A polymorphic cDNA probe on chromosome 17q11.2 located within the NF1 gene [D17S376]

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.

Benign neurofibromas in type 1 neurofibromatosis (NF1) show somatic deletions of the NF1 gene

Neurofibromatosis type 1 (NF1) is one of the most common human autosomal dominant diseases. NF1 is characterized by café-au-lait spots (CLS), axillary freckles and Lisch nodules of the iris. Another hallmark of NF1 is the development of neurofibromas, benign tumours that arise from peripheral nerve sheaths. NF1 patients also have an increased incidence of certain malignant tumours. Malignancies in NF1 are believed to follow the 'two-hit' hypothesis, in which one allele is constitutionally inactivated while the other allele is subsequently inactivated ('second hit') at the somatic level. This hypothesis has not, however, been fully tested in the aetiology of benign neurofibromas. This is a crucial issue since it addresses not only the basic mechanism behind the genesis of neurofibromas, but may also indicate a mechanism common to many or all NF1 features. Using both NF1 intragenic polymorphisms as well as markers from flanking and more distal regions of chromosome 17, we have investigated loss of heterozygosity (LOH) in 22 neurofibromas from five unrelated NF1 patients. Eight of these tumours revealed somatic deletions involving NF1, indicating that inactivation of NF1 is associated with at least some neurofibromas.

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.

Prenatal diagnosis of neurofibromatosis type 1: from flanking RFLPs to intragenic microsatellite markers

Even though the neurofibromatosis type 1 (NF1) gene was cloned more than 3 years ago, the process of identifying mutations has not been fruitful, and genetic counselling is mainly based on the use of linked markers. Since 1990, we have analysed 130 NF1 families and have performed six prenatal diagnoses. In each case, genetic counselling has relied on linked markers and informativity was achieved in all of them. The use of intragenic microsatellite polymorphisms (IVS27AAAT2.1, IVS27AC28.4, IVS27AC33.1, and IVS38GT53.0) has increased the informativeness in our series of NF1 families to an average of 90 per cent, providing accurate diagnosis and confirmation of the disease status.

Loss of the normal NF1 allele from the bone marrow of children with type 1 neurofibromatosis and malignant myeloid disorders

BACKGROUND

Children with type 1 neurofibromatosis (NF-1) are at increased risk for malignant myeloid disorders. Analysis of the NF-1 gene (NF1) suggests that the function of its product, neurofibromin, is reduced in affected persons and that NF1 belongs to the tumor-suppressor class of recessive cancer genes. This model is consistent with evidence that neurofibromin accelerates the intrinsic guanosine triphosphate-hydrolyzing activity of the Ras family of regulatory proteins. Loss of constitutional heterozygosity has not been reported in the benign tumors associated with NF-1, however, and has only been detected in a few malignant neural-crest tumors and in some tumor-derived cell lines.

METHODS

We studied DNA extracted from the bone marrow of 11 children with NF-1 in whom malignant myeloid disorders developed and from parental leukocytes. We used a series of polymorphic markers within and near NF1 to determine whether leukemogenesis was associated with structural alterations of the gene.

RESULTS

Bone marrow samples from five patients showed loss of heterozygosity. In each case, the NF1 allele was inherited from a parent with NF-1 and the normal allele was deleted.

CONCLUSIONS

These data provide evidence of NF1 may function as a tumor-suppressor allele in malignant myeloid diseases in children with NF-1 and that neurofibromin is a regulator of ras in early myelopoiesis.

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.

Exclusion of the neurofibromatosis 1 locus in a family with inherited café-au-lait spots

We have performed linkage analysis in a small family with autosomal dominant inheritance of multiple café-au-lait spots (CLS) in order to clarify its relationship to classical von Recklinghausen disease (NF 1). We found that an affected woman had transmitted a different haplotype for markers flanking the NF1 gene to both of her affected daughters. These results exclude an allelic mutation of the NF 1 gene on chromosome 17 as the cause for inherited café-au-lait spots in this family.

Prenatal diagnosis and presymptomatic detection of neurofibromatosis type 1

A two year experience of DNA diagnosis for NF1 is presented. Twenty-three NF1 families have been analysed using 11 closely linked and intragenic markers. Prenatal testing was undertaken for six families; 11 affected subjects and their partners wished to know if they would be informative for future prenatal testing, seven of whom are so far fully informative. Presymptomatic testing was done for six subjects. Despite the availability of a series of closely linked markers, informativeness could not be achieved in all of the families tested.