Maarten Fornerod

The human homologue of yeast CRM1 is in a dynamic subcomplex with CAN/Nup214 and a novel nuclear pore component Nup88

The oncogenic nucleoporin CAN/Nup214 is essential in vertebrate cells. Its depletion results in defective nuclear protein import, inhibition of messenger RNA export and cell cycle arrest. We recently found that CAN associates with proteins of 88 and 112 kDa, which we have now cloned and characterized. The 88 kDa protein is a novel nuclear pore complex (NPC) component, which we have named Nup88. Depletion of CAN from the NPC results in concomitant loss of Nup88, indicating that the localization of Nup88 to the NPC is dependent on CAN binding. The 112 kDa protein is the human homologue of yeast CRM1, a protein known to be required for maintenance of correct chromosome structure. This human CRM1 (hCRM1) localized to the NPC as well as to the nucleoplasm. Nuclear overexpression of the FG‐repeat region of CAN, containing its hCRM1‐interaction domain, resulted in depletion of hCRM1 from the NPC. In CAN−/− mouse embryos lacking CAN, hCRM1 remained in the nuclear envelope, suggesting that this protein can also bind to other repeat‐containing nucleoporins. Lastly, hCRM1 shares a domain of significant homology with importin‐β, a cytoplasmic transport factor that interacts with nucleoporin repeat regions. We propose that hCRM1 is a soluble nuclear transport factor that interacts with the NPC.

Translocation t(6;9) in acute non-lymphocytic leukaemia results in the formation of a DEK-CAN fusion gene

The t(6;9) that characterizes a specific subtype of ANLL fuses the 3 part of a gene located on chromosome 9q34, CAN, to the 5 part of a gene located on chromosome 6p23, DEK. On the 6p- chromosome, the resulting DEK-CAN fusion gene is transcribed into a leukaemia-specific 5.5 kb chimaeric mRNA that encodes a putative DEK-CAN fusion protein. No transcription could be detected from the reciprocal CAN-DEK fusion on chromosome 9q+. Analysis of 17 t(6;9) ANLL cases showed that the translocation breakpoints occur in a single intron of 7.5 kb in the CAN gene (ICB9) and in a single intron of 9 kb in the DEK gene (ICB6). As a result, the presence of a t(6;9) in blood or bone marrow cells can be faithfully diagnosed by Southern blotting. Moreover, the result of the translocation is an invariable DEK-CAN transcript, which can be sensitively monitored by RNA-PCR. Surprisingly, a SET-CAN fusion gene was found in leukaemic cells from a patient with AUL. Like CAN, SET is located on chromosome 9q34, which explains the apparently normal karyotype of the leukaemic cells. The occurrence of a SET-CAN fusion gene indicates that CAN may be the relevant oncogene involved in leukaemogenesis, and that activation of CAN can be effectuated through fusion of its 3 part to either DEK or SET. As yet, the function of CAN, DEK or SET is unknown. None of the proteins shows consistent homology to any known protein sequences. However, preliminary localization data and analysis of sequence motifs suggested that DEK-CAN may have a role in transcription regulation. CAN contains several dimerization domains and a repeated motif that can function as an ancillary DNA-binding domain. DEK and SET are non-related proteins, but they share a stretch of acidic amino acids, which is also present in the fusion proteins.