The use of blastocyst culture to avoid inheritance of an abnormal paternal genome after ICSI

Denny Sakkas

Assisted Conception Unit, Birmingham Women's Hospital and Reproductive Biology and Genetics Group, University of Birmingham, Edgbaston, Birmingham B15 2TG, UK

The development of suitable defined sequential culture media methods that can reliably lead to high rates of blastocyst development in human in-vitro fertilization (IVF) (Gardner et al., 1998a) has brought about the dilemma of when to apply this technique. As with many new IVF techniques, a choice must be made by many reproductive units of whether or not to adopt a new technology without compromising the ultimate goal of achieving pregnancies for couples.

Gardner et al. (1998a,b) have stated that one of the most important applications of blastocyst transfer is that it will facilitate high pregnancy rates while limiting the number of embryos transferred and therefore minimize the risk of multiple gestation. Some pitfalls to the use of blastocyst culture exist. One being that, of those embryos that fail to reach the blastocyst stage in vitro, how many would have been able to give rise to offspring if transferred earlier? In addition, as not all embryos develop to the blastocyst stage, many couples who have a small number of fertilized embryos would risk not having an embryo transfer. A possible cut-off point of the number of fertilized embryos may also need to be adopted by some clinics before blastocyst culture can be offered. Blastocyst culture could, therefore, not be comfortably offered to all patients. In patients who have limited numbers of fertilized embryos, other criteria should be obtained to select the best embryos. One method is that of selecting the earliest cleaving embryos to the 2-cell stage, whereby a strict time-point after fertilization is used to choose the embryos to transfer (Shoukir et al., 1997a).

The most important benefit of blastocyst culture is that it offers an excellent, simple non-invasive means of selecting the best embryo to transfer. In conjunction with the rate of blastocyst development, embryos with a high developmental potential can be selected for transfer (Lelaidier et al., 1995Go; Shoukir et al., 1998aGo). It is this ability to select highly viable embryos by non-invasive means that may prove crucial when using blastocyst culture. This will be of greater benefit when we use assisted reproductive techniques (ART) that may compromise the quality of the embryo produced. One example where embryo quality may be diminished is when using intracytoplasmic sperm injection (ICSI).

The use of blastocyst culture may therefore have a more direct benefit in its application to assisted reproduction. There is already considerable discussion as to whether ICSI may contribute to an increase in the development of abnormal offspring (Bonduelle et al., 1998Go; Bowen et al., 1998Go). With the use of ICSI, we have become less discriminatory with the quality of paternal nuclear material we introduce into the egg. Spermatozoa possessing defective chromatin packaging (Evenson et al., 1980Go), damaged DNA (Gorzyca et al., 1993; Sakkas et al., 1997Go) and those that have not completed spermatogenesis (Fishel et al., 1995Go) are being used to treat infertile men. It has been argued for several years, that sperm defects could impair not only the fertilization process but also the fate of embryos.

The effect of various sperm anomalies on embryo quality is supported by a number of studies. Using strict criteria, a relationship between embryo quality and the degree of `normality' of a semen sample regarding morphological features has been observed (Ombelet et al., 1994Go). In clinical in-vitro fertilization (IVF), when the usual parameters of sperm quality are good, there is a strict linear relationship between cleavage and blastocyst formation. For spermatozoa with poor motility and poor morphology parameters, cleavage rate did not correlate with further embryonic development. When fertile abnormal spermatozoa are used in IVF, no correlation exists between the sperm fertility estimated by the cleavage rate and the development potential of the embryos assessed by the blastocyst formation rate. Janny and Ménézo (1994) showed that poor quality spermatozoa may lead to poor rates of blastocyst formation, but once the blastocyst stage is obtained the developmental future is equal whatever sperm quality at the origin. In the human we have recently reported that ICSI patients have a lower percentage of embryos that form blastocysts compared with patients undergoing routine IVF (Shoukir et al., 1998bGo). This, however, needs confirmation in a larger study using newly defined media.

How can we lessen the possibility that ICSI offspring will inherit a defective paternal genome? The options are, either to preselect the best spermatozoon to use for ICSI, or to use a selection process to identify the embryo that is more likely to develop into a normal offspring. Adopting the use of extended culture to the blastocyst stage offers the second possibility for ICSI patients. The use of blastocyst culture will, therefore, provide us with a number of benefits. Along with the ability to confidently select more viable embryos and limit the chances of multiple pregnancy, blastocyst culture may provide a non-invasive means of weeding out the abnormal embryos. In doing so, this may in turn lessen the chance of an errant paternal genome being inherited by ICSI offspring.

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