Identification of multiple nuclear export sequences in Fanconi anemia group A protein that contribute to CRM1-dependent nuclear export.

Department of Medical Oncology, VU University Medical Center, Amsterdam, The Netherlands.
Human Molecular Genetics (Impact Factor: 6.68). 06/2005; 14(10):1271-81. DOI: 10.1093/hmg/ddi138
Source: PubMed

ABSTRACT The Fanconi anemia (FA) pathway plays an important role in maintaining genomic stability, and defects in this pathway cause cancer susceptibility. The FA proteins have been found to function primarily in a nuclear complex, although a cytoplasmic localization and function for several FA proteins has also been reported. In this study, we investigated the possibility that FANCA, FANCC and FANCG are subjected to active export out of the nucleus. After treatment with leptomycin B, a specific inhibitor of CRM1-mediated nuclear export, the accumulation of epitope-tagged FANCA in the nucleus increased, whereas FANCC was affected to a lesser extent and FANCG showed no response. CRM1-mediated export of FANCA was further confirmed using CRM1 cotransfection, which led to a dramatic relocalization of FANCA to the cytoplasm. Five functional leucine-rich nuclear export sequences (NESs) distributed throughout the FANCA sequence were identified and characterized using an in vivo export assay. Simultaneous inactivation of three of these NESs resulted in a discrete but reproducible increase of FANCA nuclear accumulation. However, these NES mutations did not affect the ability of FANCA to complement the mitomycin C or cisplatin sensitivity of FA-A lymphoblasts. Surprisingly, mutations in the other two NESs resulted in an almost complete relocation of the protein to cytoplasm, suggesting that these motifs overlap with domains that are crucial for nuclear import. Taken together, these findings indicate that FANCA can be actively exported out of the nucleus by CRM1, revealing a new mechanism to regulate the function of the FA protein complex.

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Available from: Frank Kruyt, Jan 08, 2015
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    • "Comparable mutations of leucine residues to alanines have frequently been used for the inactivation of NES sites (Ferrer et al., 2005; Jensik et al., 2004; Wen et al., 1995). NES sites are frequently and preferentially located within α-helical structures, and the most C-terminal leucine tends to be exposed and available for interactions (La Cour et al., 2004). "
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    ABSTRACT: In a field that embraces multiple aspects of both clinical and basic research and that moves impressively fast, any answers to the questions why, what and how can we learn from a rare disease like Fanconi anemia (FA) must remain tentative and preliminary. However, there are very encouraging advances, most notably at the level of understanding the molecular basis of FA and at the level of treatment via hematopoietic stem cell transplantation. For the sake of our patients we clearly need to arrive at meaningful genotype-phenotype correlations and individualized risk profiles. This requires prospective and longterm studies carried out in close cooperation among patients, clinicians and basic scientists. There are a number of open questions, for example relating to the mechanisms of chromosomal breakage and DNA repair, to the spectrum of genetic changes that herald and promote the emergence of leukaemia and solid tumors, and to the emergence of genetically reverted cells in blood and bone marrow of FA patients. Researchers in the fields of cancer and aging should be encouraged to and are likely to benefit from the study of Fanconi anemia, as are our patients from the welcome results of such studies.
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