Parents with sick children create siblings as tissue donors

Main Category: Pregnancy / Obstetrics
Article Date: 06 May 2004 - 0:00 PDT


Parents who have children who are ill with leukaemia or anemia are going to reproductive genetic clinics in order to better conceive siblings as stem cell donors.

This approach is attracting criticism from some people. Ethicists say there is not enough information about the impact this has on these kids.

Clinics screen embryos that were created by IVF. They find out if the tissues of these embryos match those of the sick child.

The problem for parents who try to conceive naturally is that there is only a 25% chance the sibling will be a perfect match for the sick child. This means you may need to have four extra children to find the match. With IVF, multiple births are more common so you can find the match more quickly and only implant into the mother's uterus those embryos that are a match for the sibling.

In the reproductive genetic clinics they only transfer the embryos that are a perfect match into the uterus of the mother. The umbilical cord of the new matching sibling can be used to treat the sick child.

The system of screening for embryos that are a perfect match is called preimplantation genetic diagnosis (PGD).

For the last decade and a half PGD has been used to detect genetic diseases before the embryo is implanted into the mothers uterus. 1000 babies have been born after screening (PGD) for cystic fibrosis, Fanconi anemia or other genetic diseases that the sibling had.

In the UK PGD is banned because the new baby does not benefit.

You can read about the research using PGD carried out by scientists from the Reproductive Genetics Institute in Chicago in JAMA (Journal of the American Medical Association). They used PGD to tissue-type embryos from nine couples who had children with either leukemia or Fanconi anemia.

Some people say that the new sibling should decide whether he/she wants to donate more tissue when he/she is old enough.

Other people wonder whether the motives of the parents to have more children is clear. Often the doctors say that the parents wanted to have more children anyway.

What is Preimplantation Genetic Diagnosis?
From: http://www.dnapolicy.org/genetics/pgd.jhtml

Preimplantation genetic diagnosis, or PGD, is the genetic testing of embryos produced through in vitro fertilization (IVF). PGD enables physicians to identify genetic abnormalities in embryos and to select for implantation only those embryos that are found not to have the abnormality. PGD was developed so that couples at risk of having children with serious genetic disorders could increase their chances of having a child without the disorder. In the absence of PGD, couples in this situation could avoid pregnancy, undergo prenatal diagnosis and terminate the pregnancy if the fetus is found to have the disease, or face the possibility of having an affected child.

It has been more than a decade since the first reported pregnancies using PGD were achieved and some 1,000 babies have been born worldwide using these techniques. Chromosome abnormalities and single gene disorders have been diagnosed, including cystic fibrosis, Tay Sachs disease, Marfan Syndrome, muscular dystrophy, sickle cell and Fanconi anemia, and thalassaemia. More controversial is the use of PGD to test for late onset disorders, such as Alzheimer disease, or genetic susceptibility to disease, for example, breast cancer.

How is PGD Performed?

PGD is a multi-step process. First, as in all in vitro fertilization processes, eggs removed from the mother after she has been given drugs to stimulate egg production are fertilized in the laboratory. The genetic material for testing can be obtained in two ways. The most common method is to use one or two cells taken from the embryo two to four days after fertilization. Alternatively, genetic tests can be performed on cells (called polar body cells) that are cast off by the egg as it matures and is fertilized. The results of the genetic tests on the polar bodies are used to infer the genetic makeup of the fertilized egg.

Two basic techniques are used to analyze the genetic material from single cells: chromosomal analysis to assess the number or structure of chromosomes present in the cells, and DNA analysis to detect specific gene mutations. For chromosomal analysis, fluorescently labeled, chromosome-specific probes are used to visualize spots representing each copy of that chromosome present in the cell. Too few or too many spots can indicate abnormalities. For direct DNA analysis, a technique known as a polymerase chain reaction (PCR) is used to make many copies of the targeted gene, which are then examined for evidence of a specific DNA sequence.

Regardless of the methods, the results of preimplantation genetic diagnosis are used to inform the selection of embryos for transfer to a woman's uterus.

Technical and Clinical Issues

In PGD, only one or two cells are available for diagnosis, making it very technically challenging. For direct DNA analysis, both copies, or alleles, of the gene may not amplify equally in the PCR, giving inconclusive results or a misdiagnosis. Chromosomal analysis using fluorescent probes is limited primarily by the fact that only a small number of chromosomes can be visualized simultaneously and some chromosome abnormalities cannot be detected.

Because of the possibility of misdiagnosis, it often recommended that the PGD diagnosis be confirmed by subsequent CVS or amniocentesis. New techniques are being developed and evaluated to decrease the error rate and increase the number of chromosomal abnormalities that can be detected.

The risks for PGD include the possibility of a misdiagnosis and unknown long-term risks to the fetus. Also, as with IVF generally, there is no certainty that a pregnancy will occur after the embryo is implanted. With improving laboratory techniques, pregnancy rates are likely to improve. The other risks include those common to all IVF treatments, for example risks associated with the hormones used to stimulate ovulation, ectopic pregnancy, and multiple pregnancies.

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