B-Type Lamins organize 3D chromatin architecture through associations with nuclear lamina and nuclear speckles [RNA-seq]

B-type lamins (Lamin B1 and B2) are key components of the nuclear lamina and play essential roles in three-dimensional (3D) chromatin organization by anchoring Lamina-Associated Domains (LADs). Recent studies have indicated that abnormal gene expression resulting from B-type lamin loss occurs in both LAD and non-LAD regions; however, the underlying molecular mechanisms remain unclear. In this study, we demonstrate that the majority of differentially expressed genes (DEGs) in B-type lamin-depleted cells are predominantly located in non-LAD regions, which are characterized by a constitutive association with nuclear speckles. Through RNA-Seq combined with SC35 Tyramide Signal Amplification Sequencing (TSA-Seq) analysis, we find that aberrant gene regulation is closely associated with the repositioning of chromatin either toward or away from nuclear speckles. The absence of B-type lamins causes a global reorganization of chromatin rather than changes in its fine structure, leading to the disruption of constitutive Speckle-Associated Domains (cSPADs). Additionally, the loss of B-type lamins results in euchromatin deactivation and heterochromatin de-repression, which impairs cell viability by increasing apoptotic signaling and causes defective mRNA splicing. Our data reveal a novel role for B-type lamins in regulating transcriptional activity by conserving genome architecture between the nuclear lamina and nuclear speckles.