The history of preimplantation genetic diagnosis (PGD)

Today, for potential parents with genetic markers for serious hereditary conditions and diseases, conceiving a healthy child is not only possible but probable — largely due to advances in assisted reproductive technology. In particular, in-vitro fertilization (IVF) with preimplantation genetic diagnosis (PGD) can screen embryos for hundreds of different genetic diseases — greatly reducing the likelihood of passing on one of these conditions. However, this transformation in understanding genetics and reproduction is recent.

The Invention of PGD

The history of PGD is as diverse as the genetic disorders it aims to prevent. Prior to 1990, carriers of genetic diseases had few options to avoid passing them on to their children. However, as technology and science progressed, so did awareness about the difficult decisions parents-to-be faced when informed they were at risk to have a pregnancy affected by a hereditary condition. Over several decades, scientists developed a new method for detecting genetic abnormalities before implantation through IVF — PGD.

The initial investigation was built off a 1967 Nature paper by Dr. Richard Gardner and Nobel Laureate Dr. Robert Edwards, which detailed the first use of PGD to identify the sex of five-day-old rabbit embryos. These experiments paved the way for further research in the 1980s, leading to a biopsy procedure to safely extract and test genetic material from human embryos.

In 1990, clinicians at Hammersmith Hospital in London were the first to successfully use PGD to prevent a sex-linked genetic disorder from passing from a carrier parent to their child — forever changing the landscape of human genetics and reproduction.

How Does PGD Work: The Evolution of PGD

Initially, PGD was used to diagnose embryos with sex-linked diseases on the X chromosome, specifically known to affect males. Researchers relied on new technology — polymerase chain reaction (PCR) — to screen embryos for the male Y chromosome and only select female embryos (which wouldn’t be at risk for the disease) for IVF.

Although PGD could successfully detect and prevent sex-linked diseases, more complex genetic conditions — such as single gene mutations located on autosomes, which are chromosomes that do not determine sex — were undetectable because of limited technology. In 1992, reproductive technology achieved its next big milestone by refining PCR for higher accuracy along with fewer errors. Researchers could now distinguish mutations linked to single gene variants, catalyzing the first use of PGD to screen for cystic fibrosis and the resulting birth of a healthy child.

After 1992, researchers focused on improving their diagnostic techniques and expanding the list of preventable genetic diseases to include not only common and rare childhood genetic diseases, but hereditary cancer, late-onset conditions, and human leukocyte antigen (HLA) matching. By 2002, 1,000 healthy children had been born without heritable conditions because of PGD.

How Does PGD Work: Current Research

Today, the list of conditions preventable through PGD includes chromosomal rearrangements — which increases the risk for miscarriage and birth defects in offspring — and single gene variants using high-resolution techniques such as next-generation sequencing and karyomapping. Additionally, clinicians now offer parents-to-be the option of taking an expanded carrier screening panel test. This test not only identifies rare genetic anomalies prior to conceiving but detects genes that could be passed on to children in individuals without a family history of the disease.

Future Advances and Obstacles for PGD

Despite years of technological and scientific advances, PGD still has limitations and obstacles to overcome. For example, PGD is only possible when there is prior knowledge of a genetic risk, and it cannot detect all genetic conditions. Unlike cystic fibrosis — which is linked to a single gene variant — multifactorial or complex disorders are influenced by both genetic and external factors, such as lifestyle and environment. Therefore, disorders such as some forms of heart disease, mental illness, and type 2 diabetes cannot be detected or prevented in future generations using PGD.

Additionally, PGD technologies are not perfect. In rare cases, doctors may encounter obstacles when performing PGD for a specific gene. Misdiagnosis, while very uncommon, remains a slight risk. Therefore, physicians recommend prenatal testing to completely verify that the genetic condition wasn’t passed on.

The history of PGD provides a rich illustration of how scientists recognized a societal need and developed a method for parents-to-be to prevent genetic diseases from being passed on to their children. The future holds many opportunities for PGD as research continues breaking new ground.

This article was originally published 08/01/2017 on SharingHealthyGenes.com

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