Identification and characterization of neurofibromatosis type 1 (NF1) gene mutations

Neurofibromatosis type 1 (NF1) afflicts 1 in 3,000 individuals and is characterized with variable clinical presentations. The NF1 gene spans 350 kb on chromosome 17q11.2 with 60 exons. The gene exhibits high mutation rate of 1 × 104/gamete/generation and approximately 50% are sporadic new mutations. In this study, a splice site mutation and 6 gross deletions have been identified and characterized. Also, a HincII polymorphism was detected and the linkage disequilibrium was investigated. Using a protein truncation assay, we have identified a exon skipping mutation. The mutation, which consists of a G to A transition at position +1 (2056+1) of the 5' splice site of exon 12a, is associated with the loss of both exons 11 and 12a in the NF1 mRNA. The mutation inactivates the 5' splice site of exon 12a, prevents exon definition and leads to the skipping of both exons 11 and 12a. These results document the first example of a natural mutation that inactivates exon definition, and suggest that the 11-12a region of NF1 plays an important role in the activity of neurofibromin. Six gross deletions (families 7610 and 7473, patients 178, 184, 236 and 236) have been identified. The breakpoint of the deletion is located between exons 5 and 26. The deletions are unique and different from those reported previously. The deletions are not associated with unusual clinical features. The HincII polymorphism is located between exons 1 and 4a as defined by probe GE2-400 bp. Three alleles are detected: A1 (3.1 and 3.3 kb), A2 (2.4 and 4.0 kb), A3 (1.3 and 5.1 kb). The detected heterozygote forms are A1-A2, or A1-A3 and the homozygote forms A1-A1 or A2-A2. Analysis haplotypes of the 19 NF1 families indicates that the four intragenic polymorphic microsatellites are strongly linked with the NF1 disease. However, no linkage disequilibrium and founder effect was observed in this Québec population collection. In conclusion, the study shows that the analysis of NF1 gene mutation is complex. No particular founder mutation has been observed in the Québec population."--Résumé abrégé par UM