Introductory Chapter: Toward a More Comprehensive Understanding of Neurofibromatosis Type 1

1. Introduction

Recent basic research on the wide range of biological functions of neurofibromin (encoded by the neurofibromatosis 1 gene) has improved our understanding of the relationship between deficits in neurofibromin function and the molecular pathogenesis of organ-specific symptoms in neurofibromatosis type 1 (NF1) patients.

NF1 has been defined as one of the major monogenic neurocutaneous syndromes. The classical cutaneous symptoms of NF1 include pigmented skin lesions, such as café-au-lait macules (CALMs), axillary or inguinal freckles, and cutaneous neurofibromas comprising dermal, subcutaneous, and plexiform types. Neurofibromas are composed of tumorigenic Schwann cells with both germline and somatic mutation (NF1^−/−), and haploinsufficient mast cells, macrophages, perineurial cells, and fibroblasts with germline mutation (NF1^+/−), which supports the proliferation of the tumors [1]. Thus, tumorigenic diploinsufficient (null) Schwann cells (NF1^−/−) have mutations of the two alleles of the NF1 gene, which causes loss of heterogeneity (LOH). (NF1^−/−)-Schwann cells express a high level of stem cell factor, which is the c-kit ligand for mast cells and melanocytes. Haploinsufficient (NF1^+/−) mast cells, fibroblasts, and macrophages infiltrating neurofibromas express various cell growth factors to maintain the proliferation of null Schwann cells. The density of melanocytes in the CALMs is increased, and CALMs are found to be composed of both NF1^+/− and NF1^−/− melanocytes [2], so NF1^−/− melanocytes might affect the increase in the density of melanocytes in the CALMs. Although preliminary practical treatments with topical vitamin D3 and/or narrow-band UVB irradiation for the cutaneous lesions of NF1 have been reported [3], clearly effective therapies have yet to be developed.

Because germline mutation of the NF1 gene shows complete penetration, the haploinsufficient gene mutation is expressed in a wide variety of cells and tissues. Thus, in addition to the classical cutaneous lesions described above, NF1 patients exhibit a multiplicity of symptoms, such as neurological and psychiatric symptoms (epileptic seizures, cognitive dysfunction, learning disabilities, and impaired intellectual function), skeletal and bone lesions (congenital pseudoarthrosis of tibia, scoliosis, and spondylolisthesis), and endocrine disorders (short stature with or without growth hormone deficiency, central precocious puberty, and growth hormone excess).

This chapter focuses on recent understanding of molecular and genetic pathogenesis of NF1-related symptoms.

2. Pathogenesis of a multiplicity of NF1-related symptoms in association with neurofibromin dysfunction

Investigation of the precise mechanisms behind the pathogenesis of the above clinical symptoms of NF1 patients has been progressed; and the genetic, molecular, and cellular data have been accumulated. For example, epileptic seizures are mostly associated with intracranial tumors, such as optic gliomas, and because neurofibromin exerts important effects on cortical development, the impairment of its function would be associated with a higher rate of seizures in individuals with NF1 than in the normal population. In addition, because neurofibromin is involved in the adenylyl cyclase pathway, its dysfunction causes impairment of the regulation of the intracellular cAMP levels, leading to the abnormal development of the brain of NF1 patients. This should be raised as one of the etiological factors for neuropsychiatric disorders of NF1 patients. The pathogenesis of pseudoarthrosis of children with NF1 is not yet fully understood, but it has been reported that LOH is required for the development of pseudoarthrosis of children with NF1 [4]. Germline mutations in the NF1 gene cause aberrant growth and differentiation of osteoblasts and osteoclasts, which are also related to the formation of pseudoarthrosis. Dysplastic and non-dysplastic types of scoliosis are seen in NF1 patients. The pathogenesis of the early occurrence of scoliosis in children is also obscure. However, growing spinal neurofibromas may be associated with the formation and progression of scoliosis. The short stature of NF1 patients is well recognized, and intracranial tumors and skeletal abnormalities such as scoliosis are the main risk factors for short stature. Intracranial tumors are also one of the factors for growth hormone deficiency.

NF1 patients are predisposed to malignant neoplasms, such as optic pathway glioma, childhood leukemia/lymphoma, breast cancer, gastrointestinal stromal tumor, malignant peripheral nerve sheath tumor, or pheochromocytoma, which is the most serious concern about this genetic disease. The main reason for the high susceptibility of NF1 patients to malignancy is the dysfunction of neurofibromin. Thus, neurofibromin acts as the negative regulator of the expression of the oncogene Ras by converting the active GTP-bound form to the inactive GDP-bound form via the GAP (GTPase activating protein)-related domain (GRD). Mutations in the NF1 gene reduce the RAS-GAP function to cause constitutive expression of oncogene Ras, which leads to the high cellular activity of the Ras signaling pathway. This abnormal activation of the signal transduction pathway confers a predisposition to tumorigenesis because of the induction of an increase in proliferation and suppression of apoptosis of the cells. Because tumorigenic NF1 null cells follow “Knudson’s two-hit hypothesis,” this might also lead to triggering of the transformation of the tumor cells from benign to malignant phenotypes. Dysregulation of the other domains of the NF1 gene might also be associated with the malignant conversion of the NF1 cells, which remains to be investigated further.

The appearance of organ-specific symptoms varies among individual NF1 patients. This wide range of symptoms related to dysfunction of the NF1 gene markedly impairs the quality of life of NF1 patients. Because of the complex multiplicity of symptoms in NF1 patients, it is important for those involved in both clinical and basic scientific research on NF1 to more comprehensively understand the various clinical organ-specific symptoms and their possible etiological and pathophysiological mechanisms.

In this book, first, clinical symptoms that are more serious manifestations, such as skeletal problems in children, a radiologically distinct spinal neurofibromatosis tissue, seizures, or epilepsy in adults with NF1, various endocrine disorders. Second, basic investigations on NF1-induced neuronal cells, which can be produced from fibroblasts by direct conversion technologies [5], alternative splicing events of two important exons in the NF1 gene [exon 31 (former exon 23a), and exon 51 (former exon 43)], and metabolic alterations in NF1-related malignant tumors, are described. Thus, this book is intended to promote a comprehensive understanding of recent findings on the multiplicity of neurofibromin functions and various clinical symptoms that have emerged in relation to the dysregulation of neurofibromin expression. This should help clinical and basic researchers in their efforts to analyze and clarify the complex functions of neurofibromin more accurately and to develop novel therapeutic drugs.