Neurofibroma causes: Difference between revisions
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==Overview== | ==Overview== | ||
==Causes== | ==Causes== | ||
'''Neurofibromin 1 gene''' | '''Neurofibromin 1 gene''' | ||
* The [[Neurofibromin 1|NF1]] gene is composed of 60 [[exons]] spanning 350kb of genomic data, and maps to chromosomal region [[CCL7|17q11.2]].<ref name="pmid8825042">{{cite journal |author=MH Shen, PS Harper, M Upadhyaya. |title=Molecular genetics of neurofibromatosis type 1 (NF1) |journal=Journal of Medical Genetics |volume=33 |issue=1 |pages=2–17 |year=1996 |pmid= 8825042|doi=10.1136/jmg.33.1.2 |pmc=1051805}}</ref> This gene codes for neurofibromin which is a large 220-250 KDa [[cytoplasm]]ic [[protein]] that is composed of 2,818 amino acids with three alternatively spliced exons (9a, 23a and 48a) in the encoding gene. The functional part of neurofibromin is a [[GTPase activating protein|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 [[c-Raf|RAF]], [[Extracellular signal-regulated kinases|ERK1/2]], [[Phosphoinositide 3-kinase|PI3K]], PAK and [[Mammalian target of rapamycin|mTOR-S6 kinase]]. It is suspected that this increased activity of downstream RAS pathways might work together to increase cell growth and survival.<ref name="pmid16069817">{{cite journal |author=Rubin JB, Gutmann DH. |title= Neurofibromatosis type 1 - a model for nervous system tumour formation? |journal=Nature Reviews Cancer |volume=5 |issue=7 |pages=557–64 |year=2005 |pmid=16069817|doi=10.1038/nrc1653}}</ref><ref name="pmid8516298">{{cite journal |author=Johnson MR, Look AT, DeClue JE, Valentine MB, Lowy DR. |title= Inactivation of the NF1 gene in human melanoma and neuroblastoma cell lines without impaired regulation of GTP.Ras. |journal=Proceedings of the National Academy of Sciences of the USA |volume=90 |issue=12 |pages=5539–43 |year=1993 |pmid=8516298|doi=10.1073/pnas.90.12.5539 |pmc=46756}}</ref> | * The [[Neurofibromin 1|NF1]] gene is composed of 60 [[exons]] spanning 350kb of genomic data, and maps to chromosomal region [[CCL7|17q11.2]].<ref name="pmid8825042">{{cite journal |author=MH Shen, PS Harper, M Upadhyaya. |title=Molecular genetics of neurofibromatosis type 1 (NF1) |journal=Journal of Medical Genetics |volume=33 |issue=1 |pages=2–17 |year=1996 |pmid= 8825042|doi=10.1136/jmg.33.1.2 |pmc=1051805}}</ref> This gene codes for neurofibromin which is a large 220-250 KDa [[cytoplasm]]ic [[protein]] that is composed of 2,818 amino acids with three alternatively spliced exons (9a, 23a and 48a) in the encoding gene. The functional part of neurofibromin is a [[GTPase activating protein|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 [[c-Raf|RAF]], [[Extracellular signal-regulated kinases|ERK1/2]], [[Phosphoinositide 3-kinase|PI3K]], PAK and [[Mammalian target of rapamycin|mTOR-S6 kinase]]. It is suspected that this increased activity of downstream RAS pathways might work together to increase cell growth and survival.<ref name="pmid16069817">{{cite journal |author=Rubin JB, Gutmann DH. |title= Neurofibromatosis type 1 - a model for nervous system tumour formation? |journal=Nature Reviews Cancer |volume=5 |issue=7 |pages=557–64 |year=2005 |pmid=16069817|doi=10.1038/nrc1653}}</ref><ref name="pmid8516298">{{cite journal |author=Johnson MR, Look AT, DeClue JE, Valentine MB, Lowy DR. |title= Inactivation of the NF1 gene in human melanoma and neuroblastoma cell lines without impaired regulation of GTP.Ras. |journal=Proceedings of the National Academy of Sciences of the USA |volume=90 |issue=12 |pages=5539–43 |year=1993 |pmid=8516298|doi=10.1073/pnas.90.12.5539 |pmc=46756}}</ref> | ||
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Shanshan Cen, M.D. [2]
Overview
Causes
Neurofibromin 1 gene
- The NF1 gene is composed of 60 exons spanning 350kb of genomic data, and maps to chromosomal region 17q11.2.[1] 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) in the encoding gene. 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.[2][3]
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.
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.[4]
Loss of tumor suppressor function
Neurofibromas arise from nonmyelinating Schwann cells that only express the inactive version of the NF1 gene, which leads to a complete loss of expression of functional neurofibromin. 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.
Once a nonmyelinating Schwann cell has suffered inactivation of its NF1 genes, 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 differ in later development stages, but the details are unclear at this point.
References
- ↑ MH Shen, PS Harper, M Upadhyaya. (1996). "Molecular genetics of neurofibromatosis type 1 (NF1)". Journal of Medical Genetics. 33 (1): 2–17. doi:10.1136/jmg.33.1.2. PMC 1051805. PMID 8825042.
- ↑ Rubin JB, Gutmann DH. (2005). "Neurofibromatosis type 1 - a model for nervous system tumour formation?". Nature Reviews Cancer. 5 (7): 557–64. doi:10.1038/nrc1653. PMID 16069817.
- ↑ Johnson MR, Look AT, DeClue JE, Valentine MB, Lowy DR. (1993). "Inactivation of the NF1 gene in human melanoma and neuroblastoma cell lines without impaired regulation of GTP.Ras". Proceedings of the National Academy of Sciences of the USA. 90 (12): 5539–43. doi:10.1073/pnas.90.12.5539. PMC 46756. PMID 8516298.
- ↑ Zheng H, Chang L, Patel N, Yang J, Lowe L, Burns DK, Zhu Y. (2008). "Induction of Abnormal Proliferation by Nonmyelinating Schwann Cells Triggers Neurofibroma Formation". Cancer Cell. 13 (2): 117–28. doi:10.1016/j.ccr.2008.01.002. PMID 18242512.