Elucidating the impact of neurofibromatosis-1 germline mutations on neurofibromin function and dopamine-based learning

Abstract

Neurofibromatosis type 1 (NF1) is a common autosomal dominant neurologic condition characterized by significant clinical heterogeneity, ranging from malignant cancers to cognitive deficits. Recent studies have begun to reveal rare genotype-phenotype correlations, suggesting that the specific germline NF1 gene mutation may be one factor underlying disease heterogeneity. The purpose of this study was to define the impact of the germline NF1 gene mutation on brain neurofibromin function relevant to learning. Herein, we employ human NF1-patient primary skin fibroblasts, induced pluripotent stem cells and derivative neural progenitor cells (NPCs) to demonstrate that NF1 germline mutations have dramatic effects on neurofibromin expression. Moreover, while all NF1-patient NPCs exhibit increased RAS activation and reduced cyclic AMP generation, there was a neurofibromin dose-dependent reduction in dopamine (DA) levels. Additionally, we leveraged two complementary Nf1 genetically-engineered mouse strains in which hippocampal-based learning and memory is DA-dependent to establish that neuronal DA levels and signaling as well as mouse spatial learning are controlled in an Nf1 gene dose-dependent manner. Collectively, this is the first demonstration that different germline NF1 gene mutations differentially dictate neurofibromin function in the brain.

Attention deficit hyperactivity disorder (ADHD) in phenotypically similar neurogenetic conditions: Turner syndrome and the RASopathies

“…Most germane to the ‘attentional’ focus of this review, is the observation that NF1-deficient mice exhibit a deficit in non-selective and selective attentional function in the absence of hyperactivity [87], a phenotype that is associated with reduced striatal dopamine levels; both the behavioural and neurochemical phenotypes could be ameliorated by the administration of methylphenidate, a dopamine reuptake inhibitor used therapeutically in ADHD cases. Consistent with this, NF1-deficient mice perform poorly on spatial and learning tasks that are dependent upon dopaminergic function, and neural progenitor cells from NF1 patients exhibit reduced dopamine levels [88]. In terms of molecular pathways, the altered gene expression profile exhibited in the hippocampus of heterozygous NF1 knockout mice has implicated aberrant interactions between neurofibromin, the amyloid precursor protein and the dopamine receptor Drd3 as being important in NF1 psychopathology [89, 90].…”

Challenges in Drug Discovery for Neurofibromatosis Type 1-Associated Low-Grade Glioma

“…While these are not glioma cells, they provide a renewable source of human cells for cellular reprograming. Importantly, these iPSC lines were instrumental in demonstrating that different germline NF1 gene mutations result in different levels of neurofibromin expression (39). Current work is underway to use these lines as cellular substrates for glioma modeling, as part of the CTF NF1 Synodos Initiative.…”

“…Relevant to brain tumors, NF1 patients with 5′ NF1 gene mutations have higher chance developing optic gliomas compared to patients with mutations at other locations of the NF1 gene (82). Based on these types of clinical observations, human iPSCs have been used to demonstrate that different NF1 mutations lead to different levels of neurofibromin protein loss (39, 86, 87). These results suggest that not all NF1 gene mutations are equivalent.…”

Contextual signaling in cancer

“…These early-phase observations are bolstered by studies employing NF1-patient derived iPSCs and derivative neural progenitor cells. In these experiments, different NF1 germline mutations have differential effects on neurofibromin expression and function, with some mutations leading to minor reductions in neurofibromin levels and others with >70% decreases [59]. Using genetically-engineered mice designed to harbor the R681X mutation observed in a patient with an OPG or a missense mutation (G848R) found in individuals with spinal neurofibromatosis, differential effects of these mutations were found.…”