Preimplantation genetic diagnosis and screening of embryos: Technical and clinical practice update
2016年度 年次大会-講演抄録 | Symposium ～ ART NEXT GENERATION ～
学会講師 : Santiago Munné
In the last few years improvements in embryo culture, vitrification and micromanipulation techniques combined with genomic technology moved
PGD and PGS from day 3 biopsy to blastocyst biopsy, and from FISH and PCR testing of a few chromosomes and markers to whole genome screening.
Currently, the vast majority of PGS and PGD cycles performed in the US are biopsied at blastocyst stage, combined with vitrification and replacement on a thaw cycle. This is because blastocyst biopsy of 5-10 cells has been shown not to be detrimental for embryo development.
Of the indications for genetic selection of embryos, aneuploidy accounts for the vast majority of cases. Aneuploidy increases with advancing maternal age and by itself explains the decrease in implantation rates observed with advancing maternal ages. Because couples are postponing reproduction to pursue career and education, more couples become infertile by delaying reproduction. PGS v2 techniques (aCGH, SNP arrays, qPCR, Next Generation Sequencing (NGS)) can analyze all chromosome aneuploidies and combined with blastocyst biopsy, if euploid embryos
are replaced, they implant equally well at any age, therefore PGS v2 eliminates the maternal age effect on implantation.
In addition several clinical randomized trials, using PGS v2 and blastocyst biopsy, have shown a significant improvement in ongoing pregnancy rates. These trials included mostly good prognosis patients, but a larger and more comprehensive trial is about to finish and is expected to show similar results. Other indications for PGS, in addition of aneuploidy are recurrent pregnancy loss, since most aneuploidies that implant result in miscarriage.
Although all aneuploidies are detectable with PGS v2 techniques, only NGS can detect mosaicism. Mosaic embryos have two or more cell lines with
different chromosome content, and they have been reported to implant less and miscarry more than euploid embryos, but some have a normal ICM and c a n m a k e b a b i e s . T h e r e f o r e , w e r e c o m m e n d classifying them as a third category of embryos and consider them for replacement if there are no available euploid embryos.
Chromosome abnormalities are not only caused by meiotic errors and increase with maternal age but some, specially mosaicism, are iatrogenic, caused by culture conditions or treatment related, and its rate varies from center to center. This means that treatments can be improved to reduce them and enrich the pool of euploid embryos available for transfer. In this fashion, PGS can be used as quality control to improve treatments.
In addition of chromosome abnormalities, carrier screening of couples allows for the detection of hundreds of gene defects (thalassemia, Cystic fibrosis, etc.). About 2% of couples are carriers of one of the gene defects currently screened with these tests. These carrier couples have 25% chance of conceiving a baby affected by these devastating diseases. By using PGD, embryos can be screened and non-affected embryos replaced. The most advanced method for PGD is called Karyomapping and consist of analyzinghundreds of thousands of SNPs to determine the haplotypes present in the embryo and identify those that are affected, without having to develop a specific test for each family, as we used to do with PCR. In Addition, this test can be combined with PGS to screen also for chromosome abnormalities.
In addition of gene and chromosome defects, we have described the presence of mitochrondrial DNA copy number abnormalities. The test, MitoGrade, quantifies mitochondrial DNA and our prospective and retrospective studies show that above a certain threshold, euploid embryos with elevated MitoGrade levels do not implant. By screening off these embryos a further improvement in implantation rates can be obtained.
The next frontier in PGD / PGS will be to determine if whole genome sequencing of embryos as well as gene editing techniques such as CRISPR are useful and doable at the gamete / embryo level.