| ICF syndrome is a rare, autosomal-recessive human genetic disorder characterized by immunodeficiency, centromeric heterochromatin instability, and facial anomalies. Fewer than one hundred patients worldwide have been diagnosed with ICF syndrome. Most cases of ICF syndrome are due to mutations in the DNA methyltransferase gene DNMT3B, but all patients share a joint molecular-cytogenetic phenotype: complete demethylation of classical satellite DNA of class II and III, associated with a dramatic decondensation and elongation of the pericentromeric regions of chromosomes 1, 9, and 16 in stimulated peripheral lymphocytes.; Classical satellite DNA is an abundant, highly repetitive subset of the human genome which, to date, has no known function. Class II and III classical satellite DNA is normally heavily methylated and is found in long tandem arrays primarily at the pericentromeric regions of chromosomes 1, 9, and 16, the same loci affected in ICF syndrome. This thesis describes experiments I have performed to understand how demethylation of class II and III classical satellite DNA leads to ICF syndrome.; Chapter 1 relates the history and characteristics of ICF syndrome and classical satellite DNA. Chapter 2 describes experiments I performed to detect proteins that bind specifically to methylated, but not unmethylated, classical satellite DNA. Chapter 3 describes the purification of the single protein, nucleolin, determined to have this activity and shows that the intracellular distribution of nucleolin is not perturbed in ICF patients.; Chapter 4 summarizes a body of literature spanning at least thirty years that describes interactions between satellite DNA and the surface of the nucleolus. I propose that failure of nucleolin binding to the unmethylated classical satellite DNA in ICF patients prevents the association of satellite DNA with the nucleolus, leading to multiple tissue-specific deficits and the symptoms of ICF syndrome. |
