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Clinical application of pre-implantation embryo diagnosis

Head of Molecular Biology Laboratory , WU HUI-MEI

During the antenatal care of the mother in advanced maternal age or with genetic diseases, the prenatal diagnosis is commonly recommended by the physicians to evaluate the conditions of the fetus. The most commonly used prenatal diagnoses are amniocentesis and chorionic villus sampling, in both cases, the samples must be cultured and the couples will not have the results until two weeks after sampling. An ethical dilemma is unavoidable in case of a fetal abnormality, i.e., the continuation of the pregnancy. If the prospective parents choose to give birth to the child, they must realize the strains that put on a marriage into which a severely defective child has been born; on the other hand, if the couples choose to terminate the pregnancy, the risk for prospective mother is increased because of the extended pregnancy, and the decision often generates considerable levels of psychological stress and emotional vulnerability. Based on the fact that both choices could lead to unwanted consequences, the physicians and scientists are aiming at the improvement of pre-implantation genetic diagnosis (PGD).

After the union of the egg and sperm through fertilization, the fertilized egg soon divided into a multicellular embryo. PGD is a strategy to test the gender, genetic or chromosomal abnormality of the embryo and is applied by the couples receiving IVF, and blastomere biopsy was carried out when the embryo develops into 6- to 8- cell stage. The major difference between PGDand antenatal care is that the target of the former is the embryo, where is that for the latter is the fetus. With the aid of PGD, normal embryos can be selected for transfer and largely reduces the possibility of fetus with sex-linked genetic diseases or other abnormalities. In summary, PGD is destiny of artificial reproduction technologywith the advantages in reducing the physiological risk on the prospective mother, thepsychological stress on the couples, thesocial cost, and moral issues of abortion.

▲ Figure: Fluorescent in situ hybridization (FISH)
Note: In this figure, the patient was diagnosed as Down Syndrome because of chromosome 21 triplication.

PGD is a complicated diagnosis which multiple techniques are required simultaneously. For example, a sophisticated micromanipulationtechnique is a prerequisite for withdraw one or two blastomeres form the embryo without the interruption of normal development of the embryo. Molecular biotechnology is also required for the analysis of the gender, genetics and chromosomes of the embryos. The team must be proficient at these techniques and cooperate smoothly to deliver accurate diagnosis in time.

The most widely used molecular biotechnologies used are polymerase chain reaction (PCR) and fluorescent in situ hybridization (FISH). PCR is used to analyze the gender and genetic abnormalities, such as thalassemia and hemophilia. FISH is used for analyzing the gender and detecting alternations in chromosome copy numbers. The main purpose of gender analysis is to screen for sex-linked genetic diseases, which are, as the name implies, diseases linked to sex chromosomes. For example, hemophilia is an X-linked inheritance, in case of a carrier mother and an affected father, they will have a 25% chance of having an unaffected son, a 25% chance of having an affected son, a 25% chance of having an unaffected daughter and a 25% chance of having a daughter who also is a carrier. Therefore only normal embryos can be selected for transfer. The analysis of chromosome copy number is to screen particular abnormalities, for example, the copy of chromosome 21 in normal people is two, but there are three copies of ch 21 in Down Syndrome patients, and such kind of embryos have to be discarded for transfer. PGD is an important technologyin artificial reproductionand eugenics to greatly reduce the painful experiences of the couples as well as social problems. However, due to the technical thresholds such as micromanipulation for blastomere biopsy, single cell PCR and FISH, as well as high expense, this technology is not yet well-developed domestically.

On the other hand, there are still many obstacles in PGD, for example, effective diagnosis of many genetic diseases or chromosomal abnormalities are currently unavailable, which means more basic research in this filed has to be carried out for more accurate and rapid diagnosis.