ＰＧＤ　着床前遺伝子診断

着床前遺伝子診断　（ＰＧＤ）

着床前遺伝子診断(PGD)は、妊娠前に行う遺伝子異常の診断です。妊娠が成立してから行われる出生前診断と違って、着床前診断では、受精後ほんの数日の段階で胚に遺伝子以上がないかどうかを診断します。胚は体外受精によって作られ、着床前診断を経て正常な遺伝子を持つことが確認された胚だけが、子宮に移植されます。RGIでは、1990年にこの技術が実用化されて以来、何千件もの着床前遺伝子診断を行ってきました。

着床前遺伝子診断は、赤ちゃんが特定の遺伝子疾患をもって生まれてくる可能性を減らすことができます。テイ・サックス病、嚢胞性線維症、筋ジストロフィー、脆弱性X症候群など、ご両親の家系に頻発する遺伝子疾患を排除することを目的として着床前遺伝子診断をリクエストされるカップルもありますし、ダウン症、18トリソミー、13トリソミー、ターナー症候群といった染色体異常を防ぐために着床前遺伝子診断を利用される方もいます。染色体異常を排除することで、流産の可能性を減らすことも可能です。また、RGIでは、家系の中に均衡型相互転座が見つかった場合、お子さんに不均衡型相互転座が出るのを防ぐための着床前遺伝子診断も提供しています。いずれにしても、着床前遺伝子診断は、お子さんに遺伝子異常が出る可能性が高く、かつ出生前診断の後に異常の判明した胎児を中絶することは避けたい、というカップルに最適の診断法であると言えます。

着床前遺伝子診断には、極体生検、胚生検、栄養外胚葉検の三つの方法があります。極体生検では、極体(卵子が成熟し受精するにつれて不要となり、自然に排出される部分)を使って、特定の遺伝子疾患がないか検査します。この検査は、卵子の採取と受精から一日後に行われます。極体生検は、母親の卵子に遺伝病を起こす遺伝子の変異がないかどうかを調べる方法です。二つ目の方法、胚生検（割球生検とも呼ばれます）では、胚が六細胞期から八細胞期に入った段階で、細胞を一つか二つ胚から慎重に取り外されます。この検査は採卵と受精の日から数えて三日目に行われます。胚の発生の初期段階においては、全ての細胞が同じ発生能力を持っているので、細胞を一つか二つ取り去っても、胚の正常な成長に悪影響はありません。三つ目の方法、栄養外胚葉検では、胚の外周部の細胞を採取し、分析することで、特定の遺伝子疾患がないかどうかを調べます。この検査は、採卵と受精の五日後に行われます。

Preimplantation Genetic Diagnosis (PGD)

Preimplantation Genetic Diagnosis (PGD) is the diagnosis of a genetic condition prior to achievement of a pregnancy. This form of genetic testing is performed on eggs or embryos during an in vitro fertilization (IVF) cycle. Therefore, in order to have PGD, one must undergo IVF. PGD was first performed in the early 1990’s as a way for couples to prevent the pregnancy of a child with genetic disease. Currently, we are able to perform PGD for many genetic conditions including single gene disorders and chromosome abnormalities. At the Reproductive Genetics Institute, we have been performing PGD since it became available in 1990. We pioneered the polar body removal technology and are one of the most active centers offering PGD in the world. Our laboratory technicians are well trained in each of the necessary steps.

Through PGD, we are able to test for many different genetic diseases. Some of our families request PGD for a specific genetic condition running in their family, such as Tay Sachs disease, cystic fibrosis, muscular dystrophy or Fragile X syndrome. Complete list of disorders tested. Other couples request genetic testing for chromosome problems, such as Down syndrome, trisomy 18, trisomy 13 and Turner syndrome, in order to improve their chances of getting pregnant during in vitro fertilization. Finally, there are some couples that utilize PGD to decrease their chance of passing down an unbalanced chromosome translocation (if the man and/or the woman has been identified to be a carrier of a balanced translocation).

PGD for Single Gene Disorders

For families at risk for a specific genetic condition, PGD is a way to significantly reduce the risk that a baby will be affected with the condition. PGD allows the laboratory to determine prior to implantation which embryos are likely to be affected or unaffected by a particular genetic condition. Only the embryos that are predicted to be free of the genetic condition are transferred to the woman’s womb. This process greatly reduces the likelihood that a family has to make difficult decisions in a pregnancy (such as therapeutic abortion of an affected pregnancy) following prenatal testing results and may alleviate much anxiety when awaiting results.

Single Gene Disorders

PGD for Aneuploidy

Preimplantation genetic testing is currently the only way to determine if the egg or embryo contains an abnormal number of chromosomes prior to pregnancy. This may not only contribute to the prevention of the birth of children with common chromosomal abnormalities but also may contribute to the efficiency of IVF. PGD for chromosome problems can increase the chance of an embryo implanting, decrease the chance of miscarriage and decrease the likelihood of having a baby with a chromosome problem (such as Down syndrome). In essence, PGD can give your physician another way to determine which embryos are the best for transfer during an IVF cycle.

PGD for Chromosome Translocations

It is estimated that 1 in 625 individuals carry a balanced chromosomal translocation or rearrangement. Individuals that carry a balanced chromosomal translocation contain all of their genetic information; however, pieces of some chromosomes have switched places. Individuals that carry chromosomal translocations are typically healthy; however they are at an increased risk for infertility, miscarriage, stillbirth, and/or having a child with birth defects. Preimplantation genetic testing for chromosomal translocations is able to distinguish between chromosomally normal and abnormal embryos. Currently, this is the only way to determine whether the embryo is chromosomally normal or “balanced”, prior to pregnancy.