Interphase chromosomal abnormalities and mitotic missegregation of hypomethylated sequences in ICF syndrome cells

The immunodeficiency, centromeric region instability, facial anomalies (ICF) syndrome is a rare autosomal recessive disease. Usually, it is caused by mutations in the DNA methyltransferase 3B gene, which result in decreased methylation of satellite DNA in the juxtacentromeric heterochromatin at 1qh, 16qh, and 9qh. Satellite II-rich 1qh and 16qh display high frequencies of abnormalities in mitogen-stimulated ICF lymphocytes without these cells being prone to aneuploidy. Here we show that in lymphoblastoid cell lines from four ICF patients, there was increased colocalization of the hypomethylated 1qh and 16qh sequences in interphase, abnormal looping of pericentromeric DNA sequences at metaphase, formation of bridges at anaphase, chromosome 1 and 16 fragmentation at the telophase-interphase transition, and, in apoptotic cells, micronuclei with overrepresentation of chromosome 1 and 16 material. Another source of anaphase bridging in the ICF cells was random telomeric associations between chromosomes. Our results elucidate the mechanism of formation of ICF chromosome anomalies and suggest that 1qh-16qh associations in interphase can lead to disturbances of mitotic segregation, resulting in micronucleus formation and sometimes apoptosis. This can help explain why specific types of 1qh and 16qh rearrangements are not present at high frequencies in ICF lymphoid cells despite diverse 1qh and 16qh aberrations continuously being generated.