Role of Nuclear Lamins in Chromosome Ositioning and Genome Stability

Abstract

Chromosome territories are non-randomly organized in the interphase nucleus with gene rich chromosomes predominantly localized towards the nuclear interior and gene poor chromosomes towards the nuclear periphery. Such an arrangement is conserved across evolution and largely conserved across cell types. In addition, gene loci that are orders of magnitude smaller than chromosome territories are also non-randomly organized in the nucleus, with transcriptionally active genes being ‗looped out‘ of their chromosome territories. The molecular basis of a non random arrangement in the nucleus is not completely understood. Nuclear Lamins are type V intermediate filament proteins that form a meshwork beneath the inner nuclear membrane. In addition, both A and B type Lamins also show nucleoplasmic pools in the nuclear interior. In this study, we have investigated the role of Lamin A/C and Lamin B2 in the organization of chromosome territories and genome stability. This study was performed in diploid colorectal cancer cells – DLD1 and showed non overlapping functions for nuclear Lamins A/C and B2 in regulating genome stability and organization. Lamin A/C and Lamin B2 knockdown revealed specific sets of chromosomes that were transcriptionally deregulated. Depletion of Lamins A/C and B2 induced aneuploidy in otherwise diploid DLD1 cells, as revealed by an increase in the number of chromosome territories in the interphase nucleus. Aneuploid chromosome territories showed a mislocalization in the interphase nucleus in a sub-population of Lamin B2 but not Lamin A/C depleted cells, suggesting a specific role for Lamin B2 in the regulation of spatial positions of aneuploid chromosome territories. Lamin B2 depletion also showed a wide spectrum of nuclear and chromosomal instabilities ranging from micronuclei, nuclear blebbing, chromosomal losses and gains that further implicate Lamin B2 in enhancing cancer associated genomic instabilities. In contrast, Lamin A/C regulates the spatial positions of gene rich chromosome territories in association with its interacting partner Emerin. Lamin A/C functions with Emerin in positioning gene rich chromosome territories towards the nuclear interior potentially through a regulation of the organization of components of the cytoskeleton - actin and nuclear myosin I. Taken together, we have uncovered two unique roles for Lamin B2 and Lamin A/C in the maintenance of genome organization and stability - Lamin B2 functions as a sensor of chromosome numbers both in mitosis and interphase while Lamin A/C primarily functions to regulate the positions of gene rich chromosome territories in association with its interactor Emerin.