What is a translocation?
A translocation is a type of abnormality in the structure of a chromosome. There are two main types of translocations: reciprocal (also known as non-Robertsonian) and Robertsonian. Reciprocal translocation is the more common type. In a reciprocal translocation, a small portion of two separate chromosomes have “swapped” positions with each other. For example, in a translocation between chromosomes number 13 and 21, a little piece of chromosome 13 is now on chromosome 21 and a little piece of chromosome 21 is now on chromosome 13. If the swap is balanced, then the total amount of genetic material is unaffected, only the location has changed. In an unbalanced translocation, the exchange of chromosome material is unequal resulting in extra or missing genes.
Reciprocal (non-Robertsonian) translocations
Reciprocal translocations are found in about 1 in 600 newborns. If the translocation is balanced, then the individual is unaffected and is said to be a carrier. Carriers of balanced reciprocal translocations have increased risks of creating embryos which are unbalanced. Unbalanced chromosome translocations may lead to miscarriages or children with birth defects.
In this type of translocation, two chromosomes essentially fuse forming one larger chromosome. An individual with this type of translocation has only 45 chromosomes. Robertsonian translocations can be found in seen in about 1 in 1300 newborns. Carriers of Robertsonian translocations can be unaffected themselves, but also have an increased risk of creating embryos which are unbalanced and cause miscarriages or birth defects.
When fertilization occurs in a couple with a translocation, a few different possible outcomes may occur:
- The embryo may be completely normal
- The embryo may end up with a balanced amount of chromosome material and end up like the parent, essentially, a carrier of the problem
- The embryo may end up with an unbalanced amount of chromosomal material. These embryos, like the aneuploidies will be miscarried.
Detecting Translocations with PGD
Couples in which one partner has a translocation usually present with a history of recurrent miscarriage. During the evaluation, a chromosome analysis on the each partner identified the translocation abnormality.
At this point, their options are to continue to try to conceive naturally, use a donor (egg or sperm depending on which partner is the carrier), or use PGD. If they decide to try PGD, blood is obtained from the affected partner so that the translocation can be characterized.
Specific fluorescent probes are used that recognize and attach to an entire chromosome. For example, a translocation between chromosomes #4 and #11 would require whole chromosome probes for each of those chromosomes. Each probe would have a different color for easy identification. After an embryo is obtained from IVF, the polar body or blastomere (or sometimes both) are removed from the embryo. These biopsied cells must be at a certain stage in order for the process to work so they are converted in the lab. The probe is then applied and read. A normal chromosome would appear with a single color. A chromosome with a translocation will show two colors.
In this way, embryos can be identified that do not have translocations and then these can be used for transfer to the female to attempt pregnancy.