| We report the first detailed karyotypic examination of untreated lymphoblastoid cell lines (LCLs) from patients with the ICF syndrome (immunodeficiency, centromeric region instability, and facial anomalies), a DNA hypomethylation disease usually due to the DNA methyltransferase DNMT3B gene mutations. These LCLs displayed distinctive cytogenetic abnormalities specifically targeted to chromosomes (Chr) 1 and 16, namely decondensation of the juxtacentromeric heterochromatin (qh), and pericentromeric rearrangements (PRs). Since these abnormalities are apparently continuously generated, the study of ICF LCLs provides an excellent model for the generation of Chr1 and Chr16 aberrations, which are also overrepresented in different types of cancer.; We also present the first report that extremely high levels of ICF-like 1qh or 16qh abnormalities are present in all examined, untreated chorionic villus (CV) and amniotic fluid (AF) cultures from normal individuals at late passage, but not in early passage cultures. Their frequency increased with passage number, but CV cultures had significantly more aberrations at lower passages than the AF cultures. Regardless of passage number, CV DNA, but not AF DNA, was hypomethylated at satellite 2 and globally. We propose that hypomethylation of satellite 2 DNA in CV cultures increases the frequency of 1qh decondensation, and favors Chr1 PRs. However, hypomethylation of satellite 2 DNA is not always necessary for 1qh decondensation, as observed in the late passage AF cultures.; Our findings on ICF LCLs have additionally given us insight into a complex 3.3-kb repeat (D4Z4). Approximately 95% of fascioscapulohumeral muscular dystrophy (FSHD) patients have <10 copies of D4Z4 on one Chr4 homologue at 4q35. We found that many restriction endonuclease sites in D4Z4 are highly methylated in normal and FSHD cell lines and somatic tissues. However, in sperm and in ICF LCLs, D4Z4 was undermethylated, as found for heterochromatic satellite 2 DNA repeats. Our data indicate that even if hypomethylation of D4Z4 does not accompany the FSHD deletion of D4Z4 repeat copies at 4q35, its highly methylated status may help repress inappropriate expression of the postulated FSHD genes in normal skeletal muscle in conjunction with a high copy number. This may occur by D4Z4 methylation helping to maintain a heterochromatin structure. |
