A major segment of the neurofibromatosis type 1 gene: cDNA sequence, genomic structure, and point mutations

Evi-2, a common integration site involved in murine myeloid leukemogenesis

BXH-2 mice have the highest incidence of spontaneous retrovirally induced myeloid leukemia of any known inbred strain and, as such, represent a valuable model system for identifying cellular proto-oncogenes involved in myeloid disease. Chronic murine leukemia viruses often induce disease by insertional activation or mutation of cellular proto-oncogenes. These loci are identified as common viral integration sites in tumor DNAs. Here we report on the characterization of a novel common viral integration site in BXH-2 myeloid leukemias, designated Evi-2. Within the cluster of viral integration sites that define Evi-2, we identified a gene that has the potential for encoding a novel protein of 223 amino acids. This putative proto-oncogene possesses all of the structural features of a transmembrane protein. Within the transmembrane domain is a "leucine zipper," suggesting that Evi-2 is involved in either homopolymer or heteropolymer formation, which may play an important role in the normal functioning of Evi-2. Interestingly, the human homolog of Evi-2 has recently been shown to be tightly linked to the von Recklinghausen neurofibromatosis locus, suggesting a role for Evi-2 in human disease as well.

Sequence homology shared by neurofibromatosis type-1 gene and IRA-1 and IRA-2 negative regulators of the RAS cyclic AMP pathway

Neurofibromatosis type-1 (NF-1) is one of the most frequently inherited genetic disorders affecting humans. NF-1 primarily affects cells of neural crest origin and is characterized by patches of skin pigmentation (café-au-lait spots) and neurofibromas. Cloning of the human NF-1 gene shows that it encodes an 11-13 kilobase transcript that is frequently disrupted in NF-1 patients. The frequent disruption of the NF-1 gene in NF-1 patients combined with the autosomal dominant mode of inheritance of NF-1 strongly suggest that the NF-1 gene is a tumour-suppressor gene. We have now sequenced a portion of the murine NF-1 gene and show that the predicted amino-acid sequence is nearly the same as the corresponding region of the human NF-1 gene product. Northern blotting identified mouse NF-1 transcripts that are equivalent in size and complexity to those in human tissues, and Southern blotting shows that this region of the NF-1 gene is evolutionarily well conserved. Finally, computer searches identified homology between the mouse NF-1 gene and IRA-1 and IRA-2, two genes identified in Saccharomyces cerevisiae that negatively regulate the RAS-cyclic AMP pathway. These findings provide important new insights into the possible function of the NF-1 gene.

Genetic and physical map of the von Recklinghausen neurofibromatosis (NF1) region on chromosome 17

The von Recklinghausen neurofibromatosis 1 (NF1) locus has been previously assigned to the proximal long arm of chromosome 17, and two NF1 patients have been identified who have constitutional balanced translocations involving 17q11.2. We have constructed a cosmid library from a chromosome-mediated gene transfectant, KLT8, that contains approximately 10% of chromosome 17, including 17q11.2. Cosmids isolated from this library have been mapped across a panel of somatic cell hybrids, including the hybrids from the two patients, and have been localized to seven small regions of proximal 17q. We have 5 cosmids that map directly above the two NF1 translocations, and 11 cosmids that map directly below. Of these, 2 cosmids in each region are linked to the disease locus and 3 of these cosmids show no recombination. One distal cosmid, 2B/B35, detects the two NF1 translocations by pulsed-field gel analysis and has been used to produce a long-range restriction map that covers the translocations.

Evi-2, a common integration site involved in murine myeloid leukemogenesis

BXH-2 mice have the highest incidence of spontaneous retrovirally induced myeloid leukemia of any known inbred strain and, as such, represent a valuable model system for identifying cellular proto-oncogenes involved in myeloid disease. Chronic murine leukemia viruses often induce disease by insertional activation or mutation of cellular proto-oncogenes. These loci are identified as common viral integration sites in tumor DNAs. Here we report on the characterization of a novel common viral integration site in BXH-2 myeloid leukemias, designated Evi-2. Within the cluster of viral integration sites that define Evi-2, we identified a gene that has the potential for encoding a novel protein of 223 amino acids. This putative proto-oncogene possesses all of the structural features of a transmembrane protein. Within the transmembrane domain is a "leucine zipper," suggesting that Evi-2 is involved in either homopolymer or heteropolymer formation, which may play an important role in the normal functioning of Evi-2. Interestingly, the human homolog of Evi-2 has recently been shown to be tightly linked to the von Recklinghausen neurofibromatosis locus, suggesting a role for Evi-2 in human disease as well.

Progress toward the isolation and characterization of the genes causing neurofibromatosis

Neurofibromatosis 1 and neurofibromatosis 2 are clinically distinct autosomal dominant disorders that affect an estimated 1.5 million individuals throughout the world. The genetic defect in each disorder has been mapped to different chromosomes, NF1 to chromosome 17 and NF2 to chromosome 22. Progress towards the cloning of the NF1 gene has proceeded rapidly. The NF1 locus was bracketed using genetic linkage analysis on NF1 affected pedigrees. Physical mapping methods were then used to precisely map the translocation breakpoints in each of two NF1 affected individuals who harbored constitutional chromosomal translocations in the putative NF1 region of chromosome 17. The region of DNA located between the two translocations has been cloned in cosmids and yeast artificial chromosomes and a number of RNA coding sequences have been identified. The identification of the NF1 gene will depend on finding mutations in the DNA of affected individuals. In the case of NF2, progress seems to have been less rapid, in part due to the lower availability of NF2 affected pedigrees. The genetic defect has been mapped to the long arm of chromosome 22 by studies of chromosomal loss in the tumours associated with this disease. Subsequent genetic mapping has confirmed this location. Flanking DNA markers for the NF2 locus have been identified. The region of DNA between these markers is in the order of 5-10 Mb. The identification of chromosomal aberrations in patients with NF2 that involve chromosome 22 will play an important role in the identification of the NF2 gene in much the same way as they have in NF1.

The neurofibromatosis type 1 gene encodes a protein related to GAP

cDNA walking and sequencing have extended the open reading frame for the neurofibromatosis type 1 gene (NF1). The new sequence now predicts 2485 amino acids of the NF1 peptide. A 360 residue region of the new peptide shows significant similarity to the known catalytic domains of both human and bovine GAP (GTPase activating protein). A much broader region, centered around this same 360 amino acid sequence, is strikingly similar to the yeast IRA1 product, which has a similar amino acid sequence and functional homology to mammalian GAP. This evidence suggests that NF1 encodes a cytoplasmic GAP-like protein that may be involved in the control of cell growth by interacting with proteins such as the RAS gene product. Mapping of the cDNA clones has confirmed that NF1 spans a t(1;17) translocation mutation and that three active genes lie within an intron of NF1, but in opposite orientation.