**1. Introduction**

Neurofibromatosis type 1 (NF1), also known as von Recklinghausen's disease, is an autosomal dominantdisorder affecting approximately 1 in 3500people. Common to all ethnic origins, NF1 appears to occur in similar proportions for all individuals without sex or ethnic specificity. Riccardi [1] classified heterogenous neurofibromatosis disorders into eight categories. Hallmarks of NF1,the classic type of neurofibromatosis, include café-au-lait macules (CALMs), and multiple benign cutaneous neurofibromas (NFs), comprising dermal, subcutaneous, and plexiform types. Clinical diagnosis is based on the presence of two or more of the following findings: six or more CALMs (largest diameter >0.5 cm in prepubertal individuals, or >1.5 cm

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

inpostpubertalindividuals);twoormoreNFsofanytype,oroneplexiformNF;axillaryfreckling; optic glioma; two or more Lisch nodules within the iris; a distinctive osseous lesion; or firstdegree relative diagnosed with NF1 according to the preceding criteria [2]. While significant advances in understanding both the pathoetiology and genetics of NF1 have been made in the lastdecade, no therapeutic modalities are currently available for NF1patients; although several studies are currently examining various agents specifically directed at plexiform NF, such as clinical trials for sirolimus and imatinib mesylate [3].

We have previously investigated the effects of vitamin D3 (VD3) and its analogs on skin lesions of patients with NF1 and observed inhibited growth of fibroblasts primarily isolated from NFs (previously called fibroblastic cells) [4]. We next performed in vivo experiments in which we found topical application of a VD3 analog onto a CALM grafted to nude mouse skin inhibited uptake of bromodeoxyuridine into cells on the basal layer of grafted epidermis [5]. In addition, cell density of NF tissues subcutaneously grafted onto the skin of nude mice decreased significantly with direct local injection of a VD3 analog [6]. Consequently, as an initial human study, we evaluated the effect of 1 month of VD3 analog application twice a day to an NF1 patient's large pigmented plaque. We observed moderate improvement of the applied macule, including a remarkable decrease in intensity of Fontana Masson staining in the epidermal basal layer [6]. We extended our study to examine whether VD3 or its analogs inhibit growth of Schwann cells and mast cells isolated from primary NFs. We also investigated the capacity of these agents to inhibit growth of human epidermal melanocytes. Our results indicated that with the exception of Schwann cells, all cell types examined were inhibited by VD3 and its analogs in vitro.

From a molecular aspect, the NF1 gene encodes a Ras GTPase-activating protein called neurofibromin; mutations in this gene affect Ras-mitogen-activated protein kinase (MAPK) signaling [7]. Inhibition of this signaling, also known as the Ras/Raf/MEK/ERK or MEK pathway, has also been shown to be efficacious in treating human NF [8]. Therefore, we examined inhibitory effects of a mechanistic target of rapamycin (mTOR) inhibitor (rapamycin) and a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor (lovastatin) [9] on growth of Schwann cells isolated from primary NFs. Our results demonstrated that these agents significantly inhibited Schwann cell growth in vitro.

We examined the potential of other therapeutic modalities to be combined with VD3 for even greater improvement of skin lesions in patients with NF1. Previous reports indicate narrowband ultraviolet B (NB-UVB) irradiation directly induces conversion of vitamin D to VD3 in various cell types including human keratinocytes [10, 11]; thus, we analyzed whether longterm whole body NB-UVB irradiation increased serum levels of VD3 in NF1 patients. We also observed the effects of irradiation on patient skin lesions by photographing treatment areas before and after irradiation procedures. Our results showed that 6 months of irradiation or more increased patient VD3 serum levels significantly. This increase was accompanied by changes in the patients' skin color including lightening of generalized hyperpigmentation.

It has been established that laser treatments have little effect on pigmented skin lesions, such as CALMs or small freckling, of patients with NF1. Instead, we investigated the effects of laser or intense pulsed-radio frequency (IPL-RF) irradiation in combination with topical application of VD3 analogs. In one young patient, treatment with Q-switched Ruby laser irradiation in combination with a topical VD3 analog treatment resulted in almost complete disappearance of a CALM located on the nose. A similar therapy combining IPL-RF irradiation with topical VD3 application was very effective at diminishing multiple small pigmented freckles on the face and body of another patient. Herein, our recent investigative results concerning the effects of VD3 and its analogs on NF1 are described in greater detail.
