Chromosome instability in ICF syndrome: formation of micronuclei from multibranched chromosomes 1 demonstrated by fluorescence in situ hybridization.

We report on a new patient with immunodeficiency, centromeric heterochromatin instability, and facial anomalies (the ICF syndrome). Studies with traditional cytogenetic methods demonstrate that aberrations in this syndrome primarily involve the centromeric regions of chromosomes 1 and 16. We applied fluorescence in situ hybridization (FISH) using "painting" probes for chromosomes 1 and 16 to document the progression of centromeric instability from simple decondensation aberrations to the subsequent formation of complex multibranched chromosomes 1, and finally to the interphase aberrations of nuclear projections and micronuclei involving both chromosomes 1 and 16. The loss of the large multibranched chromosome 1 configurations from the cells as micronuclei suggests that the centromeric aberrations subsequently interfere with normal chromosome movement at anaphase in ICF syndrome. Circular areas of counterstained chromatin were observed by FISH in the micronuclei corresponding to the intertwined segments of centromeric heterochromatin seen involving multibranched chromosomes 1 in the patient's G-banded chromosome study. The current hypothesis of recessive inheritance for this disorder suggests that the chromosomal aberrations are not a causative event in this syndrome; however, the chromosome aberrations are clearly an important basic diagnostic criterion.