A RASopathy gene commonly mutated in cancer: the neurofibromatosis type 1 tumour suppressor

Abstract

Neurofibromatosis type 1 (NF1) is a common genetic disorder that predisposes affected individuals to tumours. The NF1 gene encodes a RAS GTPase-activating protein called neurofibromin and is one of several genes that (when mutant) affect RAS-MAPK signalling, causing related diseases collectively known as RASopathies. Several RASopathies, beyond NF1, are cancer predisposition syndromes. Somatic NF1 mutations also occur in 5-10% of human sporadic cancers and may contribute to resistance to therapy. To highlight areas for investigation in RASopathies and sporadic tumours with NF1 mutations, we summarize current knowledge of NF1 disease, the NF1 gene and neurofibromin, neurofibromin signalling pathways and recent developments in NF1 therapeutics.

Figure 1. Neurofibromatosis type 1 historical developments

From the development of diagnostic criteria to the development of ongoing clinical trials, the neurofibromatosis type 1 (NF1) field has been aided by close clinician–scientist interactions, which have been facilitated by the Children’s Tumor Foundation. Currently accepted diagnostic criteria include six or more café-au-lait macules with a minimum diameter of >5 mm in pre-pubertal subjects; two or more neurofibromas of any type or one plexiform neurofibroma; freckling in the axillary or inguinal region; optic pathway glioma; two or more Lisch nodules (iris hamartomas); a distinctive osseous lesion, such as sphenoid dysplasia or thinning of long bone cortex with or without pseudarthrosis; and a first-degree relative with NF1 according to these criteria. GEM, genetically engineered mouse; JMML, juvenile myelomonocytic leukaemia; MPNSTs, malignant peripheral nerve sheath tumours; NIH, National Institutes of Health.

Figure 2. Disease manifestations in patients with neurofibromatosis type 1: epochs in which they develop

Most plexiform neurofibromas are present at a very young age but, depending on the tumour location, may not be diagnosed until later in life if whole-body magnetic resonance imaging is not performed. Multiple hyperpigmented skin lesions (café-au-lait macules) are an early sign of neurofibromatosis type 1 (NF1) and are observable in children under 3 years of age. Young children may also present with bone dysplasia, delayed speech and delayed acquisition of motor skills. Young children with NF1 are at an increased risk of developing juvenile myelomonocytic leukaemia (JMML) and optic pathway glioma (mean age of 5 years). Later in childhood, cognitive issues surface. If the features labelled in burgundy do not occur early, they will not develop later in life. The dark blue line shows that speech, language, motor and cognitive changes are detected, as children would normally develop specific skills. Cutaneous neurofibromas typically begin to grow during puberty. Although malignant peripheral nerve sheath tumours (MPNSTs) may occur in childhood, they are most common in adult patients with NF1 over 30 years of age. Beyond cancer, it is now appreciated that generalized or specific cognitive impairment is observed in >50% of patients with NF1 (REF. 198), and many have attention deficit hyperactivity disorder. Features of autism spectrum disorder may occur, and vascular defects are common. On the basis of cloning of the NF1 gene, understanding of the related disorders and clarification of age-of-onset of individual disease manifestations, it has been suggested (since 2007) that the diagnostic criteria may need revision.

Figure 3. Neurofibromatosis type 1 signalling pathways

In the absence of negative regulation of RAS proteins, resulting from loss of neurofibromatosis type 1 (NF1, which encodes neurofibromin), GTP-bound RAS levels are increased. Therefore, signalling pathways downstream of RAS that are normally activated by receptors — including receptor tyrosine kinases, integrins and ion channels — show enhanced activation. RAS signalling pathways include the MEK–ERK signalling cascade downstream of RAF and also many other potential RAS effectors, including AF6, an F-actin and RAP1-binding protein; RAL guanine nucleotide dissociation stimulator (RALGDS), a guanine nucleotide exchange factor (GEF) for the RALA and RALB GTPases; T lymphoma invasion and metastasis-inducing protein 1 (TIAM1), an exchange factor for the GTPase RAC1; phospholipase Cε (PLCε), an isoform of the phospholipase C family; and RAS and RAB interactor 1 (RIN1), which is a RAS effector and RAB5 GEF. In addition, loss of NF1 results in deregulation of cyclic AMP levels in affected cells through poorly characterized mechanisms that may be independent of RAS and/or result from crosstalk between RAS and heterotrimeric G protein signalling. RAF and cAMP are the only effector pathways currently shown to have therapeutic potential in NF1 disease. GAP, GTPase-activating protein.

Figure 4. Neurofibromin protein structure and interacting proteins

Neurofibromin contains multiple domains (light blue). These include a cysteine–serine-rich domain (CSRD), a tubulin-binding domain (TBD), a central GTPase-activating protein-related domain (GRD), a SEC14 domain^,, a pleckstrin homology (PH) domain, a carboxy-terminal domain (CTD) and a syndecan-binding domain (SBD). The SEC14 and PH domains bind to phospholipids, and have been studied structurally. Proteins identified as neurofibromin-interacting proteins and phospholipids (ovals) are shown associated with functions ascribed to them, including intracellular trafficking (light yellow); neuronal differentiation (dark yellow); membrane localization (dark blue); actin cytoskeleton remodelling (light pink); ubiquitylation (dark pink); cell adhesion (purple) and cell signalling through nitric oxide via dimethylarginine dimethylaminohydrolase 1 (DDAH1) and RAS (turquoise). Each interacting protein is shown bound to the domain of neurofibromin with which it is believed to interact. Some proteins are known to interact with neurofibromin, but the binding site is unknown. Phosphorylation (P) sites implicated as protein kinase A substrates are shown. Descriptions of each interacting protein, binding domains and literature references are shown in Supplementary information S1 (table). APP, amyloid-β (A4) precursor protein; DPYSL2, dihydropyrimidinase-related protein 2; FAF2, FAS-associated factor 2; FAK, focal adhesion kinase; LIMK2, LIM domain kinase 2; LRPPRC, leucine-rich pentatricopeptide motif-containing protein; SCF, Skp, Cullin, F-box-containing complex; SPRED1, sprouty-related, EVH1 domain-containing protein 1; VCP, valosin-containing protein.