Neurofibroma

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Neurofibroma
Neurofibroma of the skin in a person with neurofibromatosis type I
SpecialtyNeuro-oncology

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[1] or sporadic neurofibroma[1]), while the remainder are found in persons 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 nonmyelinating-type

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.[3]

Types

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. Plexiform neurofibroma are more difficult to treat and can transform into malignant tumors. Dermal neurofibroma do not become malignant.[citation needed]

Dermal neurofibroma

Anatomy

Dermal neurofibromas (sometimes referred to as cutaneous neurofibromas) originate in nerves in the skin. Three kinds are distinguished:[4]

  • Discrete cutaneous neurofibromas: Sessile or pedunculated masses on the skin, which are fleshy and non-tender, and can vary in size.
  • Discrete subcutaneous neurofibromas: Lie below and look like bumps on the skin, which can sometimes be tender.
  • Deep nodular neurofibromas: Involving tissues and organs underneath the dermis, but otherwise resembling cutaneous and subcutaneous neurofibromas.

Age of onset

Dermal neurofibroma in a person whose lesions first appeared when he was a teenager

Dermal neurofibromas typically arise in the teenage years and are often associated with the onset of puberty. In people with neurofibromatosis type I, they tend to continue to increase in number and size throughout adulthood, although limits exist as to how big they get, and cases progress at highly variable rates.[citation needed]

Medical complications

Dermal neurofibromas can lead to stinging, itching, pain, and disfigurement.[citation needed]

No evidence of malignant transformation has been found.[2]

Plexiform neurofibroma

Anatomy

Plexiform neurofibroma of the head and neck, before and after removal

Plexiform neurofibromas can grow from nerves in the skin or from more internal nerve bundles, and can be very large (with mass in the tens of kilograms).[5] 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.[citation needed]

Age of onset

Image of neurofibroma showing myxoid appearance

Plexiform neurofibromas occur earlier in life and are thought to be congenital defects.[4]

Medical complications

Plexiform neurofibroma can cause disfigurement, neurological, and other clinical deficits.[citation needed]

Plexiform neurofibromas have the potential to cause severe clinical complications if they occur in certain areas.[6]

About 10% of plexiform neurofibromas undergo transformation into a

TP53
gene in nonmyelinating Schwann cells that also exhibit biallelic inactivation of the NF1 gene.

Cause

This section discusses the

tumorigenesis of neurofibroma in terms of genetics, cell signaling, histology and the cell cycle
.

Neurofibromin 1 gene

This picture illustrates the structure of the NF1 gene product, neurofibromin 1. This protein is 2818 amino acids long with 3 alternatively spiced exons, 9a, 23a, and 48a. The IRA domains are hypothesized to function as negative regulators of RAS, along with the GRD domain in between them.

The NF1 gene is composed of 60

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.[11]

Schwann cells

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. Nonmyelinating Schwann cells are the neoplastic element in neurofibromas. This conglomeration of nonmyelinating Schwann cells and axons is called a Remak bundle.[citation needed
]

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.[12]

Loss of tumor suppressor function

Neurofibromas arise from nonmyelinating

mutations in other cells.[citation needed
]

Once a nonmyelinating Schwann cell has suffered inactivation of its NF1 genes, it begins to proliferate rapidly. This condition is called

mitogens or survival factors that alter the developing tumor microenvironment and result in neurofibroma formation.[citation needed
]

Dermal and plexiform neurofibromas differ in later development stages, but the details are unclear at this point.[10]

Diagnosis

Histopathology of neurofibroma: A spindle cell lesion composed of slender fibroblast-like cells with storiform pattern and very low amount of stroma.[13]

Dermal neurofibromas may be 2 to 20 mm in diameter, is soft, flaccid, and pinkish-white, and frequently this soft small tumor can be invaginated, as if through a ring in the skin by pressure with the finger, a maneuver called "button-holing".[14]: 619 

A blood test for protein melanoma inhibitory activity may be used to detect the presence of neurofibromas.[15][16]

A biopsy can be used for histopathology diagnosis.

  • Neurofibroma marked for biopsy, right upper back
    Neurofibroma marked for biopsy, right upper back

Treatments

Dermal neurofibroma

Dermal neurofibromas are not usually surgically removed unless they are painful or disfiguring, because there are generally so many of them and they are not dangerous.

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."[17]

Plexiform neurofibroma

Surgery

As of 2002, the primary treatment option for plexiform neurofibroma was surgery.[18]

Removal of plexiform neurofibromas is difficult because they can be large and cross tissue boundaries. However, besides pain, plexiform neurofibromas are sometimes removed due to the possibility of malignant transformation.

The following examples show that plexiform neurofibromas can form anywhere and can make surgical resection difficult:

  • A large plexiform neurofibroma in the leg of a 6-year-old male. The authors state: "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." The authors also note, "However, complete resection is quite difficult due to invasion of the tumor into the surrounding soft tissues."[19]
  • A neurofibroma on the
    left ventricle. The neurofibroma was removed and the patient's mitral valve had to be replaced.[20]
  • A 14-year-old girl with NF1 was diagnosed with a neurofibroma involving her bladder, a rare location.[21]

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.[22]

Medications

TGF-beta, which is a growth factor that has been shown to influence the development of tumors.[23]

No effect

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

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.[24] [25]

Research

The many drug therapies under study for neurofibromas[26][27] are in various stages of research; more time will be required to determine if these are viable options for the treatment of neurofibromas.

Sirolimus, a compound that inhibits mTOR signalling, is being studied to treat plexiform neurofibromas.[28][29]

The combination of erlotinib with sirolimus was studied to treat low-grade gliomas.[30]

Early research has shown potential for using the c-kit tyrosine kinase blocking properties of imatinib to treat plexiform neurofibromas.[31][32][33]

Peginterferon alfa-2b is being studied to treat plexiform neurofibromas.[34][35][36][37]

Sorafenib is being studied for treatment of unresectable plexiform neurofibroma and low-grade astrocytomas.[38][39][40]

In vitro, tranilast, inhibits growth of neurofibroma cells.[41]

Gene therapy for the neurofibromin 1 gene represents the ultimate solution to preventing the cluster of maladies which are enabled by the mutation.[42][43] As of 2006, therapy for NF1 tumors had not been tested due to the lack of an appropriate NF1 tumor model.[44]

See also

References

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  4. ^ a b Yamashiroya VK (August 2002). "Chapter XVIII.11. Neurofibromatosis". Case Based Pediatrics For Medical Students and Residents. Honolulu, HI: Department of Pediatrics, University of Hawaii, John A. Burns School of Medicine.
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  28. ^ Clinical trial number NCT00652990 for "Sirolimus to Treat Plexiform Neurofibromas in Patients With Neurofibromatosis Type I" at ClinicalTrials.gov
  29. ^ Clinical trial number NCT00634270 for "A Phase II Study of the mTOR Inhibitor Sirolimus in Neurofibromatosis Type 1 Related Plexiform Neurofibromas (Protocol 102)" at ClinicalTrials.gov
  30. ^ Clinical trial number NCT00901849 for "Tarceva/Rapamycin for Children With Low-Grade Gliomas With or Without Neurofibromatosis Type 1 (NF1)" at ClinicalTrials.gov
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  35. ^ Clinical trial number NCT00396019 for "Study of PEG-Intron for Plexiform Neurofibromas" at ClinicalTrials.gov
  36. ^ Clinical trial number NCT00846430 for "Medical Treatment of "High-Risk" Neurofibromas" at ClinicalTrials.gov
  37. ^ "A Phase II Trial of Peginterferon Alpha-2b (Pegintron) for Neurofibromatosis Type 1 Related Unresectable, Symptomatic or Life-Threatening Plexiform Neurofibromas", National Institutes of Health study 08-C-0130
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External links
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