Preimplantation diagnosis (PID)

The preimplantation diagnosis (PID) can be considered an earlier form of pre-natal diagnostics. Cell biological and molecular genetic assays are used to decide whether or not an embryo should be implanted into the uterus. In order to perform a pre-implantation diagnosis, an in vitro fertilisation procedure is necessary.

The biopsy of polar bodies of the egg cell is an exceptional variant of the preimplantation diagnosis. The polar body diagnosis is an examination method used for indirect genetic analysis of the egg cells, and is not subject to the regulatory guidelines of the Austrian law on reproduction. If this less invasive examination method is not sufficient to induce pregnancy, or to avoid a miscarriage or stillbirth, in medically indicated cases the Austrian law on reproduction allows for preimplantation diagnosis and therefore a direct examination of the embryo can be performed.

The following explanations are intended to clarify the difference and applications of the two techniques. Both treatments are available at the Kinderwunschzentrum under consideration of the legal framework.

Polar body diagnosis (PBD)

This is not an examination of the embryo, but rather an examination of the polar bodies. The polar bodies of the egg cell are produced during the maturation divisions of the egg cell and are not required for further development. In the context of artificial fertilisation, indirect conclusions can be made about the genetic health of the egg cells (although not of the sperm cells).

Trophectoderm biopsy (TB)

A trophectoderm biopsy is employed if the less invasive polar body diagnosis technique is not sufficient to induce a pregnancy, or to avoid a miscarriage or still birth, or to prevent a genetic disease. In this technique, the cells of the embryo that eventually develop into the placenta (so-called trophectoderm or trophoblast cells) are collected by biopsy. The cells that develop into the child (embryoblasts or inner cell mass) remain untouched by the biopsy or examination. As opposed to polar body diagnosis, the trophectoderm diagnosis offers the possibility of identifying changes to the paternal genetic material.

Foto PKB

Under what circumstances and for which patients is a polar body diagnosis medically advisable?

The main applications of polar body diagnosis are the identification of the following:
1) Incorrect chromosomal distribution (diagnosis of aneuploidy) in egg cells
2) Maternal translocations (exchange of chromosomal sections)
3) Changes to individual genes (for carriers of genetic diseases)

We recommend polar body diagnosis in the following cases:

  • Reduced ovarian reserve (elevated FSH values): An elevated basal FSH value is linked to a higher chance of incorrect chromosomal distribution, independent of age.
  • Elevated ovarian reserve: If ovarian stimulation causes an above-average number of egg cells to mature, there is a higher risk of incorrect chromosomal distribution compared to a normal response to the stimulation.
  • Advanced age (>37 years), and the consequent risk of chromosomal deviations in the child.
  • Known genetic disorders (balanced translocations) of the woman
  • Previous pregnancies involving chromosomal disorders of the children
  • Carriers of a severe genetic disorder

Performing a polar body biopsy

A few hours after performing the ICSI procedure the covering of the fertilised egg cell is opened by a laser. By using a capillary tube both polar bodies can be extracted in most cases and sent for genetic analysis. Previous experience has shown that roughly 1 % of egg cells are damaged by a biopsy and cannot be used for subsequent culture.

Under what circumstances and for which patients is a trophectoderm biopsy medically advisable?

The main application of trophectoderm diagnostics is the identification of the following:
1) Incorrect chromosomal distribution (aneuploidy diagnostic) in embryos
2) Changes of individual genes ( in the case of carriers of a genetic disease)

We are allowed to perform the trophectoderm biopsy method under the following circumstances:

  • If three or more preceding IVF procedures did not result in implantation of the embryos into the placenta, and the conclusion was reached that a chromosomal abnormality of the embryos was the cause.
  • If three or more miscarriages led to the conclusion that chromosomal disorders of the children were the likely cause.
  • If one parent is a carrier of a genetic disease which can cause miscarriages or stillbirths, or could cause a genetic disease in the child, and permission is granted by the scientific board for genetic analysis and therapy (WAGG).

Performing a trophectoderm biopsy

The egg cell is fertilised using the ICSI method and further cultured until day 5/6 (blastocyst stage) with subsequent cryo-storage of the biopsied embryos. Using a laser, the hull of the embryo is opened and roughly 5 cells are removed with a capillary tube and sent for genetic analysis. It is possible that in some cases the removed cells are not suitable for genetic analysis, or that biopsied embryos with sub-optimal quality do not survive cryo-storage and thawing in the next cycle.

Examination and health of the embryo

Does polar body diagnosis always indicate if the transferred embryo is healthy?

The polar body diagnosis exclusively determines the genetic health of the egg cell and therefore only analyses the maternal component of the embryo. Incorrect chromosomal distributions can also develop during embryo culture or be derived from sperm cells and these cases are not identified by polar body diagnosis. However, over 90% of embryonic chromosome abnormalities are of maternal origin. Incorrect chromosome distributions which arise during embryo culture can often be recognised by optical appearance. The combination of polar body diagnosis and blastocyst culture can in most cases identify the embryos which have the highest potential for a successful pregnancy and ultimately develop into a healthy child.

Does trophectoderm diagnosis always indicate if the transferred embryo is healthy?

The trophectoderm diagnosis is an analysis of the embryo, which can exclude genetic changes of the embryo with high probability. There is a remaining risk that the examination gives a normal result even though a genetic change is present in the embryo (alternatively a normal embryo can give an abnormal genetic result). This can be caused by a so-called genetic mosaic (meaning genetically altered cells are present along with unchanged cells). However, the risk is lower during trophectoderm diagnosis than during performance of a biopsy in an earlier cell stage of the embryo. As a result of the method used in trophectoderm diagnosis there is a remaining risk of roughly 2-3 % for an incorrect diagnosis. After pregnancy commences we recommend a prenatal screening to exclude an incorrect diagnosis.

Which genetic diagnosis is used for the cell to be analysed?

In the case of chromosomal abnormalities (suspicion of aneuploidy or translocation) the array-CGH diagnosis is performed. Array-CGH is an established procedure that allows the identification of increases or reductions of the entire chromosome complement of the embryo. The genetic material of the biopsied cell is compared with that of a normal cell. The array-CGH method tests for the absence of chromosomes, or presence of chromosomal splinters or superfluous chromosomes.

Figure 1 shows the array-CGH profile of a normal embryo (or polar body) with a normal (euploid) chromosome complement, as the black line remains within the red and green threshold levels.

Figure 2 shows the array-CGH profile of an embryo (or polar body) with altered chromosomal material. In the case of an examination of the polar body this indicates the loss of chromosome 12 of the egg cell, as the polar body shows the addition of genetic material to chromosome 12 (indirect analysis of the egg cell). In the case of embryo examination this result would indicate the acquisition of genetic material on chromosome 12 (direct analysis of the embryo).

 

For carriers of a severe genetic disease it is necessary to perform an exact analysis of particular sections of the genetic material as the sequence of the DNA building blocks which cause said disease must be determined. The polymerase chain reaction (PCR) is the method of choice for the precise analysis of defined sections of the genetic material. For the examination of changes of individual genes, a family-specific molecular genetic diagnosis is established in each case, with which the known genetic change can be identified.