Preimplantation genetic diagnosis: an update on current technologies and ethical considerations

The aim of reproductive medicine is to support the birth of healthy children. Advances in assisted reproductive technologies and genetic analysis have led to the introduction of preimplantation genetic diagnosis (PGD) for embryos. Indications for PGD have been a major topic in the fields of ethics and law. Concerns vary by nation, religion, population, and segment, and the continued rapid development of new technologies. In contrast to the ethical augment, technology has been developing at an excessively rapid speed. The most significant recent technological development provides the ability to perform whole genome amplification and sequencing of single embryonic cells by microarray or next‐generation sequencing methods. As new affordable technologies are introduced, patients are presented with a growing variety of PGD options. Simultaneously, the ethical guidelines for the indications for testing and handling of genetic information must also rapidly correspond to the changes.     

Reliable preimplantation genetic diagnosis in thawed human embryos vitrified at cleavage stages without biopsy

Purpose  

To evaluate preimplantation genetic diagnosis (PGD) efficiency in thawed human embryos vitrified without biopsy.     

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.     

Preimplantation genetic diagnosis and 'savior siblings'.

From its emergence, preimplantation genetic diagnosis (PGD) has been opposed by religious, feminist, and disability-rights advocates. PGD has developed, however, to extend beyond genetic diagnosis of embryos to diagnose chromosomal abnormalities. Evidence shows that PGD is safe, children born after in vitro fertilization (IVF) and PGD having no higher rate of birth defects than children of normal pregnancies. Laws may accommodate PGD directly or indirectly, but some prohibit PGD totally or except to identify sex-linked genetic disorders. When children suffer severe genetic disorders and require stem-cell transplantation, compatible donors may be unavailable. Then, IVF and PGD of resulting embryos may identify some whose gestation and birth would produce unaffected newborns, and placental and cord blood from which stem-cells compatible for implantation in sick siblings can be derived. Ethical issues concern conscientious objection to direct participation, discarding of healthy but unsuitable embryos, and valuing savior siblings in themselves, not just as means to others' ends.     

植入前遗传学诊断/筛查技术指征进展

植入前遗传学诊断/筛查(PGD/PGS)技术发展多年,其指征始终存在争议。PGD指征较为明确,单基因遗传病、染色体异常人群、人类白细胞抗原(HLA)配型为其适用人群。PGS的指征争议较多,主要面向反复流产、反复植入失败、高龄人群,目的是提高妊娠率及活产率。然而第一代PGS技术[PGS#1,卵裂球活检及荧光原位杂交(FISH)-PGS]技术未显示明显效果,甚至降低了妊娠率及活产率。第二代PGS技术(PGS2.0)增加了严重男性因素不育为指征,其核心为囊胚活检及全染色体筛查(CCS),对上述人群的临床效果较为明显,降低了流产风险并提高了成功率及活产率。PGS2.0已极大地改变了辅助生殖技术(ART)面貌,可能成为未来生殖中心对所有患者的一个常规项目。目前仍然需要多中心前瞻性随机病例对照研究重新评估PGS。     

Preimplantation genetic diagnosis

Since its introduction in 1990, preimplantation genetic diagnosis (PGD) has allowed embryo genetic analysis prior to transfer into the uterus during an in vitro fertilisation (IVF) cycle. The commonest indications are for detection of single-gene, X-linked, structural chromosomal and mitochondrial disorders as well as for Human Leucocyte Antigen matching. Traditional cytogenetic and molecular techniques of fluorescent in situ hybridization and polymerase chain reaction are complemented by newer technologies e.g. array comparative genomic hybridization and whole genome amplification. Following a successful PGD cycle, pregnancy complications are comparable to those in standard IVF/intra-cytoplasmic sperm injection (ICSI) cycles, which are mostly related to preterm delivery due to multiple pregnancies. There are similar rates of congenital malformations, neonatal morbidity and developmental outcomes in children born after PGD compared with those born after IVF/ICSI. Clinical outcome data is collected by the Preimplantation Genetics International Society and the European Society for Human Reproduction and Embryology PGD Consortium.     

Analysis of sex chromosomes in preimplantation genetic diagnosis for X-chromosome-linked disorders

Preimplantation genetic diagnosis (PGD) is diagnostic tool to avoid inheritance of genetic disease by transferring unaffected embryos. Recently, PCR and FISH have been mainly applied to the diagnosis of single gene disorders and chromosomal abnormalities, respectively. Since with PGD, only a few cells are available for genetic tests, both gene and chromosomes analysis have to be obtained from the same, limited material. Cell recycling makes it possible to obtain the information on genes as well as chromosomes from the same cells. Therefore cell recycling is an acceptable strategy where in PGD targets large proportions of embryos severe chromosomal abnormalities. The responsible genes of the X-linked disorder and numerical abnormalities of sex chromosomes should be analyzed simultaneously. Gender information is definitely useful because only male affected embryos should be avoided for transfer.     

Preimplantation genetic diagnosis of alpha-thalassemia-SEA using novel multiplex fluorescent PCR

Purpose  

Preimplantation genetic diagnosis (PGD) is an alternative to prenatal diagnosis (PND) giving couples at risk a chance to start a pregnancy with a disease-free baby. This study aimed to develop a new PGD protocol for alpha-thalassemia^-SEA mutation, the commonest Mendelian disorder.     

父源性染色体异常胚胎植入前遗传学诊断的最新研究进展

染色体异常是导致男性不育的一个重要原因。不同类型染色体异常的生殖细胞进行减数分裂时形成不同类型的配子,且各种异常配子所比例的实际值与理论值不相符。另外,正常胚胎率与正常配子率也不一致。本文对染色体结构异常(相互易位、罗伯逊易位、倒位、染色体复杂性重组)和数目异常(性染色体数目异常、常染色体数目异常)男性的正常精子率以及正常胚胎率进行综述。     

染色体易位的植入前遗传学诊断

目的 观察染色体平衡易位和罗伯逊(罗氏)易位基因携带者夫妇进行植入前遗传学诊断(PGD)后的胚胎染色体遗传特征和胚胎着床、妊娠情况,探讨PGD在染色体易位基因携带者夫妇实现正常生育中的意义.方法 用荧光原位杂交(FISH)技术对36对夫妇的胚胎进行PGD,其中14例为染色体平衡易位(平衡易位组),22例为染色体罗氏易位(罗氏易位组),并对诊断结果和胚胎着床、妊娠情况进行分析.结果 36例患者共活检胚胎253个,成功诊断胚胎225个,成功率为88.9%(225/253),获得可供移植的正常或平衡的胚胎共58个.平衡易位组和罗氏易位组PGD后胚胎着床率分别为36%(5/14)和14%(6/44),临床妊娠率分别为4/9和26%(5/19).结论 PGD可有效诊断胚胎染色体平衡易位和罗氏易位,避免反复流产和不必要的非意愿性终止妊娠,并获得理想的胚胎着床率和临床妊娠率.     

A review of, and commentary on, the ongoing second clinical introduction of preimplantation genetic screening (PGS) to routine IVF practice

Purpose

Current re-introduction of “improved” preimplantation genetic screening (PGS#2) raises the question whether PGS#2 is ready for routine clinical application.

Methods

We assessed available evidence via review of published data for years 2005–2012, and review of currently ongoing registered clinical trials, based on searches under appropriate key words in PubMed, MEDLINE, Cochrane Database System Review and Google Scholar and http://www.ClinicalTrials.gov. In absence of prospective clinical trials, and due to limited available data, individual publications/ongoing studies are assessed.

Results

PGS#2 offers significant improvements in accuracy of aneuploidy diagnosis over PGS#1. By moving embryo biopsy from day-3 after fertilization (6–8 cell stage) to trophectoderm biopsy at blastocyst stage (day 5–6), PGS#2, however, adds additional co-variables to the analysis of efficacy of the procedure, which have special relevance for women with diminished ovarian reserve (DOR), who usually produce small egg and embryo numbers. Limited published data, claiming efficacy of PGS#2, as well as ongoing clinical trials, do not consider these additional co-variables, do not analyze outcomes by intent to treat and, therefore, have to be considered biased in patient selection.

Conclusions

Here reached conclusions are based on absence of adequate data rather than affirmative outcome assessments. They, therefore, are subject to change at any future date with generation of significant new data. Premature introduction of PGS#1 caused significant damage to patients. As currently no reliable PGS#2 data are available to suggest improvements in IVF outcomes, to avoid a repeat of the PGS#1 experience, PGS#2 should be considered experimental until data show otherwise.     

Preimplantation genetic diagnosis and the welfare of the child

Preimplantation genetic diagnosis is a form of very early prenatal diagnosis. The technique combines assisted reproductive technology with molecular genetics and cytogenetics to allow the identification of abnormalities in embryos prior to implantation. Since its introduction in 1990 this approach has been applied to an increasing number of single gene disorders, chromosomal rearrangements, and more recent indications such as aneuploidy screening and HLA matching. Since its inception the technology has attracted much attention: geneticists have expressed concerns about the robustness and validity of diagnosis based on single cell analysis, perinatologists were anxious about the effect of embryo biopsy on normal fetal development; and philosophers and ethicists have argued the cases for and against embryo selection. This article attempts to highlight the difficult choices and ethical challenges confronting patients and clinicians in an effort to balance the recognition of parental autonomy with the obligation of clinics to consider the welfare of any child born as a result of this treatment.     

Preimplantation genetic screening: "established" and ready for prime time?

Unless attempts to improve pregnancy rates and/or diminish miscarriage rates through preimplantation genetic screening (PGS) are applied to only carefully selected patients, they will fail. Because specific PGS indications have remained undefined, PGS should be considered an experimental procedure.