Loss of lamin B1 results in prolongation of S phase and decondensation of chromosome territories.

Nuclear lamin B1 (LMNB1) constitutes one of the major structural proteins in the lamina mesh. We silenced the expression of LMNB1 by RNA interference in the colon cancer cell line DLD-1 and showed a dramatic redistribution of H3K27me3 from the periphery to a more homogeneous nuclear dispersion. In addition, we observed telomere attrition and an increased frequency of micronuclei and nuclear blebs. By 3D-FISH analyses, we demonstrated that the volume and surface of chromosome territories were significantly larger in LMNB1-depleted cells, suggesting that LMNB1 is required to maintain chromatin condensation in interphase nuclei. These changes led to a prolonged S phase due to activation of Chk1. Finally, silencing of LMNB1 resulted in extensive changes in alternative splicing of multiple genes and in a higher number of enlarged nuclear speckles. Taken together, our results suggest a mechanistic role of the nuclear lamina in the organization of chromosome territories, maintenance of genome integrity and proper gene splicing.

CKAP2 ensures chromosomal stability by maintaining the integrity of microtubule nucleation sites.

Integrity of the microtubule spindle apparatus and intact cell division checkpoints are essential to ensure the fidelity of distributing chromosomes into daughter cells. Cytoskeleton-associated protein 2, CKAP2, is a microtubule-associated protein that localizes to spindle poles and aids in microtubule stabilization, but the exact function and mechanism of action are poorly understood. In the present study, we utilized RNA interference to determine the extent to which the expression of CKAP2 plays a role in chromosome segregation. CKAP2-depleted cells showed a significant increase of multipolar mitoses and other spindle pole defects. Notably, when interrogated for microtubule nucleation capacity, CKAP2-depleted cells showed a very unusual phenotype as early as two minutes after release from mitotic block, consisting of dispersal of newly polymerized microtubule filaments through the entire chromatin region, creating a cage-like structure. Nevertheless, spindle poles were formed after one hour of mitotic release suggesting that centrosome-mediated nucleation remained dominant. Finally, we showed that suppression of CKAP2 resulted in a higher incidence of merotelic attachments, anaphase lagging, and polyploidy. Based on these results, we conclude that CKAP2 is involved in the maintenance of microtubule nucleation sites, focusing microtubule minus ends to the spindle poles in early mitosis, and is implicated in maintaining genome stability.

Genetic amplification of the NOTCH modulator LNX2 upregulates the WNT/ß-catenin pathway in colorectal cancer.

Chromosomal copy number alterations (aneuploidy) define the genomic landscape of most cancer cells, but identification of the oncogenic drivers behind these imbalances remains an unfinished task. In this study, we conducted a systematic analysis of colorectal carcinomas that integrated genomic copy number changes and gene expression profiles. This analysis revealed 44 highly overexpressed genes mapping to localized amplicons on chromosome 13, gains of which occur often in colorectal cancers (CRC). RNA interference (RNAi)-mediated silencing identified eight candidates whose loss-of-function reduced cell viability 20% or more in CRC cell lines. The functional space of the genes NUPL1, LNX2, POLR1D, POMP, SLC7A1, DIS3, KLF5, and GPR180 was established by global expression profiling after RNAi exposure. One candidate, LNX2, not previously known as an oncogene, was involved in regulating NOTCH signaling. Silencing LNX2 reduced NOTCH levels but also downregulated the transcription factor TCF7L2 and markedly reduced WNT signaling. LNX2 overexpression and chromosome 13 amplification therefore constitutively activates the WNT pathway, offering evidence of an aberrant NOTCH-WNT axis in CRC.