Screening oocytes and preimplantation embryos for aneuploidy

Is the hypothesis of preimplantation genetic screening (PGS) still supportable? A review

The hypothesis of preimplantation genetic diagnosis (PGS) was first proposed 20 years ago, suggesting that elimination of aneuploid embryos prior to transfer will improve implantation rates of remaining embryos during in vitro fertilization (IVF), increase pregnancy and live birth rates and reduce miscarriages. The aforementioned improved outcome was based on 5 essential assumptions: (i) Most IVF cycles fail because of aneuploid embryos. (ii) Their elimination prior to embryo transfer will improve IVF outcomes. (iii) A single trophectoderm biopsy (TEB) at blastocyst stage is representative of the whole TE. (iv) TE ploidy reliably represents the inner cell mass (ICM). (v) Ploidy does not change (i.e., self-correct) downstream from blastocyst stage. We aim to offer a review of the aforementioned assumptions and challenge the general hypothesis of PGS. We reviewed 455 publications, which as of January 20, 2017 were listed in PubMed under the search phrase < preimplantation genetic screening (PGS) for aneuploidy>. The literature review was performed by both authors who agreed on the final 55 references. Various reports over the last 18 months have raised significant questions not only about the basic clinical utility of PGS but the biological underpinnings of the hypothesis, the technical ability of a single trophectoderm (TE) biopsy to accurately assess an embryo’s ploidy, and suggested that PGS actually negatively affects IVF outcomes while not affecting miscarriage rates. Moreover, due to high rates of false positive diagnoses as a consequence of high mosaicism rates in TE, PGS leads to the discarding of large numbers of normal embryos with potential for normal euploid pregnancies if transferred rather than disposed of. We found all 5 basic assumptions underlying the hypothesis of PGS to be unsupported: (i) The association of embryo aneuploidy with IVF failure has to be reevaluated in view how much more common TE mosaicism is than has until recently been appreciated. (ii) Reliable elimination of presumed aneuploid embryos prior to embryo transfer appears unrealistic. (iii) Mathematical models demonstrate that a single TEB cannot provide reliable information about the whole TE. (iv) TE does not reliably reflect the ICM. (v) Embryos, likely, still have strong innate ability to self-correct downstream from blastocyst stage, with ICM doing so better than TE. The hypothesis of PGS, therefore, no longer appears supportable. With all 5 basic assumptions underlying the hypothesis of PGS demonstrated to have been mistaken, the hypothesis of PGS, itself, appears to be discredited. Clinical use of PGS for the purpose of IVF outcome improvements should, therefore, going forward be restricted to research studies.

Technical Update: Preimplantation Genetic Diagnosis and Screening

OBJECTIVE

To update and review the techniques and indications of preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS).

OPTIONS

Discussion about the genetic and technical aspects of preimplantation reproductive techniques, particularly those using new cytogenetic technologies and embryo-stage biopsy.

OUTCOMES

Clinical outcomes of reproductive techniques following the use of PGD and PGS are included. This update does not discuss in detail the adverse outcomes that have been recorded in association with assisted reproductive technologies.

EVIDENCE

Published literature was retrieved through searches of The Cochrane Library and Medline in April 2014 using appropriate controlled vocabulary (aneuploidy, blastocyst/physiology, genetic diseases, preimplantation diagnosis/methods, fertilization in vitro) and key words (e.g., preimplantation genetic diagnosis, preimplantation genetic screening, comprehensive chromosome screening, aCGH, SNP microarray, qPCR, and embryo selection). Results were restricted to systematic reviews, randomized controlled trials/controlled clinical trials, and observational studies published from 1990 to April 2014. There were no language restrictions. Searches were updated on a regular basis and incorporated in the update to January 2015. Additional publications were identified from the bibliographies of retrieved articles. Grey (unpublished) literature was identified through searching the websites of health technology assessment and health technology-related agencies, clinical practice guideline collections, clinical trial registries, and national and international medical specialty societies.

VALUES

The quality of evidence in this document was rated using the criteria described in the Report of the Canadian Task Force on Preventive Health Care. (Table 1) BENEFITS, HARMS, AND COSTS: This update will educate readers about new preimplantation genetic concepts, directions, and technologies. The major harms and costs identified are those of assisted reproductive technologies.

SUMMARY

Preimplantation genetic diagnosis is an alternative to prenatal diagnosis for the detection of genetic disorders in couples at risk of transmitting a genetic condition to their offspring. Preimplantation genetic screening is being proposed to improve the effectiveness of in vitro fertilization by screening for embryonic aneuploidy. Though FISH-based PGS showed adverse effects on IVF success, emerging evidence from new studies using comprehensive chromosome screening technology appears promising. Recommendations 1. Before preimplantation genetic diagnosis is performed, genetic counselling must be provided by a certified genetic counsellor to ensure that patients fully understand the risk of having an affected child, the impact of the disease on an affected child, and the benefits and limitations of all available options for preimplantation and prenatal diagnosis. (III-A) 2. Couples should be informed that preimplantation genetic diagnosis can reduce the risk of conceiving a child with a genetic abnormality carried by one or both parents if that abnormality can be identified with tests performed on a single cell or on multiple trophectoderm cells. (II-2B) 3. Invasive prenatal or postnatal testing to confirm the results of preimplantation genetic diagnosis is encouraged because the methods used for preimplantation genetic diagnosis have technical limitations that include the possibility of a false result. (II-2B) 4. Trophectoderm biopsy has no measurable impact on embryo development, as opposed to blastomere biopsy. Therefore, whenever possible, trophectoderm biopsy should be the method of choice in embryo biopsy and should be performed by experienced hands. (I-B) 5. Preimplantation genetic diagnosis of single-gene disorders should ideally be performed with multiplex polymerase chain reaction coupled with trophectoderm biopsy whenever available. (II-2B) 6. The use of comprehensive chromosome screening technology coupled with trophectoderm biopsy in preimplantation genetic diagnosis in couples carrying chromosomal translocations is recommended because it is associated with favourable clinical outcomes. (II-2B) 7. Before preimplantation genetic screening is performed, thorough education and counselling must be provided by a certified genetic counsellor to ensure that patients fully understand the limitations of the technique, the risk of error, and the ongoing debate on whether preimplantation genetic screening is necessary to improve live birth rates with in vitro fertilization. (III-A) 8. Preimplantation genetic screening using fluorescence in situ hybridization technology on day-3 embryo biopsy is associated with decreased live birth rates and therefore should not be performed with in vitro fertilization. (I-E) 9. Preimplantation genetic screening using comprehensive chromosome screening technology on blastocyst biopsy, increases implantation rates and improves embryo selection in IVF cycles in patients with a good prognosis. (I-B).

Routine morphological scoring systems in assisted reproduction treatment fail to reflect age-related impairment of oocyte and embryo quality

Routine morphological scoring systems in assisted reproduction treatment are based on parameters that presumably correlate with the biological quality of gametes and embryos, including chromosome abnormalities. Maternal age is a key factor predicting pregnancy and live birth, and it is therefore of considerable interest to identify age-related indicators of oocyte and embryo quality in assisted reproduction treatment. The purpose of this study was to examine whether routine morphological scoring systems reflect age-related impact on oocyte and embryo quality among 4587 couples undergoing their first assisted reproduction treatment. This study assessed over 43,000 oocytes, 25,000 embryos and 7900 transferred embryos and analysed the associations among the following parameters: number of oocytes retrieved, oocyte quality, including maturity, fertilization rates, embryo quality, based on morphological features, and treatment outcome. Advanced chronological age was found to be associated with fewer oocytes retrieved, fewer embryos available for cryopreservation, as well as lower pregnancy, implantation, live birth rates and a higher miscarriage rate. No age-related correlation was found between fertilization rates, oocyte or embryo quality. Routinely-used morphological scoring systems, such as assessment of blastomere count, shape and fragmentation, fail to reflect age-related impact on oocyte and embryo quality.

Preimplantation genetic diagnosis for beta-thalassemia using single-cell DNA analysis for codons 17 and 26 of beta-globin gene

BACKGROUND

Preimplantation genetic diagnosis (PGD) of monogenic autosomal hereditary disorders following assisted conception usually involves the removal of one or two blastomeres from preimplantation embryos. However, the amount of DNA from a single blastomere is insufficient to amplify the region of interest. Hence, the whole genome amplification (WGA) method is performed prior to amplifying the genes of interest before analysis of DNA material through polymerase chain reaction (PCR).

METHODS

In the present study we report that WGA from a single blastomere extracted from unwanted preimplantation human embryos (obtained from 10 infertile couples) could positively yield microgram quantities of amplified DNA allowing PCR analysis for codons 17 and 26 of the beta-globin gene that cause the beta-thalassemia disorder. We developed a rapid and highly specific technique of single-cell PCR to amplify a specific region on the beta-globin gene for codon 17 (AAG-->TAG) and codon 26 (GAG-->AAG) by using single-cell PCR.

RESULTS

About 249 bp of amplicon for codon 17 and about 200 bp of amplicon for codon 26 were successfully amplified. No mutations were observed. Analyzed embryos were not transferred back to patients because the embryos used as samples were wasted embryos.

CONCLUSIONS

Compared to other approaches for prenatal diagnosis, PGD is rapid and suitable as a noninvasive clinical tool for identifying genetic disorders for the purpose of reducing selective miscarriages and moral dilemmas. We opine that DNA extraction and amplification can be successfully performed by using single-cell PCR to diagnose genetic diseases before pregnancy.

Single-cell DNA and FISH analysis for application to preimplantation genetic diagnosis

The goal of preimplantation genetic diagnosis (PGD) is to avoid transfer of embryos affected with a specific genetic disease or condition. This unit describes the steps involved in amplifying DNA from a single blastomere and specific assays for detecting a variety of DNA mutations. For some assays, whole-genome amplification by primer-extention preamplification (PEP) is performed prior to analysis. Support protocols describe the biopsy of one or two blastomeres from the developing preimplantation embryo, isolation for further investigation of all blastomeres from embryos shown to have the mutant allele, and isolation of single lymphocytes or lymphoblastoid cells as models for single-cell DNA analysis. A procedure for FISH analysis on single interphase blastomeres is provided along with a support protocol for probe validation that is recommended as a preliminary step before performing any PGD FISH analysis.

Successful pregnancy outcomes after preimplantation genetic diagnosis (PGD) for carriers of chromosome translocations

Reciprocal translocations are found in about 1 in 500 people, whereas Robertsonian translocations occur with a prevalence of 1 in 1000. Balanced carriers of these rearrangements, although phenotypically normal, may present with infertility, recurrent miscarriage, or offspring with an abnormal phenotype after segregation of the translocation at meiosis. Once the translocation has been identified, prenatal diagnosis can be offered, followed by termination of pregnancies with chromosome imbalance. Couples who have suffered repeated miscarriage or those who have undergone termination of pregnancy as a result of the translocation carrier status of one partner are looking increasingly to preimplantation genetic diagnosis (PGD) as a way of achieving a normal pregnancy. Similarly, infertile couples in which one partner is a translocation carrier may request PGD to ensure transfer of normal embryos after in vitro fertilization. Translocation PGD has been applied successfully in several centres worldwide and should now be considered as a realistic treatment option for translocation carriers who do not wish to trust to luck for a successful natural outcome.

High rates of embryonic loss, yet high incidence of multiple births in human ART: is this paradoxical?

Humans have low natural fecundity, as the probability of establishing a viable conception in any one menstrual cycle is 20-25% for a healthy, fertile couple. There are numerous underlying causes for this low rate of human fertility, not the least of which are intrinsic abnormalities within the oocyte and/or embryo, which likely account for greater than 50% of failed conceptions. During assisted reproduction technology (ART) interventions, controlled ovarian stimulation is used to obtain several oocytes in attempts to increase the likelihood of having at least one developmentally competent embryo available for transfer. However, current techniques for identifying the competent embryo(s) are by no means perfect. These limitations, coupled with pressures to maximize the chance of pregnancy, typically result in the transfer of multiple embryos. Not surprisingly, this practice has resulted in an unacceptably high rate of multiple pregnancies arising from ART. During the last few years, concerted efforts have focused on reducing these rates. Programs for ART are developing patient-specific policies, restricting the number of embryos to transfer. In addition, strategies are being adopted to improve the accuracy for selecting viable embryos for transfer. One such strategy involves further refinement of morphological criteria associated with improved viability by considering, for example, pronuclei disposition, nucleolar organization, and identification of the fast-cleaving embryos with only mononucleate blastomeres. Another strategy employs pre-implantation genetic diagnosis (PGD) whereby a biopsied blastomere is tested for ploidy using fluorescence in situ hybridization (FISH). A final strategy involves extending the duration of culture to the blastocyst stage, thereby allowing self-selection of those embryos capable of proceeding to blastulation and exclusion of those less viable embryos that succumb to developmental arrest. Together, these strategies are enabling fewer embryos of higher quality to be transferred. Accordingly, the overall pregnancy rate from ART continues to increase, while the rate of triplet and higher order multiple births continues to decline. Nevertheless, the high incidence of intrinsic developmental anomalies in human oocytes inevitably will continue to result in a high degree of embryonic loss in ART.

A pregnancy following PGD for X-linked dominant [correction of X-linked autosomal dominant] incontinentia pigmenti (Bloch-Sulzberger syndrome): case report

Incontinentia Pigmenti (Bloch-Sulzberger syndrome) is a rare multisystem, ectodermal disorder associated with dermatological, dental and ocular features, and in <10% of cases, severe neurological deficit. Pedigree review suggests X-linked dominance with lethality in affected males. Presentation in female carriers is variable. Following genetic counselling, a mildly affected female carrier diagnosed in infancy with a de novo mutation was referred for preimplantation sexing, unusually selecting for male gender, with an acceptance of either normality or early miscarriage in an affected male. Following standard in-vitro fertilization and embryo biopsy, fluorescence in situ hybridization (FISH) unambiguously identified two male and two female embryos. A single 8-cell, grade 4 male embryo was replaced. A positive pregnancy test was reported 2 weeks after embryo transfer, although ultrasonography failed to demonstrate a viable pregnancy. Post abortive fetal tissue karyotyping diagnosed a male fetus with trisomy 16. This is an unusual report of preimplantation genetic diagnosis (PGD) being used for selection of males in an X-linked autosomal dominant disorder and demonstrates the value of PGD where amniocentesis or chorion villus sampling followed by abortion is not acceptable to the patient. This case also demonstrates the importance of follow-up prenatal diagnosis.

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.