Alternative splicing of the neurofibromatosis type I pre-mRNA

Abstract

NF1 (neurofibromatosis type I) is a common genetic disease that affects one in 3500 individuals. The disease is completely penetrant but shows variable phenotypic expression in patients. NF1 is a large gene, and its pre-mRNA undergoes alternative splicing. The NF1 protein, neurofibromin, is involved in diverse signalling cascades. One of the best characterized functions of NF1 is its function as a Ras-GAP (GTPase-activating protein). NF1 exon 23a is an alternative exon that lies within the GAP-related domain of neurofibromin. This exon is predominantly included in most tissues, and it is skipped in CNS (central nervous system) neurons. The isoform in which exon 23a is skipped has 10 times higher Ras-GAP activity than the isoform in which exon 23a is included. Exon 23a inclusion is tightly regulated by at least three different families of RNA-binding proteins: CELF {CUG-BP (cytosine-uridine-guanine-binding protein) and ETR-3 [ELAV (embryonic lethal abnormal vision)-type RNA-binding protein]-like factor}, Hu and TIA-1 (T-cell intracellular antigen 1)/TIAR (T-cell intracellular antigen 1-related protein). The CELF and Hu proteins promote exon 23a skipping, while the TIA-1/TIAR proteins promote its inclusion. The widespread clinical variability that is observed among NF1 patients cannot be explained by NF1 mutations alone and it is believed that modifier genes may have a role in the variability. We suggest that the regulation of alternative splicing may act as a modifier to contribute to the variable expression in NF1 patients.

Figure 1

Schematic representations of neurofibromin protein domains and alternative exons. (A) Neurofibromin protein with important domains and the exons which encode them highlighted. Protein domains are shown as grey or white boxes. The MAP domain is found within the CSRD. (B) Neurofibromin protein with naturally occurring alternative exons shown as white boxes. Exon 23a is found within the GAP related domain.

Figure 2

Schematics of NF1 exon 23a endogenous splicing patterns and exon 23 regulation. (A) Exon 23a is included in most tissues and skipped in the central nervous system. Constitutive exons 23 and 24 are shown as white boxes and alternative exon 23a is shown as a grey box. Introns are shown as straight black lines. (B) Exon 23a splicing regulation. Constitutive exons 23 and 24 are shown as white boxes and alternative exon 23a is shown as a grey box. Introns are shown as straight black lines. Proteins that act as splicing factors are shown as circles. Hu proteins promote exon 23a skipping by binding to intronic splicing silencer elements located upstream and downstream of exon 23a, given by a textured rectangle. TIA-1 and TIAR proteins promote exon 23a inclusion by compete with Hu proteins for binding of the downstream sequence element. CELF proteins promote skipping of exon 23a by binding UG-rich elements upstream.