Preimplantation Genetic Diagnosis PGD
Preimplantation Genetic Diagnosis (PGD) is performed for medical purposes in cases where the procedure can help increase the chances of the mother carrying a healthy pregnancy to full term. The goal of PGD is to identify chromosomally abnormal embryos, thus prevent recurrent miscarriages and avoid having a child affected with the most common aneuploidy disorders
There are three sources of genetic abnormalities in early embryo
- Genetically inherited diseases – PGD helps families reduce the transmission of monogenic disorders. The Most common single gene disorders that PGD has been used for are: Cystic fibrosis, Tay-Sachs disease, Spinal Muscular Atrophy( SMA), Hemophilia, Sickle Cell Disease, Duchennese Muscular Dystrophy, Thalassemia.
- Meiotic errors i.e. chromosomal aneuploidies (missing or additional numbers of chromosomes) occurring during gametogenesis ( PGS)
- Mitotic disturbances – chromosomal aneuploidies resulting from malsegregations after abnormal cell cleavages(PGS)
PGD for HLA Haplotyping
Except single gene disorders PGD can also be used for HLA Haplotyping. The human leukocyte antigen (HLA) system or complex is a gene complex encoding the major histocompatibility complex (MHC) proteins in humans. .HLA genes are highly polymorphic, which means that they have many different alleles, allowing them to fine-tune the adaptive immune system.
What Does it Mean to Test Positive for HLA? – Normal results are negative, which means you don’t have HLA-B27 in your blood. A positive test means HLA-B27 was found in your blood. You may have a higher-than-average risk of developing or having certain autoimmune diseases, such as ankylosing spondylitis and arthritis. Via PGD leukemia specific alleles of the HLA-DR4 can also be detected. But its also worth to mention, that the etiology of leukemia as well as some other diseases are unknown, though only a genetic susceptibility can only be suggested.
Chromosomal Abnormalities and PGS Screening
The goal of PGS is to identify chromosomally abnormal embryos, thus prevent recurrent miscarriages and avoid having a child affected with the most common aneuploidy disorders – Down Syndrome (trisomy 21), Edwards Syndrome (trisomy 18), Patau Syndrome (trisomy 13), Turner syndrome (monosomy X), and Klinefelter syndrome (gonosomal trisomy XXY). These trisomies originate from meiotic errors that may arise during the formation of gametes, especially in women. In terms of preimplantation diagnosis, they are very dangerous because they do not affect early embryo development and may lead to the birth of disabled child. Therefore, PGS is indicated if meiotic errors are expected, for example in the cases of advanced maternal age and severe male factor infertilities (after TESE – surgical spermatozoa retrieval from testes).
Why Genetic Testing?
According to recent researches Among the PGD + aneuploidy-screened group (n = 355 blastocysts), only 25.6 % of embryos were both Single Gene Disorder (SGD)-negative (or carriers) and euploid; thus the majority of embryos were ineligible for transfer due to the high prevalence of aneuploidy. Despite a young mean age (33-39 y), 49.9 % of Blastocysts were aneuploid. The majority of patients (53.2 %) had ≥1 blastocyst that was Single Gene Disorder (SGD)-unaffected but aneuploid; without screening, these unaffected but aneuploid embryos would likely have been transferred resulting in implantation failure, pregnancy loss, or a pregnancy affected by chromosomal aneuploidy. Despite the transfer of nearly half the number of embryos in the aneuploidy-screened group the implantation rate was higher (75 % vs. 53.3 %) and miscarriage rate lower (20 % vs. 40 %) (although not statistically significant). 24-chromosome aneuploidy screening when performed concurrently with PGD provides valuable information for embryo selection, and notably improves single embryo transfer rates.