|
|
To analyze for the presence of a genetic defect of an embryo, it is necessary to remove either the first polar body of an unfertilized egg and/or 1 or 2 cells from each embryo. This is called an egg or embryo biopsy and is usually done before insemination occurs, or 3 days after fertilization. Biopsy of at the 6-10 cell stage does not adversely affect preimplantation development. At this stage each cell has a full complement of chromosomes. Normally only a single cell is removed from each embryo, since it is expected to be identical to all of the other cells in the embryo. Occasionally, it is necessary to remove a second cell from an embryo, if a signal is not found in the first, for instance. FISH analysis has been used for preconception diagnosis by using the first and second polar bodies as an indicator of the genetic status of the oocyte. A disadvantage of polar body analysis alone is that it does not take into account paternal aneuploidies.
The analysis of the biopsied cell(s) utilizes one of two techniques:
- Fluorescence in–situ hybridization (FISH): The biopsied cell(s) are fixed to a glass slide and heated and cooled and their DNA is “labeled” with colored fluorescent dyes called probes (little pieces of DNA that are a match for the chromosomes being tested), one for each chromosome to be identified. At present 8 of the 23 chromosomes can be identified. Once complete, the embryologist counts the colors under a high-powered microscope and is able, in most cases, to distinguish normal from abnormal cells. The process takes about a day. Normal embryos will either be transferred into the uterus on day 4 following egg retrieval, or left in extended culture and transferred on day 5 as blastocysts. The cells utilized for PCR are no longer viable, and will not be replaced into the embryo(s), but may then be stored for future investigation.
- Polymerase chain reaction (PCR): A technique which amplifies the number of copies of specific regions of DNA in order to produce enough DNA to be sufficiently tested. DNA is double-stranded (except in some viruses), and the two stands pair up in a very specific way. A gene’s “building-block sequence” is the specific order of appearance of 4 different deoxyribonucleotides within a segment of DNA. These 4 components are: adenine (A), thymadine (T), cytosine (C), and guanine (G). The arrangement of this 4-letter alphabet generates a gene sequence. In this technique DNA is heated (denaturization) in order to separate the 2 strands. Primers are added and then the DNA is cooled in order to allow double-strands to form again. An enzyme is then added in cycles, which can “read” the gene sequence, and result in multiplication of DNA. PCR is utilized to diagnose gene-specific diseases, as well as disease-causing viruses and/or bacteria, or to link a criminal suspect to a crime.
What disorders are RBA currently capable of examining embryos for?
RBA is currently capable of examining embryos from in vitro fertilization utilizing either in-house FISH analysis or PCR in collaboration with other centers for the following disorders: - Age related aneuploidies
- Cystic fibrosis
- Tay-Sachs
- Hemophilia A&B
- Retinitis pigmentosa
- Sickle cell disease
- Thalassemia
- Alport’s syndrome
- Alpha 1 antitrypsin deficiency
- Fragile X
- Duchenne muscular dystrophy
- Myotonic dystrophy
- Lesch-Nyhan syndrome
- Rhesus (RhD)
- Marfan’s syndrome
- Familial adenamatous polyposis coli
- Huntington’s disease
- X-linked disease by sex determination: (sex selection for the purpose of family planning or balancing is NOT offered at RBA)
- Translocations
In addition, other IVF centers capable of performing embryo biopsy have the option of sending cells to the RBA laboratory for FISH analysis.
|
