Alternative titles; symbols
HGNC Approved Gene Symbol: FXR2
Cytogenetic location: 17p13.1 Genomic coordinates (GRCh38) : 17:7,591,230-7,614,897 (from NCBI)
Fragile X syndrome (300624) is directly associated with the FMR1 gene (309550) at Xq27.3. FMR1 is an RNA-binding protein, and mutations in the gene are believed to result in the absence or reduced expression of the protein or a functionally impaired mutant protein. FMR1 protein (FMRP) and the fragile X-related proteins 1 and 2 (FXR1; 600819, and FXR2) form a family with functional similarities, such as RNA binding, polyribosomal association, and nucleocytoplasmic shuttling.
Using a yeast 2-hybrid screen of a brain cDNA library with FMR1 as bait, followed by screening of a fetal brain library, Zhang et al. (1995) isolated a cDNA encoding FXR2. Sequence analysis predicted that the 673-amino acid protein, with high N-terminal homology to FMR1 and FXR1 and approximately 60% identity overall, contains 2 ribonucleoprotein K homology (KH) domains, which are involved in RNA binding. Northern blot analysis indicated that FXR2 is expressed as a 3.0-kb transcript in HeLa cells and in mouse brain. Immunoblot analysis showed that FXR2 is expressed as a 95-kD protein, although the predicted molecular weight was 74 kD. Immunofluorescence microscopy revealed that FXR2, like FMR1 and FXR1, is expressed in the cytoplasm.
Using immunohistochemistry, Tamanini et al. (1997) observed high expression of FMR1, FXR1, and FXR2 in adult cerebellar neurons, especially in Purkinje cell cytoplasm, and in cortical and brainstem neuron cytoplasm and proximal dendrites. In contrast, FMR1, but not FXR1 or FXR2, expression was absent in the brain of a fragile X patient. Examination of 18-week normal and fragile X fetal brains showed an expression pattern of the proteins in neurons that paralleled the adult pattern. In testis, FMR1 is expressed in spermatogonia cytoplasm; FXR1 is expressed in spermatogonia and also in cells inside the seminiferous tubules corresponding to maturing spermatogenic cells; and FXR2, at low intensity, is expressed throughout the seminiferous tubules. The expression pattern was unchanged in the testis of a fragile X patient. In fetal testis, FMR1 and FXR1 are expressed in all normal primordial germ cells, but FMR1 is present in only some fragile X germ cells. FXR2 is strongly expressed in interstitial cells of both fragile X and control testis.
Kirkpatrick et al. (2001) identified several motifs that are shared between FXR1, FXR2, and FMR1, including a nuclear localization signal, a nuclear export signal, a KH domain, and an arginine/glycine-rich (RGG) box. In addition, FXR1 and FXR2 contain 2 unique nucleolar targeting sequences (NoSs).
Protein binding analysis by Zhang et al. (1995) demonstrated that full-length or N-terminal FXR2 binds to FMR1 or FXR1 and to itself. Each of these 3 proteins can form heteromers with the others, and each can also form homomers. Immunoprecipitation analysis established that FMR1 and FXR2 also form complexes in HeLa cells.
FMR1, FXR1, and FXR2 form a family with functional similarities, such as RNA binding, polyribosomal association, and nucleocytoplasmic shuttling. Using several FMR1 deletion mutants in coimmunoprecipitation experiments, Siomi et al. (1996) found that amino acids 171 to 211 of FMR1 were sufficient for its interaction with FXR2, and that FMR1 was not required for association of FXR1 or FXR2 with the 60S ribosomal subunit. FXR1 and FXR2 associated with 60S ribosomal subunits in cells lacking FMR1 and in cells derived from a fragile X syndrome patient.
Tamanini et al. (1999) found that FMR1 and FXR1 proteins shuttle between cytoplasm and nucleoplasm, while FXR2 protein shuttles between cytoplasm and nucleolus. In additional studies, Tamanini et al. (2000) showed that FXR2 protein contains in its C-terminal part a stretch of basic amino acids 'RPQRRNRSRRRRFR' that resembles the NoS of the viral protein Rev. This particular sequence is also present within exon 15 of the FXR1 gene, which undergoes alternative splicing. Cells which were transfected with constructs of FXR1 protein and FXR2 protein isoforms with the potential NoS and also treated with the nuclear export inhibitor leptomycin B showed a nucleolar localization; expressed constructs lacking the NoS showed signal in the nucleoplasm outside the nucleoli. The authors hypothesized that the intranuclear distribution of FXR2 protein and FXR1 protein isoforms is likely to be mediated by a similar NoS localized in their C-terminal regions. This domain is absent in some FXR1 protein isoforms as well as in all FMR1 protein isoforms, suggesting functional differences for this family of proteins, possibly related to RNA metabolism in different tissues.
Darnell et al. (2009) demonstrated that FXR1P and FXR2P KH2 domains bound kissing complex RNA ligands with the same affinity as the FMRP KH2 domain, although other KH domains did not. RNA ligand recognition by this family was highly conserved, as the KH2 domain of the single Drosophila ortholog of FMRP also bound kissing complex RNA. Kissing complex RNA was able to displace FXR1P and FXR2P from polyribosomes as it did for FMRP, and this displacement was FMRP-independent. Darnell et al. (2009) suggested that all 3 family members may recognize the same binding site on RNA mediating their polyribosome association, and that they may be functionally redundant with regard to this aspect of translational control. In contrast, FMRP was unique in its ability to recognize G-quadruplexes, suggesting the FMRP RGG domain may play a nonredundant role in the pathophysiology of fragile X syndrome.
Kirkpatrick et al. (2001) determined that the FXR2 gene contains 17 exons and spans 14 to 38 kb. The 5-prime untranslated region of the FXR2 gene overlaps the CG-rich promoter region of the SHBG gene (182205) and is transcribed in the opposite orientation.
By PCR analysis of a hybrid cell line and by FISH, Zhang et al. (1995) mapped the FXR2 gene to chromosome 17p13.1. Kirkpatrick et al. (2001) mapped the mouse Fxr2 gene to chromosome 11.
Bontekoe et al. (2002) generated an Fxr2 knockout mouse model. No pathologic differences between knockout and wildtype mice were found in brain or testis; however, their behaviors were distinctive. Fxr2 knockout mice were hyperactive (i.e., traveled a greater distance, spent more time moving, and moved faster) in the open-field test, were impaired on the rotarod test, had reduced levels of prepulse inhibition, displayed less contextual conditioned fear, were impaired at locating the hidden platform in the Morris water task, and were less sensitive to a heat stimulus. The authors implicated a role for Fxr2 in central nervous system function.
Zhang et al. (2008) found that Fmr1/Fxr2 double-knockout mice and Fmr1-knockout/Fxr2-heterozygous mice exhibited a loss of rhythmic activity in a light-dark cycle, and that Fmr1- or Fxr2-knockout mice displayed a shorter free running period of locomotor activity in total darkness. Molecular analysis and in vitro electrophysiologic studies suggested essentially normal function of cells in the suprachiasmatic nucleus in Fmr1/Fxr2 double-knockout mice. However, the cyclical patterns of abundance of several core clock mRNAs were altered in the livers of double-knockout mice. Fxr2 alone or Fmr1 and Fxr2 together enhanced Per1 (602260)- or Per2 (603426)-mediated Bmal1 (ARNTL; 602550)-Npas2 (603347) transcriptional activity in a dose-dependent manner. Zhang et al. (2008) concluded that FMR1 and FXR2 are required for rhythmic circadian behavior.
Bontekoe, C. J. M., McIlwain, K. L., Nieuwenhuizen, I. M., Yuva-Paylor, L. A., Nellis, A., Willemsen, R., Fang, Z., Kirkpatrick, L., Bakker, C. E., McAninch, R., Cheng, N. C., Merriweather, M., Hoogeveen, A. T., Nelson, D., Paylor, R., Oostra, B. A. Knockout mouse model for Fxr2: a model for mental retardation. Hum. Molec. Genet. 11: 487-498, 2002. [PubMed: 11875043] [Full Text: https://doi.org/10.1093/hmg/11.5.487]
Darnell, J. C., Fraser, C. E., Mostovetsky, O., Darnell, R. B. Discrimination of common and unique RNA-binding activities among fragile X mental retardation protein paralogs. Hum. Molec. Genet. 18: 3164-3177, 2009. [PubMed: 19487368] [Full Text: https://doi.org/10.1093/hmg/ddp255]
Kirkpatrick, L. L., McIlwain, K. A., Nelson, D. L. Comparative genomic sequence analysis of the FXR gene family: FMR1, FXR1, and FXR2. Genomics 78: 169-177, 2001. [PubMed: 11735223] [Full Text: https://doi.org/10.1006/geno.2001.6667]
Siomi, M. C., Zhang, Y., Siomi, H., Dreyfuss, G. Specific sequences in the fragile X syndrome protein FMR1 and the FXR proteins mediate their binding to 60S ribosomal subunits and the interactions among them. Molec. Cell. Biol. 16: 3825-3832, 1996. [PubMed: 8668200] [Full Text: https://doi.org/10.1128/MCB.16.7.3825]
Tamanini, F., Bontekoe, C., Bakker, C. E., van Unen, L., Anar, B., Willemsen, R., Yoshida, M., Galjaard, H., Oostra, B. A., Hoogeveen, A. T. Different targets for the fragile X-related proteins revealed by their distinct nuclear localizations. Hum. Molec. Genet. 8: 863-869, 1999. [PubMed: 10196376] [Full Text: https://doi.org/10.1093/hmg/8.5.863]
Tamanini, F., Kirkpatrick, L. L., Schonkeren, J., van Unen, L., Bontekoe, C., Bakker, C., Nelson, D. L., Galjaard, H., Oostra, B. A., Hoogeveen, A. T. The fragile X-related proteins FXR1P and FXR2P contain a functional nucleolar-targeting signal equivalent to the HIV-1 regulatory proteins. Hum. Molec. Genet. 9: 1487-1493, 2000. [PubMed: 10888599] [Full Text: https://doi.org/10.1093/hmg/9.10.1487]
Tamanini, F., Willemsen, R., van Unen, L., Bontekoe, C., Galjaard, H., Oostra, B. A., Hoogeveen, A. T. Differential expression of FMR1, FXR1 and FXR2 proteins in human brain and testis. Hum. Molec. Genet. 6: 1315-1322, 1997. [PubMed: 9259278] [Full Text: https://doi.org/10.1093/hmg/6.8.1315]
Zhang, J., Fang, Z., Jud, C., Vansteensel, M. J., Kaasik, K., Lee, C. C., Albrecht, U., Tamanini, F., Meijer, J. H., Oostra, B. A., Nelson, D. L. Fragile X-related proteins regulate mammalian circadian behavioral rhythms. Am. J. Hum. Genet. 83: 43-52, 2008. [PubMed: 18589395] [Full Text: https://doi.org/10.1016/j.ajhg.2008.06.003]
Zhang, Y., O'Connor, J. P., Siomi, M. C., Srinivasan, S., Dutra, A., Nussbaum, R. L., Dreyfuss, G. The fragile X mental retardation syndrome protein interacts with novel homologs FXR1 and FXR2. EMBO J. 14: 5358-5366, 1995. [PubMed: 7489725] [Full Text: https://doi.org/10.1002/j.1460-2075.1995.tb00220.x]