Chromosoma (2005) 114: 118–126 DOI 10.1007/s00412-005-0343-7 RESEARCH ARTICLE David Gisselsson . Chunbo Shao . Cathy M. Tuck-Muller . Suzana Sogorovic . Eva Pålsson . Dominique Smeets . Melanie Ehrlich Interphase chromosomal abnormalities and mitotic missegregation of hypomethylated sequences in ICF syndrome cells Received: 21 December 2004 / Revised: 29 March 2005 / Accepted: 29 March 2005 / Published online: 27 April 2005 # Springer-Verlag 2005 Abstract The immunodeficiency, centromeric region in- stability, facial anomalies (ICF) syndrome is a rare auto- somal recessive disease. Usually, it is caused by mutations in the DNA methyltransferase 3B gene, which result in decreased methylation of satellite DNA in the juxtacen- tromeric heterochromatin at 1qh, 16qh, and 9qh. Satellite II-rich 1qh and 16qh display high frequencies of abnor- malities 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 peri- centromeric 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 chromo- somes. Our results elucidate the mechanism of formation of ICF chromosome anomalies and suggest that 1qh–16qh associations in interphase can lead to disturbances of mi- totic 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. Abbreviations DAPI: diamidinophenylindole . DNMT3B: DNA methyltransferase 3B . EBV: Epstein–Barr virus . FISH: fluorescence in situ hybridization . ICF: immunodeficiency, centromeric instability, facial anomalies . LCL: lymphoblastoid cell line . Sat2: classical satellite II DNA . MN: micronuclei . PBS: phosphate-buffered saline Introduction The immundeficiency, centromeric region instability, facial anomalies (ICF) syndrome is a rare autosomal recessive disease. Since it was first described by Hulten et al. (1978), about 35 cases have been reported in the literature. It has recently been shown that most ICF patients exhibit in- activating mutations of the DNA methyltransferase 3B gene (DNMT3B), leading to invariant hypomethylation of a lim- ited number of genomic regions (Kondo et al. 2000), in particular the heterochromatic regions containing tandemly repeated sequences of classical satellite II (Sat2) and III (Sat3) (Jeanpierre et al. 1993; Hansen et al. 1999; Xu et al. 1999). Immunologically, ICF patients show hypogamma- globulinaemia but with B-cells present (Ehrlich 2003). It has been suggested that this condition is mediated by the dysregulation of lymphogenesis genes (Ehrlich et al. 2001) and defective B-cell differentiation, leading to the accu- mulation of immature B-cells with an increased rate of apoptosis upon in vitro activation (Blanco-Betancourt et al. 2004). Because of a strong predisposition for systemic infectious diseases, the majority of ICF patients die before reaching their teens. Peripheral blood lymphocytes from ICF patients are cytogenetically characterized by abnormalities of the long Sat2-rich pericentromeric heterochromatin regions of chro- mosome 1 and 16, and, to a much lesser extent, also of the Communicated by S. Gerbi D. Gisselsson . E. Pålsson Department of Clinical Genetics, University Hospital, 221 85 Lund, Sweden C. Shao . M. Ehrlich (*) Human Genetics Program and Department of Biochemistry, Tulane Medical School, New Orleans, LA, 70112, USA e-mail: ehrlich@tulane.edu Tel.: +1-504-9882449 Fax: +1-504-9881763 C. M. Tuck-Muller . S. Sogorovic Department of Medical Genetics, University of South Alabama, Mobile, AL, 36688, USA D. Smeets Department of Human Genetics, University Medical Center St. Radboud, Nijmegen, The Netherlands