neurofibromatosis-1 ( nf1 ) , also known as von recklinghausen disease , is an inherited , autosomal dominant disease phenotypically characterized by multiple caf au lait spots , lisch nodules , freckling , and development of neurofibromas , and in some cases , their malignant derivatives 1 .
also , gastrointestinal stromal tumors ( gists ) arise in nf1 patients 150 times more frequently than in the general population 2 . with an estimated birth incidence of 1:3000 in western countries ,
it was postulated that nf1 is caused by a loss - of - function of tumor suppressor gene leading to the inactivation of neurofibromin that regulates cell proliferation and cell differentiation through inhibitory regulation of the ras oncogene , a gtpase 3 - 7 .
gastrointestinal stromal tumor ( gist ) is the most common mesenchymal neoplasm of the gastrointestinal tract .
gists are believed to originate from the interstitial cells of cajal , pacemaker cells that regulate peristalsis in the digestive tract 8 .
an oncogenic ( mutational ) activation of kit or pdgfra tyrosine kinases has been identified in a great majority of gists 9 .
however , no gist - specific kit or pdgfra mutations were identified in gist associated with nf1 , carney triad ( ct ) , carney - stratakis syndrome ( css ) , in pediatric gists , or in a small subset ( < 10% ) of sporadic adult gists 10 - 14 .
molecular mechanisms underlying the development of kit / pdgfra - wild type ( wt ) gists are not well understood .
recently , gists associated with ct ( triad of gastric gist , pulmonary chondroma , and extra - adrenal paraganglioma ) and css ( hereditary gist paraganglioma syndrome ) have been shown to have a loss of succinate dehydrogenase subunit b ( sdhb ) expression 15 , 16 .
sdhb , a mitochondrial protein , is one of the ubiquitously expressed proteins functioning at the interface of tricarboxylic acid cycle and electron transport chain .
it is believed that loss of sdhb expression leads to a tumorigennic mechanism involving the stabilization of hypoxia inducing factors ( hif ) , which in turn upregulates genes that promote adaptation of cells in low - oxygen environments by inducing angiogenesis and glycolysis 17 - 20 .
data on sdhb expression in nf1-associated gists are limited and only three cases were tested and turned to be sdhb- positive 15 . the aim of this study was to evaluate sdhb expression in a significant number of nf1-associated gists and test a hypothesis that a lack of sdhb expression , unlike in ct- and css - associated tumors and pediatric gists , is not involved in pathogenesis of nf1-associated gists .
a total of 23 nf1-associated gists , from years 1970 - 1999 , were obtained from the armed forces institute of pathology ( afip ) tissue repository .
immunostaining was performed with leica bond - max automatic immunostainer ( leica , bannockburn , il ) using a commercially available sdhb mouse monoclonal antibody 21a11 ( abcam ab14714 , cambridge , ma ) .
heat induced epitope retrieval ( high ph , edta - based buffer , ph 9.0 , leica bond - max ) was applied for 25 minutes .
leica bond - max avidin - biotin free polymer system was used in the detection according to the recommended procedure .
diaminobenzidine was used as the chromogen , following the blocking of endogenous peroxidase with 3% hydrogen peroxide diluted in phosphate buffer .
only stains with positive internal controls ( granular cytoplasmic staining in vascular endothelia and pericytes ) were considered valid .
one case with unsuccesful immunostaining ( internal control negative ) was removed from the study .
1 ) . in the analyzed cohort , there were 8 ( 36.4% ) male and 14 ( 63.6% ) female patients .
tumor sizes varied from 0.9 to 10 cm ( median , 3.5 cm ) , and 15 patients presented with multiple tumors .
most of these gists had mitotic counts < 5 per 50 hpfs , and only two tumors contained more than 5 mitoses per 50 hpfs .
gist - specific kit exon 9 , 11 , 13 , and 17 , and pdgfra exon 12 , 14 , and 18 mutations were not found as previously reported 11 .
in this study , we demonstrate consistent sdhb expression in neurofibromatosis 1 ( nf1)-associated gists . clinicopathologic features of these gists include patient female predominance , small intestinal location , common multiplicity , kit and dog 1 expression , and lack of gist - specific kit and pdgfra mutations , as previously reported 10 , 11 .
lack of gist - specific kit and pdgfra mutations has also been reported in pediatric gists , carney triad ( ct ) and carney - stratakis ( cs ) syndrome - associated gists , and in a small subset of adult gists 10 - 14 .
lack of sdhb expression has been reported in carney triad ( ct ) and carney - stratakis ( cs ) syndrome - associated gists and more recently in pediatric and adult sporadic gists showing morphologic and clinical features similar to ct- and cs - gists .
it has been proposed that based on sdhb expression , gists could be divided into 2 distinctive subtypes : sdhb- positive ( type 1 ) and sdhb - negative ( type 2 ) tumors 15 .
type 1 gists can occur in adults anywhere in the gastrointestinal tract , have equal sex distribution , generally have kit or pdgfra mutations , and , in majority , respond to imatinib treatment .
type 2 gists occur in childhood or young adulthood nearly exclusively in the stomach , show a striking predilection for the female sex and epitheliod morphology .
these tumors lack kit or pdgfra mutations and show a limited response to imatinb inhibition 14 . based on sdhb - expression status ,
nf1-associated gists belong to type 1 category . however , they differ from type 1 gist characteristics by restriction of occurrence to intestines , common tumor multiplicity , female predominance , and consistent lack of gist - specific kit and pdgra mutations .
similar to sdhb type 2 tumors , they do not respond well to imatinib treatment 21 - 23 .
therefore , clinicopathologically nf1-associated gists deserve a separation from other type 1 ( sdhb - positive ) gists .
although imatinib mesylate , kit and pdgfra tyrosine kinase inhibitor was successfully used for treatment of advanced and disseminated gists 24 , such a treatment is ineffective or minimally effective in kit / pdgfra - wild type gists , as well as those with certain mutations , such as pdgfra asp842val substitution 25 .
thus , genotyping of gists is considered a standard clinical test in all disseminated gists and tumors with a substantial risk of developing metastatic disease . in conclusion ,
nf1-associated gists express sdhb , similar to a majority of adult gists . in view of different kit / pdgfra mutation status and prospect to targeted treatment , simple categorization of gists into type 1 and type 2 based on sdhb expression is inadequate . a classification based on both sdhb expression status and other clinicopatholopgic factors : kit / pdgfra mutation and nf1 syndrome status
characterize gists more accurately allowing subdivision of sdhb - positive tumors into different categories based on these clinicopathologic factors ( table 1 ) .
 gastrointestinal stromal tumor ( gist ) is the most common mesenchymal tumor of the digestive tract . 
 gists develop with relatively high incidence in patients with neurofibromatosis-1 syndrome ( nf1 ) . 
 mutational activation of kit or pdgfra is believed to be a driving force in the pathogenesis of familial and sporadic gists . unlike those tumors 
 , nf1-associated gists do not have kit or pgdfra mutations . 
 similarly , no mutational activation of kit or pdgfra has been identified in pediatric gists and in gists associated with carney triad and carney - stratakis syndrome . 
 kit and pdgfra - wild type tumors are expected to have lesser response to imatinib treatment . 
 recently , carney triad and carney - stratakis syndrome -associated gists and pediatric gists have been shown to have a loss of expression of succinate dehydrogenase subunit b ( sdhb ) , a krebs cycle / electron transport chain interface protein . 
 it was proposed that gists can be divided into sdhb- positive ( type 1 ) , and sdhb - negative ( type 2 ) tumors because of similarities in clinical features and response to imatinib treatment . in this study , 
 sdhb expression was examined immunohistochemically in 22 well - characterized nf1-associated gists . 
 all analyzed tumors expressed sdhb . 
 based on sdhb - expression status , nf1-associated gists belong to type 1 category ; however , similarly to sdhb type 2 tumors , they do not respond well to imatinib treatment . 
 therefore , a simple categorization of gists into sdhb - positive and - negative seems to be incomplete . 
 a classification based on both sdhb expression status and kit and pdgfra mutation status characterize gists more accurately and allow subdivision of sdhb - positive tumors into different clinico - genetic categories . 
Introduction
Materials and Methods
Results
Discussion