In order to keep the chromosome number of 46 in the human constant from generation to generation the diploid number has to undergo reduction during male as well as female gametogenesis. However, the resulting gametes do not always contain the correct 23-chromosome haploid set. At fertilization this may result in aneuploid zygotes. Through abnormalities arising at fertilization normal gametes may give rise to chromosomally abnormal embryos. Finally, it is possible that a perfectly normal zygote will become a mosaic embryo through aberrant mitosis during the first cleavage divisions. With the introduction of IVF and other ART techniques it has become possible to monitor cytogenetic abnormalities in gametes, zygotes and early embryos. Human preimplantation embryos used for research purposes often originate within the context of infertility treatment and are not suitable for either transfer to the patient or cryopreservation. They therefore do not reflect the normal situation. However, it has become possible to study normal oocytes from couples with male factor infertility and vice versa. Furthermore, the introduction of preimplantation genetic diagnosis (PGD) has made embryos from couples with a normal fertility available and has allowed more unbiased studies to be carried out. The various types of fluorescent in situ hybridisation now available have led to reliable estimates of aneuploidy rates in individual blastomeres, and the contribution of chromosomal mosaicism to early embryonic death to be determined. Much is expected of the more complete picture of the embryonic chromosomal complement that will be offered by single cell array-Comparative Genomic Hybridisation (array-CGH). These technical developments in cytogenetics have resulted in a better understanding of how chromosomal abnormalities may explain the natural limits of human fecundity. This has also clear implications for where the limits of success may lie for IVF.Recently, a number of papers have pointed out the relationship between ART and epigenetic defects. Whether these are patient or technique related still needs to be sorted out. The SIG RG is also interested in epigenetic phenomena, as these have an important impact on the correct development of embryos both in vitro and in vivo.
Back to top
So far it has been very difficult to clearly establish the genetic and epigenetic consequences of ART. One reason for this is our poor understanding of human gametogenesis, which itself is linked to the absence of a human model. We propose that one way to decipherer human gametogenesis is the genetic approach and more precisely the genetics of infertility. This implies a common research effort between all the actors of the field, clinicians, geneticists, embryologists and researchers. Genetic aspects of human reproduction are studied in the clinical practice as well as at the basic level using experimental animal models. Usually clinical recognition is attempted by the detection of mutations associated with an infertile phenotype of the male or female partner. Mutations can be present at the chromosomal, which are microscopically visible, as well as at the gene level studied using molecular techniques. Presently, the most commonly found genetic defects underlying male infertility are different types of chromosome aberrations and Y chromosome microdeletions in one of the three AZF regions. While female reproductive failure may also be caused by chromosome aberrations, particularly in X chromosome or by chromosome translocations, the most common conditions associated with female infertility seemd to be complex in genetic etiology. The best known molecular genetic defects associated with male infertility are mutations in the CFTR gene and with female infertility mutations in the FMR1, FOXL2 and BMP15 gene. However, these represent rare causes of infertility. From animal model studies more than 3000 genes are expected to cause male and/or female infertility if functionally disrupted. Consequently, genetic networks controlling gametogenesis and the complete reproduction cycle have to be unravelled before it is possible to come to an understanding of the molecular basis of genetic infertility. It is popular to study these networks in the mouse. However, the function in reproduction of the genes studied might be different in the human. Prominent examples are mutations of the FSH receptor (FSHR) gene causing different pathologies in mouse and human and the DAZ gene family on the Y-chromosome which is absent in the mouse but has an essential function in male fertility in the human. So far, no mutation has been described with a non-syndromic phenotype affecting spermatogenesis exclusively, but with the emergence of new genetic technologies, such as microarray analysis and genome sequencing, this field is becoming very active. Recently, four mutated genes have been identified as responsible for different infertility phenotypes: CATSPER1in asthenozoospermic patients; SPATA16 and DPY19L2 in globozoospermic patients and AURKC in macrocephalia or enlarged headed spermatozoa. It is clear that in the near future, many more genes will be identified, which will change the way to take care of our patients. Besides FISH, the introduction of molecular techniques at the single cell level as a research tool has contributed tremendously to the study of the genetic content of single cells, be it oocytes, sperms, zygotes or blastomeres. This has given answers regarding very fundamental questions (for instance with respect to the start of embryonic genome expression and the regulation of genomic imprinting during gametogenesis) as well as applications with respect to PGD.
The first clinical application of PGD was reported in the early 1990s. The field has seen immense progression mainly in the techniques used for either chromosomal or molecular diagnosis. We have seen two-colour FISH evolve to five colour FISH, and now molecular karyotyping using array-CGH at the single cell level. Conventional PCR was replaced by fluorescent PCR, then multiplex PCR and whole genome amplification. Some groups are already using SNP arrays for PGD and genome sequencing is around the corner. Since the early days, the number of PGD cycles carried out and the number of clinics offering PGD has grown year by year. In 1997 it was decided to bring these activities together in the ESHRE PGD Consortium.
Back to top