Mouse neurofibromatosis type 1 cDNA sequence reveals high degree of conservation of both coding and non-coding mRNA segments

Functional restoration of mouse Nf1 nonsense alleles in differentiated cultured neurons

Neurofibromatosis type 1 (NF1), one of the most common autosomal dominant genetic disorders, is caused by mutations in the NF1 gene. NF1 patients have a wide variety of manifestations with a subset at high risk for the development of tumors in the central nervous system (CNS). Nonsense mutations that result in the synthesis of truncated NF1 protein (neurofibromin) are strongly associated with CNS tumors. Therapeutic nonsense suppression with small molecule drugs is a potentially powerful approach to restore the expression of genes harboring nonsense mutations. Ataluren is one such drug that has been shown to restore full-length functional protein in several models of nonsense mutation diseases, as well as in patients with nonsense mutation Duchenne muscular dystrophy. To test ataluren's potential applicability to NF1 nonsense mutations associated with CNS tumors, we generated a homozygous Nf1^R683X/R683X-3X-FLAG mouse embryonic stem (mES) cell line which recapitulates an NF1 patient nonsense mutation (c.2041 C > T; p.Arg681X). We differentiated Nf1^R683X/R683X-3X-FLAG mES cells into cortical neurons in vitro, treated the cells with ataluren, and demonstrated that ataluren can promote readthrough of the nonsense mutation at codon 683 of Nf1 mRNA in neural cells. The resulting full-length protein is able to reduce the cellular level of hyperactive phosphorylated ERK (pERK), a RAS effector normally suppressed by the NF1 protein.

Pathogenic noncoding variants in the neurofibromatosis and schwannomatosis predisposition genes

Neurofibromatosis type 1 (NF1), type 2 (NF2), and schwannomatosis are a group of autosomal dominant disorders that predispose to the development of nerve sheath tumors. Pathogenic variants (PVs) that cause NF1 and NF2 are located in the NF1 and NF2 loci, respectively. To date, most variants associated with schwannomatosis have been identified in the SMARCB1 and LZTR1 genes, and a missense variant in the DGCR8 gene was recently reported to predispose to schwannomas. In spite of the high detection rate for PVs in NF1 and NF2 (over 90% of non-mosaic germline variants can be identified by routine genetic screening) underlying PVs for a proportion of clinical cases remain undetected. A higher proportion of non-NF2 schwannomatosis cases have no detected PV, with PVs currently only identified in around 70%-86% of familial cases and 30%-40% of non-NF2 sporadic schwannomatosis cases. A number of variants of uncertain significance have been observed for each disorder, many of them located in noncoding, regulatory, or intergenic regions. Here we summarize noncoding variants in this group of genes and discuss their established or potential role in the pathogenesis of NF1, NF2, and schwannomatosis.

Neurofibromin Structure, Functions and Regulation

Neurofibromin is a large and multifunctional protein encoded by the tumor suppressor gene NF1, mutations of which cause the tumor predisposition syndrome neurofibromatosis type 1 (NF1). Over the last three decades, studies of neurofibromin structure, interacting partners, and functions have shown that it is involved in several cell signaling pathways, including the Ras/MAPK, Akt/mTOR, ROCK/LIMK/cofilin, and cAMP/PKA pathways, and regulates many fundamental cellular processes, such as proliferation and migration, cytoskeletal dynamics, neurite outgrowth, dendritic-spine density, and dopamine levels. The crystallographic structure has been resolved for two of its functional domains, GRD (GAP-related (GTPase-activating protein) domain) and SecPH, and its post-translational modifications studied, showing it to be localized to several cell compartments. These findings have been of particular interest in the identification of many therapeutic targets and in the proposal of various therapeutic strategies to treat the symptoms of NF1. In this review, we provide an overview of the literature on neurofibromin structure, function, interactions, and regulation and highlight the relationships between them.

Mice with missense and nonsense NF1 mutations display divergent phenotypes compared with human neurofibromatosis type I

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by the occurrence of nerve sheath tumors and considerable clinical heterogeneity. Some translational studies have been limited by the lack of animal models available for assessing patient-specific mutations. In order to test therapeutic approaches that might restore function to the mutated gene or gene product, we developed mice harboring NF1 patient-specific mutations including a nonsense mutation (c.2041C>T; p.Arg681*) and a missense mutation (c.2542G>C; p.Gly848Arg). The latter is associated with the development of multiple plexiform neurofibromas along spinal nerve roots. We demonstrate that the human nonsense NF1(Arg681*) and missense NF1(Gly848Arg) mutations have different effects on neurofibromin expression in the mouse and each recapitulates unique aspects of the NF1 phenotype, depending upon the genetic context when assessed in the homozygous state or when paired with a conditional knockout allele. Whereas the missense Nf1(Gly848Arg) mutation fails to produce an overt phenotype in the mouse, animals homozygous for the nonsense Nf1(Arg681*) mutation are not viable. Mice with one Nf1(Arg681*) allele in combination with a conditional floxed Nf1 allele and the DhhCre transgene (Nf1(4F/Arg681*); DhhCre) display disorganized nonmyelinating axons and neurofibromas along the spinal column, which leads to compression of the spinal cord and paralysis. This model will be valuable for preclinical testing of novel nonsense suppression therapies using drugs to target in-frame point mutations that create premature termination codons in individuals with NF1.

A randomized placebo-controlled lovastatin trial for neurobehavioral function in neurofibromatosis I

Objective

Lovastatin has been shown to reverse learning deficits in a mouse model of Neurofibromatosis Type 1 (NF1), a common monogenic disorder caused by a mutation in the Ras‐MAPK pathway and associated with learning disabilities. We conducted a randomized double‐blind placebo‐controlled trial to assess lovastatin's effects on cognition and behavior in patients with NF1.

Method

Forty‐four NF1 patients (mean age 25.7+/−11.6 years; 64% female) were randomly assigned to 14 weeks of lovastatin (N = 23; maximum dose of 80 mg/day for adult participants and 40 mg/day for children) or placebo (N = 21). Based on findings in the mouse model, primary outcome measures were nonverbal learning and working memory. Secondary outcome measures included verbal memory, attention, and self/parent‐reported behavioral problems, as well as tolerability of medication. Participants also underwent neuroimaging assessments at baseline and 14 weeks, to determine whether neural biomarkers were associated with treatment response. Linear mixed models assessed for differential treatment effects on outcome measures.

Results

Twelve participants dropped from the study prior to completion (8 placebo, 4 lovastatin), resulting in 32 completers (15 placebo, 17 lovastatin). Lovastatin was well‐tolerated, with no serious adverse events. Differential improvement favoring lovastatin treatment was observed for one primary (working memory; effect size f² = 0.70, P < 0.01) and two secondary outcome measures (verbal memory, f² = 0.19, P = 0.02, and adult self‐reported internalizing problems, f² = 0.26, P = 0.03). Exploratory moderator analyses revealed that higher baseline neural activity in frontal regions was associated with larger treatment effects.

Interpretation

These preliminary results suggest beneficial effects of lovastatin on some learning and memory functions, as well as internalizing symptoms in patients with NF1.

Optic glioma and precocious puberty in a girl with neurofibromatosis type 1 carrying an R681X mutation of NF1: case report and review of the literature

BACKGROUND

Neurofibromatosis type 1 (NF1) is a common autosomal dominant genetic disorder with an extremely variable phenotype. In childhood NF1 can be associated with optic glioma and central precocious puberty; the latter is more common when the optic chiasm is affected. The mutational spectrum of the NF1 gene is wide and complex; R681X is a rare severe mutation of the NF1 gene known to cause truncation of neurofibromin, with only ten reported cases in the literature so far.

CASE PRESENTATION

We describe a girl with NF1 associated with early central precocious puberty appearing at 2.5 years of age and optic glioma affecting the optic chiasm as seen on magnetic resonance imaging (MRI). Genetic analysis confirmed the presence of R681X. Therapy with a gonadotropin-releasing hormone agonist was instituted with good response to therapy. The lesions on MRI were stable and no significant vision impairment was present during the 6 years of follow-up.

CONCLUSION

Of the ten reported cases of NF1 due to R681X, one has presented with optic glioma and none with precocious puberty. Thus, to our knowledge, this is the first reported case of this mutation presenting with precocious puberty. We believe that this is a contribution to the few reports on the phenotype of this mutation and to the future elucidation of genotype-phenotype correlations of this disease.

Neurofibromatosis type 1 alternative splicing is a key regulator of Ras signaling in neurons

Neurofibromatosis type I (Nf1) is a GTPase-activating protein (GAP) that inactivates the oncoprotein Ras and plays important roles in nervous system development and learning. Alternative exon 23a falls within the Nf1 GAP domain coding sequence and is tightly regulated in favor of skipping in neurons; however, its biological function is not fully understood. Here we generated mouse embryonic stem (ES) cells with a constitutive endogenous Nf1 exon 23a inclusion, termed Nf1 23aIN/23aIN cells, by mutating the splicing signals surrounding the exon to better match consensus sequences. We also made Nf1 23aΔ/23aΔ cells lacking the exon. Active Ras levels are high in wild-type (WT) and Nf1 23aIN/23aIN ES cells, where the Nf1 exon 23a inclusion level is high, and low in Nf1 23aΔ/23aΔ cells. Upon neuronal differentiation, active Ras levels are high in Nf1 23aIN/23aIN cells, where the exon inclusion level remains high, but Ras activation is low in the other two genotypes, where the exon is skipped. Signaling downstream of Ras is significantly elevated in Nf1 23aIN/23aIN neurons. These results suggest that exon 23a suppresses the Ras-GAP activity of Nf1. Therefore, regulation of Nf1 exon 23a inclusion serves as a mechanism for providing appropriate levels of Ras signaling and may be important in modulating Ras-related neuronal functions.

Screening in silico predicted remotely acting NF1 gene regulatory elements for mutations in patients with neurofibromatosis type 1

Neurofibromatosis type 1 (NF1), a neuroectodermal disorder, is caused by germline mutations in the NF1 gene. NF1 affects approximately 1/3,000 individuals worldwide, with about 50% of cases representing de novo mutations. Although the NF1 gene was identified in 1990, the underlying gene mutations still remain undetected in a small but obdurate minority of NF1 patients. We postulated that in these patients, hitherto undetected pathogenic mutations might occur in regulatory elements far upstream of the NF1 gene. In an attempt to identify such remotely acting regulatory elements, we reasoned that some of them might reside within DNA sequences that (1) have the potential to interact at distance with the NF1 gene and (2) lie within a histone H3K27ac-enriched region, a characteristic of active enhancers. Combining Hi-C data, obtained by means of the chromosome conformation capture technique, with data on the location and level of histone H3K27ac enrichment upstream of the NF1 gene, we predicted in silico the presence of two remotely acting regulatory regions, located, respectively, approximately 600 kb and approximately 42 kb upstream of the NF1 gene. These regions were then sequenced in 47 NF1 patients in whom no mutations had been found in either the NF1 or SPRED1 gene regions. Five patients were found to harbour DNA sequence variants in the distal H3K27ac-enriched region. Although these variants are of uncertain pathological significance and still remain to be functionally characterized, this approach promises to be of general utility for the detection of mutations underlying other inherited disorders that may be caused by mutations in remotely acting regulatory elements.

Alternative splicing of the neurofibromatosis type I pre-mRNA

NF1 (neurofibromatosis type I) is a common genetic disease that affects one in 3500 individuals. The disease is completely penetrant but shows variable phenotypic expression in patients. NF1 is a large gene, and its pre-mRNA undergoes alternative splicing. The NF1 protein, neurofibromin, is involved in diverse signalling cascades. One of the best characterized functions of NF1 is its function as a Ras-GAP (GTPase-activating protein). NF1 exon 23a is an alternative exon that lies within the GAP-related domain of neurofibromin. This exon is predominantly included in most tissues, and it is skipped in CNS (central nervous system) neurons. The isoform in which exon 23a is skipped has 10 times higher Ras-GAP activity than the isoform in which exon 23a is included. Exon 23a inclusion is tightly regulated by at least three different families of RNA-binding proteins: CELF {CUG-BP (cytosine-uridine-guanine-binding protein) and ETR-3 [ELAV (embryonic lethal abnormal vision)-type RNA-binding protein]-like factor}, Hu and TIA-1 (T-cell intracellular antigen 1)/TIAR (T-cell intracellular antigen 1-related protein). The CELF and Hu proteins promote exon 23a skipping, while the TIA-1/TIAR proteins promote its inclusion. The widespread clinical variability that is observed among NF1 patients cannot be explained by NF1 mutations alone and it is believed that modifier genes may have a role in the variability. We suggest that the regulation of alternative splicing may act as a modifier to contribute to the variable expression in NF1 patients.

Ultrastructural synaptic changes associated with neurofibromatosis type 1: a quantitative analysis of hippocampal region CA1 in a Nf1(+/-) mouse model

Neurofibromatosis type 1 (NF1) is one of the most frequently diagnosed autosomal dominant inherited disorders resulting in neurological dysfunction, including an assortment of learning disabilities and cognitive deficits. To elucidate the neural mechanisms underlying the disorder, we employed a mouse model (Nf1(+/-) ) to conduct a quantitative analysis of ultrastructural changes associated with the NF1 disorder. Using both serial light and electron microscopy, we examined reconstructions of the CA1 region of the hippocampus, which is known to play a central role in many of the dysfunctions associated with NF1. In general, the morphology of synapses in both the Nf1(+/-) and wild-type groups of animals were similar. No differences were observed in synapse per neuron density, pre- and postsynaptic areas, or lengths. However, concave synapses were found to show a lower degree of curvature in the Nf1(+/-) mutant than in the wild type. These results indicate that the synaptic ultrastructure of Nf1(+/-) mice appears relatively normal with the exception of the degree of synaptic curvature in concave synapses, adding further support to the importance of synaptic curvature in synaptic plasticity, learning, and memory.

Lovastatin as treatment for neurocognitive deficits in neurofibromatosis type 1: phase I study

In a neurofibromatosis type 1 murine model, treatment with lovastatin reversed cognitive disabilities. We report on a phase I study examining the safety and tolerability of lovastatin in children with neurofibromatosis type 1. Twenty-four children with neurofibromatosis type 1 underwent a dose-escalation protocol for 3 months to identify the maximum tolerated dose and potential toxicity. Minimal side effects were evident, and no child experienced dose-limiting toxicity. Cognitive evaluations were completed before and after treatment, and the results suggested improvement in areas of verbal and nonverbal memory. Additional analyses, using reliable change indices, indicated improvements exceeding those of test-retest or practice effects in some participants. These observations may be analogous to the improvements observed in a neurofibromatosis type 1 murine model treated with lovastatin, although further study and replication are required. The safety and preliminary cognitive results support the need for a larger phase II trial in this population.

Molecular and cellular mechanisms of learning disabilities: a focus on NF1

Neurofibromatosis Type I (NF1) is a single-gene disorder characterized by a high incidence of complex cognitive symptoms, including learning disabilities, attention deficit disorder, executive function deficits, and motor coordination problems. Because the underlying genetic cause of this disorder is known, study of NF1 from a molecular, cellular, and systems perspective has provided mechanistic insights into the etiology of higher-order cognitive symptoms associated with the disease. In particular, studies of animal models of NF1 indicated that disruption of Ras regulation of inhibitory networks is critical to the etiology of cognitive deficits associated with NF1. Animal models of Nf1 identified mechanisms and pathways that are required for cognition, and represent an important complement to the complex neuropsychological literature on learning disabilities associated with this condition. Here, we review findings from NF1 animal models and human populations affected by NF1, highlighting areas of potential translation and discussing the implications and limitations of generalizing findings from this single-gene disease to idiopathic learning disabilities.

A distinct set of Drosophila brain neurons required for neurofibromatosis type 1-dependent learning and memory

Nonspecific cognitive impairments are one of the many manifestations of neurofibromatosis type 1 (NF1). A learning phenotype is also present in Drosophila melanogaster that lack a functional neurofibromin gene (nf1). Multiple studies have indicated that Nf1-dependent learning in Drosophila involves the cAMP pathway, including the demonstration of a genetic interaction between Nf1 and the rutabaga-encoded adenylyl cyclase (Rut-AC). Olfactory classical conditioning experiments have previously demonstrated a requirement for Rut-AC activity and downstream cAMP pathway signaling in neurons of the mushroom bodies. However, Nf1 expression in adult mushroom body neurons has not been observed. Here, we address this discrepancy by demonstrating (1) that Rut-AC is required for the acquisition and stability of olfactory memories, whereas Nf1 is only required for acquisition, (2) that expression of nf1 RNA can be detected in the cell bodies of mushroom body neurons, and (3) that expression of an nf1 transgene only in the alpha/beta subset of mushroom body neurons is sufficient to restore both protein synthesis-independent and protein synthesis-dependent memory. Our observations indicate that memory-related functions of Rut-AC are both Nf1-dependent and -independent, that Nf1 mediates the formation of two distinct memory components within a single neuron population, and that our understanding of Nf1 function in memory processes may be dissected from its role in other brain functions by specifically studying the alpha/beta mushroom body neurons.

The neurofibromatosis type I pre-mRNA is a novel target of CELF protein-mediated splicing regulation

The CUG-BP and ETR-3 like factors (CELF) are a family of six highly conserved RNA-binding proteins that preferentially bind to UG-rich sequences. One of the key functions of these proteins is to mediate alternative splicing in a number of tissues, including brain, heart and muscle. To fully understand the function of CELF proteins, it is important to identify downstream targets of CELF proteins. In this communication, we report that neurofibromatosis type I (NF1) exon 23a is a novel target of CELF protein-mediated splicing regulation in neuron-like cells. NF1 regulates Ras signaling, and the isoform that excludes exon 23a shows 10 times greater ability to down-regulate Ras signaling than the isoform that includes exon 23a. Five of the six CELF proteins strongly suppress the inclusion of NF1 exon 23a. Over-expression or siRNA knockdown of these proteins in cell transfection experiments altered the levels of NF1 exon 23a inclusion. In vitro binding and splicing analyses demonstrate that CELF proteins block splicing through interfering with binding of U2AF⁶⁵. These studies, combined with our previous investigations demonstrating a role for Hu proteins and TIA-1/TIAR in controlling NF1 exon 23a inclusion, highlight the complex nature of regulation of this important alternative splicing event.

Molecular, genetic, and cellular pathogenesis of neurofibromas and surgical implications

Neurofibromatosis 1 (NF1) is a common autosomal dominant disease characterized by complex and multicellular neurofibroma tumors. Significant advances have been made in the research of the cellular, genetic, and molecular biology of NF1. The NF1 gene was identified by positional cloning. The functions of its protein product, neurofibromin, in RAS signaling and in other signal transduction pathways are being elucidated, and the important roles of loss of heterozygosity and haploinsufficiency in tumorigenesis are better understood. The Schwann cell was discovered to be the cell of origin for neurofibromas, but understanding of a more complicated interplay of multiple cell types in tumorigenesis, specifically recruited heterogeneous cell types such as mast cells and fibroblasts, has important implications for surgical therapy of these tumors. This review summarizes the most recent NF1 and neurofibroma literature describing the pathogenesis and treatment of nerve sheath tumors. Understanding the biological underpinnings of tumorigenesis in NF1 has implications for future surgical and medical management of neurofibromas.

Neurofibromatosis type 1: New insights into neurocognitive issues

Neurofibromatosis type 1 (NF1) is a neurocutaneous disorder with a prevalence of approximately 1 in 3500 people. Academic difficulties and school failure are the most common reported complication of NF1 in childhood and are present in 40% to 60% of the cases. They are often the most significant cause of lifetime morbidity in this population. Recent advances in the recognition and characterization of the cognitive phenotype in NF1 patients have provided a better understanding of the neuropsychologic deficits that account for the impairments in cognitive performance and social interaction. Additionally, recent advances in the understanding of molecular and cellular mechanisms underlying the cognitive deficits in NF1, as well as developments in neuroimaging and molecular genetic techniques are starting to yield a global and integrative picture of the molecular, cellular, and brain system processes affected by this condition. This review focuses on these advances, as well as recent preclinical studies that point towards potential pharmacologic interventions for NF1 patients.

Trap RACK1 with Ras to mobilize Src signaling at syndecan-2/p120-GAP upon transformation with oncogenic ras

HiTrap-syndecan-2/p120-GAP and HiTrap-syndecan-2/RACK1 affinity columns were applied to reveal that Src tyrosine kinase was highly expressed in BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q(61)K)] of shrimp Penaeus japonicus. Both columns were effective to isolate Src tyrosine kinase. The selective molecular affinity for Src was found to be stronger with HiTrap-syndecan-2/RACK1, as revealed with competitive RACK1 to dislodge Src from HiTrap-syndecan-2/p120-GAP. We thus challenged the syndecan-2/p120-GAP and syndecan-2/RACK1 with GTP-K(B)-Ras(Q(61)K). The reaction between RACK1 and syndecan-2 was sustained in the presence of mutant Ras proteins, but not the reaction between p120-GAP and syndecan-2. In the presence of syndecan-2, GTP-K(B)-Ras(Q(61)K) exhibited sufficient reactivity with p120-GAP to discontinue the reaction between p120-GAP and syndecan-2. But the interference of mutant Ras disappeared when Src tyrosine kinase was introduced to stabilize the syndecan-2/p120-GAP complex. On the other hand, in the absence of syndecan-2, GTP-K(B)-Ras(Q(61)K) was found to react with RACK1. The reaction between GTP-K(B)-Ras(Q(61)K) and RACK1 could provide a mechanism to deprive RACK1 for the organization of syndecan-2/RACK1 complex and to facilitate the formation of syndecan-2/p120-GAP complex, as well as to provide docking sites for Src signaling upon transformation with oncogenic ras.

Dimerize RACK1 upon transformation with oncogenic ras

From our previous studies, we learned that syndecan-2/p120-GAP complex provided docking site for Src to prosecute tyrosine kinase activity upon transformation with oncogenic ras. And, RACK1 protein was reactive with syndecan-2 to keep Src inactivated, but not when Ras was overexpressed. In the present study, we characterized the reaction between RACK1 protein and Ras. RACK1 was isolated from BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q61K)] of shrimp Penaeus japonicus and RACK1 was revealed to react with GTP-K(B)-Ras(Q61K), not GDP-K(B)-Ras(Q61K). This selective interaction between RACK1 and GTP-K(B)-Ras(Q61K) was further confirmed with RACK1 of human placenta and mouse RACK1-encoded fusion protein. We found that RACK1 was dimerized upon reaction with GTP-K(B)-Ras(Q61K), as well as with 14-3-3beta and geranylgeranyl pyrophosphate, as revealed by phosphorylation with Src tyrosine kinase. We reported the complex of RACK1/GTP-K(B)-Ras(Q61K) reacted selectively with p120-GAP. This interaction was sufficient to dissemble RACK1 into monomers, a preferred form to compete for the binding of syndecan-2. These data indicate that the reaction of GTP-K(B)-Ras(Q61K) with RACK1 in dimers may operate a mechanism to deplete RACK1 from reaction with syndecan-2 upon transformation by oncogenic ras and the RACK1/GTP-Ras complex may provide a route to react with p120-GAP and recycle monomeric RACK1 to syndecan-2.

P120-GAP associated with syndecan-2 to function as an active switch signal for Src upon transformation with oncogenic ras

BALB/3T3 cells transfected with plasmids pcDNA3.1-[S-ras(Q(61)K)] of shrimp Penaeus japonicus were applied to reveal a complex of p120-GAP/syndecan-2 being highly expressed upon transformation. Of interest, most of the p120-GAP/syndecan-2 complex was localized at caveolae, a membrane microdomain enriched with caveolin-1. To confirm the molecular interaction between syndecan-2 and p120-GAP, we further purified p120-GAP protein from mouse brains by using an affinity column of HiTrap-RACK1 and expressed mouse RACK1-encoded fusion protein and mouse syndecan-2-encoded fusion protein in bacteria. We report molecular affinities exist between p120-GAP and RACK1, syndecan-2 and RACK1 as well as p120-GAP and syndecan-2. The selective affinity between p120-GAP and syndecan-2 was found to be sufficient to detach RACK1. The p120-GAP/syndecan-2 complex was demonstrated to keep Src tyrosine kinase in an activated form. On the other hand, the syndecan-2/RACK1 complex was found to have Src in an inactivated form. These data indicate that the p120-GAP/syndecan-2 complex at caveolae could provide a docking site for Src to transmit tyrosine signaling, implying that syndecan-2/p120-GAP functions as a tumor promoter upon transformation with oncogenic ras of shrimp P. japonicus.

Mouse models of neurofibromatosis type I: bridging the GAP

Neurofibromatosis type I (NF1) is an autosomal dominant disorder caused by mutations in the NF1 gene, leading to a variety of abnormalities in cell growth and differentiation, and to learning disabilities. The protein encoded by NF1, neurofibromin, has several biochemical functions and is expressed in a variety of different cell populations. Hence, determination of the molecular and cellular mechanisms that underlie the different NF1 symptoms is difficult. However, studies using mouse models of NF1 are beginning to unravel the mechanisms that underlie the various symptoms associated with the disease. This knowledge will aid the development of treatments for the different pathological processes associated with NF1.

Molecular and cellular mechanisms underlying the cognitive deficits associated with neurofibromatosis 1

Neurofibromatosis 1 is one of the most common single-gene disorders affecting neurologic function in humans. Mutations in the NF1 gene cause abnormalities in cell growth and differentiation and lead to a variety of learning disabilities. Neurofibromin has several biochemical functions, such as Ras-guanosine triphosphatase activity, adenylate cyclase modulation, and microtubule binding, all of which could be critical for brain function. We review how studies in mouse models are helping to unravel the molecular and cellular mechanisms underlying cognitive deficits in neurofibromatosis 1. These studies suggest that the learning disabilities associated with neurofibromatosis 1 are caused by excessive Ras activity that leads to increased gamma-aminobutyric acid (GABA(A)) inhibition and to decreased long-term potentiation. These findings have brought us closer than ever to the development of possible treatments for the learning disabilities associated with neurofibromatosis 1.

Neurofibromin and NF1 gene analysis in composite pheochromocytoma and tumors associated with von Recklinghausen's disease

Composite tumor of pheochromocytoma and neuroblastoma, or ganglioneuroma, or ganglioneuroblastoma (composite pheochromocytoma), also known as mixed neuroendocrine and neural tumor, are sometimes combined with neurofibromatosis type 1 (NF1). To better understand the relationship between NF1 and composite pheochromocytoma, an immunohistochemical study using anti-neuro-fibromin that is an NF1 gene product and DNA sequence of NF1 Exon 31 were carried out in five cases of composite pheochromocytoma and in various tumors from five patients with NF1. Neurofibromin was not expressed in Schwann cells and sustentacular cells of composite pheochromocytomas and was very weakly or negatively expressed in neurofibroma of NF1 patients. However, it was strongly expressed in ganglionic cells and pheochromocytoma cells of the composite pheochromocytomas and also in mucosal ganglioneuromas, a gangliocytic paraganglioma, and in pheochromocytomas from the patients with NF1. Although there was no mutation in NF1 Exon 31, it could not be ruled out that there were mutations in other sites of the NF1 gene. Neurofibromin insufficiency may induce abnormal proliferation of Schwann cells in composite pheochromocytomas as well as in neurofibromatosis.

GTPase stimulation in shrimp Ras(Q(61)K) with geranylgeranyl pyrophosphate but not mammalian GAP

BALB/3T3 cells were transformed by transfection with DNA encoding the mutated ras(Q(61)K) from shrimp Penaeus japonicus (Huang et al., 2000). The GTPase-activating protein (GAP) in the cytosol fraction was significantly expressed and degraded, compared to untransformed cells on the western blot. To understand this in more detail, the interaction of the bacterially expressed shrimp Ras (S-Ras) with GAP was investigated using GAP purified from mouse brains. SDS-polyacrylamide gel electrophoresis revealed the monomers of the purified GAP to have a relative mass of 65,000. Since the purified GAP was bound to the Ras conjugated affinity sepharose column with high affinity and its GTP hydolysis activity upon binding with tubulin was suppressed, the purified enzyme was concluded to be neurofibromin-like. The purified GAP enhanced the intrinsic GTPase activity of the S-Ras, to convert it into the inactive GDP-bound form, in agreement with findings for GTP-bound K(B)-Ras in vitro. To compare the effects between isoprenoids and GAP on the GTP-hydrolysis of Ras, we applied the GTP-locked shrimp mutant S-Ras(Q(61)K) and GTP-locked rat mutant K(B)-ras(Q(61)K). Radioassay studies showed that geranylgeranyl pyrophosphate at microg level catalyzed the GTP hydrolysis of S-Ras(Q(61)K) and K(B)-ras(Q(61)K) competently, but not farnesyl pyrophosphate or the purified GAP. The present study provides the view that the geranylgeranyl pyrophosphate at carboxyl terminal CAAX assists GTP hydrolysis to Ras proteins probably in a manner similar to the substrate assisted catalysis in GTPase mechanism.

Molecular analysis of the 5'-flanking region of the neurofibromatosis type 1 (NF1) gene: identification of five sequence variants

Dideoxy fingerprinting was used to analyse the 5' flanking region of the neurofibromin (NF1) gene in a panel of 380 neurofibromatosis type 1 (NF1) patients. Five polymorphisms/rare variants were identified at positions -412, - 402, + 16, + 25 and + 132, but control data indicated that these were unlikely to be of pathological significance. Promoter mutations in the NF1 gene are not, therefore, a common cause of NF1. This notwithstanding, a reporter gene assay was performed to determine if these variants could affect the expression of the NF1 gene, and all three changes in the 5'-untranslated region (UTR) (+ 16, + 25, + 132) were found to be associated with a 60-70% increase in reporter gene expression.

Molecular and cellular mechanisms of cognitive function: implications for psychiatric disorders

Recent studies on the molecular and cellular basis of learning and memory have brought us closer than ever to understanding the mechanisms of synaptic plasticity and their relevance to memory formation. Genetic approaches have played a central role in these new findings because the same mutant mice can be studied with molecular, cellular, circuit, and behavioral tools. Therefore, the results can be used to construct models that cut across levels of analytical complexity, forging connections from the biochemistry of the modified protein to the behavior of the mutant mice. These findings are not only improving our understanding of learning and memory, they are also enriching our understanding of cognitive disorders, such as neurofibromatosis type I. Mechanisms underlying long-term changes in synaptic function are likely to be at the heart of many cognitive and emotional processes in humans. Therefore, molecular and cellular insights into learning and memory undoubtedly will have a profound impact on the understanding and treatment of psychiatric disorders.

Genomic characterization of the Neurofibromatosis Type 1 gene of Fugu rubripes

The genomic structure of the Neurofibromatosis Type1 (NF1) gene of Fugu rubripes was investigated by sequence analysis of two overlapping cosmids. The Fugu NF1 gene spans 27 kb and is 13 times smaller than the human counterpart owing primarily to reduced intron size. The predicted amino acid sequence is highly related to that of human neurofibromin, exhibiting an overall similarity of 91.5%. Nearly all exons described for the human NF1 gene could be identified, except exon 12b and the alternatively spliced exons 9br and 48a. With the exception of the splice acceptor site in front of exon 16, all splice sites are in identical positions to those found in the human gene. Intron 1, which is 100-140 kb long in humans, spans 2575 bp in the Fugu NF1 gene. Another large intron of the human NF1 gene, intron 27b (45-50 kb), is 3942 bp of size in Fugu. Sequences related to the OMgp gene (Oligodendrocyte-Myelin-glycoprotein) or the EVI2A gene (ecotropic viral integration site), which are inserted into human NF1 intron 27b, were not detected in the corresponding Fugu intron. However, a single exon gene with similarity to the human EVI2B gene has been found on the reverse strand of Fugu intron 27b. This suggests that the human EVI2B gene and the Fugu gene in intron 27b have a common ancestor. We found the expression of this inserted gene in liver and kidney, but not in brain tissue of Fugu rubripes.

[Evaluation of cancer risk through genetic analysis?]

BACKGROUND

The recent literature of familial cancer, specifically related to germline mutations of RB1, p53, NF1, ATM, BRCA1, Mismatch repair genes and APC is reviewed.

RESULTS AND CONCLUSIONS

Germline mutations do not relate to an increased tumor risk of any single tissue, but instead to spectra of neoplastic diseases. The genetic background plays a major role in modifying the cancer risk. Therefore, mass screening for mutations of single genes seems to be without practical value. Only in combination with an adequate and informative family history can molecular genetic analysis significantly support the care for the individual. Comparison of the data of patients inheriting germline mutations and the experience from the corresponding "knockout" mouse demonstrate that only the p53 and APC knockout mice are useful models of human disease.

Murine Brca2: sequence, map position, and expression pattern

Mutations in the human BRCA2 gene are responsible for about 45% of hereditary early onset breast cancer. Recently, the human BRCA2 gene was cloned, and several germline mutations were identified. Here we describe the cloning of the mouse homologue of BRCA2. The mouse cDNA sequence predicts a 3328-amino-acid Brca2 protein, 90 amino acids shorter than the human protein. The overall identity between the mouse and the human proteins is 59%, while the similarity is 72%. At the nucleotide level the homology is 74%. By comparing the amino acid sequences of the two homologues we have identified five highly conserved novel domains that may be functionally significant. Brca2 has been mapped to the distal end of mouse chromosome 5, a region of the mouse genome that contains other genes that also map to human chromosome 13q12-q13, confirming the conservation of this linkage group between the two species. Expression of Brca2 was detected in midgestation embryos and adult testis, thymus, and ovary.

Cloning, chromosomal mapping and expression pattern of the mouse Brca2 gene

A proportion of human breast cancers result from an inherited predisposition to the disease. Mutations in the BRCA2 gene confer a high risk of breast cancer and are responsible for almost half of these cases. The recent cloning of the human BRCA2 gene has revealed that it encodes a large protein having little significant homology to known proteins. Here we describe the mouse Brca2 gene. The gene maps to mouse chromosome 5, consistent with its location on human chromosome 13q12. We have sequenced cDNA for the entire 3329 amino acid Brca2 protein and this has revealed that, like Brca1, Brca2 is relatively poorly conserved between humans and mice. Brca2 is transcribed in a diverse range of mouse tissues, and the pattern of expression is strikingly similar to that of Brca1. Taken together, our data highlight some intriguing similarities between two genes involved in inherited breast cancer susceptibility.

The murine homolog of the human breast and ovarian cancer susceptibility gene Brca1 maps to mouse chromosome 11D

The recently cloned human breast and ovarian cancer susceptibility gene, BRCA1, is located on human chromosome 17q21. We have isolated murine genomic clones containing Brca1 as a first step in generating a mouse model for the loss of BRCA1 function. A mouse genomic library was screened using probes corresponding to exon 11 of the human BRCA1 gene. Two overlapping mouse clones were identified that hybridized to human BRCA1 exons 9-12. Sequence analysis of 1.4 kb of the region of these clones corresponding to part of human exon 11 revealed 72% nucleic acid identity but only 50% amino acid identity with the human gene. The longest of the mouse Brca1 genomic clones maps to chromosome 11D, as determined by two-color fluorescence in situ hybridization. The synteny to human chromosome 17 was confirmed by cohybridization with the mouse probe for the NF1-gene. This comparative study confirms that the relative location of the BRCA1 gene has been conserved between mice and humans.

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.

Neurofibromatosis type 1: pathology, clinical features and molecular genetics

Neurofibromatosis type 1 (NF1) or von Recklinghausen neurofibomatosis, is a common heritable neurocutaneous disorder. This disorder appears to affect all races, with a prevalence estimated to be 1 in 3000. Approximately half of all cases of NF1 represent new mutations. The characteristics of NF1, which include cafe-au-lait spots, neurofibromas, Lisch nodules, optic glioma, osseous lesions, macrocephaly, short stature and mental retardation suggest that the genetic lesion affects the proper development of multiple organ systems. Within the past few years, the gene causing NF1 has been identified and the protein encoded by this gene, neurofibromin, has been the subject of detailed investigation. The NF1 gene spans over 350 kb of genomic DNA and encodes a protein product of 2818 amino acids. Neurofibromin is expressed in many different tissues. It is now known that one role of neurofibromin is as a GTPase activating protein (GAP), very likely in the same pathway of signal transduction as ras. Absence of neurofibromin in mice homozygously mutant for the NF1 gene results in profound developmental abnormalities. In mice that are heterozygous for NF1, an accelerated onset of tumor formation is observed. Combined with studies of tumors from NF1 patients showing homozygous deletions in the NF1 gene, these data suggest a role for NF1 as a "tumor suppressor". Evidence suggesting other roles played by neurofibromin, in control of proliferation in some situations and differentiation in others, is gradually bringing the previously hazy picture of this genetic disorder into sharper focus.

Ras-GTP regulation is not altered in cultured melanocytes with reduced levels of neurofibromin derived from patients with neurofibromatosis 1 (NF1)

As derivatives of the neural crest, epidermal melanocytes are supposed to be clinically affected by NF1 gene defects. The NF1 gene shares sequence homology with the p120 GTPase activating protein (p120-GAP) and neurofibromin has been shown to participate in Ras-regulation. By immunoprecipitation and Western blotting, neurofibromin was found to be expressed in melanocytes from the unaffected skin and café au lait macules of NF1 patients, but the intensity of the neurofibromin band was decreased compared to control cultures. The Ras-GTP/Ras-GDP ratios of NF1 derived melanocyte cultures were comparable to those derived from healthy donors. Furthermore, the total GAP-activity of cell lysates was not altered in NF1 melanocyte cultures compared to controls. However, lysates of proliferating melanocytes, both from NF1 patients and from healthy donors, showed an about 2-fold higher GAP-activity than poorly growing cells. Neurofibromin contributed approximately one third of total GAP-activity, in both control and NF1 melanocytes, indicating that it is not the major regulator of Ras in these cells. These results suggest that the function of neurofibromin in melanocytes is not limited to regulation of Ras activity.

Genomic organization of the neurofibromatosis 1 gene (NF1)

Neurofibromatosis 1 maps to chromosome band 17q11.2, and the NF1 locus has been partially characterized. Even though the full-length NF1 cDNA has been sequenced, the complete genomic structure of the NF1 gene has not been elucidated. The 5' end of NF1 is embedded in a CpG island containing a NotI restriction site, and the remainder of the gene lies in the adjacent 350-kb NotI fragment. In our efforts to develop a comprehensive screen for NF1 mutations, we have isolated genomic DNA clones that together harbor the entire NF1 cDNA sequence. We have identified all intron-exon boundaries of the coding region and established that it is composed of 59 exons. Furthermore, we have defined the 3'-untranslated region (3'-UTR) of the NF1 gene; it spans approximately 3.5 kb of genomic DNA sequence and is continuous with the stop codon. Oligonucleotide primer pairs synthesized from exon-flanking DNA sequences were used in the polymerase chain reaction with cloned, chromosome 17-specific genomic DNA as template to amplify NF1 exons 1 through 27b and the exon containing the 3'-UTR separately. This information should be useful for implementing a comprehensive NF1 mutation screen using genomic DNA as template.

Tumour predisposition in mice heterozygous for a targeted mutation in Nf1

Human neurofibromatosis type 1 is a dominant disease caused by the inheritance of a mutant allele of the NF1 gene. In order to study NF1 function, we have constructed a mouse strain carrying a germline mutation in the murine homologue. Heterozygous animals do not exhibit the classical symptoms of the human disease, but are highly predisposed to the formation of various tumour types, notably phaeochomocytoma, a tumour of the neural crest-derived adrenal medulla, and myeloid leukaemia, both of which occur with increased frequency in human NF1 patients. The wild-type Nf1 allele is lost in approximately half of the tumours from heterozygous animals. In addition, homozygosity for the Nf1 mutation leads to abnormal cardiac development and mid-gestational embryonic lethality.

Targeted disruption of the neurofibromatosis type-1 gene leads to developmental abnormalities in heart and various neural crest-derived tissues

The neurofibromatosis (NF1) gene shows significant homology to mammalian GAP and is an important regulator of the ras signal transduction pathway. To study the function of NF1 in normal development and to try and develop a mouse model of NF1 disease, we have used gene targeting in ES cells to generate mice carrying a null mutation at the mouse Nf1 locus. Although heterozygous mutant mice, aged up to 10 months, likely attributable to a severe malformation of the heart. Interestingly, mutant embryos also display hyperplasia of neural crest-derived sympathetic ganglia. These results identify new roles for NF1 in development and indicate that some of the abnormal growth phenomena observed in NF1 patients can be recapitulated in neurofibromin-deficient mice.

Two NF1 mutations: frameshift in the GAP-related domain, and loss of two codons toward the 3' end of the gene

Neurofibromatosis type 1 (NF1) is one of the most common autosomal dominant disorders, and is due to mutations within the NF1 gene on chromosome 17q11.2. Only the middle 400 amino acids of the associated protein (neurofibromin) have a known function, comprising a GTPase-activating-protein (GAP) domain. The large gene size and the fact that approximately half of cases are due to new mutation render mutation analysis difficult. NF1 direct mutation characterization is important for development of DNA diagnostic procedures, analysis of phenotype/genotype correlations, and delineation of functions for specific domains of neurofibromin. We report two mutations detected using PCR amplification of individual exons followed by heteroduplex analysis. One is a single base deletion in exon 24 which is predicted to result in a protein truncated early in the GAP-related domain. The other is a 6-bp deletion in exon 39 which is predicted to result in loss of two amino acids in the mature protein near the carboxy-terminus. The exon 24 mutant allele was shown to be expressed by RNA PCR analysis. The exon 39 mutation suggests that those two amino acids are important in neurofibromin function, perhaps indicating a functional domain.

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.