Comparison of FISH PRINS, and conventional and fluorescent PCR for single-cell sexing: suitability for preimplantation genetic diagnosis

The occurrence of aneuploidy in human: lessons from the cytogenetic studies of human oocytes

During the last 4 decades, the cytogenetic investigation of human oocytes has never stopped to progress, according to the advents of new technologies. Both karyotyping and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes. However, these studies display a great variability in results, which may be essentially attributable to the limitations of these techniques when applied to human oocytes. The most relevant analysis have led to the estimate that 15-20% of human oocytes present chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies in human oocytes has also been clearly evidenced by recent reports based on large sample of oocytes or polar bodies, whereas most of initial studies have failed to confirm any relationship between maternal age and aneuploidy in human oocytes.

Cytogénétique des ovocytes humains : 40 ans de progrès

Chromosomal abnormalities account for the majority of pre- and post- implantation embryo wastage in humans. Most of these abnormalities result from maternal meiotic errors, which preferentially occur during the first meiotic division. Consequently, the cytogenetic analysis of human oocytes has then been considered as a highly valuable source of data for the investigation of both the occurrence and the origin of chromosomal abnormalities in human. During the last 4 decades, the cytogenetic analysis of human oocytes has never stopped progressing, according to the advents of new technologies. Both karyotyping and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes. However, these studies display a great variability in results, which may be essentially attributable to the limitations of these techniques when applied to human oocytes. The most relevant analysis have led to the estimate that 15-20% of human oocytes present chromosome abnormalities, and they have emphasized the implication of both whole chromosome non-disjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidy in human oocytes has also been clearly evidenced by recent reports based on large sample of oocytes or polar bodies.

The chromosomal analysis of human oocytes. An overview of established procedures

The cytogenetic survey of mature human oocytes has been and remains a subject of great interest because of the prevalence of aneuploidy of maternal origin in abnormal human conceptuses, and the lack of understanding about the non-disjunction processes in human meiosis. The first attempts to analyse the chromosomal content of human female gametes were made in the early 1970s, and led to limited data because of the paucity of materials and the inadequacy of the procedure used. The years to follow brought a resurgence of interest in this field, because of the development of human IVF techniques which made oocytes unfertilized in vitro available for cytogenetic analysis. Numerous studies have since been performed. However, the difficulties in obtaining good chromosome preparations and of performing accurate chromosome identification have reduced the viability of these studies, resulting in large variations in the reported incidences of chromosomal abnormalities. The further introduction of new procedures for oocyte fixation and the screening of large oocyte samples have allowed more reliable data to be obtained and to identify premature chromatid separation as a major mechanism in aneuploidy occurrence. The last decade has been privileged to witness the adaptation of molecular cytogenetic techniques to human oocytes, and thus various powerful procedures have been tried not only on female gametes, but also on polar bodies, involving sequential and multicolour fluorescent in situ hybridization (FISH) labelling, comparative genomic hybridization (CGH), spectral karyotyping and alternative methods such as primed in situ labelling (PRINS) and peptide nucleic acid (PNA) techniques. A large body of data has been obtained, but these studies also display a great variability in the frequency of abnormalities, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. However, molecular cytogenetic approaches have also evidenced the co-existence of both whole chromosome non-disjunction and chromatid separation in maternal aneuploidy. In addition, the extension of these techniques to oocyte polar body materials has provided additional data on the mechanism of meiotic malsegregation. Improvements of some of these techniques have already been reported. The further development of new approaches for the in situ analysis of human meiosis will increase the impact of cytogenetic investigation of human oocytes in the understanding of aneuploidy processes in humans.

Comparison between fluorescence in situ hybridization (FISH) and quantitative-fluorescent polymerase chain reaction (QF-PCR) for the detection of aneuploidies in single blastomeres

OBJECTIVES

The aim of our investigation was to compare the efficiencies of the fluorescence in situ hybridization (FISH) and the quantitative-fluorescent PCR (QF-PCR) methods for the detection of sexing and numerical chromosome disorders in single blastomeres collected from the same preimplantation human embryos.

METHODS

FISH analysis was carried out on 145 blastomeres from the 79 surplus embryos with probes specific for chromosomes 13, 18, 21, X, and Y. QF-PCR was performed with each one or two of the primers specific for the same chromosomes on 151 blastomeres from the same embryos obtained from patients undergoing IVF treatment.

RESULTS

Analyses were possible on 135 blastomeres (93%) by FISH and on 117 blastomeres (77%) by QF-PCR. Of 65 embryos, which could be analyzed by both methods, 20 embryos (31%) were diagnosed as abnormal.

CONCLUSION

The present study shows that FISH tests are more accurate than QF-PCR assays for the detection of numerical chromosome disorders when performed on single blastomeres.

Prepregnancy testing for single-gene disorders by polar body analysis

Preventive measures for single-gene disorders are currently based on carrier screening in pregnancy and prenatal diagnosis. Although this has been extremely effective for preventing new cases of common inherited conditions, the major limitation is still termination of 25% of wanted pregnancies following detection of affected fetuses. To overcome this important problem, we developed a method for prepregnancy genetic testing that involves DNA analysis of the first and second polar bodies, which are extruded during maturation and fertilization of oocytes. We offered this option to 28 couples at risk for having children with single-gene disorders. Fifty clinical cycles were performed from these patients for the following conditions: 20 for cystic fibrosis, 18 for thalassemia, 6 for sickle cell disease, 2 each for Gaucher disease and LCHAD (long-chain 3-hydroxyacyl-COA dehydrogenase deficiency), and 1 each for hemophilia B and phenylketonuria. Oocytes obtained from these patients using in vitro fertilization procedures (IVF) were tested by a sequential multiplex nested PCR analysis of the first and second polar body to detect the gene involved simultaneously with linked polymorphic markers. A total of 191 of 399 oocytes with predicted genotype were mutation free and preselected for fertilization and transfer. In all but three cycles, one to three unaffected embryos with predicted unaffected genotypes were transferred, resulting in 20 pregnancies, from which 19 healthy children have been born. The follow-up analysis of embryos resulting from oocytes with predicted affected genotype, confirmed the diagnosis in 97% of cases, demonstrating the reliability of prepregnancy diagnosis of single-gene defects by polar body analysis.

Preimplantation genetic diagnosis using fluorescent polymerase chain reaction: results and future developments

PURPOSE

Fluorescent polymerase chain reaction (PCR) is a multipurpose technique that can be used for diagnosing sex, single-gene defects, and trisomies as well as determining DNA fingerprints from single cells. However, its effectiveness must be assessed before clinical preimplantation genetic diagnosis (PGD) application.

METHODS

Single and multiplex fluorescent PCR was applied to single cells and blastomeres.

RESULTS

Fluorescent PCR can be used to diagnose sex from blastomeres and has been successfully applied in a clinical PGD sexing program resulting in a confirmed pregnancy. A further major advantage of fluorescent PCR is the ability to multiplex, providing multiple diagnoses and DNA fingerprints with a high reliability (approximately 75% for trisomy, 86% for DNA fingerprint) and good accuracy (70-80%). Allele dropout in multiplex PCR is approximately 20% per allele and does not appear to be associated with the fragment size.

CONCLUSIONS

Fluorescent PCR is a powerful technique for PGD, and the effects of allele dropout must be considered, particularly in multiplex PCR.

Determining the sex of bovine embryos using polymerase chain reaction results: a six-year retrospective study

Knowing the sex of embryos produced for use in an embryo transfer program can assist the dairy producer in managing his resources more effectively. A reliable procedure for accomplishing this goal is to apply PCR technology to the biopsy of an embryo. A description is provided of how the technique has been applied on a large scale in a commercial setting in western Canada between 1992 and 1997. A total of 4,183 embryos was biopsied over a 6-yr period. The sex was determined with more than 90% of the embryos. The results showed that there was a seasonal variation in the sex ratio, with more females being recorded in the period of least light (October to March), than in April to September. While both sire and embryo quality affected the sex ratio, the differences were too small to be of value in most breeding programs. Pregnancy rates with fresh sexed embryos (58 to 71%) were comparable to those with fresh unsexed embryos. The results following freezing and thawing of sexed embryos were low (37 to 66%) but sufficient to be viable commercially. When the sex assigned by PCR was verified by fetal sexing at 60 d of gestation, the error rate was 7%. This study demonstrates that sexing of embryos can be carried out on a large scale. Demand for quick, reliable determination of sex can be met in a cost effective manner. The pregnancy rates achieved with embryos after biopsy are suitable for use in a commercial setting.