Deletions and a translocation interrupt a cloned gene at the neurofibromatosis type 1 locus

An 18-locus linkage map of the pericentromeric region of the human X chromosome: genetic framework for mapping X-linked disorders

We report a high-resolution genetic linkage map of the region Xp11.4 to Xq13.3, spanning the centromere of the X chromosome and encompassing approximately 30 cM. This 18-locus map is composed of 11 intervals that are spaced on average about 3 cM apart. Markers incorporated into the map together detect 19 distinct polymorphisms and include five genes (TIMP, SYP, AR, CCG1, PGK1), the OATL1 cluster, the hypervariable locus DXS255, the centromeric locus DXZ1, and 10 other anonymous DNA segments. Given that this map spans roughly one-fifth of the length of the X chromosome and includes many loci currently used in both diagnosis and mapping of X-linked disorders, it should be useful for genetic counseling and for guiding efforts to clone disease genes in this region.

The gene for von Recklinghausen neurofibromatosis (NF1) maps to the pericentromeric region of chromosome 17 in Chinese families

Linkage analysis of six Chinese families with neurofibromatosis type 1 (NF1) confirms the location of the NF1 gene to the region of the proximal long arm of chromosome 17, as in Caucasian populations. The diagnosis of NF1 was made according to internationally accepted criteria. The markers used were D17S71, D17S58, D17S33, and EVI2A. The overall odds in favor of NF1 lying within this linkage group in the families studied are over 150,000:1, with a maximum location score of 5.112 for the interval D17S58-EVI2A.

Gastrointestinal manifestations of type 1 neurofibromatosis (von Recklinghausen's disease)

Gastrointestinal involvement in von Recklinghausen's disease occurs in three principal forms: hyperplasia of the submucosal and myenteric nerve plexuses and mucosal ganglioneuromatosis which leads to disordered gut motility; gastrointestinal stromal tumours showing varying degrees of neural or smooth muscle differentiation; and a distinctive glandular, somatostatin-rich carcinoid of the periampullary region of the duodenum that contains psammoma bodies and which may be associated with phaeochromocytoma. This review describes the histopathological features of these lesions and discusses potential pitfalls in their differential diagnosis. Their accurate identification has significant implications for clinical management and may even provide the first pointer to the diagnosis of neurofibromatosis.

Transformation effector and suppressor genes

Much has been learned about the molecular basis of cancer from the study of the dominantly acting viral and cellular oncogenes and their normal progenitors, the proto-oncogenes. More recent studies have resulted in the isolation and characterization of several genes prototypic of a second class of cancer genes. Whereas oncogenes act to promote the growth of cells, members of this latter class of genes act to inhibit cellular growth and are believed to contribute to the tumorigenic phenotype only when their activities are absent. This new class of cancer genes is referred to by a number of different names including; anti-oncogenes, recessive oncogenes, growth suppressor genes, tumor suppressor genes and emerogenes. Although only a few of these cancer genes have been identified, to date, it is likely that many additional genes of this class await identification. A third class of genes, necessary for the development of the cancer phenotype, is comprised of the transformation effector genes. These are normal cellular genes that encode proteins that cooperate with or activate oncogene functions and thereby induce the development of the neoplastic phenotype. The inactivation of transformation effector functions would therefore inhibit the ability of certain dominantly acting oncogenes to transform cells. The approaches outlined here describe functional assays for the isolation and molecular characterization of transformation effector and suppressor genes.

242 breakpoints in the 200-kb deletion-prone P20 region of the DMD gene are widely spread

Using whole cosmids as probes, we have mapped 242 DMD/BMD deletion breakpoints located in the major deletion hot spot of the DMD gene. Of these, 113 breakpoints were mapped more precisely to individual restriction enzyme fragments in the distal 80 kb of the 170-kb intron 44. An additional 12 breakpoints are distributed over the entire region, with no significant local variation in frequency. Furthermore, deletion sizes vary and are not influenced by the positions of the breakpoints. This argues against a predominant role of one or a few specific sequences in causing frequent rearrangements. It suggests that structural characteristics or a more widespread recombinogenic sequence makes this region so susceptible to deletion. Our study revealed several RFLPs, one of which is a 300-bp insertion/deletion polymorphism. Abnormally migrating junction fragments are found in 81% of the precisely mapped deletions and are highly valuable in the diagnosis of carrier females.

Molecular basis of mouse developmental mutants

Collapse

Chromosome-specific recombinant DNA libraries from the fungus Aspergillus nidulans

Development of physical genomic maps is facilitated by identification of overlapping recombinant DNA clones containing long chromosomal DNA inserts. To simplify the analysis required to determine which clones in a genomic library overlap one another, we partitioned Aspergillus nidulans cosmid libraries into chromosome-specific subcollections. The eight A. nidulans chromosomes were resolved by pulsed field gel electrophoresis and hybridized to filter replicas of cosmid libraries. The subcollections obtained appeared to be representative of the chromosomes based on the correspondence between subcollection size and chromosome length. A sufficient number of clones was obtained in each chromosome-specific subcollection to predict the overlap and assembly of individual clones into a limited number of contiguous regions. This approach should be applicable to many organisms whose genomes can be resolved by pulsed field gel electrophoresis.

Identification and characterization of the gene for neurofibromatosis type 1

Elucidation of the partial genomic structure and DNA sequence of the gene responsible for neurofibromatosis type 1, and discovery of clues to its function, have led to new opportunities not only for understanding this particular disease process, but also for clarifying signalling pathways involved in cellular growth and differentiation.

Complex pattern of immunoglobulin mu gene expression in normal and transgenic mice: nonoverlapping regulatory sequences govern distinct tissue specificities

Analysis of normal mice and transgenic mice carrying a rearranged immunoglobulin mu gene revealed a more complex tissue-specific pattern of mu gene expression than anticipated from previous observations. Expression of the endogenous mu locus and the mu transgene was detected both in lymphoid tissues and in skeletal muscle. Analysis of the expression pattern of mu transgenes containing intragenic deletions or point mutations in binding sites for Oct transcription factors (OCTA sites) indicated that distinct regulatory sequences control lymphoid- and muscle-specific mu gene expression. Consistent with previous transfection experiments, mu gene expression in lymphoid tissues is dependent on the intragenic enhancer and the OCTA site in the promoter. However, neither of these regulatory sequences is required for mu gene expression in skeletal muscle that is governed by a muscle-specific control region located 3' of the enhancer. An "off-state" of the mu transgene was observed only in liver and embryonal fibroblasts, whereas enhancer-dependent mu transgene expression was detected at low levels in other nonlymphoid tissues. From these data we suggest a model for the regulation of tissue-specific mu gene expression in which a "ubiquitous" competence for basal transcription is up-regulated in lymphoid and muscle tissues by distinct cell type-specific regulatory sequences and down-regulated in liver and fibroblastic cells by putative negative sequence elements.

Mammalian genome mapping: lessons and prospects

The recent emphasis on human genome mapping has stimulated the development of gene maps in close to thirty mammalian species. Animal gene maps provide an invaluable resource for genetic analysis and manipulation of phenotypic characters, as well as a retrospective glimpse at the patterns and processes of genome evolution. An empirical strategy for developing new gene maps in mammals by emphasizing two important classes of index or anchor marker loci is presented.

Increased melanogenesis in cultured epidermal melanocytes from patients with neurofibromatosis 1 (NF 1)

Melanocyte cultures from the normally pigmented skin of patients with neurofibromatosis 1 (NF 1) have a higher melanin content than those from the skin of healthy donors. An additional increase in the amount of melanin per cell was found in 5 out of 6 lines of melanocytes derived from café au lait macules of NF 1 patients. Omission of the tumor promoter phorbol-12-myristate-13-acetate from the culture medium brings about a comparable increase in the melanin content in all three kinds of melanocyte cultures. Cultures of NF 1 melanocytes show a higher tyrosine hydroxylase activity than those of control melanocytes, and incorporate larger amounts of dihydroxyphenylalanine than the latter. We conclude that melanogenesis in epidermis melanocytes is affected by defective alleles of the NF 1 gene. Our findings do not contradict the hypothesis that the defect underlying NF 1 impairs the inhibition of a wild-type RAS oncogene by interfering with the GTPase-activating function of the NF 1 gene product.

Antibody against neurofibromatosis type 1 gene product reacts with a triton-insoluble GTPase activating protein toward ras p21

Cellular fractionation of GTPase activating protein (GAP) activity using bovine cerebral cortex revealed that about half of GAP activity was found in membrane fraction. GAP activity of membrane was not solubilized with 0.5% (v/v) triton X-100 and was immunoprecipitated with antibody against carboxy-terminus of neurofibromatosis type 1 (NF1) gene product. In contrast, soluble GAP activity was precipitated with antibody against GAP but not with anti-NF1. These results suggest that NF1 gene product is a GTPase activating protein toward ras p21 with completely different intracellular distribution from that of GAP.

A linkage map of mouse chromosome 8: further definition of homologous linkage relationships between mouse chromosome 8 and human chromosomes 8, 16, and 19

Using an interspecific cross, a mouse chromosome 8 linkage map spanning 72 cM has been defined by the segregation of restriction fragment length variants. Linkage and genetic distance were established for 10 loci by analysis of 114 meiotic events and indicated the following gene order: (centromere)-Insr-3.5 cM-Plat-26.3 cM-Crryps/Mel/Jund-3.5 cM-Junb/Ucp-10.5 cM-Mt-1-27.2 cM-Acta2-0.9 cM-Aprt. These data provide further definition of mouse chromosome 8 linkage relationships and the relationship between segments of this chromosome and human chromosomes 8, 16, and 19.

Chromosomal in situ hybridization using yeast artificial chromosomes

Large DNA fragment cloning methods using yeast artificial chromosomes (YACs) have vastly improved the strategies for constructing physical maps of regions of complex genomes, as well as for isolating and cloning genes important for human disease. We present here a simple and rapid method for carrying out in situ hybridization to metaphase chromosomes using isolated YAC clones by labeling DNA directly in agarose gel slices. Nonisotopic labeling and chromosomal in situ hybridization can be used to determine the chromosomal localization of individual YAC clones on human metaphase chromosomes. This method can also be used to characterize YAC clones consisting of single fragments from those that contain concatamerized, and thus artifactual, inserts. This technique also offers a valuable tool to study consistent translocations in neoplastic diseases by identifying YACs that span a specific chromosomal breakpoint.

Identification of a gene located at chromosome 5q21 that is mutated in colorectal cancers

Recent studies have suggested the existence of a tumor suppressor gene located at chromosome region 5q21. DNA probes from this region were used to study a panel of sporadic colorectal carcinomas. One of these probes, cosmid 5.71, detected a somatically rearranged restriction fragment in the DNA from a single tumor. Further analysis of the 5.71 cosmid revealed two regions that were highly conserved in rodent DNA. These sequences were used to identify a gene, MCC (mutated in colorectal cancer), which encodes an 829-amino acid protein with a short region of similarity to the G protein-coupled m3 muscarinic acetylcholine receptor. The rearrangement in the tumor disrupted the coding region of the MCC gene. Moreover, two colorectal tumors were found with somatically acquired point mutations in MCC that resulted in amino acid substitutions. MCC is thus a candidate for the putative colorectal tumor suppressor gene located at 5q21. Further studies will be required to determine whether the gene is mutated in other sporadic tumors or in the germ line of patients with an inherited predisposition to colonic tumorigenesis.

How does p21ras transform cells?

Oncogenic forms of p21ras are found in a wide range of human tumors. However, the mechanism by which p21ras transforms remains obscure. Genetic evidence has identified a domain of p21ras that is involved with interaction with an effector molecule required for transformation. Two proteins, GAP and the tumor suppressor NF1, interact with p21ras in this region but it is an unresolved puzzle whether either of these is the an unresolved puzzle whether either of these is the effector. After interaction with an effector, two downstream events--activation of protein kinase C and another pathway--are necessary for induction of DNA synthesis by oncogenic p21ras; however, morphological transformation does not require activation of protein kinase C.

cDNA sequence and genomic structure of EV12B, a gene lying within an intron of the neurofibromatosis type 1 gene

The gene responsible for neurofibromatosis type 1 (NF1), one of the more common inherited human disorders, was identified recently, and segments of it were cloned. Two translocation breakpoints that interrupt the NF1 gene in NF1 patients flank a 60-kb segment of DNA that contains the EV12A locus (previously reported as the EV12 locus), the human homolog of a mouse gene, Evi-2A, implicated in retrovirus-induced murine myeloid tumors. EVI2A lies within an intron of the NF1 gene and is transcribed from telomere toward centromere, opposite to the direction of transcription of the NF1 gene. Here we describe a second locus, EVI2B, also located between the two NF1 translocation breakpoints. Full-length cDNAs from the EV12B locus detect a 2.1-kb transcript in bone marrow, peripheral blood mononuclear cells, and fibroblasts. Sequencing studies predict an EV12B protein of 448 amino acids that is proline-rich and contains an N-terminal signal peptide, an extracellular domain with four potential glycosylation sites, a single hydrophobic transmembrane domain, and a cytoplasmic hydrophilic domain. At the level of genomic DNA the EV12B locus lies within the same intron of the NF1 gene as EV12A and contains a 57-bp 5' exon that is noncoding, an 8-kb intron, and a 2078-bp 3' exon that includes the entire open reading frame. EV12B is transcribed in the same direction as EV12A; its 5' exon lies only 4 kb downstream from the 3' exon of the EV12A locus. In the mouse the 5' exon of the homologous gene, Evi-2B, lies approximately 2.8 kb from the 3' end of Evi-2A, in the midst of a cluster of viral integration sites identified in retrovirus-induced myeloid tumors; thus, Evi-2B may function as an oncogene in these tumors.

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

Images

Mutations in the p53 tumor suppressor gene in human cutaneous squamous cell carcinomas

In this study, we analyzed 10 human squamous cell carcinomas (SCCs) for alterations in the p53 tumor suppressor gene in exons 4 through 9 by single-strand conformation polymorphism (SSCP) analysis. We found that 2 of 10 SCCs displayed unusual SSCP alleles at exon 7 of the p53 gene. Subsequent cloning and sequencing of PCR-amplified exon 7 DNA from these two tumors revealed that one had a G----A transition at the first position of codon 244, predicting a glycine-to-serine amino acid change, while the other tumor exhibited a G----T base change at the second nucleotide of codon 248, predicting an arginine-to-leucine substitution. Because the mutations in the p53 tumor suppressor gene in both tumors were located opposite potential pyrimidine dimer sites (C-C), it is consistent with these mutations having been induced by the ultraviolet radiation present in sunlight. These studies demonstrate that inactivation of the p53 tumor suppressor gene, as well as activation of ras oncogenes, may be involved in the pathogenesis of some human skin cancers.

Molecular and cytogenetic analysis of tumors in von Recklinghausen neurofibromatosis

Von Recklinghausen neurofibromatosis (NF1) is a common autosomal dominant disorder mapped to 17q11.2 and typically characterized by the occurrence of neural crest-derived tumors. The gene has recently been cloned using reverse genetics or "positional cloning" approaches. Its function, however, remains unknown. We have performed cytogenetic and molecular analyses on 9 malignant tumors from NF1 patients to look for loss of alleles or chromosome rearrangements involving chromosome 17 to test the hypothesis that the NF1 gene acts as a recessive "tumor suppressor" gene. Loss of alleles on this chromosome was detected for 3 of 9 malignant tumors. Two peripheral nerve sheath tumors showed allele loss at informative loci on both the long and short arms of chromosome 17. In contrast, a glioblastoma with focal gliosarcoma showed loss of heterozygosity on the short arm of chromosome 17 only, and not at loci on the long arm. One nerve sheath tumor was previously shown by direct sequence analysis to have a point mutation at the TP53 locus at 17p13. These data support a role for the TP53 gene or other genes on the short arm of chromosome 17 in at least some malignancies in NF1. Six other neurofibrosarcomas showed no allele loss at informative loci on chromosome 17. Cytogenetic analysis was performed on 7 tumors, including 2 with allele loss. The two tumors with allele loss showed abnormal karyotypes while all others were normal. Southern blot and pulsed-field gel analysis using probes within or closely linked to the NF1 locus detected no gross deletions or rearrangements in the tumors studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal bone pathology of acoustic neurinoma (unilateral and bilateral) in relation to the internal auditory canal surgery

Temporal bone pathology of 3 cases with large unilateral acoustic neurinoma (AN) and 2 cases with large bilateral AN (neurofibromatosis 2; NF2) were studied. One case with unilateral AN was diagnosed as neurofibromatosis 1 (NF1). Unilateral AN had distinct borderlines at the fundus in 5 of 12 examination points between tumor tissues and nerve fibers, but only 1 of 12 was distinct around the porus acousticus. Bilateral AN showed no distinct borderlines at the fundus and around the porus acousticus. These findings suggest that clear separation of the VIIth or VIIIth nerve from large tumors is very difficult, especially in cases of bilateral AN. Geniculate ganglion cells of tumor origin on ears were fairly well preserved in 2 cases with unilateral AN and 1 case with bilateral AN. Well preserved geniculate ganglion cells could be dependent on blood supply not from the internal auditory artery.

Physical analysis of murine albino deletions that disrupt liver-specific gene regulation or mesoderm development

Complementation analyses of radiation-induced deletion mutations involving the albino (c) locus in Chromosome (Chr) 7 of the mouse have identified several loci, in addition to c, that have important roles in development. The "mesoderm-deficient" (msd) and "hepatocyte-specific developmental regulation-1" (hsdr-1) loci, which are proximal and tightly linked to c, are important in the formation of mesoderm and in the regulation of liver- and kidney-specific induction of various enzymes and proteins, respectively. Cloning deletion-breakpoint-fusion fragments caused by lethal albino deletions that genetically define the extents of the msd and hsdr-1 loci is one way of generating molecular probes for studying the gene(s) involved in these phenotypes. The distal breakpoints of five such deletions were positioned on a long-range (PFGE) map of approximately 1.7 Mb of wild-type DNA surrounding the c, D7Was12, and Emv-23 loci. In addition, the distal breakpoints of two viable albino deletions, which remove part of the tyrosinase gene and extend distally, were localized in the vicinity of the lethal deletion breakpoints. Therefore, the viable deletions can be exploited to generate additional DNA probes that should facilitate the isolation of breakpoint clones from chromosomes carrying lethal deletions defining hsdr-1 and msd.

Current concepts of neurocutaneous disorders

The neurocutaneous diseases are a loosely bound group of clinical entities that were initially considered to have dysplastic and/or neoplastic changes of the nervous system and skin, though other organ systems are frequently involved. During the last several decades a variety of additional diseases have been included in this disease category despite their not having any known cutaneous abnormality. The major neurocutaneous syndromes are considered in this review.

The molecular basis of genetic disease

The pace of localization and characterization of genes affected in human genetic disorders is quickening. Many important genes were localized or characterized recently: genes for in cystic fibrosis, NF-2, Marfan's syndrome and xeroderma pigmentosum, to name a few. Also, in the past 15 months, the CFTR gene affected in cystic fibrosis has been isolated, the first disease gene to be isolated without use of previous cytogenetic clues, such as deletions or translocations in sporadic cases. Other examples should follow, although we have been disappointed to date by the difficulties encountered in the isolation of Huntington's disease gene which was localized a number of years ago to distal chromosome 4p. It is still very difficult to isolate a disease gene without critical cytogenetic information. New improved techniques for finding the desired expressed sequences in a large cloned segment of human DNA are needed. Our ability to find mutant alleles of a given sequence has expanded greatly with the recent technical advances in denaturing gradient gel electrophoresis, chemical cleavage, and single-stranded conformational electrophoresis. One would predict that information derived from the human genome project will have a major impact upon the isolation of further disease genes. As whole regions of human chromosomes or indeed entire chromosomes are physically mapped and cloned as continuous, overlapping YACs (yeast artificial chromosomes), isolation of disease genes will become easier and easier.(ABSTRACT TRUNCATED AT 250 WORDS)

A 90 kb DNA deletion associated with neurofibromatosis type 1

A deletion of 90 kb of DNA has been identified in a patient with neurofibromatosis type 1, using pulsed field gel electrophoresis. The deletion lies between probes 17L1A and AC5 in the critical region of chromosome 17 and represents the only molecular alteration found by PFGE in a series of 90 unrelated patients. The subject showing the deletion is an isolated case, shows typical clinical features, and represents one of the first examples of a molecular deletion to be found in this disorder.

The NF1 locus encodes a protein functionally related to mammalian GAP and yeast IRA proteins

The von Recklinghausen neurofibromatosis locus, NF1, encodes a protein with homology restricted to the catalytic region of the RAS GTPase-activating protein, GAP, and with extensive homology to the IRA1 and IRA2 gene products of the yeast S. cerevisiae. A segment of the NF1 cDNA gene, expressed in yeast, can complement loss of IRA function and can inhibit both wild-type and mutant activated human H-ras genes that are coexpressed in yeast. Yeast expressing the NF1 segment have increased H-ras GTPase-stimulating activity. These studies indicate that the NF1 gene product can interact with RAS proteins and demonstrate structural and functional similarities and differences among the GAP, IRA1, IRA2, and NF1 proteins.

The GAP-related domain of the neurofibromatosis type 1 gene product interacts with ras p21

The neurofibromatosis type 1 (NF1) protein contains a region of significant sequence similarity to ras p21 GTPase-activating protein (GAP) and the yeast IRA1 gene product. A fragment of NF1 cDNA encoding the GAP-related domain (NF1 GRD) was expressed, immunoaffinity purified, and assayed for effects on N-ras p21 GTPase activity. The GTPase of wild-type ras p21 was stimulated by NF1 GRD, but oncogenic mutants of ras p21 (Asp-12 and Val-12) were unaffected, and the GTPase of an effector mutant (Ala-38) was only weakly stimulated. NF1 GRD also down-regulated RAS function in S. cerevisiae. The affinity of NF1 GRD for ras p21 was estimated to be 250 nM: this is more than 20-fold higher than the affinity of GAP for ras p21. However, its specific activity was about 30 times lower. These kinetic measurements suggest that NF1 may be a significant regulator of ras p21 activity, particularly at low ras p21 concentrations.

The catalytic domain of the neurofibromatosis type 1 gene product stimulates ras GTPase and complements ira mutants of S. cerevisiae

Sequencing of the neurofibromatosis gene (NF1) revealed a striking similarity among NF1, yeast IRA proteins, and mammalian GAP (GTPase-activating protein). Using both genetic and biochemical assays, we demonstrate that this homology domain of the NF1 protein interacts with ras proteins. First, expression of this NF1 domain suppressed the heat shock-sensitive phenotype of yeast ira1 and ira2 mutants. Second, this NF1 domain, after purification as a glutathione S-transferase (GST) fusion protein, strongly stimulated the GTPase activity of yeast RAS2 and human H-ras proteins. The GST-NF1 protein, however, did not stimulate the GTPase activity of oncogenic mutant ras proteins, H-rasVal-12 and yeast RAS2Val-19 mutants, or a yeast RAS2 effector mutant. These results establish that this NF1 domain has ras GAP activity similar to that found with IRA2 protein and mammalian GAP, and therefore may also regulate ras function in vivo.

A chromosome jump crosses a translocation breakpoint in the von Recklinghausen neurofibromatosis region

The von Recklinghausen neurofibromatosis (NFI) gene has been previously localized to the region 17q11.2 by genetic analysis. Consistent with this, two NFI patients have been described with autosomal translocations with breakpoints in 17q11.2, and these represent presumed markers for the location of the NFI gene. Recent work has defined the two breakpoints on a physical map, and they lie less than 100 kb apart. To characterize further the distance between these breakpoints and clone additional DNA, a chromosome jump was made from a DNA fragment that maps between the breakpoints. The end of the jump crosses one of the NFI translocation breakpoints and detects that breakpoint on Southern analysis, placing the probe less than 15 kb telomeric to this breakpoint. Pulsed field analysis with the jump clone allows revision of the previous NFI region map and indicates that the two breakpoints lie no more than 60 kb apart. This jump clone will be useful for further mapping, breakpoint cloning, analysis of patient DNA, and the search for transcripts in the NFI region.

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.

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

Overlapping cDNA clones from the translocation breakpoint region (TBR) gene, recently discovered at the neurofibromatosis type 1 locus and found to be interrupted by deletions and a t(17;22) translocation, have been sequenced. A 4 kb sequence of the transcript of the TBR gene has been compared with sequences of genomic DNA, identifying a number of small exons. Identification of splice junctions and a large open reading frame indicates that the gene is oriented with its 5' end toward the centromere, in opposition to the three known active genes in the region. PCR amplification of a subset of the exons, followed by electrophoresis of denatured product on native gels, identified six variant conformers specific to NF1 patients, indicating base pair changes in the gene. Sequencing revealed that one mutant allele contains a T----C transition changing a leucine to a proline; another NF1 allele harbors a C----T transition changing an arginine to a stop codon. These results establish the TBR gene as the NF1 gene and provide a description of a major segment of the gene.