Epigenomic Mechanisms of Centromere Function and Chromosome Rearrangements
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The centromere is essential for chromosome segregation and genome stability. It is the site of kinetochore assembly and chromosome attachment to the spindle microtubules, and it is important for chromosome movement during mitosis and meiosis. Normal human chromosomes have one centromere, but genome rearrangements that occur with instability, aging, and disease often result in chromosomes with two centromeres, called dicentrics. Nearly seventy-five years ago, Barbara McClintock demonstrated that dicentric chromosomes in plants are associated with instability through mitotic "breakage-fusion-bridge" cycles. However, human dicentrics are unusually stable due to the poorly understood phenomenon of centromere inactivation. Centromere inactivation has been primarily studied in patient-derived dicentrics, limiting the derivation of a molecular pathway. Key centromere and kinetochore proteins are not present at inactive centromeres, but beyond these observations, the process of centromere inactivation is unclear. Epigenetic and sequence-dependent factors are known to contribute to centromere specification, but requirements for centromere assembly, maintenance, and suppression remain obscure. The aims of this research were to (1) determine the mechanism(s) by which de novo dicentric chromosomes are stabilized, (2) ascertain the factors influencing the involvement of specific chromosomes in de novo fusions, and (3) establish the epigenomic, temporal, and mechanistic basis of centromere inactivation. To uncover the mechanistic foundations of these processes, we developed in vitro cell culture systems to study the formation and stabilization of de novo dicentrics. We demonstrate that transient disruption of human telomere structure non-randomly produces dicentric fusions involving acrocentric chromosomes. This finding is notable since the most prevalent rearrangement in humans involves the acrocentrics and is called Robertsonian translocation (ROB). In some cases, centromere inactivation occurs by an apparently epigenetic mechanism. In other dicentrics, the size of the centromeric DNA array is reduced compared to the same array before dicentric formation. Many functional dicentrics persist for months after formation. Our results indicate that dicentric human chromosomes undergo alternative fates after formation across a broad temporal window. During transient telomere disruption, we observed a dramatic change in nucleolar appearance. Nucleolar proteins did not coalesce into condensed structures, but appeared dispersed throughout the nucleus. This surprising alteration in nucleolar organization and nuclear architecture suggests remodeling of the nucleolus and subsequent effects on nucleolar-associated chromosomes, such as the acrocentrics, could contribute to the high incidence of ROB formation. Further studies and development of additional cell culture systems will allow us to evaluate current models of centromere assembly and disassembly and the importance of chromatin organization to centromere function and genome architecture.
DepartmentGenetics and Genomics
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