Focus: Brave New World

fall 2008 • Harvard Science Review 21

By Peter Cai

Preimplantation genetic diagnosis (PGD), also known as embry-

onic screening, allows parents and doctors to screen fertilized embryos and pre-fertilized oocytes for numerous characteristics, particularly genetically inherited diseases. PGD is an extension of in vitro fertilization (IVF). In IVF, oocytes are obtained, fertilized outside of the body, and implanted in the woman’s uterus. While IVF facilitates fertilization of eggs and implantation of embryos, PGD goes a step further than simply assisting couples to become pregnant. It screens these embryos and selects only those with desired traits to be implanted and eventually grow into a baby: scientists look for genetic flaws and actively select embryos for implan-tation that are disease-free.

However, the development of PGD opens up a Pandora’s Box of opportu-nities to dictate a child’s future traits. As PGD becomes more widespread and successful, it presents a moral dilemma: where is the line between creating a healthy child and trying to mold a “perfect” child?

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Development of PGD

The development of embryonic screening began in 1967 with Edwards and Gardner’s ability to determine the gender of live rabbit blastocysts while retaining their viability (1). Their ability to harvest embryos and identify future traits without harming the embryos set the stage for application of PGD in humans. In 1989, Coutelle et al. estab-lished a preimplantation diagnostic test for cystic fibrosis for human embryos (2), and in 1992, Handyside et al.’s IVF and PGD testing led to the successful birth of a healthy, cystic-fibrosis-free girl (3).

Throughout the past 2 decades, PGD has made rapid progress as the development of efficient PCR tech-niques has allowed for more varied, accurate, and cheaper diagnoses. Since Handyside’s first PGD baby, the list of genetic conditions which can be tested is rapidly expanding; the UK’s Human Fertilisation and Embryology Authority currently lists over 60 conditions that are testable by PGD (4).

“

EMBRYONIC SCREENINGSELECTING FOR THE PERFECT CHILD

“However, the development of PGD opens up a Pandora’s Box of opportunities to dictate a child’s

future traits. ”

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Focus: Brave New World

22 Harvard Science Review • fall 2008

The science behind PGDEmbryonic screening has 3 main

components: obtaining an embryo, taking a biopsy of it, and completing a genetic analysis of the biopsy. The procedure for obtaining embryos for PGD is identical to that used in IVF. Controlled ovarian stimulation uses a set of timed hormone supplements to stimulate production of surplus oocytes for harvesting. Finally, to fertilize the oocyte and create an embryo, sperm is directly injected into the oocyte, a pro-cedure called intracytoplasmic sperm injection (5).

One of the most controversial pro-cedural components of PGD is the biopsy. Depending on the personal, religious, and legal definitions of which embryonic stage constitutes a living be-ing, the following three biopsy options present varying scientific and moral trade-offs.

The first and most popular biopsy

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The top picture shows a polar body biopsy, which occurs 14-20 hours after normal fertilization. The bottom pic-ture depicts cleavage-stage biopsy, which happens about 72 hours after fertilization has occurred.

method (used in 94% of embryon-ic screens) is the “cleavage-stage biopsy” where 1 or 2 cells from a blastomere (8 cell or later develop-mental stage) are sectioned out for analysis. The ob-vious advantage to this method is that it allows analysis of the DNA contrib-uted by both the mother and the father, since the cells were taken

from a fertilized embryo. Nonetheless, there are several scientific and moral cons to this method.

Scientifically, this sectioning disrupts development and can cause the biop-sied cell to develop abnormally, defeat-ing the purpose of the screen. Also, chromosomal mosaicism is common at this stage, which can make results of the tests irrelevant (5). Cleavage stage biopsies obtain far less tissue than the other two techniques, forcing scientists to rely on diagnostic techniques that present their own problems. Although it is a preferable method from a scien-tific perspective, it has been rejected on a moral basis by individuals or countries that consider these embryos to be living and thus forbidden to probing, selec-tion, and disposal (6).

The second available method is called a “polar body biopsy.” Oocytes develop via a two-step meiotic oogenesis, during which uneven division of cytoplasm

results in one large oocyte and two smaller polar body byproducts. (Figure needed here). Since the polar bodies are waste products of oogenesis and not needed for embryo development, polar body (PB) biopsies provide a very safe alternative to taking biopsies of devel-oping embryos. First used by Verlinsky (7), a polar body biopsy bypasses the ethical concern regarding the status of embryos as living beings, which is why it is used in countries such as Germany where cleave-stage embryo selection is banned. Unfortunately, this biopsy method can be misleading because it only provides information on mater-nally transmitted diseases (the egg has yet to be fertilized and does not contain DNA from the sperm) and genetic in-formation may be degraded, since polar bodies are effectively “junk.” However, the ethical and safety benefits of PB biopsies have encouraged reproductive researchers to continue to develop reli-able PB-based diagnoses (8).

The last biopsy method, “blastocyst biopsy,” presents many of the moral challenges that cleavage stage biopsy presents, but overcomes the limited tis-sue problem by taking a biopsy later in development (figure of human embryo development). While the additional tis-sue may lead to more accurate genetic diagnoses, since blastocysts are later in development, fewer are available for screening. Furthermore, the delaying the biopsy limits the time before im-plantation and the time for diagnosis (9).

Genetic ScreeningThere are a number of screens that

can be used during PGD to detect ge-netic abnormalilties. Fluorescent in situ

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Focus: Brave New World

fall 2008 • Harvard Science Review 23

—Peter Cai ’10 is a Molecular Cellular Biology concentrator in Adams House.

hybridization (FISH) probes chromo-somes with fluorescent DNA probes specific for various chromosomal segments and is particularly useful for detecting abnormal chromosome ar-rangements such as trisomy 21 which causes Down’s syndrome (10).

To detect for more specific genetic abnormalities, the polymerase chain reaction (PCR) amplifies a cells’ lim-ited amount of genetic material to an amount sufficient for further testing (10). While traditionally a very robust assay, the use of PCR in PGD exac-erbates the traditional problems of PCR due to the extremely low amount of template available in PGD (in the case of a cleavage stage biopsy, a single strand). The high number of PCR cycles required makes amplifica-tion of contaminants and misleading results more likely. Furthermore, a phenomenon called allele drop out where random non-amplification of one of the alleles in a heterozygous sample occurs can lead to many false positives and negatives depending on which allele failed to amplify. PCR and subsequent DNA analysis can provide more specific information than FISH regarding specific disease causing ge-netic mutations (10).

Despite all these challenges, embry-onic screening has been hailed a success for helping otherwise infertile couples bear healthy children. Out of 4748 PD attempts, 754 babies have been born, and since screening eliminates embryos carrying chromosomal abnormalities that would likely abort a pregnancy, “The overall pregnancy rate per transfer is 23.3%, much higher than the rate in IVF patients in the comparable age group (average age, >39 years) without PGD.” For couples using IVF, PGD led to a fourfold reduction of spontaneous abortion (11).

ConclusionWhile these scientific successes

validate embryonic screening as a nec-essary assistive reproductive technol-ogy, controversy still lurks nearby. The question of what stage of the embryo is considered a living organism has led to the various biopsy options discussed above. However, recent findings have also suggested that PGD may have det-rimental effects on the children, causing parents to question whether the screen is really worthwhile (12). The success of the human genome project and the increasing accuracy and availability of PGD suggest that embryonic screen-

ing may one day go beyond simply helping parents bear healthy children and instead be used for sex selection and choosing specific traits (13). These possibilities of selecting embryos for ideal children is not far from the limits of current science and threatens to eliminate the much beloved surprise of having children.

References(1) Edwards RG, Gardner RL. Sexing of live rabbit blastocysts. Nature (May 1967) 214: 576-577. (2) Coutelle C, Williams C, Handyside A, Hardy K, Winston R, Williamson R. Genetic analysis of DNA from single human oocytes: a model for preimplan-tation diagnosis of cystic fibrosis. BMJ (July 1989) 299: 22-24. (3) Handyside AH, Lesko JG, Tarín JJ, Winston RM, Hughes MR. Birth of a normal girl after in vitro fer-tilization and preimplantation diagnostic testing for cystic fibrosis. NEJM (Sept 1992) 327: 905-909. (4) UK HEFA. http://www.hfea.gov.uk/en/910.html (5) Preimplantation diagnosis. Delhanty JD. Prenat Diagn (Dec 1994) 14: 1217-27. (6) Fasouliotis, JS, and Schenker, JG. “Preimplanta-tion genetic diagnosis principles and ethics.” Human Reproduction, Vol. 13 No. 8, p 2238-2245, 1998. (7) Verlinsky Y, Ginsberg N, Lifchez A, Valle J, Moise J, Strom CM. Analysis of the first polar body: preconception genetic diagnosis. (Oct 1990) Human Reproduction 5: 826-829. (8) Altarescu G, Brooks B, Eldar-Geva T, Margal-ioth EJ, Singer A, Levy-Lahad E, Renbaum P. Polar Body-Based Preimplantation Genetic Diagnosis for N-Acetylglutamate Synthase Deficiency. Fetal Diagn Ther (August 2008) 24: 170-176. (9) Ogilvie, C.M., et al. “Preimplantation Genetic Diagnosis.” Journal of Histochemistry and Cytochem-istry. Vol 53(3): 255-260, 2005. (10) Dreesen J, Drüsedau M, Smeets H, de Die-Smulders C, Coonen E, Dumoulin J, Gielen M, Evers J, Herbergs J, Geraedts J. Validation of Preimplan-tation Genetic Diagnosis by PCR analysis: genotype comparison of the blastomere and corresponding embryo, implications for clinical practice. Mol Hu Reprod (Sept 2008). (11) Yury Verlinsky Ph.D.a, Jacques Cohen Ph.D.b, Santiago Munne Ph.D.b, Luca Gianaroli M.D.c, Joe Leigh Simpson M.D.d, Anna Pia Ferraretti M.D.c and Anver Kuliev M.D., Ph.D. Over a decade of experience with preimplantation genetic diagnosis: A multicenter report. Fertility and Sterility (August 2004) 82: 292-294. (12) Neelanjana M, Sabaratnam A. Malignant condi-tions in children born after assisted reproductive technology. Obstet Gynecol Surv (Oct 2008) 63: 669-76. (13) Sex selection for social purposes in Israel: quest for the “perfect child” of a particular gender or centuries old prejudice against women? Landau R. J Med Ethics. (Sept 2008) 34: e10.

A four-cell embryo has undergone fluorescence in situ hybridization with probes at chromosomes 13, 16, 18, 21 and 22. In the original color picture, each of the probes would be a different color.

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