Poly-FISH: a technique of repeated hybridizations that improves cytogenetic analysis of fetal cells in maternal blood.

Prenatal diagnosis of fetal chromosomal abnormalities using interphase fetal nucleated erythrocytes (FNRBCs) separated from maternal peripheral blood can be technically challenging due to the limited number of FNRBCs available for analysis, the limited number of probes that can be used simultaneously, and low FISH efficiency on the formaldehyde-fixed and immunohistochemically stained interphase FNRBCs. We developed a technique of sequential FISH analysis that involves removal of the previous hybridized probe under denaturing conditions, and rehybridization with different probes to improve FISH efficiency. This technique facilitates the analysis of multiple chromosome-specific probes on the same nuclei. Results from our experiments show that FISH can be performed at least nine times on the same interphase nucleus and at least three different probes can be used simultaneously. Thus, theoretically, at least 24 different chromosomes can be analysed on a single interphase fetal cell isolated from maternal blood. We have termed this technique 'Poly-FISH', and have successfully diagnosed trisomy 21, triploidy, and other chromosome abnormalities in FNRBCs using this technique.