Identification of a novel FRMD7 splice variant and functional analysis of two FRMD7 transcripts during human NT2 cell differentiation

PURPOSE FERM domain containing protein 7 (FRMD7) mutations are associated with X-linked idiopathic congenital nystagmus (ICN). The purpose of this study is to identify a novel splice variant of FRMD7 (FRMD7-S) in both humans and mice with a shortened exon 4 relative to the original form of FRMD7 (FRMD7-FL),and to detect the role of FRMD7-FL and FRMD7-S in the process of neuronal development. METHODS The splice variant of FRMD7 was identified by PCR. Expression levels of hFRMD7-FL and hFRMD7-S transcripts in developing human fetal brain were tested by RT-PCR, and expression levels in the human pluripotent embryonic carcinoma NTera 2/cl.D1 (NTERA-2; NT2) cell line with all-trans retinoic acid (ATRA) or bone morphogenetic protein-2 (BMP-2) treatment were tested by real-time qPCR. hemaglutinin (HA)-tagged recombinant plasmids DNA encoding hFRMD7-FL and Myc-tagged recombinant plasmids DNA encoding hFRMD7-S were used to transiently transfect the human NT2 cells. Further, immunofluorescence experiments were performed to determine the co-localization of the two fusion proteins. Finally, using co-immunoprecipitation analyses, we demonstrated that FRMD7-FL and FRMD7-S interacted with each other. RESULTS A novel splice variant of FRMD7 (FRMD7-S) with a shortened exon 4 relative to the original form of FRMD7 (FRMD7-FL) was identified from the cDNA of the human NT2 cell line and mouse fetal brain. The FRMD7 transcripts showed similar tissue distributions and were upregulated following all trans retinoic acid (ATRA)-induced differentiation of NT2 cells. FRMD7-FL and FRMD7-S co-localized and co-immunoprecipitated with each other. Further, overexpression of FRMD7-FL in NT2 cells resulted in altered neurite development and upregulation of FRMD7-S. CONCLUSIONS Although the significance of the 45 bp deletion remains unknown, our observations suggest that the FRMD7 isoforms may play a significant role during neuronal differentiation and development.