Preimplantation Genetic Diagnosis - The Newest ART

PGD or pre-implantation genetic diagnosis is a new technique, which marries the recent spectacular advances in molecular genetics and assisted reproductive technology. Pre-implantation genetic diagnosis enables physicians to identify genetic diseases in the embryo, prior to implantation, before pregnancy is established. PGD was first developed for patients who were at risk of having children with serious genetic disorders, which often discouraged them having their own biological children. These couples are often faced with attempting a type of "Russian Roulette" to have children, many times having to confront the difficult decision to terminate an affected pregnancy. Consider a woman known to be carrying an X-linked disease such as hemophilia with a 50% risk of an affected male in each pregnancy. Her daughters have a 50% risk of being carriers, but are unlikely to be clinically affected. She may not wish to become pregnant if she has to make decisions about an affected child in a viable pregnancy. However, she would become pregnant if she knew she had conceived a daughter, and with preimplantation diagnosis this possibility becomes a reality. PGD thus eliminates the need for possible pregnancy termination after prenatal diagnosis of a genetically affected fetus.

Research has shown that it is possible at three days after fertilization to remove one or two cells from an 8-10 celled embryo without affecting its further development. Embryos can be sexed on the basis of the presence or absence of a DNA fragment specific for the Y chromosome and in 1990 two sets of twin girls were born to five couples at risk of passing on an X linked disorder. Subsequently, a number of babies have been born after pre-implantation genetics has ruled out the diagnosis of a number of genetic diseases such as cystic fibrosis, Tay Sachs disease and Lesch Nyhan syndrome.

Sexing the embryo to avoid X linked diseases such as hemophilia and duchenne muscular dystrophy remains the commonest reason for pre-implantation diagnosis, now optimally carried out by the molecular cyto genetic technique of FISH (fluorescent in situ hybridization) with DNA probes derived from the X and Y chromosomes.

Techniques

How is PGD done? After IVF, on the third day, the eight-cell embryo is biopsed to obtain blastomeres (single cells) for molecular diagnosis. An embryo biopsy is done using micromanipulators under the visual control provided by an inverted tissue culture microscope. The embryo is held in position using a holding pipette, while a fine needle is used to drill a hole through the zona pellucida (the shell or the outer layer of the embryo) using acid Tyrode's or with a laser. A single cell is then removed by gentle suction. The cell (called a blastomere) is then available for genetic diagnosis.

Analysis of genetic material (DNA) from a single cell is performed either using a technique called FISH (fluorescent in situ hybridization) or PCR (polymerase chain reaction). FISH utilizes fluorescent probes, which are specific for a given chromosome, and therefore allows one to screen embryos for chromosomal normality. PCR on the other hand allows one to amplify (multiply) a selected DNA sequence of interest, so that it can be analyzed. While the genetic analysis on the single cell, is being performed which can take 4 - 24 hours, the embryos are kept in culture and allowed to further divide. Once the appropriate molecular diagnosis is made, unaffected embryos can be transferred back into the uterus in the IVF cycle.

PGD is now also being used in order to increase pregnancy rates for older infertile women. One of the reasons older women have a poorer pregnancy rate is because their embryos are often chromosomally abnormal, because of the fact they have older eggs (which may have genetic defects). PGD allows the doctor to select only the chromosomally normal embryos, so that only these can be transferred back into the uterus, resulting in a higher pregnancy rate.

PGD for sex selection - right or wrong

While PGD represents the cutting edge of reproductive technology, and gives us an idea of what may be possible for the future, it also raises a number of worries and concerns, especially in India, where people are worried that it may be used for sex-selection.

PGD is emotionally a very touchy area, because not only are we dealing with human embryos - the very start of new life, but we are studying their basic blueprint - their genes - the stuff of which humanity is made. Obviously, this is likely to cause people to take very strong views on what is right and what is wrong - so that they start thinking with their hearts rather than their heads! Many people confuse PGD with genetic engineering. A familiar refrain is we shouldn't be doing any of this because scientists are becoming too big for their boots - they are trying to play God by tinkering with the genes, and it is far better that they leave this entire field well alone, since we will never be able to understand any of it - it is beyond human wisdom. This is a common knee-jerk reaction, which precludes further rational debate.

The other viewpoint is - Why not? If man can improve on Nature, then why should he not try? After all, building a house is simply man's way of improving on nature - and if we can improve man himself, then why not? Seen in this light, then studying the molecular genetics of the human embryo would be the ultimate goal of all medicine. In the past, doctors used to treat adults. In the beginning of the 20th century, we started treating children, and the field of pediatrics was born. We can now treat the fetus - and the future patient of the 21st century will be the embryo - this is a logical progression!

If we allow people to choose when to have babies; how many to have; and even to terminate pregnancies if they inadvertently get pregnant, then why not allow them to select the sex of their child, if it is possible?

We should allow patients freedom to choose for themselves - medical technology should empower them with choices they can make for themselves! A common criticism against PGD for sex selection is that it will cause an unbalanced sex ratio. In reality, PGD will allow couples to balance the sex ratio in their families, rather than unbalance it! For example, take a couple with a baby girl, who wants to have a second baby. If they leave things up to chance, half of them will have a second baby girl - causing unbalanced interfamily sex ratios! PGD will allow them to make sure that they have a balanced sex ratio in their family, if they so desire. Seen in this light, PGD is perhaps the ultimate form of family planning there is!

Credits: Dr. Aniruddha Malpani, MD and DR. Anjali Malpani,