Rapid visualization of metaphase chromosomes in single human blastomeres after fusion with in-vitro matured bovine eggs

BasePhasing: a highly efficient approach for preimplantation genetic haplotyping in clinical application of balanced translocation carriers

BACKGROUND

Preimplantation genetic testing (PGT) has already been applied in chromosomally balanced translocation carriers to improve the clinical outcome of assisted reproduction. However, traditional methods could not further distinguish embryos carrying a translocation from those with a normal karyotype prior to implantation.

METHODS

To solve this problem, we developed a method named "Chromosomal Phasing on Base level" (BasePhasing), which based on Infinium Asian Screening Array-24 v1.0 (ASA) and a specially phasing pipeline. Firstly, by comparing the number of single nucleotide polymorphism (SNP) loci in different minor allele frequencies (MAFs) and in 2Mbp continuous windows of ASA chip and karyomap-12 chip, we verified whether ASA could be adopted for genome-wide haplotype linkage analysis. Besides, the whole gene amplification (WGA) of 3-10 cells of GM16457 cell line was used to verify whether ASA chip could be used for testing of WGA products. Finally, two balanced translocation families were utilized to carry out BasePhasing and to validate the feasibility of its clinical application.

RESULTS

The average number of SNP loci in each window of ASA (473.2) was twice of that of Karyomap-12 (201.2). The coincidence rate of SNP loci in genomic DNA and WGA products was about 97%. The 5.3Mbp deletion was detected positively in cell line GM16457 of both genomic DNA and WGA products, and haplotype linkage analysis was performed in genome wide successfully. In the two balanced translocation families, 18 blastocysts were analyzed, in which 8 were unbalanced and the other 10 were balanced or normal chromosomes. Two embryos were transferred back to the patients successfully, and prenatal cytogenetic analysis of amniotic fluid was performed in the second trimester. The results predicted by BasePhasing and prenatal diagnosis were totally consistent.

CONCLUSIONS

Infinium ASA bead chip based BasePhasing pipeline shows good performance in balanced translocation carrier testing. With the characteristics of simple operation procedure and accurate results, we demonstrate that BasePhasing is one of the most suitable methods to distinguish between balanced and structurally normal chromosome embryos from translocation carriers in PGT at present.

Risk of chromosomal aberration in spermatozoa during intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) has become critical for the treatment of severe male infertility. The principal feature of ICSI is the direct injection of spermatozoon into an oocyte, which facilitates the production of fertilized embryos regardless of semen characteristics, such as sperm concentration and motility. However, the chromosomal integrity of ICSI zygotes is degraded compared to that of zygotes obtained via in vitro fertilization. This chromosomal damage may occur due to the injection of non-capacitated, acrosome-intact spermatozoa, which never enter the oocytes under natural fertilization. Furthermore, it is possible that the in vitro incubation and pre-treatment of spermatozoa during ICSI results in DNA damage. Chromosomal aberrations in embryos induce early pregnancy losses. However, these issues may be overcome by embryo production using gametes with guaranteed chromosomal integrity. Because conventional chromosome analysis requires fixing cells to obtain the chromosome spreads, embryos cannot be produced using the nucleus that has been analyzed. On the other hand, genome cloning using androgenic or gynogenic embryos provides an additional nucleus for chromosome analysis following embryo production. Thus, this review aims to highlight the hazardous nature of chromosomal aberrations in sperm during ICSI and to introduce a method for the prezygotic examination for chromosomal aberrations.

Mapping allele with resolved carrier status of Robertsonian and reciprocal translocation in human preimplantation embryos

Reciprocal translocations (RecT) and Robertsonian translocations (RobT) are among the most common chromosomal abnormalities that cause infertility and birth defects. Preimplantation genetic testing for aneuploidy using comprehensive chromosome screening for in vitro fertilization enables embryo selection with balanced chromosomal ploidy; however, it is normally unable to determine whether an embryo is a translocation carrier. Here we report a method named "Mapping Allele with Resolved Carrier Status" (MaReCs), which enables chromosomal ploidy screening and resolution of the translocation carrier status of the same embryo. We performed MaReCs on 108 embryos, of which 96 were from 13 RecT carriers and 12 were from three RobT carriers. Thirteen of the sixteen patients had at least one diploid embryo. We have confirmed the accuracy of our carrier status determination in amniotic fluid karyotyping of seven cases as well as in the live birth we have thus far. Therefore, MaReCs accurately enables the selection of translocation-free embryos from patients carrying chromosomal translocations. We expect MaReCs will help reduce the propagation of RecT/RobT in the human population.

Reciprocal Translocation Carrier Diagnosis in Preimplantation Human Embryos

Preimplantation genetic diagnosis (PGD) is widely applied in reciprocal translocation carriers to increase the chance for a successful live birth. However, reciprocal translocation carrier embryos were seldom discriminated from the normal ones mainly due to the technique restriction. Here we established a clinical applicable approach to identify precise breakpoint of reciprocal translocation and to further distinguish normal embryos in PGD. In the preclinical phase, rearrangement breakpoints and adjacent single nucleotide polymorphisms (SNPs) were characterized by next-generation sequencing following microdissecting junction region (MicroSeq) from 8 reciprocal translocation carriers. Junction-spanning PCR and sequencing further discovered precise breakpoints. The precise breakpoints were identified in 7/8 patients and we revealed that translocations in 6 patients caused 9 gene disruptions. In the clinical phase of embryo analysis, informative SNPs were chosen for linkage analyses combined with PCR analysis of the breakpoints to identify the carrier embryos. From 15 blastocysts diagnosed to be chromosomal balanced, 13 blastocysts were identified to be carriers and 2 to be normal. Late prenatal diagnoses for five carriers and one normal fetus confirmed the carrier diagnosis results. Our results suggest that MicroSeq can accurately evaluate the genetic risk of translocation carriers and carrier screen is possible in later PGD treatment.

Pregnancy derived from human zygote pronuclear transfer in a patient who had arrested embryos after IVF

Nuclear transfer of an oocyte into the cytoplasm of another enucleated oocyte has shown that embryogenesis and implantation are influenced by cytoplasmic factors. We report a case of a 30-year-old nulligravida woman who had two failed IVF cycles characterized by all her embryos arresting at the two-cell stage and ultimately had pronuclear transfer using donor oocytes. After her third IVF cycle, eight out of 12 patient oocytes and 12 out of 15 donor oocytes were fertilized. The patient's pronuclei were transferred subzonally into an enucleated donor cytoplasm resulting in seven reconstructed zygotes. Five viable reconstructed embryos were transferred into the patient's uterus resulting in a triplet pregnancy with fetal heartbeats, normal karyotypes and nuclear genetic fingerprinting matching the mother's genetic fingerprinting. Fetal mitochondrial DNA profiles were identical to those from donor cytoplasm with no detection of patient's mitochondrial DNA. This report suggests that a potentially viable pregnancy with normal karyotype can be achieved through pronuclear transfer. Ongoing work to establish the efficacy and safety of pronuclear transfer will result in its use as an aid for human reproduction.

Benefits and drawbacks of preimplantation genetic diagnosis (PGD) for reciprocal translocations: lessons from a prospective cohort study

Preimplantation genetic diagnosis (PGD) using fluorescence in situ hybridisation probes was carried out for 59 couples carrying reciprocal translocations. Before treatment, 85% of pregnancies had resulted in spontaneous miscarriage and five couples had achieved a healthy live-birth delivery. Following treatment, 33% of pregnancies failed and 21 of 59 couples had a healthy live-born child. The accuracy of diagnosis was 92% (8% false abnormal and 0% false normal results). The overall incidence of 2:2 alternate segregation products was 44%; however, products consistent with 2:2 adjacent segregation were ~twice as likely from male heterozygotes, and those with 3:1 disjunction were three times more likely from female heterozygotes. Our results indicate that up to three stimulation cycles per couple would give an ~50% chance of a successful live birth, with the risk of miscarriage reduced to the level found in the general population. In our study, 87% of all normal/balanced embryos available were identified as being suitable for transfer. We conclude that PGD provides benefit for couples with high-risk translocations by reducing the risk of miscarriage and avoiding a pregnancy with an unbalanced form of the translocation; however, for fertile carriers of translocations with a low risk of conceiving a chromosomally unbalanced offspring, natural conception may be a more viable option.

Reproductive outcome after PGD in couples with recurrent miscarriage carrying a structural chromosome abnormality: a systematic review

BACKGROUND

Preimplantation genetic diagnosis (PGD) has been stated to improve live birth rates compared with natural conception in couples with recurrent miscarriage (RM) carrying a structural chromosome abnormality. It is unclear to what extent this claim can be substantiated by evidence. A systematic review of the literature was performed on the reproductive outcome of these couples after natural conception or after PGD.

METHODS

MEDLINE, EMBASE and the Cochrane database were searched until April 2009. Trials, patient series and case reports describing reproductive outcome in couples with RM carrying a structural chromosome abnormality after natural conception and/or after PGD were included. Since no randomized controlled trials or non-randomized comparative studies were found, separate searches for both groups were conducted. Primary outcome measure was live birth rate per couple. Secondary outcome measure was miscarriage rate per couple.

RESULTS

Four observational studies reporting on the reproductive outcome of 469 couples after natural conception and 21 studies reporting on the reproductive outcome of 126 couples after PGD were found. After natural conception, live birth rate per couple varied between 33 and 60% (median 55.5%) after parental chromosome analysis; miscarriage rate ranged from 21 to 40% (median 34%). After PGD, live birth rate per couple varied between 0 and 100% (median 31%) after parental chromosome analysis; miscarriage rate ranged from 0 to 50% (median 0%).

CONCLUSIONS

Currently, there are insufficient data indicating that PGD improves the live birth rate in couples with RM carrying a structural chromosome abnormality.

Conversion and non-conversion approach to preimplantation diagnosis for chromosomal rearrangements in 475 cycles

Due to the limitations of preimplantation genetic diagnosis (PGD) for chromosomal rearrangements by interphase fluorescent in-situ hybridization (FISH) analysis, a method for obtaining chromosomes from single blastomeres was introduced by their fusion with enucleated or intact mouse zygotes, followed by FISH analysis of the resulting heterokaryons. Although this allowed a significant improvement in the accuracy of testing of both maternally and paternally derived translocations, it is still labour intensive and requires the availability of fertilized mouse oocytes, also creating ethical issues related to the formation of interspecies heterokaryons. This method was modified with a chemical conversion procedure that has now been clinically applied for the first time on 877 embryos from PGD cycles for chromosomal rearrangements and has become the method of choice for performing PGD for structural rearrangements. This is presented within the context of overall experience of 475 PGD cycles for translocations with pre-selection and transfer of balanced or normal embryos in 342 (72%) of these cycles, which resulted in 131 clinical pregnancies (38%), with healthy deliveries of 113 unaffected children. The spontaneous abortion rate in these cycles was as low as 17%, which confirms an almost five-fold reduction of spontaneous abortion rate following PGD for chromosomal rearrangements.

Calyculin-A improves chromosome condensation for cytogenetic analysis of blastomeres from bovine and murine eight-cell stage embryos

This study evaluated the serine/threonine phosphatase inhibitor calyculin-A for rapid, efficient induction of premature chromosome condensation (PCC) in blastomeres obtained from Day 3 bovine and Day 2 murine eight-cell stage embryos, and its potential for use in cytogenetic analysis. Experiment 1 tested calyculin-A duration (0, 60, 120, and 180min) to induce PCC in bovine blastomeres. More blastomeres that underwent PCC had chromosomes suitable for cytogenetic analysis if treated for 120 or 180min (P<0.005). Experiment 2 compared doses of calyculin-A (0, 10, 50, and 100nM) on bovine blastomeres; calyculin-A (50nM, 120min) induced PCC suitable for cytogenetic analysis in the greatest number of blastomeres when compared to other doses (52.5%; P<0.005). Effects of calyculin-A (50nM) on murine blastomeres at durations of 0, 60, 90, and 120min to induce PCC were tested in Experiment 3, with 90min inducing the highest frequency of condensed chromosomes suitable for cytogenetic analysis (34%; P<0.05). Finally, Experiment 4 evaluated calyculin-A treated bovine embryos under optimal conditions (50nM, 120min) for use in gender and cytogenetic analysis. Whole chromosome paint probes were successfully hybridized to chromosomes along with 4',6-diamidino-2-phenylindole dihydrochloride hydrate (DAPI) counterstaining, allowing detection of embryo gender (54% F:46% M) and ploidy of individual blastomeres within embryos (64% diploid:36% mixoploid embryos). In conclusion, we inferred that calyculin-A was useful for rapid induction of PCC, producing chromosome spreads suitable for cytogenetic analysis of blastomeres in G1 or G2/M phase of the cell cycle.

Preimplantation genetic diagnosis for complex chromosome rearrangements

Complex chromosome rearrangements (CCR) are structural rearrangements that involve more than two breakpoints. The most common ones involve three chromosomes and three breakpoints, but double translocations can also occur. Theoretically, the potential for chromosome imbalance in the gametes of CCR carriers is higher than for simple translocations, and thus they are at an even higher risk of recurrent miscarriage. Preimplantation genetic diagnosis (PGD) has been shown to significantly reduce the risk of repeated pregnancy loss in translocation carriers, and thus it is well indicated for CCR. Here we describe different approaches for PGD of CCRs. After PGD of five couples carriers of CCR, only 6.4% of embryos were found normal or balanced, but the transfer of these six embryos into four couples, resulted in two pregnancies, one with normal non-identical twins, and the other with a balanced karyotype. Thus the implantation rate was 50%, and the pregnancy per retrieval was 40%, with 0% spontaneous abortions and 0% unbalanced offspring. This compares favorably with a prior history of spontaneous abortions, unbalanced offspring, and low fertility.

Spectral karyotyping (SKY): applications in prenatal diagnostics

The method of spectral karyotyping (SKY) is based on a combination of the technologies of charge-coupled device imaging and spectrometry. The engineering feasibility has been realized in the SpectraCube system from Applied Spectral Imaging Inc., and it allows the simultaneous identification of all 24 human chromosomes. This is performed by characterizing the spectral signature of every image pixel in relation to a fluorochrome combinatorial library translating the image and spectral information into chromosome classification. Applications for SKY include pre- and postnatal characterization of certain numerical and structural rearrangements and complex karyotypes and highly informative analysis of sample materials with only single or few cells available for investigation.

Preimplantation genetic diagnosis: present and future

PURPOSE

Preimplantation genetic diagnosis (PGD) was developed more than a decade ago and aims to identify embryos free of genetic disease attributed either to gene mutations or chromosome errors. The purpose of this article is to provide an update on the current status and future prospects of PGD.

METHODS

Review of studies employing different strategies for the detection of single gene defects, and chromosome abnormalities, both structural and numerical in the context of PGD.

RESULTS

Amplification of several DNA fragments is feasible via multiplex PCR for the PGD of single gene disorders, whilst current FISH protocols employ up to 10 probes to identify embryos with a normal chromosome complement. New methods are being developed which will enable the assessment of the entire chromosome complement of embryonic blastomeres.

CONCLUSIONS

PGD has come a long way since its first application, and has become very accurate and reliable. Technical advances in the field of preimplantation genetics mean that PGD holds great promise for the future.

Fluorescence In Situ Hybridization on Single Cells

Fluorescence in situ hybridization (FISH) is the technique of choice for preimplantation genetic diagnosis (PGD) selection of female embryos in families with X-linked disease, for which there is no mutation-specific test. FISH with target-specific DNA probes is also the primary technique used for PGD detection of chromosome imbalance associated with Robertsonian translocations, reciprocal translocations, inversions, and other chromosome rearrangements, because the DNA probes, labeled with different fluorochromes or haptens, detect the copy number of their target loci. The methods described outline strategies for PGD for sex determination and chromosome rearrangements. These methods are assessment of reproductive risks, the selection of suitable probes for interphase FISH, spreading techniques for blastomere nuclei, and in situ hybridization and signal scoring using directly labeled and indirectly labeled probes.

The status of human nuclear transfer

Human therapeutic cloning is a recently emerged application of somatic cell nuclear transfer (SCNT), which is currently being performed to produce patient-specific stem cell lines for future stem cell therapies. The advantages in producing human nuclear transfer (NT) embryos to derive NT stem cell lines are that these can be tailor-made (i.e., are autologous in nature) for the patient and may overcome the need to administer life-long immunosuppression following stem cell transplantation. Although the rationale for using NT embryos is not for reproductive purposes, human NT remains clouded in ethical, moral, and religious controversies. The recent retraction of high-impact factor publications in the field of human NT from a research group in South Korea has placed stem cell research in a delicate situation. These heavily publicized issues may hinder the progress of this research and may threaten to bring current research to a complete halt. This review outlines the recent status of human NT, its continuing progress and the difficulties the field faces. Of most concern are the ethical issues, which surround obtaining human oocytes for research. Recent evidence suggests that failed-to-fertilize oocytes are poor sources for human SCNT, but obtaining fresh, viable oocytes may be even more problematic. The current status of human SCNT is outlined in this review with particular reference made to, lessons learnt from animal research, the oocyte dilemma and optimization of human NT.

Molecular cytogenetic characterization of chromosome 9-derived material in a human thyroid cancer cell line

The incidence of papillary thyroid carcinoma (PTC) increases significantly after exposure of the head and neck region to ionizing radiation, yet we know neither the steps involved in malignant transformation of thyroid epithelium nor the specific carcinogenic mode of action of radiation. Such increased tumor frequency became most evident in children after the 1986 nuclear accident in Chernobyl, Ukraine. In the eight years following the accident, the average incidence of childhood PTCs (chPTC) increased 70-fold in Belarus, 200-fold in Gomel, 10-fold in the Ukraine and 50-fold in Tschnigov, Kiev, Rovno, Shitomyr and Tscherkassy compared to the rate of about 1 tumor incidence per 106 children per year prior to 1986 (Likhtarev et al., 1995; Sobolev et al., 1997; Jacob et al., 1998). To study the etiology of radiation-induced thyroid cancer, we formed an international consortium to investigate chromosomal changes and altered gene expression in cases of post-Chernobyl chPTC. Our approach is based on karyotyping of primary cultures established from chPTC specimens, establishment of cell lines and studies of genotype-phenotype relationships through high resolution chromosome analysis, DNA/cDNA micro-array studies, and mouse xenografts that test for tumorigenicity. Here, we report the application of fluorescence in situ hybridization (FISH)-based techniques for the molecular cytogenetic characterization of a highly tumorigenic chPTC cell line, S48TK, and its subclones. Using chromosome 9 rearrangements as an example, we describe a new approach termed 'BAC-FISH' to rapidly delineate chromosomal breakpoints, an important step towards a better understanding of the formation of translocations and their functional consequences.

The importance of aneuploidy screening in reciprocal translocation carriers

The purpose of this study is to investigate the aneuploidy rate and the mosaicism of chromosomes not involved in reciprocal translocations. Aneuploidy screening (AS) (13, 16, 18, 21 and 22) was performed as a re-analysis on fixed blastomeres from 126 embryos already analysed in preimplantation genetic diagnosis (PGD) cycles of eight female and five male reciprocal translocation carriers who had not achieved a pregnancy. A successful diagnosis for AS was achieved in 91.3% of embryos; 30.9% were euploid and 60.3% were aneuploid for the five chromosomes analysed. Of the embryos, 8.7% were euploid for AS and normal-balanced for the translocation and 22.2% were euploid for AS but unbalanced for the translocation; 8% of the embryos were aneuploid for AS but normal-balanced for the translocation and 52.4% were aneuploid for AS and also unbalanced for the translocation. At least 58.7% of the embryos were mosaic regarding mosaicism for the chromosomes involved and not involved in the translocations. Six of the 16 embryos transferred in the PGD cycles were aneuploid for the AS study; four of them were also mosaics. AS should be performed in reciprocal translocation carriers after segregation analysis in PGD.

Patterns of ovarian response to gonadotropin stimulation in female carriers of balanced translocation

OBJECTIVE

To determine whether maternal balanced translocation is a risk factor for poor ovarian response to controlled ovarian hyperstimulation (COH).

DESIGN

A retrospective analysis.

SETTING

Private IVF center.

PATIENT(S)

All couples presenting to a single center for preimplantation genetic diagnosis (PGD) for autosomal balanced translocation in either partner from 1995 through 2001.

INTERVENTION(S)

Cycle parameters and embryology outcomes were examined and compared between two groups: 61 cycles in 46 women with balanced translocations compared with 42 cycles in 32 women whose male partner had a balanced translocation.

MAIN OUTCOME MEASURE(S)

Response to ovarian stimulation.

RESULT(S)

In couples undergoing IVF/PGD to avoid transmission of an unbalanced karyotype, a significantly higher proportion of women carrying balanced translocations (female carrier) responded very poorly (E2 on the day of hCG <1,000 pg/mL) to ovarian stimulation compared to women whose partner had a balanced translocation (male carrier) (23.0% vs. 7.1%).

CONCLUSION(S)

In couples undergoing IVF/PGD for balanced translocation, the risk for poor response to ovarian stimulation may be increased when the female partner carries the balanced translocation compared to when the male partner carries the translocation. Given significant embryo attrition due to chromosomal imbalance, aggressive stimulation should be considered if the patient does not have risk factors for ovarian hyperstimulation syndrome (OHSS).

Analysis of chromosome segregation during preimplantation genetic diagnosis in both male and female translocation heterozygotes

Individuals carrying translocations suffer from reduced fertility or spontaneous abortions and seek help in form of assisted reproductive technology (ART) and preimplantation genetic diagnosis (PGD). While most translocations are relatively easy to detect in metaphase cells, the majority of embryonic cells biopsied in the course of in vitro fertilization (IVF) procedures are in interphase. These nuclei are thus unsuitable for analysis by chromosome banding or painting using fluorescence in situ hybridization (FISH). Thus several methods have been devised to detect translocation imbalance through FISH in single cells for purpose of PGD, among them polar body chromosome painting, interphase FISH with combination of subtelomeric and centromeric probes, breakpoint spanning probes, and cell conversion. Results with PGD indicate a significant decrease in spontaneous abortions, from 81% before PGD to 13% after PGD. They also indicate very high rates of chromosome abnormalities in embryos from translocation carriers, 72% for Robertsonian translocations and 82% for reciprocal translocations. Sperm analysis was found to be a good predictor of IVF and PGD outcome, with samples with more than 60% abnormal forms indicating poor prognosis. Similarly, the predictability from first PGD cycle results for future cycles was 90%. In summary, PGD can help translocation carriers to achieve viable pregnancies, but the success of the process is conversely related to the baseline of unbalanced gametes.

Preimplantation Genetic Diagnosis:

Preimplantation genetic diagnosis (PGD) can provide genetic information on embryos obtained through in vitro fertilization (IVF), allowing implantation of embryos identified as unaffected with a given genetic or chromosomal disorder. With the availability of increasingly sophisticated genetic testing, its use has advanced from the selection of female embryos for the prevention of X-linked genetic diseases to testing for single gene disorders via PCR. Recently, PGD has also been used in the setting of assisted reproductive technology to select for chromosomally normal embryos in an effort to increase the rates of implantation and successful pregnancy. As the number of patients undergoing IVF increases, the indications for its use broadens, and more mutations underlying genetic disorders are identified, PGD is becoming more widespread. As this evolution continues, recognition of the limitations of PGD, as well as ethical concerns regarding use and misuse of this technology, need to be considered by patients, clinicians, and policy makers.

Negligible interchromosomal effect in embryos of Robertsonian translocation carriers

It has been suggested that translocations, and perhaps other chromosome rearrangements, disturb meiotic disjunction of chromosome pairs not involved in the translocation, resulting in non-disjunction in those chromosomes (interchromosomal effect) and predisposition to trisomy offspring. Other reports have suggested an increased risk of mosaicism and chaotic embryos in translocation carriers. This study was designed to determine if such interchromosomal effects are producing significantly more chromosome abnormalities than those expected from unbalanced gametes. For that purpose, two groups of PGD patients were compared, Robertsonian translocation carriers (RBT) and carriers of X-linked diseases (XLI), of similar age. Both groups were analysed by FISH with similar DNA probes. The results indicate that overall, the higher rate of chromosome abnormalities in the RBT group was solely due to unbalanced gametes and not to an interchromosomal effect or higher incidence of mosaicism. If unbalanced and normal were combined, this proportion was 53% in XLI and 59% in RBT. However, when specific RBT translocations were studied, only a slight increase in embryos with aneuploidy for chromosome 22 was found for the t(13;14) translocation carriers, while a higher rate of post-zygotic abnormalities was observed in the more rare RBT. In conclusion, the overall rate of non-translocation related abnormalities was not increased in the RBT group compared with the control group, but a slight interchromosomal effect may exist, as some Robertsonian translocations may be more prone to produce mosaic and chaotic embryos.

Preimplantation genetic diagnosis and chromosome analysis of blastomeres using comparative genomic hybridization

Numerical chromosome errors are known to be common in early human embryos and probably make a significant contribution to early pregnancy loss and implantation failure in IVF patients. Over recent years fluorescent in situ hybridization (FISH) has been used to document embryonic aneuploidies. Many IVF laboratories perform preimplantation genetic diagnosis (PGD) with FISH to select embryos that are free from some aneuploidies in an attempt to improve implantation, pregnancy and live birth rates in particular categories of IVF patients. The usefulness of FISH is limited because only a few chromosomes can be detected simultaneously in a single biopsied cell. Complete karyotyping at the single cell level can now be achieved by comparative genomic hybridization (CGH). CGH enables not only enumeration of all chromosomes but gives a more complete picture of the entire length of each chromosome and has demonstrated that chromosomal breakages and partial aneuploidies exist in embryos. CGH has provided invaluable information about the extent of mosaicism and aneuploidy of all chromosomes in early human conceptuses. CGH has been applied to clinical PGD and has resulted in the birth of healthy babies from embryos whose full karyotype was determined in the preimplantation phase.

Advances in preimplantation genetic diagnosis

Strategies for preimplantation genetic diagnosis (PGD) have become increasingly complex. For single gene disorders it is now usual for several DNA fragments to be simultaneously amplified using multiplex-PCR. This allows redundant diagnostic loci to be analyzed, reducing the chance of misdiagnosis due to allele dropout (ADO). Additionally, hypervariable 'fingerprinting' loci can be amplified, revealing the presence of DNA contaminants. Chromosomal screening has also increased in complexity. Current FISH techniques investigate up to nine chromosomes per cell and are offered to an increasingly wide range of patients, including women of advanced reproductive age and those with a history of repeated spontaneous abortion. Technical limitations, which preclude a full assessment of all chromosomes using FISH, have encouraged the development alternative tests. These include nuclear conversion, comparative genomic hybridization (CGH) and the use of DNA microarray 'chip' technology. This paper discusses technical innovations that have improved the scope and accuracy of PGD, as well as the emergence of new indications for PGD that are sometimes considered controversial (e.g. HLA-typing).

Implications of cloning technique for reproductive medicine

The birth of Dolly following the transfer of mammary gland nuclei into enucleated eggs established cloning as a feasible technique in mammals, but the moral implications and high incidence of developmental abnormalities associated with cloning have induced the majority of countries to legislate against its use with human gametes. Because of such negative connotations, restrictive political reactions could jeopardize the therapeutic and scientific promise that certain types of cloning may present. For example, in addition to its proposed use as a way of generating stem cells, the basic technique of nuclear transplantation has proven useful in other ways, including its application to immature eggs as a new approach to the prevention of the aneuploidy common in older women, and for some recent advances in preimplantation genetic diagnosis. Thus, while attempts at reproductive cloning in man would seem premature and even dangerous at present, this field will require rational rather than emotional reactions as a basis for legislation if the therapeutic promise of stem cell research and the experimental potential of nuclear transplantation techniques are to be fully realized.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD) is an exciting new approach for the prevention of transmission of genetic disorders between generations. The use of genetically screened, healthy embryos to establish a pregnancy avoids the need for termination of an affected pregnancy, a procedure which can be traumatic physically and emotionally for potential patients, and is sometimes not available when needed. PGD usually follows the processes of ovarian hyperstimulation and in vitro fertilization (IVF). After carrying out appropriate genetic tests, only normal embryos are transferred to the patient's uterus to achieve an unaffected pregnancy and the birth of a healthy infant.

The role of preimplantation genetic diagnosis in women of advanced reproductive age

PURPOSE OF REVIEW

More than half of in-vitro fertilization patients are of advanced reproductive age and at risk for producing offspring with age-related aneuploidies, which contribute significantly to spontaneous abortions and implantation failure.

RECENT FINDINGS

Fluorescent in-situ hybridization analysis of thousands of oocytes and preimplantation embryos obtained from these patients revealed an aneuploidy rate of over 50%, suggesting practical relevance of preimplantation genetic diagnosis for aneuploidy to women of advanced reproductive age. The overall preimplantation genetic diagnosis experience for age-related aneuploidies comprising more than 3000 clinical cycles indicates the positive impact of preselection and transfer of aneuploidy-free embryos on implantation and pregnancy rates and outcome of pregnancies in women of advanced reproductive age.

SUMMARY

These patients will need to be informed about preimplantation genetic diagnosis availability, in order use this option to improve their relatively poor chances of becoming pregnant, especially with the current tendency of limiting the number of transferred embryos to avoid complications due to multiple pregnancies. This may contribute significantly to improving standards of assisted reproduction technology, substituting the current practice of selection of embryos for transfer using morphological parameters with the preselection of aneuploidy-free embryos with a higher potential to result in pregnancy.

Predictive value of sperm fluorescence in situ hybridization analysis on the outcome of preimplantation genetic diagnosis for translocations

OBJECTIVE

To determine whether the proportion of abnormal sperm is predictive of the proportion of abnormal embryos from couples in which the males are translocation carriers.

DESIGN

Controlled clinical study.

SETTINGS

Private in vitro fertilization (IVF) center.

PATIENT(S)

Eleven cases of reciprocal translocation male carriers.

INTERVENTION(S)

Blood sample and sperm sample collection from each male partner. Embryo biopsy of the embryos produced in each cycle.

MAIN OUTCOME MEASURE(S)

Fluorescence in situ hybridization on lymphocyte slides to characterize each translocation case, then fluorescent in situ hybridization (FISH) with specific probes for each of the sperm samples. Preimplantation genetic diagnosis of the translocations in the 11 cases.

RESULTS

A correlation was found between the percentage of abnormal gametes and the percentage of abnormal embryos, and a predictive equation is proposed for this relationship: A = -55 + (1.9 x B), where A is the percentage of abnormal embryos and B the percentage of abnormal sperm.

CONCLUSION(S)

The predictive value of the sperm analysis was established. Patients with 65% or less chromosomally abnormal sperm have a good chance at conceiving; patients with higher rates would need to produce 10 or more good quality embryos to have reasonable chances of conceiving.

Cytogenetics in reproductive medicine: the contribution of comparative genomic hybridization (CGH)

Cytogenetic research has had a major impact on the field of reproductive medicine, providing an insight into the frequency of chromosomal abnormalities that occur during gametogenesis, embryonic development and pregnancy. In humans, aneuploidy has been found to be relatively common during fetal life, necessitating prenatal screening of high-risk pregnancies. Aneuploidy rates are higher still during the preimplantation stage of development. An increasing number of IVF laboratories have attempted to improve pregnancy rates by using preimplantation genetic diagnosis (PGD) to ensure that the embryos transferred to the mother are chromosomally normal. This paper reviews some of the techniques that are key to the detection of aneuploidy in reproductive samples including comparative genomic hybridization (CGH). CGH has provided an unparalleled insight into the nature of chromosome imbalance in human embryos and polar bodies. The clinical application of CGH for the purposes of PGD and the future extensions of the methodology, including DNA microarrays, are discussed.

Methods in preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD) is a new strategy, orientated toward primary prevention of congenital anomalies in couples with reproductive risk, such as advanced maternal age, carriers of chromosomal abnormalities, and carriers of monogenic conditions. For these patients, PGD is an acceptable alternative to prenatal diagnosis, mainly in those countries where pregnancy interruption is forbidden by law. PGD effectively avoids the implications linked to traditional prenatal diagnosis. Centres that provide medical servicies on reproductive biomedicine are responsible for the development and improvement of this new prevention strategy. Thanks to advances in micromanipulation techniques, associated with recent progress in molecular genetics, PGD may be employed for any genetic condition in the future.

Cytogenetic analysis of human blastocysts

The human blastocyst is key to understanding the aetiology of constitutional chromosome abnormalities in our species.

OBJECTIVES

To investigate the range and incidence of chromosome abnormalities in a large series of human blastocysts, using classic cytogenetic techniques.

METHODS

Using thymidine, cell division is synchronized in spare five-to-eight-day-old human blastocysts generated by IVF. A simple acetic acid disaggregation step produces discrete metaphases for G-band analysis. Subsequent FISH analysis of both metaphase and interphase nuclei allows further exploration of an abnormality detected by G-banding, including the investigation of any mosaicism.

RESULTS

A total of 438 blastocysts have been prepared. Where analysis was possible, 3% appeared polyploid (mainly tetraploid), 29% were diploid : tetraploid mosaics and 68% were uniformly diploid. Abnormalities observed include triploidy, trisomy 16, trisomy 2, trisomy for unidentifiable D-group chromosome, mosaic trisomy 3, and mosaic trisomy 3 and trisomy 7.

CONCLUSION

Comparison of results with existing data from both first trimester pregnancies and cleavage stage embryos suggests significant loss of haploid and monosomic embryos, as well as loss of some trisomies, prior to the blastocyst stage. It appears that the general range and incidence of most main groups of constitutional abnormalities observed in the first trimester (including mosaic forms) are in place by the blastocyst stage.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD) is an evolving technique that provides a practical alternative to prenatal diagnosis and termination of pregnancy for couples who are at substantial risk of transmitting a serious genetic disorder to their offspring. Samples for genetic testing are obtained from oocytes or cleaving embryos after in vitro fertilization. Only embryos that are shown to be free of the genetic disorders are made available for replacement in the uterus, in the hope of establishing a pregnancy. PGD has provided unique insights into aspects of reproductive genetics and early human development, but has also raised important new ethical issues about assisted human reproduction.

Nuclear transfer for full karyotyping and preimplantation diagnosis for translocations

Preimplantation genetic diagnosis (PGD) for chromosomal disorders is currently performed by interphase fluorescence in-situ hybridization (FISH) analysis. This technique is known to have limitations for detecting some translocations and complete karyotyping, so visualization of chromosomes in single cells is required to improve the PGD accuracy for chromosomal disorders. To achieve this, single blastomeres were fused with enucleated or intact mouse zygotes, followed by fixing the resulting heterokaryons at the metaphase of the first cleavage division, or treating them with okadaic acid to induce premature chromosome condensation. This method allowed a significant improvement in the accuracy of testing both maternally- and paternally-derived translocations. In all, 437 blastomeres were tested, achieving full karyotyping in as many as 383 (88%), making it possible to pre-select only normal embryos or those with balanced chromosomal complements for transfer. Overall, PGD for translocations was applied in 94 clinical cycles, resulting in 66 transfers and 20 (30.3%) clinical pregnancies, with healthy deliveries of 15 children. Fifty-two of these cycles were performed using a nuclear transfer (conversion) technique, which resulted in 38 transfers of balanced or normal embryos, demonstrating that the technique is accurate and reliable for karyotyping single blastomeres for PGD of translocations.

Preimplantation genetic diagnosis for aneuploidy screening in early human embryos: a review

Embryonic aneuploidies may be responsible for pregnancy failure in many IVF patients. In recent years, fluorescent in situ hybridisation (FISH) for multiple chromosomes has been used to document a high frequency of chromosomal errors and aneuploidy in human preimplantation embryos and, after embryo biopsy, to select embryos that are more likely to implant. Such studies suggest that women with recurrent miscarriage and advanced maternal age may benefit most from preimplantation genetic diagnosis with aneuploidy screening (PGD-AS). The success of PGD-AS is likely to be enhanced by new technologies, such as comparative genomic hybridisation, which enable full karyotyping of single cells.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis is essentially an alternative to prenatal diagnosis, in which genetic testing is performed on embryos before a clinical pregnancy is established. Preimplantation genetic diagnosis has been applied to patients carrying chromosomal rearrangements, such as translocations, in which it has been proven to decrease the number of spontaneous abortions and prevent the birth of children affected with chromosome imbalance. Preimplantation genetic diagnosis techniques have also been applied to increase implantation rates, reduce the incidence of spontaneous abortion and prevent trisomic offspring in women of advanced maternal age undergoing fertility treatment. A third group of patients receiving preimplantation genetic diagnosis are those at risk of transmitting a single gene disorder to their children. The number of monogenic disorders that have been diagnosed in preimplantation embryos has increased each year. Recent protocols have tended to be more complex and more reliable than previous methods, making greater use of multiplex polymerase chain reaction. As well as an expansion in the variety of disorders for which preimplantation genetic diagnosis is offered, new indications have been reported including the use of human leukocyte antigen histocompatibility typing and the application of preimplantation genetic diagnosis to late onset diseases.

Preimplantation genetic screening and human implantation

In recent years, preimplantation genetic screening (PGS) has been used and recommended to increase the implantation rate in older women or in couples with previous assisted reproduction (ART) failures, to try to increase pregnancy rates in couples with recurrent abortions, to prevent the transmission of chromosome anomalies to the offspring of carriers of balanced chromosomal rearrangements, or even to try to decrease the incidence of trisomic births in older women. So far, PGS has contributed to increase the implantation rate in older women; however, the rate of clinical pregnancies has not increased, either in older women or in couples with previous ART failures. In couples with recurrent abortions, the pregnancy rate seems to increase, but only when the woman is young (< or =35). In carriers of balanced reorganizations, the prognosis is poor. Attempts to decrease the birth of trisomic children to older women are difficult to evaluate. This absence of relevant results is not related to the technique itself, which is quite safe, but to other still largely unknown factors.

Preimplantation genetic diagnosis of numerical and structural chromosome abnormalities

The causes of the decline in implantation rates observed with increasing maternal age are still a matter for debate. Data from oocyte donation strongly suggest that in women of advanced reproductive age, the ability to become pregnant is largely unaffected while oocyte quality is compromised. The incidence of chromosomal abnormalities in embryos is considerably higher than that reported in spontaneous abortions, suggesting that a sizable percentage of chromosomally abnormal embryos are eliminated before any prenatal diagnosis. Such loss may partly account for the decline in implantation in older women. Because of the correlation between aneuploidy and reduced implantation, it has been postulated that selection of chromosomally normal embryos could reverse this trend. Preimplantation genetic diagnosis (PGD) for aneuploidy had three objectives relevant to the present paper: (i) to increase rates of implantation, (ii) to reduce risks of spontaneous abortion, and (iii) to avoid chromosomally abnormal births. Implantation rates did not increase when only five chromosomes were analysed in blastomeres. With eight chromosomes, a significant increase in implantation was achieved. PGD can significantly reduce the incidence of spontaneous abortion. In our clinic, a significant decrease in spontaneous abortions was found, from 23 to 11% after PGD. Currently in cases diagnosed at Saint Barnabas, 0.8% chromosomally abnormal conceptions have been observed after PGD versus an expected 3.2% in a control age-matched group. It seems clear that PGD reduces the possibility of trisomic conceptions under all conditions. If a couple's main interest is to improve their chances of conceiving (improve implantation), then one should consider maternal age and number of available embryos. Improvements in conception after PGD again increase after 37 years of age with eight or nine probes. Carriers of translocations are at a high risk of miscarriage or chromosomally unbalanced offspring, and a high proportion have secondary infertility. PGD of translocations has been approached through a variety of methods, here reviewed, and has resulted in a significant reduction in spontaneous abortions. However, implantation rates in translocation carriers are directly correlated with the proportion of normal gametes, and male patients with 70% or more unbalanced spermatozoa have great difficulty in achieving pregnancy with PGD.

Preimplantation genetic diagnosis of pericentric inversions

Inversions are structural chromosome abnormalities that may be associated with infertility, multiple miscarriage and chromosomally unbalanced offspring. Preimplantation genetic diagnosis (PGD) with subtelomeric probes was used to select for transfer only those embryos that were normal or balanced for three pericentric inversions. In contrast to previous protocols the present procedure allows the detection of unbalanced embryos that might arise from U-recombination in the inverted region. Additionally, aneuploidy screening was carried out in two cases by a second round of fluorescent in situ hybridization (FISH) with centromeric probes. Of the three couples that underwent the procedure one became pregnant twice. The first pregnancy delivered a healthy and chromosomally normal baby and the second pregnancy is ongoing with triplets.

Preimplantation genetic diagnosis: applications for molecular medicine

Preimplantation genetic diagnosis is an alternative to prenatal diagnosis for the detection of genetic disorders. Tests are conducted on single cells biopsied from embryos before they are implanted, allowing the selection of unaffected embryos before a pregnancy has been established. Thus, the issue of pregnancy termination is circumvented. The use of preimplantation genetic diagnosis might have a significant impact on in vitro fertilization success rates as well as allowing the diagnosis of inherited disease.

Multilocus genetic analysis of single interphase cells by spectral imaging

Numerical chromosome aberrations are detrimental to early embryonic, fetal and perinatal development of mammals. When fetuses carrying a chromosomal imbalance survive to term, an aberrant gene dosage typically leads to stillbirth or causes a severely altered phenotype. Aneuploidy of any of the 24 chromosomes will negatively impact on human development, and a preimplantation and prenatal genetic diagnosis test should thus score as many chromosomes as possible. Since cells available for analysis are likely to be in interphase, we set out to develop a rapid enumeration procedure based on hybridization of chromosome-specific probes and spectral imaging detection. The probe set was chosen to allow the simultaneous enumeration of ten chromosome types and was expected to detect more than 70% of all numerical chromosome aberrations responsible for spontaneous abortions, i.e., human chromosomes 9, 13, 14, 15, 16, 18, 21, 22, X, and Y. Cell fixation protocols were optimized to achieve the desired detection sensitivity and reproducibility. We were able to resolve and identify ten separate chromosomal signals in interphase nuclei from different types of cells, including lymphocytes, uncultured amniocytes, and blastomeres. In summary, this study demonstrates the strength of spectral imaging, allowing us to construct partial spectral imaging karyotypes for individual interphase cells by assessing the number of each of the target chromosome types.

Effect of chromosomal translocations on the development of preimplantation human embryos in vitro

OBJECTIVE

To determine the reliability of a new technique for single human blastomere karyotyping during clinical cases for preimplantation genetic diagnosis of translocations.

DESIGN

Controlled clinical study.

SETTING

Preimplantation genetic diagnosis and IVF program.

PATIENT(S)

Nineteen preimplantation genetic diagnosis cases with 11 types of translocations (10 reciprocal and one Robertsonian) involving chromosomes 1, 5, 7, 8, 9, 11, 12, 13, 14, 15, 16, 18, 20, 21, and 22.

INTERVENTION(S)

Blastomere biopsy followed by blastomere nucleus conversion into metaphase chromosomes. Fluorescent in situ hybridization (whole chromosome painting) was used for the detection of chromosomally unbalanced preimplantation human embryos.

MAIN OUTCOME MEASURE(S)

Percentage of informative metaphase plates and effect of unbalanced translocations on preimplantation embryo development.

RESULT(S)

Informative metaphases were obtained for 84% of the blastomeres. Analysis of preimplantation development of the resulting embryos showed that an unbalanced chromosomal complement does not affect embryo ability to reach the blastocyst stage in vitro.

CONCLUSION(S)

For the translocations tested, there is no evident selection against chromosomally unbalanced embryos at the preimplantation stage of embryo development.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD) includes a variety of techniques that have been developed to detect the transmission to the offspring of genetic diseases or of chromosome abnormalities by couples at risk before a pregnancy is established, to avoid these couples the risk of recurrent abortions and/or of repeated terminations of pregnancy. Candidate couples are carriers of gene mutations or of structural chromosome rearrangements, or with recurrent spontaneous abortions of unknown origin. Diagnostic procedures include different modalities of gene amplification using the polymerase chain reaction (PCR) or of fluorescent in situ hybridization (FISH). Embryo biopsies are carried out at the 6-8 cell stage. Healthy embryos are transferred on day 4 or at the blastocyst stage. By now, several hundred healthy children have been born using PGD, and only one diagnostic error has been reported.

Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis (PGD) was developed in the UK over 10 years ago. There are now more than 40 centres worldwide carrying out PGD and 150 babies have been born after genetic testing on day 3 of development, at the cleavage stage. This review covers the current status of PGD, the technology used and the types of genetically determined diseases for which testing has been developed.

Outcome of preimplantation genetic diagnosis of translocations

OBJECTIVE

To review 35 cases of preimplantation genetic diagnosis (PGD) of translocations with several methods, including telomeric probes.

DESIGN

Retrospective study.

SETTING

Clinical IVF laboratory.

PATIENT(S)

Thirty-five couples with one partner carrying a chromosomal translocation.

INTERVENTION(S)

PGD of translocation after polar-body or embryo biopsy.

MAIN OUTCOME MEASURE(S)

Pregnancy outcome.

RESULT(S)

Several trends were observed. First, PGD can achieve a statistically significant reduction in spontaneous abortion, from 95% to 13%. Second, the chances of achieving pregnancy are correlated with 50% or more of the embryos being chromosomally normal. Third, patients with robertsonian translocations produced fewer abnormal gametes and more pregnancies than did patients with reciprocal translocations. Fourth, a new fluorescence in situ hybridization protocol for PGD of translocations, which involves applying telomeric probes, has proved adequately reliable with a 6% average error rate.

CONCLUSION(S)

PGD of translocations achieves a statistically significant reduction in spontaneous abortion, both for polar-body and blastomere biopsy cases. Pregnancy outcome depended on the number of normal embryos available for transfer, with patients having <50% abnormal embryos achieving the most pregnancies. Because robertsonian translocations caused fewer abnormal embryos than reciprocal translocations, they also resulted in higher rates of implantation.

Preimplantation genetic diagnosis

Embryo biopsy for preimplantation genetic diagnosis can be performed on the oocyte/zygote, cleavage stage embryo, or blastocyst, but the majority of centres perform cleavage stage biopsy. Single-cell diagnosis is undertaken by the polymerase chain reaction or fluorescent in-situ hybridization. Technical difficulties have arisen with preimplantation genetic diagnosis, such as allele dropout and chromosomal mosaicism. However, it is hoped that these difficulties can be overcome in the future with the advent of new techniques.

Preimplantation genetic diagnosis of a reciprocal translocation t(3;11)(q27.3;q24.3) in siblings

Preimplantation genetic diagnosis (PGD) was performed in two couples to avoid chromosomally unbalanced progeny in a family in which a brother and a sister carry an identical maternally inherited balanced translocation t(3;11)(q27.3;q24.3). Embryos were biopsied 3 days after fertilization and blastomeres were analysed by fluorescent in-situ hybridization (FISH). Embryos were classified as unbalanced or normal/balanced. In the first case, the male carrier and his wife underwent one IVF/PGD treatment cycle. In all, 18 embryos were analysed. Of those, 15 revealed an unbalanced karyotype. For one embryo, results were not conclusive, from one embryo results were contradictory and one embryo was classified as normal/balanced and subsequently transferred. A singleton pregnancy was achieved. The PGD analysis was confirmed at 16 weeks gestation by amniocentesis. At term, a healthy girl with a balanced karyotype was born. Pregnancy and delivery were without complications. In the second case, the female carrier and her husband underwent two IVF/PGD treatment cycles. During the first cycle, three embryos were analysed. One embryo revealed an unbalanced karyotype and two embryos were designated a normal/balanced karyotype and transferred but no pregnancy was achieved. During the second PGD cycle three embryos were analysed. Of those, none appeared suitable for transfer. The couple decided not to undergo further treatment. Our results indicate that for individuals carrying a reciprocal translocation PGD is a feasible approach to obtain embryos with a normal chromosome balance and to avoid both spontaneous and induced abortion.