FISH for Sexing and Inherited Chromosomal Abnormalities

Any X-linked disease, for which no specific single-cell polymerase chain reaction (PCR) test is available, may be considered for PGD sex selection. By using DNA probes for the sex chromosomes and an autosome as a hybridization control, the sex of embryos can be determined and only female embryos are transferred to the woman. For this particular application FISH is more advantageous and robust than PCR; it can additionally detect potential aneuploidy for the analyzed chromosomes while the problem with allele dropout (ADO) associated with PCR is entirely avoided. The first application of FISH in PGD was for sex selection and it remains the method of choice for this indication. Nevertheless, sexing and transfer of only female embryos cannot be regarded as the optimal treatment for carriers of an X-linked disorder as theoretically half of the discarded male embryos are unaffected. On the other hand, the workup of a new diagnostic test for every X-linked disease is very

Fig. 1 Embryo biopsy. Embryo biopsy with removal of one blastomere at cleavage stage. (Courtesy of Dr. Jose Inzunza, Karolinska Institutet.) (View this art in color at

expensive and time-consuming, thus sexing is still considered a reasonable option for these patients.

Fluorescent in situ hybridization is also used for PGD of inherited structural chromosomal abnormalities. Previously, the limiting factor was the need to design unique probes for every case as most structural chromosome abnormalities are ''private'' with patient-specific breakpoints. However, this has to a large extent been solved by the recent publication and subsequent commercial availability of subtelomeric probes from all chromosomes. The majority of reported cases of PGD for structural chromosomal abnormalities concern carriers of balanced translocations.1-3-1 Two types of translocations are known: Robertsonian translocations, the centric fusion of two acrocentric chromosomes, and reciprocal translocations, an exchange of segments from at least two chromosomes. To discriminate between embryos showing balanced and unbalanced chromosome content, a minimum dual-color FISH assay with one DNA probe from each q-arm of the chromosomes involved is required for Robertsonian translocations (Fig. 3). Generally, a three-color FISH assay is used for reciprocal translocations with one probe from each side of the breakpoint on either of the chromosomes involved and one probe located anywhere on the other chromosome. However, this diagnostic approach does not allow distinguishing between normal and balanced embryos. To be able to discriminate between them, the DNA probes used need to span or flank the translocation breakpoints. Such a strategy will be restricted by the time required to develop specific probes for each translocation carrier and the additional costs. Also, as both normal and balanced embryos will result in normal offspring it is ethically questionable whether only normal embryos should be transferred and not balanced translocation carriers. Preimplantation genetic diagnosis has also been successfully applied for other structural chromosomal abnormalities including inversions and deletions.[4]

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook

Post a comment