Mutations affecting mRNA splicing are the most common molecular defects in patients with neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is one of the mostcommon inherited disorders in humans and is causedby mutations in the NF1gene. To date, the majority ofthe reported NF1mutations are predicted to result inprotein truncation, but veryfew studieshavecorrelatedthe causative NF1mutation with its effect at the mRNAlevel. We have applied a whole NF1cDNA screeningmethodology to the study of 80 unrelated NF1 patientsand have identified 44 different mutations, 32 beingnovel, in 52 of these patients. Mutations were detectedin 87% of the familial cases, but in 51% of the sporadicones. At least 15 of the 80 NF1 patients (19%) hadrecurrent mutations. The study shows that in 50% of thepatients in whom the mutations were identified, theseresulted in splicing alterations. Most of the splicingmutations did not involve the conserved AG/GTdinucleotides of the splice sites. One frameshift, twononsense and two missense mutations were alsoresponsible for alterations in mRNA splicing. Thelocation and type of mutation within the NF1gene, andits putative effect at the protein level, do not indicateany relationship to any specific clinical feature of NF1.The high proportion of aberrant spliced transcriptsdetected in NF1 patients stresses the importance ofstudying mutations at both the genomic and RNA level.It is possible that part of the clinical variability in NF1could be due to mutations affecting mRNA splicing,which is the most common molecular defect in NF1.INTRODUCTIONNeurofibromatosis type 1 (NF1) (MIM 162200) is one of themost common inherited disorders in humans with a prevalenceof 1 in 3000 individuals. NF1 is an autosomal dominantdisorder fully penetrant at the age of 5 years, but with a vari-able clinical expression, even among members of the samefamily. The main clinical features of NF1 are cafe au lait spots(CLSs), cutaneous neurofibromas and Lisch nodules (1,2). TheNF1 gene was mapped to chromosome 17q11.2 and was posi-tionally cloned in 1990 (3–5). It spans 350 kb of genomicDNA, contains 60 exons (6,7) and is transcribed ubiquitouslyto an mRNA of 11–13 kb that encodes for a protein, neuro-fibromin, of 2818 amino acids (8). The only region of neuro-fibromin with a well-defined function and structure is a centraldomain with a high similarity to ras-specific GTPase-activating proteins (GAPs), called the GAP-related domain(GRD), which downregulates ras activity (9–11).The NF1 gene has one of the highest mutation ratesdescribed for any human disorder ( 1 10