Genetic studies in early embryos with emphasis on preimplantation genetic diagnosis

University dissertation from Stockholm : Karolinska Institutet, Department of Molecular Medicine

Abstract: It has been estimated that as much as 70-80% of all human conceptions do not develop to term. A majority of clinically recognisable pregnancies, which are miscarried, contain chromosome abnormalities. However, data from very early embryo development are difficult to obtain. The recent development of single-cell fluorescent in situ hybridisation (FISH) analysis makes it possible to study chromosome abnormalities and segregation in early embryos. Using preimplantation genetic diagnosis (PGD) for carriers of structural chromosome abnormalities, we were able to perform chromosome studies in embryos as well as evaluate a new diagnostic tool. These experiences were also successfully implemented in a clinical service. A new single needle approach to obtain blastomere biopsies from human preimplantation embryos was tested and subsequently applied to PGD. The method was first evaluated in a mouse system and shown to be compatible with a high degree of in vitro and in vivo development of biopsied mouse embryos. Successful human PGD was performed when this method was applied in our human PGD programme. By using FISH, the presence of aneuploidy and mosaicism in normally fertilised freeze-thawed human embryos of good morphology were studied. The study shows a high degree of numerical chromosome abnormalities and 72% of the embryos displayed one or more abnormal blastomeres for the chromosomes studied. These data show a slightly higher incidence of abnormal embryos compared to those obtained with FISH in non-cryopreserved embryos and confirm that the majority of preimplantation embryos fertilised in vitro contain abnormal blastomeres. The investigation on mosaicism and aneuploidy in preimplantation embryos was continued with the detailed analysis of embryos from translocation carriers undergoing PGD. A high number of mosaic/chaotic embryos (73%) for the chromosomes involved in the translocation were found. A second hybridisation with two additional probes not involved in the translocation showed that the degree of mosaicism in each embryo differed between the two hybridisations. The difference in the average degree of mosaicism for all embryos was 65% regarding the chromosomes involved in the translocation as compared to 35% for two control chromosomes. The chromosome segregation pattern in embryos from carriers of structural chromosome abnormalities was investigated. The distribution of balanced and unbalanced gametes was for the first time studied in a female inversion carrier and the analysis showed that half of the analysed embryos were balanced. Alternate segregation mode was the most common mode of segregation in Robertsonian (85% of the embryos) and reciprocal (50% of the embryos) translocation carriers. Carriers of structural chromosome abnormalities are at high risk of having children with a severe handicap. Assays for the selection of balanced embryos was established using FISH. The first cases of PGD in order to avoid chromosomal imbalance in the progeny of two carriers of chromosome abnormalities, a deletion on chromosome 22q11.2 (DiGeorge syndrome) and an inversion of chromosome 5, respectively, were described. In addition, 11 PGD cycles for carriers of Robertsonian and reciprocal translocations were presented. As these patients are at high risk of having children with a severe handicap, PGD may be an attractive alternative as it allows only unaffected embryos to be selected for transfer to the uterus and the need to terminate the pregnancy is thereby avoided.

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