Pre-implantation Genetic Testing (PGD, PGS)

Pre-implantation Genetic testing is the diagnostic of genetic and chromosomal abnormalities of IVF embryos after removing cells from these embryos before they are transferred to the uterus, in order to ensure that children are born free of hereditary diseases.

The term PGD, pre-implantation genetic diagnosis, is often used to refer to any pre-implantation genetic testing performed on an embryo before embryo transfer. However, the terms PGD and PGS are different.

PGD, preimplantation genetic diagnosis

Method involves removing cells from embryo to test it for a specific genetic defect (disease) before transferring the embryo to the uterus and is recommended only for those individuals or couples who have or carry a specific genetic disease and are thus at risk for passing this condition on to their child.

There are various ways to determine that a male or female partner may carry a genetic abnormality: a couple may have previously given birth to a child with a birth defect or serious medical condition; a family member(s) may carry a genetic mutation; or a condition may be found during routine, pre-treatment genetic screening.

PGS, preimplantation genetic screening

Method for testing for overall chromosomal normalcy in embryo. PGS is not looking for a specific disease diagnosis - it is screening the embryo for normal chromosome numbers.

Chromosome count is an important indication of embryo health. Too few or too many chromosomes may indicate a number of problematic conditions, including, for example, early miscarriages or Down Syndrome.

Some platforms are available for 24-chromosome embryonic aneuploidy screening, including quantitative real time polymerase chain reaction(qPCR), array Comparative Genomic Hybridization (aCGH) and single nucleotide polymorphism microarrays (SNP). Some studies have determined that the error rate using aCGH technology is about 5,7-7%. Although qPCR remains the most rapid protocol, this method is still very expensive. SNP is very precise, but needs very long time period for analysis performing.

AVA Clinic genetic laboratory performs prenatal genetic screening (PGS) by Next Generation Sequencing (NGS).

 

NGS provides a new approach with potential advantages including reduced costs and enhanced precision. It allows parallel and customizable analysis of multiple embryos on one chip.

 

NGS is an emerging technology which provides an unprecedented high-throughput, and highly parallel and base pair resolution data for genetic analysis. NGS provided a unique ability to identify compound heterozygosity due to the highly parallel single stranded nature of the sequence data. Furthermore, both single gene disorder and aneuploidy screening could be performed in parallel.

 

PGS and PGD by Next Generation Sequencing (NGS) recommended for:
Women of advanced age (older than 37).
Implantation failure after several attempts with IVF.
Couples with a medical history of repeated miscarriages.
Abnormalities in spermatozoa meiosis (usually are associated with other sperm quality abnormalities).
Couples at risk of transmitting chromosomal translocations or inherited genetic diseases (also single gene disorder).

 

* Several studies have shown that about 50% of human preimplantation embryos from IVF are chromosomally abnormal. This rate increase to about 80% in women over 42 years old. Chromosomes in eggs from older women have a significantly increased rate of abnormalities.

 

To a great extent, chromosomal abnormalities are responsible for failure of implantation of IVF embryos. Chromosomal abnormalities are also responsible for about 70% of miscarriages in early pregnancy.

 

Procedure

 

Prenatal Genetic Screening (PGS) by Next Generation Sequencing (NGS) is a laboratory diagnostic procedure that allows us to study the DNA of embryos in order to select those with a correct genetic load.

 

This procedure is done as part of IVF/ICSI programmes.

 

The purpose of PGS is to analyse embryo cells in the laboratory following in vitro fertilisation and before embryos are transferred to the maternal uterus.

 

A biopsy of embryo is performed and the embryo cells are analysed by using cytogenetic molecular diagnosis technique (NGS).

 

This technique allows us to distinguish between the healthy embryos and those which are affected.

 

The doctor can therefore transfer only those which will result in a pregnancy with a completely healthy baby.

 

NGS is an emerging technology which provides an unprecedented high-throughput, and highly parallel and base pair resolution data for genetic analysis. NGS provided a unique ability to identify compound heterozygosity due to the highly parallel single stranded nature of the sequence data. Furthermore, both single gene disorder and aneuploidy screening could be performed in parallel.

 

Many platforms are available for 24-chromosome embryonic aneuploidy screening, including qPCR, aCGH, and SNP arrays. NGS provides a new approach with potential advantages including reduced costs and enhanced precision. It allows parallel and customizable analysis of multiple embryos on one chip.

 

Targeted NGS provides an accurate high-throughput methodology for 24-chromosome aneuploidy screening and genotyping. Although qPCR remains the most rapid protocol (4 hours), this NGS method may represent a more cost-effective solution and soon become routine when blastocyst vitrification is performed.

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