Induction of polyploidy by nuclear fusion mechanism upon decreased expression of the nuclear envelope protein LAP2β in the human osteosarcoma cell line U2OS

Background: Polyploidy has been recognized for many years as an important hallmark of cancer cells. Polyploid cells can arise through cell fusion, endoreplication and abortive cell cycle. The inner nuclear membrane protein LAP2 beta plays key roles in nuclear envelope breakdown and reassembly during mitosis, initiation of replication and transcriptional repression. Here we studied the function of LAP2 beta in the maintenance of cell ploidy state, a role which has not yet been assigned to this protein. Results: By knocking down the expression of LAP2 beta, using both viral and non-viral RNAi approaches in osteosarcoma derived U2OS cells, we detected enlarged nuclear size, nearly doubling of DNA content and chromosomal duplications, as analyzed by fluorescent in situ hybridization and spectral karyotyping methodologies. Spectral karyotyping analyses revealed that near-hexaploid karyotypes of LAP2 beta knocked down cells consisted of not only seven duplicated chromosomal markers, as could be anticipated by genome duplication mechanism, but also of four single chromosomal markers. Furthermore, spectral karyotyping analysis revealed that both of two near-triploid U2OS sub-clones contained the seven markers that were duplicated in LAP2 beta knocked down cells, whereas the four single chromosomal markers were detected only in one of them. Gene expression profiling of LAP2 beta knocked down cells revealed that up to a third of the genes exhibiting significant changes in their expression are involved in cancer progression. Conclusions: Our results suggest that nuclear fusion mechanism underlies the polyploidization induction upon LAP2 beta reduced expression. Our study implies on a novel role of LAP2 beta in the maintenance of cell ploidy status. LAP2 beta depleted U2OS cells can serve as a model to investigate polyploidy and aneuploidy formation by nuclear fusion mechanism and its involvement in cancerogenesis.