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Neurofibromin (NF-1) is a protein that is encoded in humans, in the ''NF1''
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
. ''NF1'' is located on chromosome 17. Neurofibromin, a
GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are a ...
that negatively regulates
RAS/MAPK pathway The MAPK/ERK pathway (also known as the Ras-Raf-MEK-ERK pathway) is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell. The signal starts when a signaling m ...
activity by accelerating the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of Ras-bound GTP. ''NF1'' has a high mutation rate and mutations can alter cellular growth control, and
neural development The development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. The field ...
, resulting in various presentations (depending on the specific
allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...
expressed) such as Watson syndrome and
neurofibromatosis type 1 Neurofibromatosis (NF) refers to a group of three distinct genetic conditions in which tumors grow in the nervous system. The tumors are non-cancerous (benign) and often involve the skin or surrounding bone. Although symptoms are often mild, e ...
(NF1, also known as von Recklinghausen syndrome). Symptoms of NF1 include disfiguring
cutaneous Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different d ...
neurofibroma A neurofibroma is a benign nerve-sheath tumor in the peripheral nervous system. In 90% of cases, they are found as stand-alone tumors (solitary neurofibroma, solitary nerve sheath tumor or sporadic neurofibroma), while the remainder are found in ...
s (CNF), café au lait pigment spots, plexiform neurofibromas (PN), skeletal defects, optic nerve gliomas, life-threatening
malignant peripheral nerve sheath tumor A malignant peripheral nerve sheath tumor (MPNST) is a form of cancer of the connective tissue surrounding peripheral nerves. Given its origin and behavior it is classified as a sarcoma. About half the cases are diagnosed in people with neurofi ...
s (MPNST),
pheochromocytoma Pheochromocytoma is a rare tumor of the adrenal medulla composed of chromaffin cells and is part of the paraganglioma (PGL) family of tumors, being defined as an intra-adrenal PGL. These neuroendocrine tumors can be sympathetic, where they relea ...
, attention deficits, learning deficits and other
cognitive disabilities There are a variety of disabilities affecting cognitive ability. This is a broad concept encompassing various Intellectual disability, intellectual or cognitive deficits, including intellectual disability (formerly called ''mental retardation'') ...
.


Gene

''NF1'' was cloned in 1990 and its product neurofibromin was identified in 1992. Neurofibromin, a
GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are a ...
, primarily regulates the protein Ras. ''NF1'' is located on the long arm of
chromosome 17 Chromosome 17 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 17 spans more than 84 million base pairs (the building material of DNA) and represents between 2.5 and 3% of the total DN ...
, position q11.2 ''NF1'' spans over 350- kb of genomic DNA and contains 62 exons. 58 of these exons are constitutive and 4 exhibit
alternative splicing Alternative splicing, alternative RNA splicing, or differential splicing, is an alternative RNA splicing, splicing process during gene expression that allows a single gene to produce different splice variants. For example, some exons of a gene ma ...
( 9a, 10a-2, 23a, and 28a). The genomic sequence starts 4,951- bp upstream of the
transcription start site Transcription is the process of copying a segment of DNA into RNA for the purpose of gene expression. Some segments of DNA are transcribed into RNA molecules that can encode proteins, called messenger RNA (mRNA). Other segments of DNA are transc ...
and 5,334-bp upstream of the
translation Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...
initiation codon, with the length of the
5' UTR The 5′ untranslated region (also known as 5′ UTR, leader sequence, transcript leader, or leader RNA) is the region of a messenger RNA (mRNA) that is directly Upstream and downstream (DNA), upstream from the initiation codon. This region is im ...
being 484-bp long. There are three genes that are present within
intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...
27b of ''NF1''. These genes are '' EVI2B'', ''EVI2A'' and ''OMG'', which are encoded on the opposite strand and are transcribed in the opposite direction of ''NF1.'' ''EVI2A'' and ''EVI2B'' are human homologs of the ''Evi-2A'' and ''Evi-2B'' genes in mice that encode proteins related to
leukemia Leukemia ( also spelled leukaemia; pronounced ) is a group of blood cancers that usually begin in the bone marrow and produce high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or '' ...
in mice. ''OMG'' is a membrane glycoprotein that is expressed in the human
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
during
myelination Myelination, or myelinogenesis, is the formation and development of myelin sheaths in the nervous system, typically initiated in late prenatal neurodevelopment and continuing throughout postnatal development. The term ''myelinogenesis'' is also ...
of
nerve cells A neuron (American English), neurone (British English), or nerve cell, is an excitable cell that fires electric signals called action potentials across a neural network in the nervous system. They are located in the nervous system and help to ...
.


Promoter

Early studies of the ''NF1'' promoter found that there is great homology between the human and mouse ''NF1'' promoters. The major transcription start site has been confirmed, as well as two minor transcription start sites in both the human and mouse gene. The major transcription start is 484-bp upstream of the translation initiation site. The
open reading frame In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...
is 8,520-bp long and begins at the translation initiation site. ''NF1'' exon 1 is 544-bp long, contains the 5' UTR and encodes the first 20
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s of neurofibromin. The ''NF1'' promoter lies within a CpG island that is 472-bp long, consisting of 43 CpG dinucleotides, and extends into the start of exon 1. This CpG Island begins 731-bp upstream of the promoter and no core promoter element, such as a TATA or CCATT box, has been found within it. Although no core promoter element has been found, consensus binding sequences have been identified in the 5' UTR for several
transcription factors In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fun ...
such as Sp1 and AP2. A
methylation Methylation, in the chemistry, chemical sciences, is the addition of a methyl group on a substrate (chemistry), substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replac ...
map of five regions of the promoter in both mouse and human was published in 1999. This map showed that three of the regions (at approximately – 1000, – 3000, and – 4000) were frequently methylated, but the
cytosine Cytosine () (symbol C or Cyt) is one of the four nucleotide bases found in DNA and RNA, along with adenine, guanine, and thymine ( uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attac ...
s near the transcription start site were unmethylated. Methylation has been shown to functionally impact Sp1 sites as well as a
CREB CREB-TF (CREB, cAMP response element-binding protein) is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements (CRE), thereby increasing or decreasing the transcription of the genes. CREB was first des ...
binding site. It has been shown that the CREB site must be intact for normal promoter activity to occur and methylation at the Sp1 sites may affect promoter activity. Proximal ''NF1'' promoter/5' UTR methylation has been analyzed in tissues from NF1 patients, with the idea that reduced transcription as a result of methylation could be a "second hit" mechanism equivalent to a
somatic mutation A somatic mutation is a change in the DNA sequence of a somatic cell of a multicellular organism with dedicated reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. Unlike germline muta ...
. There are some sites that have been detected to be methylated at a higher frequency in tumor tissues than normal tissues. These sites are mostly within the proximal promoter; however, some are in the 5' UTR as well and there is a lot of interindividual variability in the cytosine methylation in these regions.


3' UTR

A study in 1993 compared the mouse ''NF1''
cDNA In genetics, complementary DNA (cDNA) is DNA that was reverse transcribed (via reverse transcriptase) from an RNA (e.g., messenger RNA or microRNA). cDNA exists in both single-stranded and double-stranded forms and in both natural and engin ...
to the human transcript and found that both the untranslated regions and coding regions were highly conserved. It was verified that there are two ''NF1'' polyadenylated transcripts that differ in size because of the length of the
3' UTR In molecular genetics, the three prime untranslated region (3′-UTR) is the section of messenger RNA (mRNA) that immediately follows the translation (biology), translation termination codon. The 3′-UTR often contains regulatory regions that P ...
, which is consistent with what has been found in the mouse gene. A study conducted in 2000 examined whether the involvement of the 3' UTR in post-transcriptional gene regulation had an effect on the variation of ''NF1'' transcript quantity both spatially and temporally. Five regions of the 3' UTR that appear to bind proteins were found, one of which is HuR, a
tumor antigen Tumor antigen is an antigenic substance produced in tumor cells, i.e., it triggers an immune response in the host. Tumor antigens are useful tumor markers in identifying tumor cells with diagnostic tests and are potential candidates for use in ...
. HuR binds to AU-rich elements which are scattered throughout the 3' UTR and are thought to be negative regulators of transcript stability. This supports the idea that post-transcriptional mechanisms may influence the levels of ''NF1'' transcript.


Mutations

''NF1'' has one of the highest mutation rates amongst known human genes, however mutation detection is difficult because of its large size, the presence of
pseudogene Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Pseudogenes can be formed from both protein-coding genes and non-coding genes. In the case of protein-coding genes, most pseudogenes arise as superfluous copies of fun ...
s, and the variety of possible mutations. The ''NF1'' locus has a high incidence of ''de novo'' mutations, meaning that the mutations are not inherited maternally or paternally''.'' Although the mutation rate is high, there are no mutation "hot spot" regions. Mutations tend to be distributed within the gene, although exons 3, 5, and 27 are common sites for mutations. The Human Gene Mutation Database contains 1,347 ''NF1'' mutations, but none are in the "regulatory" category. There have not been any mutations conclusively identified within the promoter or untranslated regions. This may be because such mutations are rare, or they do not result in a recognizable
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
. There have been mutations identified that affect splicing, in fact 286 of the known mutations are identified as splicing mutations. About 78% of splicing mutations directly affect splice sites, which can cause aberrant splicing to occur. Aberrant splicing may also occur due to mutations within a splicing regulatory element.
Intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...
ic mutations that fall outside of splice sites also fall under splicing mutations, and approximately 5% of splicing mutations are of this nature.
Point mutation A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequences ...
s that effect splicing are commonly seen and these are often substitutions in the regulatory sequence. Exonic mutations can lead to deletion of an entire exon, or a fragment of an exon if the mutation creates a new splice site. Intronic mutations can result in the insertion of a cryptic exon, or result in
exon skipping In molecular biology, exon skipping is a form of RNA splicing used to cause cells to “skip” over faulty or misaligned sections (exons) of genetic code, leading to a truncated but still functional protein despite the genetic mutation. Mechanis ...
if the mutation is in the conserved 3' or 5' end.


Protein

''NF1'' encodes neurofibromin (NF1), which is a 320- kDa protein that contains 2,818 amino acids. Neurofibromin is a
GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are a ...
(GAP) that negatively regulates
Ras pathway Ras, from "Rat sarcoma virus", is a protein family, family of related proteins that are expressed in all animal cell lineages and organs. All Ras protein family members belong to a class of protein called small GTPase, and are involved in trans ...
activity by accelerating
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of Ras-bound
guanosine triphosphate Guanosine-5'-triphosphate (GTP) is a purine nucleoside triphosphate. It is one of the building blocks needed for the synthesis of RNA during the transcription process. Its structure is similar to that of the guanosine nucleoside, the only di ...
(GTP). Neurofibromin localizes in the
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
; however, some studies have found neurofibromin or fragments of it in the
nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...
. Neurofibromin does contain a
nuclear localization signal A nuclear localization signal ''or'' sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysin ...
that is encoded by exon 43, but whether or not neurofibromin plays a role in the nucleus is currently unknown. Neurofibromin is ubiquitously expressed, but expression levels vary depending on the tissue type and developmental stage of the organism. Expression is at its highest level in adult
neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
s,
Schwann cell Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include Satellite glial cell, satellite ...
s,
astrocyte Astrocytes (from Ancient Greek , , "star" and , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of en ...
s,
leukocytes White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...
, and oligodendrocytes. The catalytic
RasGAP RAS p21 protein activator 1 or RasGAP (Ras GTPase activating protein), also known as RASA1, is a 120-kDa cytosolic human protein that provides two principal activities: * Inactivation of Ras (protein), Ras from its active Guanosine triphosphate, ...
activity of neurofibromin is located in a central portion of the protein, that is called the GAP-related domain (GRD). The GRD is closely homologous to RasGAP and represents about 10% (229 amino acids) of the neurofibromin sequence. The GRD is made up of a central portion called the minimal central catalytic domain (GAPc) as well as an extra domain (GAPex) that is formed through the coiling of about 50 residues from the N- and C- terminus. The Ras-binding region is found in the surface of GAPc and consists of a shallow pocket that is lined by conserved amino acid residues. In addition to the GRD, neurofibromin also contains a Sec14 homology-like region as well as a pleckstrin homology-like (PH) domain. Sec14 domains are defined by a
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
binding pocket that resembles a cage and is covered by a helical lid portion that is believed to regulate
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
access. The PH-like region displays a protrusion that connects two beta-strands from the PH core that extend to interact with the helical lid found in the Sec14 domain. The function of the interaction between these two regions is presently unclear, but the structure implies a regulatory interaction that influences the helical-lid conformation in order to control ligand access to the lipid binding pocket.


Function

Through its NF1-GRD domain, neurofibromin increases the rate of GTP hydrolysis of Ras, and acts as a
tumor suppressor A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell (biology), cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results ...
by reducing Ras activity. When the Ras-Nf1 complex assembles, active Ras binds in a groove that is present in the neurofibromin catalytic domain. This binding occurs through Ras switch regions I and II, and an arginine finger present in neurofibromin. The interaction between Ras and neurofibromin causes GAP-stimulated hydrolysis of GTP to GDP. This process depends on the stabilization of residues in the Ras switch I and switch II regions, which drives Ras into the confirmation required for enzymatic function. This interaction between Ras and neurofibromin also requires the transition state of GDP hydrolysis to be stabilized, which is performed through the insertion of the positively charged arginine finger into the Ras active site. This neutralizes the negative charges that are present on GTP during phosphoryl transfer. By hydrolyzing GTP to GDP, neurofibromin inactivates Ras and therefore negatively regulates the Ras pathway, which controls the expression of genes involved in apoptosis, the cell cycle, cell differentiation or migration. Neurofibromin is also known to interact with
CASK A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids ...
through syndecan, a protein which is involved in the KIF17/ABPA1/CASK/LIN7A complex, which is involved in trafficking GRIN2B to the synapse. This suggests that neurofibromin has a role in the transportation of the NMDA receptor subunits to the synapse and its membrane. Neurofibromin is also believed to be involved in the synaptic ATP-PKA-cAMP pathway, through modulation of
adenylyl cyclase Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction: :A ...
. It is also known to bind the caveolin 1, a protein which regulates p21ras, PKC and growth response factors.


Isoforms

There are currently five known
isoforms A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences. While many perform the same or similar biological roles, some isoforms have uniqu ...
of neurofibromin (II, 3, 4, 9a, and 10a-2) and these isoforms are generated through the inclusion of
alternative splicing Alternative splicing, alternative RNA splicing, or differential splicing, is an alternative RNA splicing, splicing process during gene expression that allows a single gene to produce different splice variants. For example, some exons of a gene ma ...
exons (9a, 10a-2, 23a, and 48a) that do not alter the reading frame. These five isoforms are expressed in distinct tissues and are each detected by specific
antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...
. * Neurofibromin type II, also named GRD2 (domain II-related GAP), results from the insertion of exon 23a, which causes the addition of 21 amino acids in the 5' region of the protein. Neurofibromin type II is expressed in Schwann cells and has reduced GAP activity. * Neurofibromin type 3 (also called isoform 3' ALT) contains exon 48a which results in the insertion of 18 amino acids into the 3' terminal. * Neurofibromin type 4 contains exons 23a and 48a, which results in the insertion of 21 amino acids in the 5' region, and 18 amino acids in the 3' terminal. * Neurofibromin 9a (also referred to as 9br), includes exon 9a which results in the insertion of 10 amino acids in the 5' region. This isoform shows little neuronal expression and may play a role in memory and learning mechanisms. * An isoform with insertion of exon 10a-2 has been studied introduces a transmembrane domain. The inclusion of exon 10a-2 causes the insertion of 15 amino acids in the 5' region. This isoform is expressed in most human tissues, therefore it likely performs a housekeeping function in intracellular membranes. It has been suggested that the quantitative differences in expression between the different isoforms may be related to the phenotypic variability of neurofibromatosis type 1 patients.


RNA editing

In the ''NF1''
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
, there is a site within the first half of the GRD where mRNA editing occurs.
Deamination Deamination is the removal of an amino group from a molecule. Enzymes that catalysis, catalyse this reaction are called deaminases. In the human body, deamination takes place primarily in the liver; however, it can also occur in the kidney. In s ...
occurs at this site, resulting in the conversion of
cytidine Cytidine (symbol C or Cyd) is a nucleoside molecule that is formed when cytosine is attached to a ribose ring (also known as a ribofuranose) via a β-N1-glycosidic bond. Cytidine is a component of RNA. It is a white water-soluble solid that is ...
into
uridine Uridine (symbol U or Urd) is a glycosylated pyrimidine analog containing uracil attached to a ribose ring (or more specifically, a ribofuranose) via a β-N1- glycosidic bond. The analog is one of the five standard nucleosides which make up nuc ...
at
nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...
3916. This deamination changes an
arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...
codon Genetic code is a set of rules used by living cells to translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished by the ribosome, which links prote ...
(CGA) to an in-frame translation
stop codon In molecular biology, a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in messenger RNA correspond to the additio ...
(UGA). If the edited transcript is translated, it produces a protein that cannot function as a tumor suppressor because the
N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
of the GRD is truncated. The editing site in ''NF1'' mRNA was shown to have high homology to the ApoB editing site, where double stranded mRNA undergoes editing by the ApoB
holoenzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
. ''NF1'' mRNA editing was believed to involve the ApoB holoenzyme due to the high homology between the two editing sites, however studies have shown that this is not the case. The editing site in ''NF1'' is longer than the sequence required for ApoB mediated mRNA editing, and the region contains two
guanidine Guanidine is the compound with the formula HNC(NH2)2. It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine predominantly in patients experi ...
s which are not present in the ApoB editing site.


Clinical significance

Mutations in ''NF1'' are primarily associated with
neurofibromatosis type 1 Neurofibromatosis (NF) refers to a group of three distinct genetic conditions in which tumors grow in the nervous system. The tumors are non-cancerous (benign) and often involve the skin or surrounding bone. Although symptoms are often mild, e ...
(NF1, also known as von Recklinghausen syndrome). NF1 is the most common single gene disorder in humans, occurring in about 1 in 2500–3000 births worldwide. NF1 is an autosomal dominant disorder, but approximately half of NF1 cases arise from ''de novo'' mutations. NF1 has high phenotypic variability, with members of the same family with the same mutation displaying different symptoms and symptom intensities. Café-au-lait spots are the most common sign of NF1, but other symptoms include lisch nodules of iris,
cutaneous Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different d ...
neurofibroma A neurofibroma is a benign nerve-sheath tumor in the peripheral nervous system. In 90% of cases, they are found as stand-alone tumors (solitary neurofibroma, solitary nerve sheath tumor or sporadic neurofibroma), while the remainder are found in ...
s (CNF), plexiform neurofibromas (PN), skeletal defects, optic nerve gliomas, life-threatening
malignant peripheral nerve sheath tumor A malignant peripheral nerve sheath tumor (MPNST) is a form of cancer of the connective tissue surrounding peripheral nerves. Given its origin and behavior it is classified as a sarcoma. About half the cases are diagnosed in people with neurofi ...
s (MPNST), attention deficits, learning deficits and other
cognitive disabilities There are a variety of disabilities affecting cognitive ability. This is a broad concept encompassing various Intellectual disability, intellectual or cognitive deficits, including intellectual disability (formerly called ''mental retardation'') ...
. In addition to
neurofibromatosis type I Neurofibromatosis type I (NF-1), or von Recklinghausen syndrome, is a complex multi-system neurocutaneous disorder caused by a subset of genetic mutations at the neurofibromin 1 (''NF1'') locus. Other conditions associated with mutation of the ...
, mutations in ''NF1'' can also lead to
juvenile myelomonocytic leukemia Juvenile myelomonocytic leukemia (JMML) is a rare form of chronic leukemia (cancer of the blood) that affects children, commonly those aged four and younger. The name JMML now encompasses all diagnoses formerly referred to as juvenile chronic mye ...
s (JMML), gastrointestinal stromal tumors (GIST), Watson syndrome, astrocytic neoplasms, phaeochromocytomas and
breast cancer Breast cancer is a cancer that develops from breast tissue. Signs of breast cancer may include a Breast lump, lump in the breast, a change in breast shape, dimpling of the skin, Milk-rejection sign, milk rejection, fluid coming from the nipp ...
. No effective therapy NF1 yet exists. Instead, people with neurofibromatosis are followed by a team of specialists to manage symptoms or complications. However, in April, 2020, the FDA approved selumetinib (brand name Koselugo) for the treatment of pediatric patients 2 years of age and older with neurofibromatosis type 1 (NF1) who have symptomatic, inoperable plexiform neurofibromas (PN).


Model organisms

A lot about of our knowledge on the biology of NF1 came from
model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
s including the fruit fly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'', the zebrafish ''
Danio rerio The zebrafish (''Danio rerio'') is a species of freshwater ray-finned fish belonging to the family Danionidae of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (a ...
'' and the mouse ''
Mus musculus The house mouse (''Mus musculus'') is a small mammal of the rodent family Muridae, characteristically having a pointed snout, large rounded ears, and a long and almost hairless tail. It is one of the most abundant species of the genus ''Mus (genu ...
,'' which all contain an NF1
ortholog Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a speci ...
in their genome (no NF1 ortholog exists in the nematode ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a Hybrid word, blend of the Greek ''caeno-'' (recent), ''r ...
''.) Research based on these preclinical models has already proven its efficacy as multiple clinical assays have been initiated subsequently regarding
neurofibromatosis type 1 Neurofibromatosis (NF) refers to a group of three distinct genetic conditions in which tumors grow in the nervous system. The tumors are non-cancerous (benign) and often involve the skin or surrounding bone. Although symptoms are often mild, e ...
-related plexiform neurofibromas, gliomas, MPNST and neurocognitive disorders.


Mouse models

In 1994, the first NF1 genetically engineered
knockout mice A knockout mouse, or knock-out mouse, is a genetically modified mouse (''Mus musculus'') in which researchers have inactivated, or " knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. They are importan ...
were published: homozygosity for the ''Nf1'' mutation (''Nf1-/-'') induced severe developmental cardiac abnormalities that led to embryonic lethality at early stages of the development, pointing out that NF1 plays a fundamental role in normal development. On the contrary, Nf1
heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
animals (''Nf1+/-'') were viable but predisposed to form different types of
tumors A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
. In some of these tumor cells, genetic events of
loss of heterozygosity In genetics, loss of heterozygosity (LOH) is a type of genetic abnormality in diploid organisms in which one copy of an entire gene and its surrounding chromosomal region are lost. Since diploid cells have two copies of their genes, one from each ...
(LOH) were observed, supporting that NF1 functions as a
tumor suppressor gene A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell (biology), cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results ...
. The development of several other NF1 mouse models has also allowed the implementation of
preclinical research In drug development, preclinical development (also termed preclinical studies or nonclinical studies) is a stage of research that begins before clinical trials (testing in humans) and during which important feasibility, iterative testing and dr ...
to test the therapeutic potential of targeted pharmacologic agents, such as sorafenib (VEGFR, PDGFR and RAF kinases inhibitor) and everolimus (mTORC inhibitor) for the treatment of NF1 plexiform neurofibromas, sirolimus (rapamycin) (mTORC inhibitor) for MPNSTs, or
lovastatin Lovastatin, sold under the brand name Mevacor among others, is a statin medication, to treat high blood cholesterol and reduce the risk of cardiovascular disease. Its use is recommended together with lifestyle changes. It is taken by mouth. ...
(HMG-CoA reductase inhibitor), and alectinib (ALK inhibitor) for NF1 cognitive and learning disabilities. In 2013, two conditional knockout mouse models, called ''Dhh-Cre;Nf1flox/flox'' (which develops neurofibromas similar to those found in NF1 patients) and ''Mx1-Cre;Nf1flox/flox'' (which develops myeloproliferative neoplasms similar to those found in NF1 juvenile myelomonocytic leukemia/JMML) were used to study the effects of the specific
MEK inhibitor A MEK inhibitor is a chemical or drug that inhibits the mitogen-activated protein kinase enzymes MEK1 and/or MEK2. They can be used to affect the MAPK/ERK pathway which is often overactive in some cancers. (See MAPK/ERK pathway#Clinical signifi ...
PD032590 on tumor progression. The inhibitor demonstrated a remarkable response in tumor regression and in hematologic improvement. Based on these results, phase I and later phase II
clinical trial Clinical trials are prospective biomedical or behavioral research studies on human subject research, human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel v ...
s were then conducted in children with inoperable NF1-related plexiform neurofibromas, using Selumetinib, an oral selective
MEK inhibitor A MEK inhibitor is a chemical or drug that inhibits the mitogen-activated protein kinase enzymes MEK1 and/or MEK2. They can be used to affect the MAPK/ERK pathway which is often overactive in some cancers. (See MAPK/ERK pathway#Clinical signifi ...
used previously in several advanced adult neoplasms. The children enrolled in the study benefited from the treatment without suffering from excessive toxic effects, and treatment induced partial responses in 72% of them. These unprecedented and promising results from the phase II SPRINT trial,ClinicalTrials.gov Identifier: NCT01362803 https://clinicaltrials.gov/ct2/show/NCT01362803 led, first in 2018, both the
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respo ...
(FDA) and the
European Medicines Agency The European Medicines Agency (EMA) is an agency of the European Union (EU) in charge of the evaluation and supervision of pharmaceutical products. Prior to 2004, it was known as the European Agency for the Evaluation of Medicinal Products ...
to grant selumetinib an '' Orphan Drug Status'' for the treatment of
neurofibromatosis type 1 Neurofibromatosis (NF) refers to a group of three distinct genetic conditions in which tumors grow in the nervous system. The tumors are non-cancerous (benign) and often involve the skin or surrounding bone. Although symptoms are often mild, e ...
, and then, a few months later in 2019, FDA to grant a '' Breakthrough Therapy Designation'' to the inhibitor.


''Drosophila melanogaster''

The ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'' ortholog gene of human NF1 (dNF1) has been identified and cloned in 1997. The gene is slightly more compact than its human counterpart but still remains one of the largest genes of the fly genome. It encodes a protein 55% identical and 69% similar to human neurofibromin over its entire 2,802 amino acid length. It comprises an IRA-related central segment containing the catalytic GAP-related domain (GRD), which are both highly similar to their human counterparts. Also, other conserved regions exist both up- and downstream of this domain. dNF1, like its human counterpart, is mainly expressed in the developing and adult nervous system and primarily controls the MAPK RAS/ERK signaling pathway. Through the use of several
mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It i ...
''null'' alleles of dNF1 that have been generated, its role has been progressively elucidated. dNF1 functions to regulate organism growth and whole-body size (first elucidated by the
rescue Rescue comprises responsive operations that usually involve the saving of life, removal from danger, liberation from restraint, or the urgent treatment of injury, injuries after an incident. It may be facilitated by a range of tools and equipm ...
study of The et al. 1997), synaptic growth,
neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...
function, circadian clock and rhythmic behaviors,
mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
function, and learning (also found in The) including associative
learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, Attitude (psychology), attitudes, and preferences. The ability to learn is possessed by humans, non-human animals, and ...
and
long-term memory Long-term memory (LTM) is the stage of the Atkinson–Shiffrin memory model in which informative knowledge is held indefinitely. It is defined in contrast to sensory memory, the initial stage, and short-term or working memory, the second stage ...
. Large scale genetic and functional screens have also led to the identification of dominant modifier genes responsible for the dNF1-associated defects. The et al. 1997 found the size defect to be rescuable by
transgenic A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques, from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the ...
modification by either a working NF1 or a
protein kinase A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them ( phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a f ...
– but this works only during development and not in adulthood. Interestingly, whole-body size deficits, learning defects and aberrant RAS/ERK signaling are also key features of the NF1 condition in humans, and are all due to a deregulation of the anaplastic lymphoma kinase ALK-NF1- RAS/ERK signaling pathway in flies. Pharmacological treatment using a highly-specific ALK inhibitor corrected all these defects in flies and this therapeutic approach was later successfully validated in a preclinical mouse model of NF1 by treating mice with Alectinib, suggesting it represents a promising therapeutic target.


See also

* Merlin (protein) * SPRED1 gene


References


Further reading

* * * * * * *


External links


GeneReviews/NCBI/NIH/UW entry on Neurofibromatosis 1

Human Gene NF1 (uc002hgf.1)

neurofibromin 1
from
GeneCards GeneCards is a database of human genes that provides Genomics, genomic, Proteomics, proteomic, Transcriptomics, transcriptomic, Genetics, genetic and functional information on all known and predicted human genes. It is being developed and maintai ...

Database of RNA editing
{{Tumor suppressor genes Proteins Genes mutated in mice Tumor suppressor genes