Characterization of a constitutive heterochromatin abnormality in Roberts syndrome

Abstract

<p>Roberts syndrome (RS) is a rare autosomal recessive developmental disorder that is characterized by severe pre- and post-natal growth retardation, tetraphocomelia and various craniofacial abnormalities. In some patients with Roberts syndrome (RS+) but not others (RS-), RS is associated with a cytogenetic abnormality which presents as an unusual "puffing" of the constitutive heterochromatin and nucleolar organizing regions. The decondensed appearance of the constitutive heterochromatin suggests an alteration in chromatin structure in RS+. Two main questions were addressed in this study; (1) are altered levels of DNA methylation associated with the constitutive heterochromatin abnormality of RS+ chromosomes and (2) is the timing of DNA replication of constitutive heterochromatin, relative to the rest of the genome, altered in RS+ lymphoblast cells? Levels of methylated cytosine residues of RS+ and RS- heterochromatin DNA, assessed using MspI and HpaII isoschizomer restriction enzyme digestion and Southern blot hybridization with a repetitive probe (D15Z1) to the heterochromatin DNA, were found to be significantly lower (p = 0.048) in RS+ fibroblast cell strains compared to control and RS- cell strains. In addition to this, D15Z1 DNA methylation levels differed significantly (p ≤ 0.05) at early and late passage in fibroblast cell strains of all three groups, suggesting that decreased levels of DNA methylation were associated with an in vitro aging effect. The chromosome replication patterns of the constitutive heterochromatin regions of chromosomes 1, 9, 16 and Y were investigated using terminal 5-bromodeoxyuridine labelling and an S phase subclassification system. A significant delay (p < 0.001 for male cell lines; p<0.05 for female cell lines) was observed in the timing of RS+ constitutive heterochromatin. These results suggest that the cytogenetic abnormality of the constitutive heterochromatin of RS+ cells is associated with alterations in DNA conformation and synthesis.</p>