation. Given the potential for epigenetic modifications to arise following
ooplasm donation, the safety and efficacy of this method need to be evaluated in a suitable animal model.
Key words: assisted reproduction/mitochondrial heteroplasmy/ooplasm donation
Introduction
Cytoplasmic transfer between human oocytes (ooplasm donation) and germinal vesicle transfer, which represents a complete
cytoplasmic exchange, have been performed recently as a
means to try to improve the outcome of assisted reproduction
methods (Cohen et al., 1997, 1998; Zhang et al., 1999). These
procedures have been performed essentially in the absence of
any basic research to evaluate either the efficacy or the potential
risk of the methods. Traditionally, the adaptation of novel
scientific techniques for application to human IVF methods
involves a ‘try-it-and-see’ approach. Initially, development of
culture media for human oocyte and embryo culture involved
applying the experience of mouse studies to the human
(Bavister et al., 1995; review). This strategy was successful
for limited endpoints, such as successful fertilization, embryo
cleavage and, ultimately, pregnancy and live birth following
embryo transfer. In contrast to this strategy, ICSI, developed
for the alleviation of male infertility, had not been investigated
in an animal model prior to its use for human IVF (Van
Steirteghem et al., 1993).
Cohen and colleagues first reported the use of ooplasm
donation in humans as a means of obtaining pregnancy in a
group of women they categorize as having ‘recurrent implantation failure’ (Cohen et al., 1997, 1998). As pregnancies were
obtained after this procedure, an ad hoc explanation of the
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biological basis has been that a small amount of injected
cytoplasm restores some unknown property lacking in the
recipient oocyte. Mitochondria, present in cytoplasm and
responsible for respiratory processes, provide one possible
basis for this effect (Barritt et al., 2001a). Cohen’s group has
recently reported that two out of 15 children born as a result
of this technique still have traces of donor mitochondrial DNA
in their blood cells at 1 year of age (Barritt et al., 2001a).
A potential for adverse effects of cytoplasm transfer may
exist, based on studies undertaken in rodent models. Surprisingly, these studies have received little or no attention in the
context of human oocyte cytoplasm transplantation. In fact,
the debate concerning the safety of the method has centred on
the possible effect of mitochondrial heteroplasmy (Cummins,
2001). However, possible epigenetic effects exerted upon
maternal or paternal genomes by ‘foreign’ cytoplasm must
also be taken into account. Numerous studies have revealed
that different genotypes of mice modify maternal and paternal
genomes differently (Baldacci et al., 1992; Reik et al., 1993;
Renard et al., 1994; Pardo-Manuel de Villena et al., 1997,
2000; Roemer et al., 1997; Latham and Sapienza, 1998;
Pickard et al., 2001) either during oogenesis (in the case of the
maternal genome) or during the period immediately following
fertilization. These differences have been revealed either
through simple breeding studies (wherein a striking incidence
© European Society of Human Reproduction and Embryology
Ooplasm donation
of embryo lethality is observed), through methylation effects
on inherited transgenes, or through microsurgical techniques
similar in outcome (i.e. maternal pronuclear exchange between
oocytes of different strains) to the cytoplasm transfer procedure.
The latter procedures have resulted in abnormalities in gene
expression, morphology and physiology, and most disturbingly,
these defects can be transmitted to the next generation (Roemer
et al., 1997). The genes responsible for these effects are being
mapped and may soon be isolated. Other studies have revealed
genetic incompatibilities that affect blastomere integrity in the
early embryo (Hawes et al., 2001), creating the possibility that
cytoplasm transfer could possibly have immediate adverse
effects on embryo viability beyond the epigenetic defects
described above.
A number of these studies have clearly shown this effect(s)
being mediated by oocyte cytoplasm action upon pronuclei or
embryonic nuclei. For example, transfer of ooplasm from the
inbred mouse DDK strain or oocyte RNA to non-DDK oocytes
converts the oocyte to a DDK phenotype and causes postzygotic lethality (Renard et al., 1994). The recent paper by
Pickard et al. reveals an interesting co-operation between
ooplasmic factors and factors expressed post-zygotically to
affect transgene function (Pickard et al., 2001). Pronuclear
transfers with DBA/2 and C57BL/6 strains reveal that the
ooplasm strain-specifically modifies paternal pronuclei postfertilization (Latham and Solter, 1991). Such effects may reflect
the fact that much of the epigenetic information displayed in
fetal and adult mammals is not completely elaborated in the
genome, but rather is acquired ontogenetically (Latham et al.,
1995; Latham, 1999).
Extrapolating the published observations of these inbred
mouse studies to what effects may occur in children born
through the use of similar techniques is difficult. Doing so
will require knowledge of gene identities, human homologues
and allele frequencies in the population. Nevertheless, taking
into account the striking observations seen in mouse studies
is important when debating possible future implications in the
adaptation of this technique in the human, particularly as
epigenetic modifications may affect future generations. Moreover, the potential effects of cytoplasm transfer need not be
limited to those abnormalities observed in mice. There is
abundant evidence to indicate that epigenetic effects leading
to aberrant regulation of imprinted genes in humans is associated with serious disease (Falls et al., 1999). The possibility
that effects on imprinted genes could arise as a consequence
of cytoplasm transfer, and a subsequent imprinting modification
of one or both parental genomes must be considered.
Although proponents of the ooplasm donation and germinal
vesicle transfer methods may opine that it is safe and effective,
there is no significant evidence that either opinion is justified.
There are no objective criteria to determine whether repeated
failure in IVF or ICSI is due to a nuclear or cytoplasmic
defect. There are no objective criteria by which individuals
are diagnosed with ‘bad’ ooplasm and conversely no objective
criteria by which a donor oocyte is diagnosed to have ‘good’
ooplasm. Furthermore, there is no evidence that all oocytes
from a given individual are of the same quality. Even in the
event that such descriptive terms could be correctly applied,
there is no evidence that a case of ‘bad’ ooplasm can be cured
by the addition of a dollop of ‘good’ ooplasm. Indeed the
whole approach may be akin to trying to improve a bottle of
spoiled milk by adding a cup of fresh. The results obtained to
date in the clinical setting have been obtained in the absence
of suitable experimental controls, and thus it cannot be judged
that, when a pregnancy is achieved following the cytoplasm
transfer, the transfer in fact contributed to that success.
Little consideration has been given to the possibility that
some individuals may harbour genetic or epigenetic defects
(related to age, for example) that affect chromosome segregation during meiosis and possibly mitosis. If such a defect
exists in an individual, and is somehow rescued by cytoplasm
transfer, this may leave the potential for chromosome missegregation during mitotic divisions, particularly given the
non-disjunction-prone nature of early cleavage divisions (Bean
et al., 2001). Such concerns are relevant, given that two of 15
pregnancies have resulted in abortion or miscarriage of fetuses
with Turner’s syndrome (Barritt et al., 2001b).
It is an admirable goal to try to help infertile couples
produce children. The urge to do so, however, must be balanced
by a certain degree of caution, regardless of the nobility of
this incentive. Pioneering methods such as cytoplasm transfer
or germinal vesicle transfer may prove to be valuable treatments
for infertility, but should be evaluated in a suitable animal
model for both efficacy and safety. Efforts need to be taken
to identify genes that may affect epigenetic processes following
such procedures, so that appropriate screens can be applied
before the method is used. Additional efforts should be taken
to understand the genetic basis for infertility in the relevant
cases, which could lead to a more refined approach than the
currently available microsurgical methods.
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