Pre-implantation genetic diagnosis
Development in assisted reproduction and molecular genetic methods enabled to move diagnosis of genetic disorders forward to very early developmental stages of embryo. Using pre-implantation genetic diagnosis (PGD) enables examination of just three days old embryo. In relation to in vitro fertilization, where there are more embryos available than can be transferred into the uteri, this method represents a significant advantage – to transfer only such embryos that are normal in all examined parameters, thus increasing probability that pregnancy will be successful and healthy child will be born.
Disorders in number of chromosomes in embryo are often causing spontaneous miscarriages and birth of disabled children. That is why examination of disorders in number of chromosomes is most frequent indication of PGD. Examination is carried out using fluorescent in-situ hybridization method (FISH). That method enables to identify presence of chromosome even in non-splitting cell. The principle is coloured labelling of a defined segment of DNA (part of chromosome) by so-called DNA probe enabling to determine number of copies of a chromosome in cell nucleus. Monitored chromosomes are not chosen randomly, but based on knowledge of frequency of chromosomal disorders in disabled children born and miscarriages.
Increased number of embryos with numeric chromosomal abnormalities is found in couples with higher reproductive age of the mother (over 35 years) or repeated unsuccessful IVF cycles. For such patients pre-implantation genetic screening (PGS) is carried out, the examination of copies of „most problematic“ chromosomes. Those are gonosomes (X, Y), chromosomes 21, 13, 18, 16 and 22. Trizomia (presence of three copies instead of two) chromosomes 13, 18, 21 and disorder in number of gonosomes lead most often to birth of alive, but disabled offspring (trizomia of chromosome 21 – Down’s syndrome, trizomia of chromosome 18 – Edward’s syndrome, trizomia of chromosome 13 – Patau’s syndrome, XXY – Klinefelter’s syndrome, XO Turner’s syndrome) – picture 1 and 2.
Next group comprises couples with sex linked inherited diseases (for example hemophilia). In that case it is possible to select an embryo of such a sex that cannot be affected. Sanatorium Pronatal has implemented the PGD method in 2000 in cooperation with Department of Genetics and Reproduction of Veterinary Medicine Research Institute, but since 2003 all genetic examinations are beeing performed in Genetic department of Sanatorium Pronatal.
(Pic. 1) „Healthy“ blastomere (nucleus under-colored grey, color marks are signals for examined chromosomes: X-1 purple signal, Y-1 orange signal, 13-2 red signals, 18-2 light blue signals, 21-2 green signals)
(Pic. 2) Blastomere of embryo from which a foetus has affected by Edward’s syndrome could have been born (nucleus is under-colored grey, color marks are signals for examined chromosomes: X-2 purple signals, 13-2 red signals, 18-3 light blue signals, 21-2 green signals)