The most common complication (estimated to be as high as 80%) in children with NF1 is cognitive impairment (learning disability, attention deficit). NF1 gene mutation and loss of neurofibromin function results in constitutive activation of the RAS signaling pathway. Mice in which the Nf1 gene has been knocked out display defects in visuospatial learning and memory, and alterations in LTP, and both these effects have been shown to be related to activation of the RAS-MAPK pathway. Treatment of Nf1-/+ mice with lovastatin (and other drugs that restore RAS activation state to normal levels) corrects the defects in attention and learning. A long term goal of our research has been to understand the cellular mechanisms underlying cognitive impairment in NF1. Work done by us and others suggest that there is an important interaction between neurofibromin, kinesins, microtubules, and amyloid precursor protein. This led us to hypothesize that Nf1 gene mutation causes a defect in kinesin-mediated axonal and dendritic transport. We have applied an MRI imaging method (manganese enhanced MRI or MEMRI) to measure axonal transport directly in living mice. Preliminary studies suggest that there is a significant reduction in axonal transport in the olfactory pathway of Nf1-/+ mice as measured by MEMRI. Understanding how NF1 mutation leads to axonal transport defects may result in the discovery of new targets for treatment. In this research we propose to determine if the axonal transport defect in Nf1-/+ mice is related to RAS activation, or if this is a RAS-independent effect. The first strategy to answer this question is genetic. We generate mice that are doubly heterozygous for the Nf1 mutation (Nf1-/+) and for the KRAS mutation (KRAS- /+). We will determine if axonal transport is restored to normal in these mice. The second strategy is pharmacological. We shall use drugs (lovastatin, FTI-277, rapamycin) that modulate the RAS and the mTOR signaling pathways (both key pathways in tumor formation and in synaptic plasticity) and GABA-A receptor antagonists (picrotoxin). We will determine if pharmacological treatment corrects the axonal transport defect in Nf1-/+ mice. Such genetic and pharmacological rescue of RAS activation corrects the learning deficit seen in the Nf1-/+ animals, and we shall determine if the axonal transport defect is also rescued. The MEMRI method has been used to investigate axonal transport in other genetic mouse models (Alzheimer's disease). The innovation of this project lies in the application of this method to characterize axonal transport in the Nf1-/+ mouse model, and in our idea that the axonal transport deficit may contribute to the neurological symptoms of NF1. We expect that axonal transport (measured by MEMRI) will be a useful biomarker for neurological disease, and can be used as a preclinical tool for screening therapeutic drugs. Developing similar (safer) imaging methods for human use may provide a method for monitoring treatment efficacy in the clinical setting.
Cognitive dysfunction (learning disability, attention deficit disorder, behavior disorder) is the most common complication in children with neurofibromatosis type 1 (NF1). The results of our study will help us better understand what aspects of neuronal function (cellular biology) are disrupted because of NF1 gene mutation, and how drugs might work to correct cognitive defects in an Nf1 mouse model. The MRI imaging method that we have applied to the study of NF1 may be useful as a screening tool in the search for novel drugs for treatment of neurological problems in NF1, as well as in other neurogenetic disorders. This method can potentially be modified for use in human patients with NF1, and might serve as a biomarker of neurological severity, and of treatment efficacy.
