A de novo nonsense mutation in exon 28 of the neurofibromatosis type 1 (NF1) gene

Homonucleotide tracts, short repeats and CpG/CpNpG motifs are frequent sites for heterogeneous mutations in the neurofibromatosis type 1 (NF1) tumour-suppressor gene

Neurofibromatosis type 1 (NF1) is among the most common human genetic disorders, having a constellation of cutaneous and skeletal manifestations, intellectual impairment, and an increased risk for a variety of malignancies. The NF1 gene has a high spontaneous mutation rate and is also associated with a variety of sporadic cancers in the general population. While a number of laboratories are involved in a coordinated effort to identify NF1 mutations, an important gap in our knowledge is an understanding of the mechanisms responsible for NF1 mutagenesis. In this present paper we describe our analysis of the sequence environment in the NF1 gene at those sites where small deletions, insertions and nucleotide substitution mutations have been reported. Our objective was to determine whether specific nucleotide sequences commonly occur at these mutation sites within the NF1 gene. We assessed how frequently independent NF1 mutations occur at the site of short direct repeats, single nucleotide repeats (homonucleotides) and at CpG and CpNpG motifs. We have established that homonucleotide and short direct repeats are commonly involved in the majority of small deletions and insertions analysed. Substitution mutations are frequently associated with homonucleotide repeats and methylatable CpG dinucleotides and CpNpG trinucleotides. We suggest that NF1 mutations are acquired and retained by cells through an intricate balancing of repair and replication mechanisms. Such mutations may provide a proliferative advantage for that cell and its progeny.

Molecular genetics of neurofibromatosis type 1 (NF1)

Neurofibromatosis type 1 (NF1), also called von Recklinghausen disease or peripheral neurofibromatosis, is a common autosomal dominant disorder characterised by multiple neurofibromas, café au lait spots, and Lisch nodules of the iris, with a variable clinical expression. The gene responsible for this condition, NF1, has been isolated by positional cloning. It spans over 350 kb of genomic DNA in chromosomal region 17q11.2 and encodes an mRNA of 11-13 kb containing at least 59 exons. NF1 is widely expressed in a variety of human and rat tissues. Four alternatively spliced NF1 transcripts have been identified. Three of these transcript isoforms (each with an extra exon: 9br, 23a, and 48a, respectively) show differential expression to some extent in various tissues, while the fourth isoform (2.9 kb in length) remains to be examined. The protein encoded by NF1, neurofibromin, has a domain homologous to the GTPase activating protein (GAP) family, and downregulates ras activity. The identification of somatic mutations in NF1 from tumour tissues strongly supports the speculation that NF1 is a member of the tumour suppressor gene family. Although the search for mutations in the gene has proved difficult, germline mutation analysis has shown that around 82% of all the fully characterised NF1 specific mutations so far predict severe truncation of neurofibromin. Further extensive studies are required to elucidate the gene function and the mutation spectrum. This should then facilitate the molecular diagnosis and the development of new therapy for the disease.

Characterisation of germline mutations in the neurofibromatosis type 1 (NF1) gene

Neurofibromatosis type 1 is one of the most common inherited disorders with an incidence of 1 in 3000. The search for NF1 mutations has been hampered by the overall size of the gene, the large number of exons, and the high mutation rate. To date, fewer than 90 mutations have been reported to the NF1 mutation analysis consortium and the details on 76 mutations have been published. We have identified five new mutations using single strand conformation polymorphism (SSCP) and heteroduplex analysis (HA) and three intragenic deletions with the microsatellite markers. Of the five new mutations, two were in exon 27a, two in exon 45, and one in exon 49 and these include 4630delA, 4572delC, R7846X, T7828A, and one in the 3' untranslated region (3' UTR). The two nucleotide alterations in exon 27a and the one in exon 45 are predicted to produce a truncated protein.

Two single base polymorphisms in introns 41 and 16 of the NF1 gene

We have characterized two intragenic polymorphisms in the neurofibromatosis type 1 (NF1) gene by direct sequencing of PCR products. The variants for these polymorphisms were initially detected on Hydrolink gels. One of the polymorphisms involves a G to A transition in intron 41 at the 28th base upstream of exon 42 with an observed 'G'/'A' heterozygosity of 0.42. The other polymorphism is a T to C transition in intron 16 at the 16th base upstream of exon 17 with an observed 'T'/'C' heterozygosity of 0.09. In combination with other documented polymorphisms in the NF1 gene, these variants should assist in genetic analysis of NF1 families.

Molecular basis of neurofibromatosis type 1 (NF1): mutation analysis and polymorphisms in the NF1 gene

Neurobromatosis type 1 (NF1) is one of the commonest genetic disorders in humans. The gene for NF1 was cloned in 1990. The protein encoded by the gene (neurofibromin) has extensive sequence homology with GTPase-activating protein (GAP). Despite screening the whole coding region of the gene for large and medium size rearrangements and approximately 40% of the coding region of the gene for small alterations, only 45 germ-line mutations have been reported in more than 500 unrelated patients. Of these, 25 mutations involve small changes in the gene, of which 17 (68%) result in the formation of an inappropriate stop codon. A "hot spot" for mutations has not been identified. The high mutation rate at this locus and the general difficulty in identifying mutations are discussed. A complete understanding of the structure and function of the NF1 gene awaits further detailed studies of both naturally occurring and in vitro-generated mutations.