Deciphering the Molecular Orchestrators of Heterochromatin-Lamina Interactions at the Nuclear Periphery. Ashley M Karnay Abstract It is well established that the eukaryotic genome is segregated within three dimensional nuclear space, with specific genomic regions adopting subnuclear positions relative to specific nuclear landmarks. One such nuclear landmark associated is the nuclear lamina (NL) at the nuclear periphery. Positioning of specific genes at the NL correlates with enrichment in heterochromatin-associated epigenetic signatures, such as dimethylation of Lysine 9 on Histone H3 (H3K9me2), and repression of gene transcription. The dynamic coupling of peripheral positioning and gene silencing plays a key role in directing crucial developmental processes such as cardiomyocyte lineage restriction and goes awry in genetic forms of cardiomyopathy and other diseases. Genomic regions that make contact with the NL are defined as lamina-associated domains (LADs). Distributed across all chromosomes, these large domains dynamically interact with the NL to release or attach genes and regulatory elements in accordance with cell-type and differentiation state-specific gene expression programs. Specifically, loss of LADs results in precocious cardiac differentiation. How chromatin-lamina interactions are established and maintained remains poorly understood. Endogenously-encoded sequences sufficient for peripheral targeting of LADs have not been identified, strongly supporting the existence of regulatory proteins, epigenetic modifications or biomolecular processes capable of mediating chromatin-NL contacts. However, the precise orchestrators of spatial and temporal dynamics between LADs and the nuclear periphery remain elusive. By combining targeted manipulations of local transcriptional states and epigenetic signatures with single-cell microscopy and population-based genomics analysis I will test the hypothesis that the heterochromatin- associated epigenetic mark H3K9me2 is required to maintain sequestered genomic loci at the nuclear periphery while transcriptional repression serves to establish chromatin-lamina interactions. These studies will provide mechanistic insights into how chromatin states and gene activity are coupled to gene radial positioning. By probing peripheral chromatin organizational principles at the single-cell level while simultaneously analyzing higher-order genome organization at the population level, I can uniquely dissect the underlying mechanism mediating the spatial organization of the genome. Elucidating the link between nuclear organization, human development and disease etiology is crucial to understanding how these relationships impact organogenesis and disease.