Advances in the field of pre-implantation genetic diagnosis definitely give reasons for satisfaction. Year by year, scientists gain deeper insight into the possibilities of methods like Next-Generation Sequencing (NGS). They are also able to obtain more and more data with the proper level of credibility while analysing even very small quantities of material harvested from an embryo. Reporting recent achievements in this field provides other centres with the possibility to improve their everyday practice. It is also the signal of continuing exploration of the field of human genetics. Now we already know how to obtain information; however, we don’t always know how to interpret and use it. I hope that in the future we will have broader possibilities of the clinical application of the comprehensive analysis of embryonic DNA –both for the needs of PGD and potential gene therapies.
At the present state of science we can hardly imagine the implementation and application of the reports of Complete Genomics Inc.team for the needs of couples with infertility or hereditary genetic load.Actually, this issue is being discussed intensively even by the authors of the reports. Although the whole genome sequencingof embryos in itself is a step forward, it is still not a ready-to-use solution for practical medicine.
Furthermore, the Californian scientists discuss the issue of chances and risks potentially related to the analysis of de novo mutations at the stage of embryonic development. We don’t know all genetic corrective mechanisms emerging at the fetal stage although we observe their presence. Disorders occurring at the molecular level are sometimes eliminated already before birth. It may happen that de novo mutation detected by pre-implantation genetic tests will not manifest at the later stage of life or will not have any significant impact on the health condition of a given person.
With regard to the possibility to obtain complete data on the embryo’s genetic material, questions may arise relating to the directions of development of this branch of medical science and to the future. Will we select embryos for implantation on the basis of features other than those responsible for the most common diseases? Won’t we be tempted to classify embryos to better and worse categories, based on the information on their genotypes? These questions will most probably have to be answered as research progresses.
Whole genome sequencing. New possibility in pre-implantation genetic diagnosis
Anna Palko-Łabuz | 2015-02-26
Since 1978,when the first child conceived by in vitro fertilisation was born, the IVFtechnique has helped in bringing into the world over 5 million babies. There is no question that all parents want their kids to be healthy; for this reason in many specific cases pre-implantation genetic diagnosis is the vital stage of the in vitro procedure. The scientists from Complete Genomics Inc. contributed to the improvement of currently applied methods.
Pre-implantation genetic diagnosis(PGD) includes a number of techniques allowing for genetic identification of defective embryos created in the process of in vitro fertilisation. The testing of embryos is performed before they are implanted into the female uterus and allows preventing future miscarriages, postpartum injuries (both physical and mental) or abortion after positive prenatal diagnosis.
Current diagnostics uses the so-called Next-Generation Sequencing (NGS) which covers specific parts of all 24 human chromosomes. This is a direct method, much more accurate than older techniques – DNA microarray or FISH where changes are detected by specified markers or light.
TheNGStechnique may be used for detecting chromosomal abnormalities, single-gene disorders and translocations. However, scientists are continuously searching for methods allowing the analysis of even larger parts of genetic material with much higher precision. The scientists from Complete Genomics Inc. in California have proposed a new method which allows testing over 95% of the entire human genome.
Their findings have been presented recently in „Genome Research” Journal in the article „Detection and phasing of single base de novo mutations in biopsies from human in vitro fertilized embryos by advanced whole-genome sequencing”.
“Because each individual carries on average less than 100 de novo mutations, being able to detect and assign parent of origin for these mutations, which are the cause of many diseases, required this extremely low error rate,” said co-authors Brock Peters, Ph.D., and RadojeDrmanac, Ph.D., from Complete Genomics. The presence of spontaneous mutations in genetic material is related to many congenital disorders such as autism or epilepsy.
In the currently used PGD procedures, 5 to 10 cells are biopsied from the blastocyst embryo, and the DNA is amplified before sequencing is performed. Due to lack of 100 percent replication fidelity, thousands of errors occur in the amplified DNA, many of which lead to a false positive resultand are determined to be spontaneous mutations which are not inherited from parents.
In 2012 Dr.Peters and his colleagues developed the method of massive parallel sequencing based on long fragment readtechnology (LFR). In the most recent studies this method was used in the context of human IVF embryo testing.
LRFallows generating high quality replicated genome already from 10 human cells (from ca 66 pg DNA). It has been found that the applied technology allows both more accurate detection of de novo mutation in single nucleotide variant(SNV) and significant reduction in the number of mutations leading to false positive results. In the performed studies, the scientists were able to achieve approximately a 100-fold reduction of errors over currently used clinical methods.
There is no doubt that in vitro technique is extremely controversial in ethical context. However, it has to be recognised that the recent study results present the important step in research in this procedure.