Research in the Sullivan Lab is aimed at understanding mechanisms of formation and behavior of human chromosomes. Abnormalities in chromosome structure and/or number are associated with birth defects, infertility, miscarriage, mental retardation, and cancer. A major focus of the lab's research is the centromere, a specialized chromosomal locus that is involved in kinetochore function and interact with spindle microtubules. Our experiments have uncovered a unique type of chromatin (CEN chromatin) formed exclusively at the centromere via replacement of core histone H3 by the centromeric histone variant CENP-A. Centromeres have historically been considered heterochromatic, however, we found that CEN chromatin contains euchromatic modifications of H3, signifying an open or flexible chromatin conformation. We use normal and engineered centromeres to define genomic and functional elements involved in centromere specification. In addition, we are testing how DNA sequence variation impacts de novo centromere assembly.
We also study naturally-occurring and engineered human chromosomes that contain two centromeres. On these chromosomes, called dicentrics, only centromere is usually functional. Barbara McClintock, the famous cytogeneticist and Nobel prizewinner, studied dicentric chromosomes in maize (corn) in the 1930s. She described dicentrics as inherently unstable because the two centromeres usually segregated to opposite spindle poles in anaphase, causing repeated chromosome breakage and massive genome instability. However, we have shown that dicentric human chromosomes exhibit unprecedented stability because either both centromeres work together to may make a super-centromere, or one of the centromeres is shut off, so that the chromosome behaves as if it has only a single centromere. We want to understand the mechanism of centromere repression - how and when does a centromere get turned off? To answer our questions, we have developed novel experimental systems to generate dicentric chromosomes in the laboratory. Our current work includes determining how dicentric chromosomes form, defining the molecular process of centromere inactivation, and monitoring mitotic fates of dicentric chromosomes after their formation.