Region-specific FISH probes used to identify and characterize an interstitial paracentric inv(21)(q22.1q22.3)

Region-specific probes developed for the diagnosis of specific syndrome, can be adapted to elucidate the exact nature of certain chromosomal structural anomalies. We describe the use of FISH probes in characterizing a prenatally diagnosed chromosome rearrangement. An abnormal chromosome 21 was detected during amniocentesis for maternal age indication, and a similar appearing chromosome 21 was found in the mother. The exact nature of the rearrangement was not immediately evident from G-banded karyotypes. FISH was performed using a whole chromosome painting probe, as well as the region-specific probes D21S65 (21q21-22.1), D21S55 (21q22.3) and D21S1219/D21S1220 (21q22.3-qter) (Oncor). Results showed an interstitial paracentric inversion, with breakpoints in bands 21q22.1 and 21q22.3, which was identical in the mother and the fetus: 46,XX,?inv(21)(q).ish inv(21)(q22.1q22.3)(wcp+.D21S65 mv, D21S55 mv, D21S1219/D21S1220 st). In this case, FISH using region-specific probes was helpful in characterizing the inversion and aided in the genetic counselling of risk assessment for the family.