The human genome contains ~300 ribosomal gene (rDNA) repeats, organized in tandem arrays at nucleolar organizer regions (NORs), situated on the short p-arms of each of the five human acrocentric chromosomes. Despite harboring NORs that direct the assembly of one of the key functional domains of the nucleus (the nucleolus), acrocentric p-arms remain absent from the current human genome assembly. The research goal of my laboratory is to determine
how NORs are arranged at a chromosomal level and how they direct formation and regulate the function of nucleoli in human cells. The research program I propose focuses on the chromosomal context of NORs and its role in nucleolar biology. It has the following three specific aims:
(1) Determine the genomic architecture of acrocentric short-arms.
(2) Identify and characterize elements that regulate NOR activity and nucleolar morphology.
(3) Determine how the chromosomal context of NORs contributes to the genomic stability of rDNA arrays. The major advance that the three integrated components of this research program will offer is a deeper understanding of the regulation, biogenesis and morphology of the largest and arguably the most critical of all nuclear bodies in human cells. We will provide a complete description of what constitutes a NOR. We expect to identify elements within rDNA distal sequences, that influence NOR activity status, nucleolar formation and morphology. We aim to prove that anchoring of NORs in PNH is of fundamental importance. We predict it is involved in nucleolar fusion and in a highly specialized nucleolar response to DNA DSBs that is required to
maintain genetic stability of the rDNA array. This work will provide a wealth of tools for us and other workers to study the nucleolus in various cellular contexts including development, differentiation, senescence/aging and disease. The distal sequences we have identified thus far are already providing valuable tools. As an example, DJ FISH probes have allowed us for the first time to count how many NORs contribute to each nucleolus and determine what proportion of NORs are active in each cell type18. Through genetic manipulation we expect to be able to influence some of these more complex aspects of nucleolar biology and determine the subsequent effects. In this regard it should be pointed out that modulating rRNA synthesis fosters changes in the cell fate, growth and proliferation of germline stem cells in animal models37. Modulating ribosome biogenesis by non-genotoxic means is also proving to be a new modality for development of anti-cancer drugs.
