Neurofibroma

A neurofibroma is a benign nerve sheath tumor in the peripheral nervous system. Usually found in individuals with neurofibromatosis type I (NF1), an autosomal dominant genetically-inherited disease, they can result in a range of symptoms from physical disfiguration and pain to cognitive disability. Neurofibromas arise from Schwann cells that exhibit biallelic inactivation of the NF1 gene that codes for the protein neurofibromin.. This protein is responsible for regulating the RAS-mediated cell growth pathway. In contrast to schwannomas, another type of tumor arising from Schwann cells, neurofibromas incorporate many additional types of cells and structural elements in addition to Schwann cells, making it difficult to identify and understand all the mechanisms through which they originate and develop.

Genetics and Intracellular Signaling


Neurofibromas arise from Schwann cells that are homozygous for the inactive version of the NF1 gene, which leads to a complete loss of expression of neurofibromin. The NF1 gene is composed of 60 exons spanning 350kb of genomic data, and maps to chromosomal region 17qll.2. This gene codes for neurofibromin which is a large 220-250 KDa cytoplasmic protein that is composed of 2,818 amino acids with three alternatively spliced exons (9a, 23a and 48a). The functional part of neurofibromin is a GAP, or GTPase-activating protein. GAP accelerates the conversion of the active GTP-bound RAS to its inactive GDP-bound form, inactivating RAS and reducing RAS-mediated growth signaling. Loss of RAS control leads to increased activity of other signaling pathways including RAF, ERK1/2, PI3K, PAK and mTOR-S6 kinase. It is suspected that this increased activity of downstream RAS pathways might work together to increase cell growth and survival. Genes that code for proteins that regulate cell growth, such as NF1 and TP53, are referred to as tumor suppressor genes. Neurofibromin has other growth-regulatory properties besides its ability to regulate RAS activity, but these other functions are poorly understood at this time. While one defective allele may be inherited, loss of heterozygosity (LOH) must occur before a neurofibroma can form; this is called the ‘two-hit hypothesis’. This LOH happens by the same mechanisms, such as oxidative DNA damage, that causes mutations in other cells.

Origin and Development of Neurofibromas
Schwann cells are the neoplastic element in neurofibromas. There are two kinds of Schwann cells, myelinating and nonmyelinating. While myelinating Schwann cells cover large diameter (>1 micrometer) peripheral nervous system (PNS) axons with myelin, nonmyelinating Schwann cells encapsulate small diameter PNS axons with their cytoplasmic processes. This conglomeration of nonmyelinating Schwann cells and axons is called a Remak bundle. While nonmyelinating Schwann cells are the origin of neurofibromas, the mutations that make them susceptible to this transformation occur in Schwann cell precursors during early nerve development. Mutated nonmyelinating Schwann cells do not form normal Remak bundles. Instead, they fail to properly surround and segregate target axons. It is unknown at this time why, if both types of Schwann cells exhibit bilallelic inactivation of the NF1 gene, only the nonmyelinating variety give rise to neurofibromas. According to Zheng et al. in the paper Induction of Abnormal Proliferation by Nonmyelinating Schwann Cells Triggers Neurofibroma Formation, “Taken together, our data suggest that the initially expanded nonmyelinating Schwann cells are early-stage tumor cells, which are responsible for both the initiation and progression of plexiform neurofibromas.”

Once a nonmyelinating Schwann cell has suffered inactivation of its NF1 genes, through genetic mutation and DNA damage caused by environmental factors, it begins to proliferate rapidly. This condition is called hyperplasia, which is cell growth beyond what is normally seen. However, despite increased numbers of nonmyelinating Schwann cells, there is no neurofibroma yet. In order for the neurofibroma to develop, cells that are heterozygous for the NF1 gene must be recruited to the site. It has been hypothesized that the proliferating nonmyelinating Schwann cells secrete chemoattractants such as the KIT ligand, and angiogenic factors such as the heparin-binding growth factor midkine. These chemicals promote the migration of different kinds of cells that are heterozygous for the NF1 gene into the hyperplastic lesions created by the nonmyelinating Schwann cells. These cell types include fibroblasts, perineurial cells, endothelial cells, and mast cells. The mast cells then secrete mitogens or survival factors that alter the developing tumor microenvironment and result in neurofibroma formation. Dermal and plexiform neurofibromas do differ in later development stages, but the details are unclear at this point.

Once a plexiform neurofibroma has formed, there is a chance that it will undergo transformation into a malignant peripheral nerve sheath tumor (MPNST). The formation of malignant cancers from neurofibromas is associated with the loss of expression of the CDKN2A or TP53 gene in non-myelinating Schwann cells that also exhibit biallelic inactivation of the NF1 gene.

Subtypes of Neurofibromas
Neurofibromas have been subdivided into two broad categories: dermal and plexiform. Dermal neurofibromas are associated with a single peripheral nerve, while plexiform neurofibromas are associated with multiple nerve bundles. According to the World Health Organization classification system, dermal and plexiform neurofibromas are grade I tumors.

Dermal Neurofibromas
Dermal neurofibromas, sometimes referred to as cutaneous neurofibromas, typically arise in the teenage years and are often associated with the onset of puberty. They continue to increase in number and size throughout adulthood, although there are limits to how big they get. They look like lumps on or under the skin. While dermal neurofibromas can lead to stinging, itching, pain and disfiguration, there is no evidence of malignant transformation.

Plexiform Neurofibromas
Plexiform neurofibromas are often congenital defects, and are the more troublesome type. They can be very large and can cause pain, disfigurement, neurological and other clinical deficits. While dermal neurofibromas originate in nerves in the skin, plexiform neurofibromas can grow from nerves in the skin or from more internal nerve bundles.

Internal plexiform neurofibromas are very difficult to remove completely because they extend through multiple layers of tissue and the attempt would damage healthy tissue or organs. Plexiform neurofibromas also have the potential to cause severe clinical complications if they occur in certain areas. In addition, about 10% of plexiform neurofibromas undergo malignant transformation. This transformation turns the plexiform neurofibroma into a malignant peripheral nerve sheath tumor (MPNST).

Diagnosis
A blood test for protein melanoma inhibitory activity may be used to detect the presence of neurofibromas.

Treatment
According to Packer, et al. in the article titled Plexiform neurofibromas in NF1-Toward biologic-based therapy, “There are several impediments that have limited progress in designing optimal therapies for NF1-associated neurofibromas, including a more complete understanding of 1) the contribution of each cell type in a neurofibroma to its genesis and continued growth, 2) the specific consequences of absent NF1 gene function on cell growth control, and 3) the role of additional genetic and biologic factors that influence neurofibroma formation and growth.” While this article was written prior to the papers cited in the Origin and Development of Neurofibromas section, these issues have not been totally addressed. While surgery is the most common method of treating neurofibromas, there are some alternative methods as well.

Surgery
Neurofibromas are commonly treated with surgical removal. Dermal neurofibromas are not usually removed unless they are painful or disfiguring, because there are generally so many of them and they are not dangerous.

Removal of plexiform neurofibromas can be more difficult because they can be larger and cross tissue boundaries. However, besides pain, plexiform neurofibromas are sometimes removed due to the possibility of malignant transformation. One example of this can be found in the case study titled A case of isolated giant plexiform neurofibroma involving all branches of the common peroneal nerve, which discusses the removal of a large plexiform neurofibroma in the leg of a 6-year old male. The authors state (dr.cebesoy): “Our case was operated, as both the cutaneous and deep branches of the peroneal nerve were involved causing pain and numbness in the leg, and because there was a possibility for malignant transformation, as growth in the mass was realized by the family members of the patient.” In layman’s terms, they decided to remove the neurofibroma because it was causing the boy pain and numbness in his leg, and because there was a possibility that it was undergoing a malignant transformation as his family noticed an increase in the tumor's size. The authors also note, “However, complete resection is quite difficult due to invasion of the tumor into the surrounding soft tissues.” Sometimes plexiform neurofibromas form in locations that make them especially hard to access. One specific example of this can be found in a paper titled Surgical Treatment of a Left Ventricular Neurofibroma which chronicles the resection (partial removal) of a neurofibroma on the left ventricle. The neurofibroma was removed and the patient’s mitral valve had to be replaced. Another example of a neurofibroma in an unusual location is recorded in a paper titled Pelvic plexiform neurofibroma involving the urinary bladder. A 14-year-old girl with NF1 was diagnosed with a neurofibroma involving her bladder, a rare location. These examples serve to illustrate that plexiform neurofibromas can form anywhere and can make surgical resection difficult.

Radiation
Once a plexiform neurofibroma has undergone malignant transformation, radiation and chemotherapy can be used as treatment. However, radiation is generally not used as a treatment for plexiform neurofibromas because of concerns that this could actually promote malignant transformation. There has even been a documented case of a Schwannoma being induced from a neurofibroma due to radiation therapy.

CO2 Laser
CO2 lasers have been used to remove dermal neurofibromas. In a paper titled Hypertrophic Scars After Therapy with CO2 Laser for Treatment of Multiple Cutaneous Neurofibromas Ostertag et al. said this about treatment by laser: “The cosmetic disfigurement is the most important issue in the decision to treat cutaneous symptoms of neurofibromatosis. Treating patients with extensive neurofibromas with [a] CO2 laser is still the best choice. However, it is strongly advised that a test treatment be performed to judge the effectiveness of the procedure and whether the developed scar is an acceptable trade-off.”

Drug therapies
There are many drug therapies under study for neurofibromas. These are in various stages of research; more time will be required to determine if these are viable options for the treatment of neurofibromas.

Erlotinib (Tarceva) with Sirolimus
The combination of Erlotinib with Sirolimus is being studied to treat low-grade gliomas.

Imatinib (Gleevec)
Early research has shown potential for using the c-kit tyrosine kinase blocking properties of Imatinib to treat plexiform neurofibromas.

Pegylated Interferon (Peg-Intron)
Peginterferon alfa-2b is being studied to treat plexiform neurofibromas.

Sirolimus (Rapamycin)
Sirolimus is an antibiotic developed as an antifungal agent. It inhibits mTOR signalling. It is being studied to treat plexiform neurofibromas.

Sorafenib (Nexavar)
Sorafenib is being studied for treatment of unresectable plexiform neurofibroma and low-grade astrocytomas.

Tranilast (Rizaben)
In vitro, Tranilast, inhibits growth of neurofibroma cells.

Pirfenidone
Pirfenidone inhibits fibroblast growth. Studies showed no improvement over controls.

Tipifarnib
Tipifarnib (also known as drug R115777) inhibits the activation of RAS. This drug is a Farnesyltransferase inhibitor which inhibits the Ras kinase in a post translational modification step before the kinase pathway becomes hyperactive. It successfully passed phase one clinical trials but was suspended (NCT00029354) in phase two after showing no improvement over controls.

Use ACE inhibitors (status as of 2008)
In a letter to the editor of the Annals of Surgical Oncology by Dr. Hamid Namazi in 2008, ACE inhibitors were proposed as a novel treatment of neurofibromas. ACE inhibitors are currently used to treat hypertension and congestive heart failure, to avert remodeling and reinfarction after myocardial infarction, and to ameliorate diabetic nephropathy and other renal diseases. ACE inhibitors work by indirectly down regulating TGF-beta, which is a growth factor that has been shown to influence the development of tumors.

Stabilize axon-Schwann cell interaction and reduce mast cell infiltration (status as of 2008)
Based on the recent discovery that the nonmyelinating Schwann cells that make up Remak bundles are the origin of neurofibromas, it has been proposed that therapies for prevention and treatment be based on stabilizing the axon-Schwann cell interactions and reducing mast cell infiltration. As it appears that these elements are needed for neurofibroma formation, prevention or reduction could prove an effective treatment.

Gene therapy (status as of 2006)
Gene therapy for the neurofibromin 1 gene represents the ultimate solution to preventing the cluster of maladies which are enabled by the mutation.

As of 2006, therapy for NF1 tumors had not been tested due to the lack of an appropriate NF1 tumor model.

Notable Cases
Joseph Merrick, also known as the Elephant Man, lived in the 19th century and suffered severe facial deformities. In 1971, Ashley Montagu suggested in his book The Elephant Man: A Study in Human Dignity that Merrick suffered Neurofibromatosis type 1, and this has continued to be reported. Although, careful research has shown that Merrick actually suffered from Proteus syndrome not Neurofibromatosis as had been previously reported.