neurofibromatosis type 1 ( nf1 ) is an autosomal dominant genetic disorder that affects approximately 1 in 3000 births .
individuals with nf1 are predisposed to developing abnormalities in a number of body systems ; individually and cumulatively they can have a significant impact on quality of life . for example , skeletal deformities such as scoliosis and pseudarthroses , are uncommon but can be associated with considerable morbidity .
benign , malignant , and disfiguring tumors ( termed neurofibromas ) are characteristic of the condition and can be challenging to manage .
neuropsychological impairments in nf1 , such as executive dysfunction , inattention , specific learning disorder and reduced social competency , can result in reduced social participation and social isolation .
the clinical diagnosis of nf1 relies on fulfilling at least two of the seven diagnostic criteria ; caf au lait macules , skinfold freckling , neurofibromas , lisch nodules , optic pathway tumors , bone dysplasia or a family history
. notably , some of these features are congenital in origin , while some manifest over time at characteristic developmental stages . skeletal muscle and motor deficits , such as reduced muscle size , muscle weakness , and poor co - ordination
however , recent preclinical and clinical studies have indicated a primary role for the nf1 gene product , neurofibromin , in muscle growth and metabolism .
neurofibromin has been characterized as a gtpase activating protein that is a negative regulator of the ras gtpase .
loss of functional neurofibromin leads to a dose - dependent increase in ras signaling , which can profoundly affect cell proliferation , differentiation , and function [ reviewed in . in this review
we will discuss the evidence for neurological and cognitive deficits contributing to the motor / muscle phenotype .
this review also refocuses discussion on previous studies that describe a vital role for nf1 in skeletal muscle development , as well as the emerging pre - clinical models that suggest a novel regulatory role for nf1 in muscle metabolism .
children with nf1 have been shown to have a generalized decrease in intellectual function ( mean iq in the low 90 s ) as well as a higher rate of specific cognitive deficits ( i.e. attention , visuoperceptual skills , language and executive function ) .
neurocognitive studies have also frequently reported impairment in motor abilities in individuals with nf1 ( table 1 ) , including mild impairment in gross and fine motor tasks[14 - 16 ] and impairment on the beery test of visuomotor integration , which requires subjects to draw increasingly difficult shapes and figures[17 - 19 ] .
individuals with nf1 also present with deficits in a range of functional tasks that likely have significant impact on quality of life .
for example , significant impairment in balance , muscle strength , and upper limb co - ordination in nf1 has been observed using the bot-2 test of motor proficiency .
mild to moderate deficits in manual dexterity , balance , and ball handling skills , deficient motor timing and reaction time , reduced fine motor speed , and impaired handwriting in nf1 children has also been reported .
gait assessment was performed in 46 children and adolescents with nf1 ( ages 7 - 17yrs ) using the gaitrite electronic walkway , and results were compared with a battery of cognitive tests , including the wechsler intelligence scale for children and sub - tests from the cambridge automated neuropsychological test battery .
the largest correlations were found between deficits in gait width and spatial working memory ( r=0.594 , p<0.01 ) , and deficits in running speed and agility with impaired strategy generation ( r=0.549 , p<0.01 ) , supporting speculation of a link between nf1 motor deficits and abnormal central nervous system ( cns ) function . while the mechanisms of interaction between the cns and peripheral nervous system ( pns ) and motor dysfunction are unclear , structural and functional brain abnormalities are a feature of the condition that may contribute to motor impairments .
neurofibromin plays a significant role in the developing brain , with loss of expression resulting in increased cell proliferation and differentiation , ultimately impacting morphology .
a recurrent finding has been an increase in total brain volume , involving both grey and white matter .
diffusion weighted magnetic resonance imaging ( mri ) in individuals with nf1 indicates reduced white matter integrity in a number of regions closely linked to motor function .
these include the : ( 1 ) corpus callosum , ( 2 ) caudate nucleus ; a sub - region of the basal ganglia involved in goal - direct behavior and voluntary movement , and ( 3 ) thalamus ; a sub - cortical nuclear complex that receives and relays cortical and sub - cortical inputs that sub - serve sensory and motor mechanisms , as well as cognitive abilities . volumetric studies report these structures as abnormally large in nf1 cohorts , suggesting a reduced signal - to - noise ratio .
moore et al have further shown significant associations between increased corpus callosum size and reduced motor performance in children with nf1 . there is also mounting evidence for functional abnormalities within the thalamus and caudate in individuals with nf1 .
both adult and pediatric positron emission tomography studies report thalamic hypometabolism , suggesting reduced thalamic signal processing , and a recent event - related functional mri study identified abnormal right caudate activation during a spatial working memory task ; a cognitive ability significantly associated with impaired gait in nf1 .
individuals with nf1 also commonly show focal areas of high t2 signal intensity on mri , which have been associated with reduced fine motor skill .
mechanistically , one clinical study has examined sensory and motor neuropathy in nf1 as possibly contributory .
electrophysiological measures in 39 individuals with nf1 aged 10 - 56 revealed motor polyneuropathy was a rare manifestation , and while abnormalities in multimodal evoked potentials ( visual , auditory and sensory ) were seen commonly , many were associated with tumors or lesions .
further clinical studies correlating neurological assessments with strength and other motor outcomes are needed to define the influence of the cns and pns in the nf1 motor phenotype . while preclinical mouse models have been used to investigate mechanisms underlying some neurocognitive deficits ,
a relationship between cognition and motor performance has not yet been defined in these systems .
abnormalities such as the learning and attention deficits have been extensively examined in the nf1 mouse[38 - 43 ] .
for example , increased gaba release in the hippocampus has been shown to underlie learning deficits , and can be rescued by inhibition of erk signaling , while lovastatin treatment has been shown to rescue deficits in learning and attention .
however , interactions between these deficits and motor function remain unknown . the most significant motor deficit seen in this mouse line was impaired grip strength using a hanging wire test , and there was no demonstration of a neurocognitive basis for this result .
furthermore , this line failed to show any deficiencies in motor performance tests linked to cerebellar function . decreased dopamine levels in the striatum have also been identified in a further nf1 mouse model with bi - allelic nf1 inactivation in glia .
these mice exhibited reduced exploratory behaviors as well as selective and non - selective attention abnormalities , which were rescued by pharmacologic intervention to restore dopamine levels . while speculative
, abnormal dopamine levels may underlie some of the motor impairments observed in individuals with nf1 .
indeed some gait characteristics identified in a pediatric nf1 cohort , including a shorter step length and longer step time , resemble those seen in early parkinson disease ; a disorder associated with reduced dorsal striatal dopamine levels .
further insight into the interactions between neurocognitive development and motor deficits may come from more advanced mouse models of conditional double inactivation of nf1 in neurons .
in 2005 , stevenson et al . published data from an analysis of 40 individuals with nf1 using peripheral quantitative computed tomography ( pqct ) scanning .
individuals with nf1 presented with a significant reduction in muscle cross - sectional area compared to age matched controls .
comparative findings were seen in a pediatric nf1 cohort where lean tissue was measured using dual - energy x - ray absorptiometry ( dexa ) .
children with nf1 had a significantly reduced lean tissue mass . while reduced muscle size may imply a reduction in strength , muscle functional outcomes were not assessed in these primarily radiographic studies . however , in a seminal 2009 study , souza et al performed hand grip dynamometry testing of 21 subjects ( age 7 - 60 yrs ) with nf1 compared to gender , aged , and physical activity matched controls , and demonstrated a significant reduction in grip strength in the nf1 cohort .
findings of muscle functional impairment in nf1 have been subsequently reported in clinical studies ( table 2 ) . reduced strength of the hip extensor muscles has been described using hand held dynamometry .
likewise , a 2013 trial investigating lower body muscle function in nf1 children , found that jumping force ( n / kg ) and jumping power ( w / kg ) were both significantly reduced .
a cohort of 17 individuals with nf1 , along with gender , age , and bodyweight matched control subjects , underwent maximal oxygen consumption ( vo2 max ) testing , a measure of maximal aerobic exercise capacity .
individuals with nf1 had a reduced vo2 max as well as a reduced maximal systolic blood pressure .
while the authors acknowledged that there was some difficulty in recruiting activity matched controls , it remains to be thoroughly investigated whether reduced physical activity accounts for reduced exercise capacity in nf1 .
given the mounting evidence for motor and muscular deficits in nf1 , continued exercise studies of this kind are warranted , particularly those assessing quality of life outcome measures . muscle size and muscle function in nf1 .
insight gained from research into nf1 muscle function may be applicable to other related genetic diseases .
nf1 belongs to the rasopathy family of diseases , which includes costello syndrome , cardiofaciocutaneous syndrome , and noonan syndrome . in a 2012 clinical study ,
reduced grip strength and muscle weakness was identified as a common feature of them all .
it is unclear whether these conditions share a common mechanism for muscle weakness downstream of altered ras signaling .
however , if this is the case , any successful pathway - targeted interventions that improve nf1 muscle performance , may have broad clinical applicability to other rasopathies .
early evidence suggesting a critical role for nf1 in skeletal muscle development came from in vitro experiments assessing gene expression during myoblast differentiation .
levels of nf1 mrna , and neurofibromin were elevated during differentiation , and a concomitant decrease in activated p21-ras was observed .
double inactivation of nf1 was found to be embryonically lethal , and nf1 null embryos showed underdeveloped cardiac and skeletal muscle .
however , it remained unclear whether these effects on muscle were secondary to some other failure in the developmental program , and it was nt until recently that a key role for nf1 in muscle was demonstrated in an animal model .
developed a limb - specific nf1 knockout mouse ( nf1prx1 ) using a prx1-cre transgene to drive deletion of nf1 in cells of the mesenchymal lineage .
furthermore , consistent with in vitro data , analysis of mutant embryos revealed hyperactive ras / mapk signaling , and impaired myoblast differentiation in the developing limbs .
while the nf1prx1-/- mouse model demonstrated the requirement of nf1 for normal limb muscle development , prx1-cre driven recombination is not restricted to the muscle lineage .
thus , the inactivation of nf1 in other mesenchymal tissues including adipocytes , connective tissue , and bone , were potential confounders to interpretation . to overcome this ,
a skeletal muscle specific nf1 knockout mouse ( nf1myod ) was subsequently generated , by crossing the myod - cre transgenic mouse with the nf1-flox line .
homozygous nf1 inactivation in muscle was lethal in the first week of life , and while pups were born at normal bodyweight , they had stunted growth and a high rate of maternal infanticide was observed .
electron microscopy ( em ) imaging of nf1myod muscle specimens showed no evidence of cytoarchitectural abnormalities , including protein aggregates , myofibrillar disruption , or z - line streaming . while nf1prx1-/- muscle has been described as dystrophic , this may be a misnomer , as neither mouse model nor patient muscle biopsies have been characterized as having progressive loss of cytoskeletal or membrane protein integrity .
em of 3-day old nf1myod muscle samples unexpectedly revealed excessive accumulations of intramyocellular lipid , which was subsequently confirmed by oil red o staining .
this led to speculation that nf1 may have a key role in the regulation of muscle lipid metabolism .
analysis of adult nf1prx1-/- muscle samples revealed similarly elevated triglyceride levels , 10-fold that of controls . increased fatty acid synthesis may underlie these accumulations in the nf1prx1-/- mice , as a substantive increase in the expression of fatty acid synthase was observed .
metabolic dysregulation was also seen for a range of mitochondrial enzymes , including succinate dehydrogenase ( sdh ) , -hydroxyacyl - coa dehydrogenase ( bhad ) , and medium - chain acyl - coa dehydrogenase ( mcad ) .
the expression of mitochondrial fatty acid transport protein carnitine palmitoyl transferase-1 ( cpt-1 ) , and membrane transport proteins , cd36 , and fatty acid transport protein 4 ( fatp4 ) were also reduced .
for example , it is difficult to separate any primary deficits responsible , from any downstream or compensatory metabolic changes in the mature nf1prx1-/- muscle .
interestingly , analysis of neonatal nf1myod muscle showed only the intramyocellular lipid phenotype , suggesting that lipid accumulation may be the initiating factor , and that subsequent molecular and metabolic dysregulation is temporal in nature .
although the precise mechanisms remain unknown , these mouse data demonstrate a novel metabolic regulatory role for neurofibromin in muscle .
one possibility is that nf1 muscle has commonalities with the lipid storage myopathies ( lsms ) , which also present with progressive muscle weakness and muscle lipid accumulation .
if parallels are found to exist with the lsms , this may provide insight into potential interventions for nf1 .
for example , primary carnitine deficiency ( pcd ) leads to an impairment of lipid transport into the mitochondria , a resultant accumulation of lipid droplets , and muscle weakness .
pcd patients have been successfully treated with high dose l - carnitine supplementation . while such speculation is attractive to entertain , evidence for lipid accumulation in human nf1 samples has not been firmly established .
identifying lipid accumulation in human nf1 muscle biopsies will be an important goal for researchers in order to demonstrate the relevance of these murine models .
a number of historical and recent studies raise important questions regarding the role of the nf1 gene in muscle development and function . in the 1990s , gutmann et al identified cardiac and skeletal muscle isoforms of nf1 . to date , the functional importance of these isoforms remains unclear .
it is possible that they have a unique role in the regulation of muscle development and/or metabolism .
isoform - specific knockout models may be able to provide insight into the role of alternatively spliced variants of neurofibromin in muscle .
furthermore , the genetics of nf1 in muscle are yet to be elucidated . some manifestations of nf1 are associated with heterozygosity ( haploinsufficiency ) and others with double inactivation .
for example , local double inactivation has been observed in nf1 tumors as well as in tibial pseudarthrosis tissue . to date , no studies have addressed the potential double inactivation of nf1 in muscle .
myofibers are multinucleated cells where sporadic double inactivation in individual nuclei could have unpredictable effects . analyzing double inactivation in myofibers may be challenging , but fluorescent in situ hybridization for nf1 on human muscle biopsies could be a feasible approach .
one confounding factor in interpretation of clinical data is that the cognitive , motor control and psycho - social effects of nf1 may indirectly influence physical activity .
recent data indicates that children with nf1 have reduced participation in formal and informal physical activities . finding ways to accommodate this into both studies of the underlying biological weakness and strategies for exercise - based intervention may have its own challenges .
one intriguing possibility is that nf1 mutations and associated muscle weakness may increase the risk or severity of other related or unrelated conditions .
for example , spinal complications such as scoliosis can be a major source of morbidity in nf1 .
late onset scoliosis has been described in adolescents with nf1 with no underling bone abnormalities , and it is possible that weakness or hypotonia may be contributory .
in addition , two recent case reports suggest the potential for interactions between nf1 and other muscle - related genetic conditions .
one report describes an individual with digeny for mutations in nf1 and ryanodine receptor 1 , resulting in myopathy .
the other shows a previously unreported mutation in an nf1 locus leading to mitochondrial complex i deficiency , hypotonia , and developmental delay .
continued identification of clinical presentations of this kind may provide useful insight . from a therapeutic standpoint
, it will be critical to ascertain the capacity of exercise regimes to modify the muscle and motor phenotypes in nf1 .
the current literature is limited to a single case study that reports improved jumping and throwing performance in children with nf1 , following a plyometric training program . however , this study was small ( n=3 ) , the children were of variable ages and genders , and the study lacked a control cohort .
larger randomized and controlled exercise intervention studies are greatly needed to answer questions regarding the effects of exercise training on motor control , muscle size and strength , fatigue , and quality of life outcomes in individuals with nf1 . while physical therapies are often favored if they can produce significant benefits , pathway - specific pharmacological interventions remain a potential treatment for those found unresponsive to exercise .
the ras - mek - erk pathway is the canonical pathway in nf1 , but this signaling cascade is also recognized for its role in muscle .
constitutively active mek has been shown to directly bind and repress myogenic transcription factors , inhibiting myogenic differentiation in vitro .
furthermore , in a cancer setting , mek / erk inhibition has been shown to be anabolic for skeletal muscle in humans and mice .
this pathway is likely to be of particular relevance for interventions aiming to improve muscle function in nf1 .
souza et al . can be credited for initiating an explosion of activity in the field of nf1 muscle research in 2009 . since then , a range of clinical studies have confirmed reduced motor performance and/or muscle impairment in individuals with nf1 . while there are associations between neurological abnormalities and the nf1 muscle / motor phenotype , the mechanisms underlying these interactions are yet to be elucidated , and remain an area for further research .
mechanistically , recent studies using genetically modified mouse models have provided strong evidence for a metabolic regulatory role for neurofibromin in muscle , likely contributing to the phenotype .
this review has also reflected on a number of historically overlooked or potentially undervalued studies .
the key roles for nf1 in muscle development are preceded by studies showing increases in nf1 gene and protein expression during myogenic differentiation .
the findings of deficits in muscle strength are similarly preceded by radiographic studies showing decreases in muscle mass .
however , it is the studies describing muscle - specific nf1 isoforms that are perhaps the most relevant to revisit , as these isoforms may have as yet undefined roles in the nf1-muscle phenotype . in summary , there have been significant advances in our understanding due to a co - ordination of basic and clinical research studies .
 neurofibromatosis type 1 ( nf1 ) is a genetic neurocutaneous disorder with multisystem manifestations , including a predisposition to tumor formation and bone dysplasias . 
 studies over the last decade have shown that nf1 can also be associated with significant motor deficits , such as poor coordination , low muscle tone , and easy fatigability . 
 these have traditionally been ascribed to developmental central nervous system and cognitive deficits . 
 however , recent preclinical studies have also illustrated a primary role for the nf1 gene product in muscle growth and metabolism ; these findings are consistent with clinical studies demonstrating reduced muscle size and muscle weakness in individuals with nf1 . 
 currently there is no evidence - based intervention for nf1 muscle and motor deficiencies ; this review identifies key research areas where improved mechanistic understanding could unlock new therapeutic options . 
Introduction
Neurological and cognitive deficits may contribute to impaired motor skill
Reduced muscle size and impaired muscle function in NF1
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Key questions remain unanswered
Conclusion