The array-based comparative genomic hybridization (aCGH) method is a new technique that provides more detailed screening results because it examines all chromosomes, including sex chromosomes. Therefore, the selection of a fetus that is normal in terms of the number of chromosomes without deficiencies is more accurate and reliable.
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Human chromosomes consist of a total of 23 pairs, divided into 22 pairs of body chromosomes and an additional pair of sex chromosomes. XX represents the female sex, while XY represents the male sex.
What are Chromosomes?
Chromosomes are genetic codes located within the nucleus of cells (cells are the smallest units of our body tissues). They are paired like homologous chromosomes, resembling a pair of pants. Their function is to control the functioning of cells, which in turn control the functioning of various organs.
The genetic characteristics of each individual are determined by their genes. Genes are small sections of genetic code on each pair of chromosomes. Each gene performs the function of regulating cell production and functioning, and determines the unique characteristics of each individual.
Chromosomal Structure
Eggs and sperm are components of chromosomes. Each egg and sperm cell contains only a part of the chromosome. When the mother’s egg and the father’s sperm combine, they form the genetic code of the embryo. This results in chromosomes from the mother and father pairing up to form homologous pairs in the child. Ultimately, these genetic codes develop into specific individual characteristics. This is why each person is unique, even if siblings may have cells from the same parents. Each individual must develop their own specific genetic code, which is why siblings born one after the other may be different.
Prenatal Genetic Testing Methods
There are several methods available for prenatal genetic testing nowadays.
Method 1 : The first method is called FISH technique (Preimplantation genetic screening).
This method is considered outdated, and the Phyathai Sriracha Hospital no longer performs prenatal chromosomal testing using this method. The FISH technique involves counting the number of abnormal chromosomes (numerical disorders) by examining the nucleus of a single cell from the embryo. It particularly focuses on detecting abnormal chromosome pairs that are commonly found in newborn infants, such as Trisomy 13 (Patau syndrome), Trisomy 18 (Edward syndrome), and Trisomy 21 (Down syndrome). Additionally, there are also abnormalities in sex chromosomes, such as XO (where one sex chromosome is missing, leaving only one X chromosome) or the XXY group (which is typically male but may have developmental issues and be unable to have children due to the absence of sperm). Most cases of chromosomal abnormalities occur due to abnormal development of egg cells, which is more common in women over the age of 35.
Method 2: Preimplantation Genetic Screening for Diagnosing Abnormalities from Birth (Preimplantation Genetic Screening) caused by gene abnormalities.
This method involves screening for normal embryos for transfer while the abnormal embryos are not transferred. Examples of detectable abnormalities include severe types of thalassemia, where the parents are carriers but not affected, but there is a chance that the child born will have the disease because it is inherited genetically.
Currently, the diagnosis of fetal abnormalities has more efficient techniques and methods than the FISH method. We refer to this new method as the array CGH FISH technique.
Method 3: Improved Array CGH FISH Technique for Higher Accuracy
For the array CGH FISH technique used in genetic testing of embryos, the process begins with the step of fertilization to obtain embryos for examination. Initially, the eggs are stimulated to produce multiple follicles through injection medication, and the procedure follows similar steps as in the glass tube baby method.
Subsequently, the embryos are examined before implantation. The embryos are typically dissected into cell fragments for analysis of abnormalities, usually performed at 6-8 cells or at the blastocyst stage, which is around 5 days after fertilization
The dissected embryos have their cytoplasm removed, leaving only the nucleus of the cell. The nuclei are then subjected to special staining using the FISH technique to count the number of chromosomes pairs, specifically pairs 13, 18, 21, X, and Y.
The image displays the nucleus portion of the examined embryos, with two pairs each of chromosomes 13, 18, and 21. The sex chromosomes are XY, indicating male gender.
Advantages and disadvantages of chromosome testing using the FISH technique:
In cases where the female partner is over 35 years old, there is an increased risk of abnormalities in egg development. Abnormalities in chromosomes of the eggs, when fertilized with sperm, can result in embryos with abnormalities, which may either fail to implant or result in miscarriages within the first three months of pregnancy.
Performing chromosome testing on embryos before transferring them back into the uterus helps to screen for common abnormalities found in newborns. This screening reduces the miscarriage rate during the first three months of pregnancy. However, retrospective studies comparing groups that underwent chromosome testing with those that did not have shown that the group undergoing chromosome testing had a lower pregnancy rate. This is due to the potential impact on the embryos during the cell extraction process, leading to the loss of some viable embryos. Additionally, false-positive results from the testing can lead to the exclusion of normal embryos, reducing the chances of a successful pregnancy.
Performing chromosome testing on embryos before transferring them back into the uterus helps to screen for common abnormalities found in newborns. This screening reduces the miscarriage rate during the first three months of pregnancy. However, retrospective studies comparing groups that underwent chromosome testing with those that did not have shown that the group undergoing chromosome testing had a lower pregnancy rate. This is due to the potential impact on the embryos during the cell extraction process, leading to the loss of some viable embryos. Additionally, false-positive results from the testing can lead to the exclusion of normal embryos, reducing the chances of a successful pregnancy.
Therefore, to maximize the benefits of chromosome testing, it is important to select cases with indications or a higher risk of abnormalities, such as women over 38 years of age or those with a family history of Down syndrome, among others.
Furthermore, after achieving pregnancy, amniocentesis for prenatal diagnosis at 16-18 weeks is still necessary, especially for older women, as the screening test can only detect a limited number of abnormalities. However, amniocentesis can detect abnormalities in all pairs of chromosomes.
In summary, the advantages of screening for chromosomal abnormalities include reducing the risk of abnormal chromosomal conditions in individuals with high risk, such as those over 38 years old or with a family history of Down syndrome. Additionally, it allows us to determine the fetal sex. The downside is that it may increase false positive results and potentially decrease the chances of conception due to the testing process.
Phyathai Sriracha Hospital utilizes a new method called array Comparative Genomic Hybridization (array CGH) for chromosomal screening.
This method has been developed to address the limitations of traditional chromosomal testing methods. The new method provides more detailed screening results by examining all chromosomes, including the sex chromosomes. This improves the accuracy of identifying embryos with a normal number of chromosomes.
The principle behind this method involves amplifying the genetic material of the embryo to obtain hundreds of thousands of copies. Afterward, the amplified genetic material is examined to identify any abnormalities in terms of the number and missing portions of chromosomes. This is compared to the standard genetic material that we already know is normal.
This new method allows for more precise identification of chromosome abnormalities compared to the previous method, which relied on counting color dots on the nucleus. The problem with the old method was that sometimes the color did not completely fade, leading to the erroneous interpretation of the embryo having extra chromosomes. In some cases, excessive background coloration made it difficult to visualize the chromosomes. On the other hand, the array CGH method involves placing the amplified genetic material onto a microchip that contains human chromosomes. The microchip is then compared to a known normal DNA standard. By comparing the results obtained from the tested embryo’s DNA with the standard DNA on the microchip, we can determine whether there is an excess or missing amount of DNA. If the results match the standard DNA on the microchip, it indicates a normal amount of DNA, as shown in the image.
- Step 1 and 2: Take DNA samples from the fetus and a standard reference DNA for comparison, then perform simultaneous testing (control DNA).
- Step 3: Use fluorescent dyes to label the DNA of the fetus and the standard reference DNA (control DNA) to visualize which chromosomes belong to the fetus and the reference DNA. This allows for imaging and detailed analysis.
- Step 4: The DNA from both sources will pair and compete with another portion of DNA embedded in a microchip called a microarray.
- Step 5 and 6: Scan the results of DNA binding on the microchip using a specialized scanner and count the number of bindings. Then compare the bindings using computer software to determine which chromosomes of the fetus have more bindings than the standard reference DNA, indicating an excess, and which chromosomes have fewer bindings than the standard reference DNA, indicating a deficiency.
The images show the genetic strands of the fetus from the first pair of chromosomes to the last pair, revealing the absence of any portions of the chromosomes exceeding the upper green line, indicating an excess, or falling below the lower red line, indicating a deficiency
The magnified image of the first pair of chromosomes allows us to examine the details within the chromosomes more precisely than observing them in pairs, which appear as shortened segments. This enables the detection of small missing or excessive segments in specific positions
The displayed video graphically represents the genetic strands of the fetus from the first pair of chromosomes to the last pair. In a normal case, no portions of the strands exceed the upper boundary represented by the green line, indicating an excess, nor fall below the lower boundary represented by the red line, indicating a deficiency. Additionally, we can zoom in and examine the details of each chromosome pair in great detail, stretching them out to visualize the details more clearly compared to a general examination of chromosomes.
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