Project
Cognition and Behavior in Neurofibromatosis Type 1 and Legius Syndrome
Neurofibromatosis Type 1 (NF1)
is a frequent autosomal dominant genetic disorder (1/2700 newborns). It is known as a RASopathy, caused by a mutation in the NF1 gene encoding neurofibromin, which is a negative regulator of the RAS (Rat sarcoma) / MAPK (Mitogen Activated Protein Kinase) pathway. This pathway has an important role in the process of cell proliferation and differentiation. However in recent years, accumulating evidence has shown that signaling through this pathway is also important in post‐mitotic neurons for synaptic plasticity and learning and memory. A small but significant proportion of patients has been identified with a milder NF1 phenotype, including pigmentary changes and cognitive and behavioral problems but no neurofibromas, Lisch nodules or optic pathway gliomas. Some of these individuals are diagnosed with Legius Syndrome (LS). LS is caused by heterozygous loss‐of‐function SPRED1 gene mutations. The SPRED1 protein is another negative regulator of the RAS / MAPK pathway acting at the level of RAS‐RAF interaction and neurofibromin membrane recruitment.The overall objective of this thesis was to further study the consequences of NF1 on cognition (Part 1) and social behavior (Part 2) in daily life. At pediatric age, cognitive impairment is considered to be the most common complication in NF1 children, including an average left shift in IQ and several difficulties concerning visual‐spatial abilities, memory, attention and executive functioning.
In Part 1 ‐ Chapter 3, we characterized all of these cognitive problems in NF1 adults, since literature on this topic was rather limited. When comparing a sample of NF1 adults to a normative reference group, a continuum of neuropsychological deficits to adulthood was observed, except for a deficit in sustained attention. When additionally comparing these adultswith their spouses as an IQ‐matched control group, specific deficits in visualspatial abilities and memory, auditory working and long‐term memory function and several components of EF were observed, while unexpectedly no immediate recall and attention deficits were observed.
There has been hope of developing a targeted therapy for cognition and behavioral problems in NF1, especially since their reversibility with statins was demonstrated in a mouse model. In cooperation with the Erasmus Medical Center of Rotterdam, we participated in a randomized, placebo‐controlled, double blind clinical trial in NF1 children, resulting in an adequate sample size of 84 NF1 children in combination with a low attrition rate and high medication compliance. However, no long‐term effect of Simvastatin treatment (dose: <12y: 20mg, >12y: 40mg) on cognition and behavioral dysfunctioning in the daily life of NF1 children was observed (Part 1 ‐ Chapter 4).
While considerable work has focused on cognitive functioning in NF1, several research groups also observed difficulties in general and social behavior (Part 2). Literature over the last 10 years has frequently described social and communication deficits within NF1 but without integrating this within a broader ASD syndromic framework. Therefore, our objective was to specifically screen for increased ASD symptoms and clinical ASD diagnosis in NF1 children visiting the NF1 outpatient clinic of the University Hospital of Leuven (Part 2 ‐ Chapter 5). We showed that NF1 is often accompanied by ASD symptomatically, with 63% being at risk of clinically significant symptoms. Multidisciplinary child psychiatric assessment shows a minimum prevalence estimate of 26% clinical ASD co‐occurrence. Nonetheless, detailed characterization of the ASD behavioural phenotype was still lacking. However, based on in‐depth analyses of two gold standard instruments, a distinctive phenotypic profile was found with NF1+ASD children displaying overall better eye contact, better language skills and less stereotyped behavior when compared to IQ‐matched children with autism/ASD (Part 2 ‐ Chapter 6).
Although several studies have shown Executive Functioning (EF) deficits in NF1 children, findings remain inconsistent and incomplete. In a large NF1 sample, we observed deficits on five EF domains, namely inhibition, cognitive flexibility, generativity, working memory and planning. Most of these deficits were not solely attributable to their lower average IQ and/or elevated Autism Spectrum Disorder (ASD) characteristics (Part 2 ‐ Chapter 7).
Taking a step back from a clinical to preclinical point of view, we focused on characterizing and rescuing ASD core symptoms, while also assessing motor and attention problems in the Spred1‐/‐ mouse model for LS (Part 3 ‐ Chapter 8). Experiments showed social and flexibility dysfunctions associated with ASD, which could be rescued with MEK inhibitor treatment. Other findings support a motor deficit in Spred1 knock‐out mice.
To conclude, NF1 and LS are two single‐gene disorders, which can be seen as excellent experimental platforms to identify the relationship between specific human phenotypes and particular molecular and cellular mechanisms. Clearly characterizing and understanding the cognitive and behavioral difficulties of these disorders is critical to guide clinical care and offer multidisciplinary support in development and functioning. In a next step, assembling preclinical observations on these difficulties within a small‐animal (e.g. mouse) model is crucial to get insight into the pathogenesis of these problems and generate efficient translational research. The convergence of multidisciplinary andclinical support with targeted treatment(s) will be essential as we move forward into a time period of personalized care.