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تشخیص ژنتیکی قبل از لانه گزینی جنین

 (PGD)

 

اغلب بیماران سوال میکنند در مواقعی که احتمال خطر ابتلا به

بیماری خاصی بدلیل ازدواج فامیلی و.... بالا می باشد آیا راهی

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برای پاسخ به این سوال و نیز برای آشنایی با روش

 

PGD:Preimplantation Genetic Diagnosis

 

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ScienceDaily (Dec. 21, 2005) — Screening embryos for genetic diseases during in vitro fertilization offers couples the best chance for a healthy child, but a genetic glitch could potentially cause doctors to misdiagnose a small fraction of them,University of Florida researchers say 

Citing concerns about the accuracy of preimplantation genetic diagnosis, the method many practitioners use to pick the healthiest embryos during IVF, UF researchers set out to study the procedure. Their work, described during a recent meeting of the American Society of Reproductive Medicine, reveals the technique is actually highly reliable. But because there is a slim chance a genetic abnormality can cause doctors to misdiagnose embryos, some concerns still need to be addressed, the researchers said.

Preimplantation genetic diagnosis has been used for more than a decade to screen embryos for hereditary diseases such as Down syndrome and other abnormalities. To do this, one cell from an eight-cell embryo is extracted and examined for chromosomal defects.

However, as many as half these embryos spawn cells with different genetic information as they divide, giving doctors an inaccurate idea of how the embryo will continue to develop, said Larissa Kovalinskaia, M.S., a UF research associate with the College of Medicine's IVF program. While many embryos with this abnormality - called chromosomal mosaicism - stop developing early, some go on to be born. Because these embryos' cells contain different sets of chromosomes, doctors cannot always accurately screen them for genetic diseases, Kovalinskaia said.

"As more data were coming out, saying that as many as 50 percent of (IVF) embryos were mosaic, we started worrying about the accuracy of preimplantation genetic diagnosis," she said. "When you take one cell, does it represent the entire embryo? What we've shown is that we can rely on PGD."

UF researchers estimate less than 3 percent of healthy embryos are discarded as abnormal and only 1.5 percent are implanted with undetected genetic defects because of mosaicism.

To find out how big of a problem mosaicism is, the researchers developed a mathematical model using data from UF's IVF program and other programs to illustrate the different paths an embryo takes as it develops. While half of embryos turn out normal, with no mosaicism, the other half produce irregular cells as they divide.

Embryos that become mosaic after their cells divide twice typically stop progressing by the time they have accumulated eight cells. Too many of their cells have chromosomes that don't match and these mixed messages lead the embryo to stop developing, Kovalinskaia said.

Embryos with only one or two irregular cells still have a chance to develop normally though, she said. These embryos tend to not develop this abnormality until after their cells have divided a third time, typically the time when doctors perform preimplantation genetic diagnosis. This is where the problem with diagnosis occurs, researchers say.

"Our pathway concept may help to explain the observed outcomes during preimplantation diagnosis," said Kenneth Drury, Ph.D., director of the UF IVF and Andrology Laboratory and a clinical professor in the College of Medicine who was one of the researchers. "When we biopsy an eight-cell embryo, if we choose (the one) normal cell in an abnormal embryo, we may think that embryo is normal and transfer it.

"This occurs about 1 percent of the time, which means 99 percent of the time we will be accurate. Again the error is not due to a technical mistake but to natural errors occurring during early cell division in the embryo."

Extracting test cells before the embryo divides a third time could reduce the number of misdiagnoses because of mosaicism, the UF researchers suggest. Improving culture conditions could help too, Kovalinskaia said.

But the small margin of error shouldn't stop couples from having preimplantation genetic diagnosis, said Jamie Grifo, M.D., Ph.D., a professor of obstetrics and gynecology at New York University. The testing can work miracles for women prone to miscarriages or who carry genetic diseases, he said.

"If you are at risk for genetic diseases, it changes your outlook on starting a family," he said. "PGD allows them to get pregnant. It's a dramatic improvement when it works."

More than 45,000 babies are born each year with the aid of assisted reproductive technologies like IVF, according to the Centers for Disease Control and Prevention. The use of IVF has declined in recent years, but Grifo said that is largely because baby boomers are leaving their childbearing years. Technologies like preimplantation genetic diagnosis have helped too, making IVF more accurate, he said.

"There has been a lot of progress," Grifo said. "It's helped many patients."

   Embryo Biopsy Does Not Affect Early Growth And Risk Of Congenital Malformations In PGD/PGS Babies       

ScienceDaily (July 10, 2008) — A study of 70 singleton babies born after preimplantation genetic diagnosis and screening has shown that the procedure does not adversely affect their early growth and risk of congenital malformations

The lead researcher, Professor Maryse Bonduelle, told the 24th annual meeting of the European Society of Human Reproduction and Embryology in Barcelona July 8th: "These findings are very reassuring for the children, parents and clinicians involved in PDG/PGS treatments. They show that embryo biopsy in PGD and PGS had no adverse impact on singletons' growth or medical outcomes."

PGD/PGS is an invasive procedure that involves removing cells from the embryo before it is transferred to the mother's uterus and testing them for specific genetic disorders (such as cystic fibrosis and Duchenne muscular dystrophy). Doctors then select only the unaffected embryos to transfer. Until now, little was known about the effect on the early growth and clinical outcomes of children born after embryo biopsy.

Prof Bonduelle, head of the Centre for Medical Genetics at Universitair Ziekenhuis Brussel (Brussels, Belgium), and her colleagues compared 70 babies born after PGD/PGS with equal numbers of babies born after ICSI (intracytoplasmic sperm injection) or spontaneously conceived (SC).

They found no difference between the three groups with regard to weight, height and head circumference at birth and age two, or in rates of growth. PGD/PGS children were born more frequently after caesarean section than ICSI children, but had no more congenital malformations, hospital admissions or surgical interventions than did the ICSI or SC babies. Although the PGD/PGS children underwent more medical investigations, the number of abnormal results arising from these investigations was not significantly different to the other two groups.

The PGD/PGS children did have a lower body mass index (BMI) at age two than did the SC children. Prof Bonduelle said the reason for this might have something to do with the babies' diet: "We found that mothers who conceived after PGD/PGS worked less (working 63% of the time) than the ICSI mothers (working 77% of the time) and SC mothers (working 83% of the time); PGD/PGS children were less frequently in day care centres. We speculate that different, more healthy food might have been given to PGD/PGS children by the parents, given the finding that their mothers stayed at home more frequently, and this may be the reason for the healthier BMI."

"Complications of pregnancy and older parental age were more present in the PGD/PGS and ICSI groups, but these are both factors that are known to be present more frequently in mothers after assisted reproduction technology," said Prof Bonduelle. "Mothers and fathers of SC babies were significantly younger at delivery compared to the other two groups, and this also contributes to fewer pregnancy complications."

Other work published this year by the same research group has shown that there was no difference between the PGD/PGS, ICSI and SC children in other developmental areas, such as motor, mental, and socio-emotional development and language. Nor did the parents differ in terms of parental stress and health.

The researchers are continuing their study of PGD/PGS children as, so far, there are insufficient numbers to draw firm conclusions about any differences in major malformation rates. Two PGD/PGS children, one ICSI child and two SC children had major malformations. In the PGD/PGS group, one child suffered from a ventricular septum defect (a heart problem) and another child had hemivertebrae with gibbus (a malformation of the vertebrae causing a hump). One child from the ICSI group born at 39 weeks underwent surgery for an inguinal hernia (a hernia in the groin). In the SC group, a surgical intervention was performed in one child with a transposition of the great arteries and in another child with bilateral cryptorchidism.

One or more minor malformations were recorded in 24 PGD/PGS children, 23 ICSI children and 13 SC children. Skin anomalies (moles or spots and various birth marks) were the most frequently observed minor malformations in 18 ICSI children (25.7%), 14 PGD/PGS children (20%) and 10 SC children (14.3%).

In addition, the researchers excluded twins from the study because of the impact twin pregnancies can have on premature births, low birth weight, growth and malformations. "Further research on the outcome of PGD and PGS twins compared to ICSI and SC twins is mandatory," concluded Prof Bonduelle.


Adapted from materials provided by European Society for Human Reproduction and Embryology, via EurekAlert!, a service of AAAS.