title: Functional characterization of the AFF (AF4/FMR2) family of RNA-binding proteins: insights into the molecular pathology of FRAXE intellectual disability.
abstract: The AFF (AF4/FMR2) family of genes includes four members: AFF1/AF4, AFF2/FMR2, AFF3/LAF4 and AFF4/AF5q31. AFF2/FMR2 is silenced in FRAXE intellectual disability, while the other three members have been reported to form fusion genes as a consequence of chromosome translocations with the myeloid/lymphoid or mixed lineage leukemia (MLL) gene in acute lymphoblastic leukemias (ALLs). All AFF proteins are localized in the nucleus and their role as transcriptional activators with a positive action on RNA elongation was primarily studied. We have recently shown that AFF2/FMR2 localizes to nuclear speckles, subnuclear structures considered as storage/modification sites of pre-mRNA splicing factors, and modulates alternative splicing via the interaction with the G-quadruplex RNA-forming structure. We show here that similarly to AFF2/FMR2, AFF3/LAF4 and AFF4/AF5q31 localize to nuclear speckles and are able to bind RNA, having a high apparent affinity for the G-quadruplex structure. Interestingly, AFF3/LAF4 and AFF4/AF5q31, like AFF2/FMR2, modulate, in vivo, the splicing efficiency of a mini-gene containing a G-quadruplex structure in one alternatively spliced exon. Furthermore, we observed that the overexpression of AFF2/3/4 interferes with the organization and/or biogenesis of nuclear speckles. These findings fit well with our observation that enlarged nuclear speckles are present in FRAXE fibroblasts. Furthermore, our findings suggest functional redundancy among the AFF family members in the regulation of splicing and transcription. It is possible that other members of the AFF family compensate for the loss of AFF2/FMR2 activity and as such explain the relatively mild to borderline phenotype observed in FRAXE patients.
authors.affiliation: CNRS UMR 6097, Institute of Molecular and Cellular Pharmacology, University of Nice-Sophia Antipolis, 06560 Valbonne, France.
authors.forename: Mireille
authors.initials: M
authors.lastname: Melko
authors.forename: Dominique
authors.initials: D
authors.lastname: Douguet
authors.forename: Mounia
authors.initials: M
authors.lastname: Bensaid
authors.forename: Samantha
authors.initials: S
authors.lastname: Zongaro
authors.forename: Céline
authors.initials: C
authors.lastname: Verheggen
authors.forename: Jozef
authors.initials: J
authors.lastname: Gecz
authors.forename: Barbara
authors.initials: B
authors.lastname: Bardoni
mesh_terms: D000595:Amino Acid Sequence; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D005347:Fibroblasts / Q000378:metabolism; D005600:Fragile X Syndrome / Q000235:genetics* / Q000473:pathology*; D015870:Gene Expression / Q000235:genetics; D023061:Gene Order; D017930:Genes, Reporter / Q000235:genetics; D006367:HeLa Cells; D006801:Humans; D042581:Intranuclear Space / Q000378:metabolism; D008969:Molecular Sequence Data; D021381:Protein Transport; D012326:RNA Splicing / Q000235:genetics; D016601:RNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D004268:DNA-Binding Proteins; D016601:RNA-Binding Proteins