Review Cutaneous Findings in Neurofibromatosis Type 1

Bengisu Ozarslan 1 , Teresa Russo 2, Giuseppe Argenziano 2 , Claudia Santoro 3 and Vincenzo Piccolo 2,*

1 Unit, Doku Medical Center, 34381 Istanbul, Turkey; [email protected] 2 Dermatology Unit, University of Campania Luigi Vanvitelli, 80100 Naples, Italy; [email protected] (T.R.); [email protected] (G.A.) 3 Department of Woman, Neurofibromatosis Referral Centre, Child and of General and Specialised Surgery, University of Campania Luigi Vanvitelli, 80100 Naples, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-08-1566-6834; Fax: +39-08-1546-8759

Simple Summary: Neurofibromatosis type 1 (NF1) is characterized by major and minor cutaneous findings, whose recognition plays a key role in the early diagnosis of the disease. The disease affects multiple systems and clinical manifestation has a wide range of variability. Symptoms and clinical signs may occur over the lifetime, and the complications are very diverse. Although significant progress has been made in understanding the pathophysiology of the disease, no specific treatment has been defined. Multidisciplinary approach is required to provide optimum care for the patients. The aim of this paper is to provide the clinician with a complete guide of skin findings of NF1.

Abstract: Neurofibromatosis type 1 (NF1) is a complex autosomal dominant disorder associated with germline in the NF1 tumor suppressor . NF1 belongs to a class of congenital anomaly syndromes called , a group of rare genetic conditions caused by mutations in the Ras/mitogen-activated protein pathway. Generally, NF1 patients present with dermatologic manifestations. In this review the main features of café-au-lait macules, freckling, neurofibromas, juvenile xanthogranuloma, anemicus and other cutaneous findings will be discussed.   Keywords: neurofibromatosis type 1; skin; cutaneous; café-au-lait macules; neurofibroma; freckling; Citation: Ozarslan, B.; Russo, T.; nevus anemicus; juvenile xanthogranuloma Argenziano, G.; Santoro, C.; Piccolo, V. Cutaneous Findings in Neurofibromatosis Type 1. Cancers 2021, 13, 463. https://doi.org/ 10.3390/cancers13030463 1. Background Neurofibromatosis type 1 (NF1) is a complex autosomal dominant disorder associated Received: 14 December 2020 with germline mutations in the NF1 tumor suppressor gene [1]. NF1 belongs to a class Accepted: 14 January 2021 of congenital anomaly syndromes called RASopathies, a group of rare genetic conditions Published: 26 January 2021 caused by mutations in the Ras/mitogen-activated protein kinase (RAS/MAPK) path- way [2]. Generally, NF1 patients present with dermatologic manifestations (Table1). Other Publisher’s Note: MDPI stays neutral abnormalities in the nervous system and skeletal system may also develop [3]. with regard to jurisdictional claims in published maps and institutional affil- Table 1. Summary of main features of cutaneous findings of Neurofibromatosis type 1 (NF1). iations. Light to dark brown-pigmented, oval or round skin patches with regular, Cafe-au-lait Macules well-demarcated margin Small, clustered pigmented lentigines Neurofibroma Dome—shaped, pink to brown, pedunculated or sessile cutaneous tumors Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Plexiform Thickened, slightly raised and pigmeted firm masses located along the nerve; “Bag Neurofibroma of worms” appearance; hypertrichosis This article is an open access article distributed under the terms and Nevus Anemicus Hypopigmented, pale, well-defined skin patch conditions of the Creative Commons Juvenile Yellowish-pink dome-shaped granulomatous papules and nodules Attribution (CC BY) license (https:// Xantogranuloma creativecommons.org/licenses/by/ Glomus Tumor Small reddish-blue papulonodule often located at the subungual area 4.0/). Melanoma Unevenly pigmented unusual looking macule

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2. Genetic NF1 is inherited in an autosomal dominant pattern, and a single copy of a mutated or deleted NF1 gene is needed in order to be effective. Half of the cases are familial, but the remaining half arise from de novo NF1 mutations. Descendants of an NF1 patient are at a 50% risk of inheriting the altered NF1 gene; however, the disease manifestations vary extremely even within the same family members. In addition, somatic mutations in NF1 during fetal development also cause phenotype diversity, known as mosaic NF1 [4]. NF1 gene encodes neurofibromin protein which interacts with the Ras-specific guano- sine triphosphatase (GTPase), a family of proteins involved in cellular , controlling cell proliferation. Overactive Ras-GTPase signals lead to tumor growth. Neurofi- bromin has the role of downregulating the RAS/MAPK signaling pathway and suppressing cell overgrowth. Therefore, the NF1 gene is considered to be a tumor suppressor gene and has been associated with benign and malignant tumors in many different areas from nerve cells to myeloid cells [5]. A diagnosis of NF1 is clinically established by the presence of two or more of the following diagnostic criteria as formulated by the National Institute of Health (NIH) in 1988: six or more cafe-au-lait macules (CALMs) (>5 mm in children or >15 mm adults), two or more cutaneous neurofibromas (CNFs) or one plexiform neurofibroma (PNF), axillary or inguinal freckling, (OPG), two or more Lisch nodules, distinctive bony (sphenoid wing dysplasia and typical bowing of long-bone with or without pseudo-arthrosis), and first degree relative with NF1 [6]. Although it is rarely needed after early childhood, genetic testing may be required primarily in patients who do not meet the diagnostic criteria. It is also important in situations where the rapid diagnosis may affect the course of the disease and the treatment (children with a serious tumor). Additionally, a severe NF1 variant with spinal cord neurofibromas has been identified in several studies. In the lack of other diagnostic features, genetic testing is indicated to diagnose these patients and at-risk family members. Finally, prenatal or preimplantation genetic testing may be necessary for the offspring of NF1 parents [4]. However, performing genetic analyses and genotypic–phenotypic correlations are not easy for several different reasons. The NF1 gene, which has been identified so far as having more than 3000 pathological allele variants, is a large gene covering 275 kb of genomic DNA and containing almost 60 constitutive exons located at 17q11.2 [7]. There may be missense mutations/in-frame deletions, truncating/splicing mutations and large deletions in the gene. The absence of defined hot spots makes it necessary to analyze every coding exon. Numerous NF1 pseudogenes with high sequence homologies and deep intronic splicing mutations further complicate molecular analyses of the NF1 gene [8,9]. There are different methods used for genetic analysis, of which the most accurate is the multistep pathogenic variant detection protocol based on cDNA (mRNA) and genomic DNA sequence analysis. In this method, diagnostic sensitivity has been reported to be more than 95% [10]. The clinical presentation of NF1 may be multifactorial. While biallelic inactivation is necessary for the development of some clinical findings such as CALMs and neurofibromas, some manifest in haploinsufficiency of NF1. Additional genetic alterations also affect the disease course as in the malignant transformation of peripheral nerve sheath. Even though clear genotype–phenotype correlations, which are hard to establish in NF1, there are some important relations identified. One study investigating mutation and phenotype trends found that NF1 was more prevalent in patients with truncating/splicing mutations and large deletions than in those with missense mutations. They also reported more items of NF1 in patients with truncating/splicing mutations [9]. Another study showed an association between 1.4 Mb deletions that encompass the entire NF1 gene with nonfamilial facial dysmorphism, a lower IQ and possibly increased incidence of malignant peripheral nerve sheath tumors (MPNSTs). Codeletion of SUZ12, especially, within the common NF1-microdeletion has been identified to be a significant alteration for the malignant trans- formation [11]. Noonan-like features (pulmonic stenosis and short stature) and multiple CALMs, but no external PNF or symptomatic OPG have been linked to NF1 missense Cancers 2021, 13, 463 3 of 17

variations affecting p.Arg1809 [12]. A small mutation in exon 17 of the NF1 gene identified as a single amino acid NF1 deletion (c.2970–2972 delAAT) has been related to mild form of NF1 disease without visible CNFs or PNFs [13]. Koczkowska et al. reported a severe form of NF1 presenting as major PNF and symptomatic spinal neurofibromas, OPGs, other malignant transformations and skeletal abnormalities due to missense mutations affecting NF1 codons 844–848 [14]. Further investigations are needed to clarify the genetic background of NF1.

3. Mosaicism Mosaic NF1 is caused by postzygotic somatic mutation in the NF1 gene. It occurs usually with no antecedent family history. NF1 in mosaic forms can be present as gen- eralized, segmental, or gonadal [15]. Early somatic mutations that occur before tissue differentiation manifest as a generalized phenotype. The generalized mosaic pattern ap- pears clinically indistinguishable from classical nonmosaic NF1 but the symptoms tend to be milder. Blood leucocyte screening usually does not reveal the specific NF1 mutation. Late somatic mutations cause a limited distribution, classified as segmental NF1. Segmen- tal NF1 has been divided into four groups as follows: pigmentary alterations (CALMs and freckling) alone, CNFs alone, pigmentary alterations plus neurofibromas and isolated PNFs. Skin manifestation is limited to the affected area, which may cover either a narrow strip or one quadrant sometimes covers half of the body. Classically, it is unilateral, but it can be bilateral as well. CALMs may be distributed in a block-like or blaschkoid pattern whereas neurofibromas usually follow the dermatomes. A darker background may cover the affected region [16]. Maertens et al. showed biallelic inactivation of the NF1 gene in either the Schwann cells of neurofibromas or in the of CALMs in individuals with different types of segmental disease [17]. While it is known that the gonadal type can Cancers 2021, 13, x FOR PEER REVIEW 4 of 18 affect the offspring, segmental NF1 cases may be transmitted to future generations with some unknown mechanisms [15] (Figure1).

FigureFigure 1.1.Segmental Segmental arrangementarrangement ofof CALMsCALMs and and freckling freckling in in a a patient patient with with a a mosaic mosaic form form of of NF. NF.

4.4. Cafe-Au-LaitCafe-Au-Lait MaculesMacules AA CALMCALM isis a pigmented, flat flat skin skin patch patch which which is is the the most most common common initial initial manifes- mani- festationtation of ofNF-1. NF-1. CALM CALM is seen is seen in 99% in 99% of NF1 of NF1 patients. patients. The The classical classical CALM CALM lesion lesion in NF1 in NF1has a has round a round or oval or ovalshape shape with witha uniform a uniform color color of light of lightto dark to darkbrown brown and regular and regular well- well-demarcateddemarcated margins margins (“coast (“coast of California”) of California”) [18]. [ 18CALMs]. CALMs may may be observed be observed at birth, at birth, be- becomecome almost almost always always obvious obvious by by the the first first year year of of life, and may increaseincrease inin diameterdiameter andand number.number. NF1NF1 patientspatients havehave higherhigher numbersnumbers ofofCALMs CALMswidely widelydistributed distributedon on any any part part of of thethe bodybody exceptexcept forfor palms palms and and soles. soles. CALMsCALMs rarelyrarely occuroccur on on the the face. face. AlthoughAlthough oneone to to threethree macules macules may may be be present present in in up up to to 15% 15% of of the the normal normal population, population, six six or or more more of of these these lesionslesions greatergreater thanthan 0.5 cm cm in in prepubertal prepubertal children children or or greater greater than than 1.5 1.5 cm cm after after puberty puberty are arediagnostics diagnostics for forNF1 NF1 [6]. [6However,]. However, it is it also is also recommended recommended to evaluate to evaluate and and follow follow the the pa- patients with more than three CALMs, as it is less frequent to see more than three lesions tients with more than three CALMs, as it is less frequent to see more than three lesions in the normal population [19]. There is no relation between NF1 severity and number of le- sions [20]. Macules may get tanned under UV exposure and Wood′s lamp may be helpful to detect CALMs invisible to the naked eye [21]. Underlying PNF may accompany CALMs, especially those with hypertrichosis. Therefore, palpation is recommended [22]. (Figures 2–5)

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in the normal population [19]. There is no relation between NF1 severity and number of lesions [20]. Macules may get tanned under UV exposure and Wood0s lamp may be helpful to detect CALMs invisible to the naked eye [21]. Underlying PNF may accompany Cancers 2021, 13, x FOR PEER REVIEWCALMs, especially those with hypertrichosis. Therefore, palpation is recommended5 [ 22of 18] Cancers 2021, 13, x FOR PEER REVIEW 5 of 18 (Figures2–5).

FigureFigure 2. 2.Multiple Multiple typicaltypical cafe-au-laitcafe-au-lait macules macules (CALMs) (CALMs) on on the the trunk trunk of of a a young young man. man. Figure 2. Multiple typical cafe-au-lait macules (CALMs) on the trunk of a young man.

Figure 3. For this child, CALMs were detectable on lower limbs. Note that the size is larger than 5 FiguremmFigure and 3.3. theForFor typical thisthis child,child, well-demarcated CALMs were borders.detectable on on lower lower limbs. limbs. Note Note that that the the size size is islarger larger than than 5 5mm mm and and the the typical typical well-demarcated well-demarcated borders. borders.

Another type of CALM which has irregular jagged margins (“coast of Maine”) is mostly solitary and larger; arrangement in a blaschkoid pattern is suggestive of sporadic cases or other genetic disorders. Additionally, it is extraordinary for new sporadic CALMs to occur after the age of 6 [18].

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FigureFigure 4. 4.This This CALM CALM is is not not associated associated with with NF1. NF1. It It shows shows irregular irregular borders borders and and it it is is isolated. isolated. Figure 4. This CALM is not associated with NF1. It shows irregular borders and it is isolated.

Figure 5. Small CALMs of the left thigh of a young child associated with early flexural freckling. FigureFigure 5.5. SmallSmall CALMsCALMs ofof thethe leftleft thighthigh ofof aa youngyoung childchild associatedassociated withwith earlyearly flexuralflexural freckling.freckling. Another type of CALM which has irregular jagged margins (“coast of Maine”) is mostlyHistologicalAnother solitary type and findings of larger; CALM are arrangement basalwhich has inirregula a blaschkoidr jagged and pattern sometimesmargins is suggestive(“coast giant melanosomesof ofMaine”) sporadic is (macromelanasomes),mostlycases or solitary other genetic and larger; disorders. although arrangement diagnosisAdditionally in is a clinicalblaschkoid, it is extraordinary and pattern pathology isfor suggestive rarelynew sporadic necessary. of sporadic CALMs Re- cently,casesto occur or mutation other after geneticthe of age both disorders.of 6 alleles [18]. Additionally of the NF1 gene, it is hasextraordinary been detected for new in thesporadic melanocytes CALMs ofto NF1-CALMoccurHistological after the while agefindings of normal 6 [18]. areskin basal has hyperpigmentation a and [23 sometimes]. CALMs giant in NF1 melano- tend tosomes showHistological (macromelanasomes), greater findings are density although basal andhyperpigmentation diagnosis greater stem-cellis clinical and factor and sometimes pathology secretion giant (KITrarely melano- ligand) neces- ofsomessary. fibroblasts Recently, (macromelanasomes), while mutation non-NF1 of both CALMalthough alleles lesions diagnosisof th showe NF1 is thegene clinical same has melanocyteandbeen pathology detected level in rarely the compared melano- neces- tosary.cytes the Recently, of normal NF1-CALM skin. mutation A while melanocyte of normalboth alleles activationskin ofhas th ae germline responseNF1 gene mutation tohas hepatocyte been [23]. detected CALMs growth in the in factor NF1 melano- andtend KIT ligand has been hypothesized as the mechanism of melanocyte growth and mast cell cytesto show of NF1-CALM greater melanocyte while normal density skin and has greate a germliner stem-cell mutation factor [23]. secretion CALMs (KIT in NF1ligand) tend of infiltration [24,25]. tofibroblasts show greater while melanocyte non-NF1 CALM density lesions and greate showr stem-cellthe same factor melanocyte secretion level (KIT compared ligand) ofto fibroblaststhe normal while skin. non-NF1A melanocyte CALM activation lesions showresponse the tosame hepatocyte melanocyte growth level factor compared and KIT to the normal skin. A melanocyte activation response to hepatocyte growth factor and KIT

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Cancers 2021, 13, 463 6 of 17 ligand has been hypothesized as the mechanism of melanocyte growth and mast cell in- ligandfiltration has [24,25]. been hypothesized as the mechanism of melanocyte growth and mast cell in- filtration [24,25]. 5.5. FrecklingFreckling 5. Freckling FrecklingFreckling inin skinfold is one one of of the the seven seven cardinal cardinal diagnostic diagnostic features features of ofNF1. NF1. They They are aredescribed describedFreckling as small as in small skinfold (1–4 (1–4 mm), is mm), one clustered of clustered the seven pigmente pigmented cardinald macules diagnostic macules located located features mostly mostly of inNF1. axillary in They axillary and are anddescribedinguinal inguinal regions as small regions (Crowe (1–4 (Crowe mm), sign) clustered sign) [26].[ 26It ].ispigmente Itconsidered is consideredd macules pathognomonic pathognomonic located mostly and foundin and axillary found in more and in moreinguinalthan 90% than regions of 90% NF1 of patients(Crowe NF1 patients sign)by the [26]. byage the Itof is7. age consideredTh ofe development 7. The pathognomonic development of freckles of and may freckles found not be may in seen more not as bethanearly seen 90% CALMs. as of early NF1 They CALMs. patients often Theybyoccur the often byage the of occur age7. Th of bye threedevelopment the age to five of three following of freckles to five CALMs followingmay not and be CALMs precede seen as andearlythe precedeappearance CALMs. the They of appearance often occur of neurofibromasby [27].the age Typi ofcally, three [27 ]. theseto Typically, five lesions following these resembling CALMs lesions sun-inducedand resembling precede sun-inducedthefreckles appearance appear freckles offirst neurofibromas appearin the inguinal first in [27]. the region inguinalTypi [28].cally, region The these presence [ 28lesions]. The ofresembling presence only one of orsun-induced only two one small or twofrecklesfreckles—or small appear freckles—or freckling first in frecklingof the other inguinal intertriginous of other region intertriginous [28]. areas The such presence areas as suchthe of neckline, asonly the one neckline, upperor two uppereyelid, small eyelid, around the lips, under breasts in women, even a more diffuse pattern over the body freckles—oraround the lips, freckling under ofbreasts other in intertriginous women, even areas a more such diffuse as the pattern neckline, over theupper body eyelid, may may arise suspicion—are not sufficient for diagnosis [15] (Figures6 and7). aroundarise suspicion—are the lips, under not breasts sufficient in women, for diagnosis even a [15]. more (Figures diffuse 6pattern and 7) over the body may arise suspicion—are not sufficient for diagnosis [15]. (Figures 6 and 7)

FigureFigure 6.6.Crowe Crowe signsign withwith extensiveextensive frecklingfreckling of of axillae axillae in in a a patient patient affected affected by by NF1. NF1. Figure 6. Crowe sign with extensive freckling of axillae in a patient affected by NF1.

Figure 7. Precocious freckling of inguinal area of a young patient. FigureFigure 7.7. PrecociousPrecocious frecklingfreckling ofof inguinalinguinal areaarea ofof aa youngyoungpatient. patient.

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Cancers 2021, 13, 463 7 of 17 6. Lisch Nodules Lisch nodules are benign iris which are seen as densely pigmented flat 6.or Lisch slightly Nodules elevated lesions during the slit-lamp examination. These lesions are the most commonLisch NF1 nodules feature are benignin adults. iris They hamartomas appear whichearlier arethan seen neurofibromas as densely pigmented and multiple flat orLisch slightly nodules elevated are specific lesions to during NF1. They the slit-lamp do not have examination. any ophthalmologic These lesions complication, are the most no commonassociation NF1 has feature been reported in adults. with They clinical appear severity, earlier and than therefore neurofibromas no specific and treatment multiple is Lischneeded. nodules However, are specific periodical to NF1. examination They do notto detect have these any ophthalmologic benign hamartomas complication, may be use- no associationful to confirm has the been diagnosis reported of withyoung clinical NF1 patients severity, [29]. and therefore no specific treatment is needed. However, periodical examination to detect these benign hamartomas may be useful7. Neurofibromas to confirm the diagnosis of young NF1 patients [29]. is associated with many kinds of tumors but the neurofibromas 7.are Neurofibromas the most prevalent benign tumors found in the affected individual, hence the appella- tion Neurofibromatosisneurofibromatosis. is They associated also have with a manynegative kinds impact of tumors on the but life the of neurofibromasNF1 patients. Dif- are theferent most variants prevalent have benign been described tumors found related in the to NF1; affected CNFs individual, or dermal hence neurofibromas, the appellation sub- neurofibromatosis.cutaneous neurofibromas They also and have PNFs a negative [21,30]. impact While onloss the of life the of neurofibromin NF1 patients. Different allele in variantsSchwann have cells been is known described to be related the initial to NF1; trigger, CNFs their or dermal stem cell neurofibromas, factor secretion subcutaneous drive NF1- neurofibromasgene-deficient andmast PNFs cells [into21,30 the]. While neurofibro loss ofma the lesion. neurofibromin Migration allele of mast in Schwann cells leads cells to istumor known formation to be the as initial they trigger,express theirtransforming stem cell growth factor secretion factor-β (TGF driveβ NF1-gene-deficient). TGFβ boosts Nf1- mastdeficient cells Schwann into the neurofibroma cell growth and lesion. collagen Migration production. of mast Gen cellserally, leads proliferation to tumor formation of spin- asdle they cells, express Schwann transforming cells, mast growth cells factor-and vascularβ (TGFβ componen). TGFβ booststs leads Nf1-deficient to manifestation Schwann of cellthese growth tumors and [3,31,32]. collagen production. Generally, proliferation of spindle cells, Schwann cells,Thousands mast cells and of vasculardiscrete CNFs, components fleshy, leads pink to to manifestation brown pedunculated of these tumors or sessile [3,31 pap-,32]. ulonodules,Thousands depict of discretethe most CNFs, common fleshy, dermatologic pink to brownal finding pedunculated in NF1. They or sessile may papulon-be soft or odules,firm in depicttexture the ranging most commonfrom few dermatological millimeters to findingfew centimeters in NF1. They located may anywhere be soft or along firm inthe texture course ranging of peripheral from fewnerves. millimeters “Buttonhol to fewe sign” centimeters describes locatedthe pathogonomic anywhere alonginvagina- the coursetion of of the peripheral lesion with nerves. pressure. “Buttonhole Subcutaneous sign” describes neurofibromas the pathogonomic arise as nodules invagination in deep of thedermis lesion or with the pressure.subcutaneous Subcutaneous layer, usually neurofibromas presenting arise a less as nodulesobvious in appearance deep dermis than or theCNFs. subcutaneous Both cutaneous layer, and usually subcutaneous presenting neurofibromas a less obvious appearance may initially than appear CNFs. as Both slightly cu- taneousraised discolored and subcutaneous skin patches neurofibromas and most may typi initiallycally they appear become as slightly visible raised in adolescence. discolored skinThey patches may grow and mostand increase typically in they number become with visible age; acceleration in adolescence. may They occur may during grow preg- and increasenancy [15]. in number (Figures with 8–11) age; acceleration may occur during pregnancy [15] (Figures8–11).

FigureFigure 8.8. DeepDeep neurofibromaneurofibroma softsoft ofof chin.chin.

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FigureFigure 9. 9.Isolated Isolated neurofibroma of of supraclavicular supraclavicular region. region. Figure 9. Isolated neurofibroma of supraclavicular region.

Figure 10. Diffuse neurofibromas on the back. FigureFigure 10.10. DiffuseDiffuse neurofibromasneurofibromas onon thethe back.back.

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FigureFigure 11. Large 11. plexiform Large plexiform neurofibroma neurofibroma of the left shoulder. of the left shoulder.

BlueBlue redred macules and and pseudoatrophic pseudoatrophic macules macules are are bizarre bizarre variants variants of cutaneous of cutaneous neu- neurofibroma.rofibroma. If there If there is the is predominance the predominance of wide of widevessels vessels in neurofibromatous in neurofibromatous tissue located tissue locatedin papillary in papillary dermis, dermis, the lesion the may lesion appear may appearas blue-red as blue-red macules. macules. Pseudoatrophic Pseudoatrophic macules maculesoccur as occura result as aof result neurofibromatous of neurofibromatous tissue proliferation tissue proliferation damaging damaging collagen collagen in reticular in reticulardermis. These dermis. two These presentations two presentations are less frequent are less types frequent of neurofibromas types of neurofibromas and they fulfill and theythe diagnostic fulfill the diagnostic criterion of criterion NF1 [33]. of NF1 [33]. PhysicalPhysical removalremoval appearsappears toto bebe the the most most effective effective and and practical practical treatment treatment for for CNFs CNFs asas there there is is no no gold gold standard standard approach. approach. Surgical Surgical removal, removal, laser laser ablation, ablation, modified modified biopsy biopsy removal,removal, photocoagulation, photocoagulation, electrodessication electrodessication and and radiofrequency radiofrequency ablation ablation are theare modali-the mo- tiesdalities that havethat have been been developed. developed. Camouflage Camoufla andge moisturizers and moisturizers may also may provide also provide some reliefsome forrelief patients for patients [34]. In [34]. a previous In a previous study, topicalstudy, imiquimodtopical imiquimod has shown has someshown volume some volume reduc- tion.reduction. Since vascularSince vascular endothelial endothelial growth growth factor (VEGF) factor is(VEGF) known is to known be highly to be expressed highly ex- in neurofibromas,pressed in neurofibromas, Ranizumab, Ranizumab, a VEGF antibody, a VEGF was antibody, injected was into injected a tumor into and a a tumor decrease and in a volumedecrease and in pressurevolume and was pressure noticed [was35]. noticed [35]. PNFsPNFs areare specificspecific forfor NF1NF1 andand theythey affectaffect betweenbetween 30%30% andand 50%50% ofof patientspatients [[36,37].36,37]. PNFsPNFs arisearise from from the the peripheral peripheral nerve nerve sheath. sheath. They They are are often often located located along along the the nerve nerve and and maymay be be superficial superficial or or may may develop develop into into a a complex complex deep deep mass mass with with the the network network of of multiple multiple nerves.nerves. PNFs PNFs may may appear appear as as thickened, thickened, slightly slightly raised raised skin skin areas areas or firmor firm masses masses or nodules or nod- in the subcutaneous tissue. Lesions may also be present with a particular wavy “bag of ules in the subcutaneous tissue. Lesions may also be present with a particular wavy “bag worms” texture [38]. Deep lesions may infiltrate structures such as fascia, muscle or more of worms” texture [38]. Deep lesions may infiltrate structures such as fascia, muscle or internal components of the body causing disfigurement and dysfunction [36]. Unlike CNFs, more internal components of the body causing disfigurement and dysfunction [36]. Un- PNFs are congenital and superficial lesions and may be clinically visible at birth, resembling like CNFs, PNFs are congenital and superficial lesions and may be clinically visible at CALMs. Those with hyperpigmentation and hypertrichosis should be distinguished from birth, resembling CALMs. Those with hyperpigmentation and hypertrichosis should be congenital melanocytic nevus and smooth muscle Becker nevus [22]. Magnetic distinguished from congenital melanocytic nevus and smooth muscle hamartoma Becker resonance imaging (MRI) may be needed to detect deep, hidden lesions. They may expand nevus [22]. Magnetic resonance imaging (MRI) may be needed to detect deep, hidden le- during childhood and adolescence but after they grow at a fairly low rate [39]. sions. They may expand during childhood and adolescence but after they grow at a fairly PNFs are associated with increased morbidity and mortality in NF1. Distortion of low rate [39]. limbs or head, soft tissue and bone deformities, degeneration of airway or spinal cord and PNFs are associated with increased morbidity and mortality in NF1. Distortion of neurological deficits may occur due to invasion [40]. Spinal neurofibromas, particularly C2limbs nerve or head, root neurofibromas, soft tissue and bone tend todeformities, occur bilaterally, degeneration with high of airway incidence or spinal of intradural cord and invasionneurological and myelopathydeficits may inoccur NF1 due patients. to invasi Theon higher [40]. Spinal frequency neurofibromas, of C2-located particularly neurofi- bromasC2 nerve in root patients neurofibromas, with NF1 was tend attributed to occur tobilaterally, the risk ofwith trauma. high incidence There are of hypotheses intradural regardinginvasion and increased myelopathy post-traumatic in NF1 patients. fibroblast The activity higher or frequency “second hit” of C2-located mutations neurofibro- in the NF1 genemas[ 41in ].patients As we mentionedwith NF1 was before, attributed there is to also the a risk spinal of NF1trauma. subtype There which are hypotheses is associated re- withgarding large increased deletions post-traumatic in the NF1 gene . Spinal NF1activity is characterized or “second hit” by mutations multiple bilaterally in the NF1 gene [41]. As we mentioned before, there is also a spinal NF1 subtype which is associated

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located spinal and paraspinal neurofibromas with very few dermal manifestations of clas- sical NF1. This form should be differentiated due to life-threatening massive neoplastic burden [42]. Although the treatment of large tumors causing progressive neurological deficit includes surgery, the nonsurgical management remains a clinical challenge [43]. The RAF/MEK/ERK pathway is an important regulator of biology and several biologically targeted therapies (such as mTOR inhibitors, and selective MEK inhibitors) have been evaluated in clinical trials. , an oral selective inhibitor, partially contributed to reduce PNF tumor size [44].

8. Malignant Peripheral Nerve Sheath Tumors MPNSTs are aggressive which may develop in 3–15% of patients with NF1 [45]. They mostly transform from plexiform or nodular neurofibromas predominantly after puberty. Especially in NF1 affected young adults, the rapid expansion of the tumor with pain or the presence of new neurological symptoms should alert the clinician for MPNST [46]. Coffien et al. reported NF1 patients with MPNST occurring as a second malignant neoplasm especially after alkylating agent chemotherapy and irradiation [47]. Positron emission tomography imaging is shown to be a more useful diagnostic tool to reveal these malignant degenerations [40]. Although the inactivation of the loss of biallelicity of the NF1 gene in Schwann cells seems to be an important initiating factor in the development of MPNST, genomic studies show that the malignant degeneration arises as a result of some additional alterations. Somatic loss-of-function mutations or deletions of the Polycomb repressive complex 2 (PRC2) its core subunits (EED or SUZ12) have been described in NF-1-associated MPNSTs. PRC2 plays a role in chromatin compaction, cell proliferation, differentiation and stem-cell plasticity. A cooperative role of TP53, epidermal growth factor receptor (EGFR) inactivation has also been proposed to contribute MNPST transformation. CDKN2A loss has also been linked to early steps of MPNST [48]. While high-grade MPNST often metastasizes to bone and lung, low-grade tumors have a better prognosis. The main therapy remains a complete surgical excision with nega- tive margins [49]. Discussions on the benefits of adjuvant/neoadjuvant chemotherapies continue. Epirubicin-based adjuvant/neoadjuvant chemotherapy appears to show some survival benefit [50]. Although the development of radiotherapy-induced neoplasms is known to be increased in NF1 patients, radiotherapy may be useful for downsizing the tumors. On the other hand, research shows that radiotherapy benefits local control of the tumor without contributing to survival [51]. Metastatic disease remains challenging due to a lack of promising therapies. Chemother- apy is known to be the standard approach. Biological therapies combining MEK/mTOR inhibition are still under investigation in preclinical and clinical trials. Pexidartinib (tyro- sine kinase inhibitor) and sirolimus (mTOR inhibitor) combination contributed to stabilize the disease in a phase I study [52]. Additionally, immunotherapy has also opened up a method for the treatment of MPNST [53]. In terms of the future direction of the treatments, gene-based therapies targeting PRC2, the Janus Kinase (JAK) pathway and HIPPO pathway are presented as a new hope. EZH2 inhibitors—such as tazemetostat for the PRC2 complex, Tyrosine Kinase 2 (TYK2) inhibitors targeting the JAK/STAT pathway and verteporfin inhibiting the HIPPO pathway—could be considered in the combination treatments [54].

9. Optic Pathway Glioma NF1 is one of the most common heritable predisposition disorders, in which affected individuals develop low-grade classified as WHO grade I gliomas (pilocytic ). They are generally indolent neoplasms usually occurring be- fore the age of 8 located within the optic pathway [55]. OPGs occur in 15–20% of NF1 patients [56]. They are often asymptomatic until the age of 7. The tumor may arise from anywhere along the optic pathway such as nerves, chiasm, postchiasmatic tracts, and Cancers 2021, 13, x FOR PEER REVIEW 12 of 18

Cancers 2021, 13, 463 11 of 17 [56]. They are often asymptomatic until the age of 7. The tumor may arise from anywhere along the optic pathway such as nerves, chiasm, postchiasmatic tracts, and radiations. Tu- radiations.mor location Tumor strongly location affects strongly the affectssympto them symptomspectrum. spectrum.While 30–50% While of 30–50% OPGs of become OPGs becomesymptomatic, symptomatic, patients patients may present may presentwith unilat witheral unilateral proptosis, proptosis, visual visualacuity acuityloss, visual loss, visualfield defect, field defect, strabismus, strabismus, relative relative afferent afferent pupillary pupillary defect, defect, and optic and opticpapilloedema papilloedema or optic or opticnerve nerve atrophy. atrophy. Chiasmal Chiasmal optic optic pathway pathway glio gliomasmas can can present present with with precocious pubertypuberty withoutwithout anyany visualvisual symptoms.symptoms. Rarely,Rarely, headacheheadache andand vomitingvomiting maymay bebe signssigns ofof tumorstumors involvinginvolving thethe hypothalamus. hypothalamus. Children Children usually usually do do not not complain complain of visual of visual defects defects and NF1- and NF1-OPGsOPGs can be can unpredictable—as be unpredictable—as a result, a result, a routine a routine eye examination eye examination is required is required [57]. [ 57]. AlthoughAlthough prognosticprognostic featuresfeatures areare notnot clear,clear, clinicalclinical experiencesexperiences showshow thatthat girlsgirls expe-expe- riencerience visualvisual impairmentimpairment moremore thanthan boys.boys. Additionally,Additionally, postchiasmaticpostchiasmatic opticoptic pathway-pathway- locatedlocated OPGs OPGs and and tumors tumors that that occur occur before before the the age age of 2 appearof 2 appear to be to more be aggressive.more aggressive. After diagnosis,After diagnosis, performing performing an eye an examination eye examination and visual and assessmentvisual assessment with MRI with is requestedMRI is re- everyquested 3 months every 3 in months the first in year the first for follow year for up. follow up. ChemotherapyChemotherapy remainsremains aa mainstaymainstay treatment.treatment. SurgerySurgery seemsseems toto bebeproblematic problematicdue due toto thethe location of of these these tumors tumors and and radiatio radiationn is isknown known to tobe bea trigger a trigger for forsecondary secondary ma- malignancieslignancies along along with with vasculopathy. vasculopathy. mTOR mTOR andand MEK MEK inhibitors inhibitors for the for treatment the treatment of optic of opticpathway pathway gliomas gliomas are still are in still progress in progress [58]. [58].

10.10. NevusNevus AnemicusAnemicus NevusNevus anemicusanemicus (NA)(NA) isis aa congenital,congenital, cutaneouscutaneous anomalyanomaly characterizedcharacterized byby hypopig-hypopig- mented,mented, well-definedwell-defined patchespatches withwith limitedlimited vascularvascular supply.supply. While While rubbing rubbing or or heat heat causes causes rednessredness ofof normalnormal skin,skin, thethe NANA lesionlesion remainsremains pale. pale. TheThe vascularvascular structurestructure of of the the lesion lesion is is foundfound toto bebe normalnormal butbut thethe excessiveexcessive responseresponse toto catecholaminescatecholamines causescauses vasoconstriction.vasoconstriction. NANA isis alsoalso seenseen inin thethe normalnormal population,population, butbut prospectiveprospective studiesstudies suggestsuggestthat that prevalenceprevalence ofof NANA inin NF1NF1 patientspatients isis muchmuch higher—aroundhigher—around 50%,50%, affectingaffecting bothboth sexessexes equallyequally [[59–61].59–61]. TheThe lesionslesions areare usually usually multiple multiple and and mostly mostly located located on on the the trunk trunk in in varying varying sizes. sizes. The The age age ofof onsetonset is is unclear unclear as as it it may may be be difficult difficult to detectto detect the the lesion lesion clinically. clinically. The The link betweenlink between NA andNA NFand is NF not is well-established. not well-established. NF is NF associated is associated with with vascular vascular complications complications and itand has it beenhas been postulated postulated that therethat there might might be a somaticbe a somatic or second-hit or second-hit mutation mutation in NA in lesions NA lesions [62] (Figure[62]. (Figure 12). 12)

FigureFigure 12.12.Nevus Nevus anemicusanemicus ofof thethe backback inin aa femalefemale withwith NF1.NF1.

11. Juvenile Xanthogranuloma Juvenile xanthogranuloma (JXG) is the most common form of non-Langerhans-cell histiocytosis characterized by yellowish-pink dome-shaped granulomatous tumors often located on the upper side of the body. It may be solitary or multiple and may start as a pink

Cancers 2021, 13, x FOR PEER REVIEW 13 of 18

11. Juvenile Xanthogranuloma Cancers 2021, 13, 463 12 of 17 Juvenile xanthogranuloma (JXG) is the most common form of non-Langerhans-cell histiocytosis characterized by yellowish-pink dome-shaped granulomatous tumors often located on the upper side of the body. It may be solitary or multiple and may start as a macule.pink macule. The estimated The estimated prevalence prevalence is around is 15–35%around in15–35% NF1 children. in NF1 JXGchildren. may be JXG present may asbe earlypresent as CALM,as early therefore as CALM, it wastherefore suggested it was as su aggested potential as sign a potential for the diagnosis sign for the of NF1.diagnosis The lesionof NF1. may The disappear lesion may spontaneously disappear spontaneou [61,63]. Juvenilesly [61,63]. myelomonocytic Juvenile myelomonocytic leukemia (JMML) leuke- ismia a rare, (JMML) fatal disorderis a rare, of fatal childhood disorder [64 of]. Itchildhood has been suggested[64]. It has that been NF1 suggested patients withthat JXGNF1 havepatients a considerably with JXG have higher a cons riskiderably of developing higher JMML risk of than developing those with JMML only NF1than[ 65those]. There with isonly no clearNF1 consensus[65]. There on is theno clear increased consensus risk of on JMML the increased in the presence risk of of JMML JXG in in NF1 the presence patients sinceof JXG other in NF1 retrospective patients since case other series retrospectiv did not reveale case suchseries associationdid not reveal [66 ,such67]. association Although routine[66,67]. screeningAlthough forroutine JMML screening in NF1 patientsfor JMML is in not NF1 obligatory, patients theis not physicians obligatory, should the phy- be carefulsicians aboutshould the be other careful signs about of JMMLthe other (Figure signs 13 of). JMML. (Figure 13)

FigureFigure 13.13.Yellowish Yellowish papulepapule ofof thethe rightright sideside ofof thethe face,face, compatiblecompatible with the diagnosis of juvenile xanthogranuloma.xanthogranuloma.

12.12. GlomusGlomus TumorTumor GlomusGlomus tumortumor isis aa benignbenign neoplasmneoplasm thatthat arises arises from from the the neuromyoarterial neuromyoarterial glomus glomus bodybody oror thethe Suquet–HoyerSuquet–Hoyer canal.canal. Lately,Lately, the the discovery discovery of of biallelic biallelic inactivation inactivation of of the theNF1 NF1 genegeneor ora a “second “second hit” hit” that that occurs occurs specifically specifically within within the theα α-smooth-smooth muscle muscle actin-positive actin-positive cellscells of of thethe glomusglomus body body has has been been added added to to an anNF1-related NF1-related tumor tumor spectrum spectrum [ 68[68].]. It It is is often often locatedlocated atat thethe subungual subungual area, area, clinically clinically presented presented as as a a small small reddish-blue reddish-blue papulonodule papulonodule thatthat is is particularly particularly painful painful after after pressure pressure or or temperature temperature change. change. Regarding Regarding the the location, location, it canit can arise arise from from almost almost everywhere everywhere in in the the body body including including visceral visceral organs. organs. NF1-associated NF1-associated glomusglomus tumors tumors also also appear appear to to occur occur mostly mostly in in the the subungual subungual area area and and in youngin young women, women, as inas sporadic in sporadic cases cases [69]. [69]. There There are are several several case ca reportsse reports highlighting highlighting thelink the link between between multiple mul- lesionstiple lesions and NF1 and [ 70NF1,71 [70,71].]. 13. Melanoma 13. Melanoma NF1 patients have about four-fold higher risk of neoplasms [72] through dysregula- NF1 patients have about four-fold higher risk of neoplasms [72] through dysregula- tion of the RAS/MAPK pathway [73]. Although one of the main drivers in melanoma tion of the RAS/MAPK pathway [73]. Although one of the main drivers in melanoma de- development is also NF1 mutation, the incidence of melanoma in NF1 patients is slightly velopment is also NF1 mutation, the incidence of melanoma in NF1 patients is slightly elevated [74]. One review concluded that NF1 patients with melanoma were mostly elevated [74]. One review concluded that NF1 patients with melanoma were mostly younger-aged women and the tumor thickness was greater [75]. Although NF1 mutations younger-aged women and the tumor thickness was greater [75]. Although NF1 mutations have been associated with desmoplastic melanoma in the general population [76], only have been associated with desmoplastic melanoma in the general population [76], only one case has been reported as a desmoplastic melanoma but interestingly sharing some one case has been reported as a desmoplastic melanoma but interestingly sharing some morphologic characteristics with MPNSTs [77] (Figure 14). morphologic characteristics with MPNSTs [77]. (Figure 14)

Cancers 2021, 13, 463 13 of 17 Cancers 2021, 13, x FOR PEER REVIEW 14 of 18

FigureFigure 14.14. SmallSmall congenitalcongenital nevusnevus associatedassociated withwith multiplemultiple CALMsCALMs inin aa girl girl with with NF1. NF1.

14.14. OtherOther NeoplasmsNeoplasms NF1NF1 alterationalteration comes comes along along with with an an increased increased risk risk of developing of developing malignant malignant conditions condi- suchtions as such MNPST, as MNPST, leukemia, leukemia, rhabdomyosarcoma and adrenal and gland adrenal tumors. gland Pheochromocy- tumors. Pheo- tomachromocytoma is a rare benign is a rare adrenal benign medulla adrenal tumor medulla that istumor capable that of is produce capable excessive of produce amounts exces- ofsive catecholamine amounts of resultingcatecholamine in tachycardia, resulting , in tachycardia, sweating, hypertension, and flushing. sweating, The over- and allflushing. incidence The is overall low—0.1–5.7% incidence of is patients low—0.1–5.7% with NF of develop patients —mostly with NF develop pheochro- betweenmocytoma—mostly the third and between fourth decade the third of life. and However, fourth decade the specific of life. alteration However, responsible the specific for thealteration association responsible is still unknown. for the association It is usually is still solitary unknown. and benign It is usually but long-term solitary and follow-up benign isbut recommended. long-term follow-up Surgical is removal recommended. is the standard Surgical approach removal foris the the standard treatment approach [78]. for the treatmentRhabdomyosarcoma [78]. (RMS) is the most common soft-tissue in children. Spo- radicRhabdomyosarcoma RMS is more common, (RMS) but RASopathies is the most common and disorders soft-tissue with sarcoma TP53 alterations in children. such Spo- as NF1radic have RMS also is more been associatedcommon, but with RASopathies RMS. Around and 1% disorders of patients with with TP53 NF1 alterations develop RMS, such specificallyas NF1 have embryonal-type also been associated due to RASwith activation RMS. Around in this 1% form. of RMSpatients in NF1 with cases NF1 seems develop to haveRMS, early specifically onset favoring embryonal-type the pelvic due location. to RAS Careful activation clinical in this examination form. RMS and in NF1 attention cases toseems any signto have pointing early outonset possible favoring RMS the are pelvic important location. for Careful early diagnosis. clinical examination NF1-associated and RMSattention requires to any the sign same pointing treatment out protocol possible as RMS the sporadic are important cases [for79]. early diagnosis. NF1- associatedRecently, RMS several requires reports the same have treatment depicted protocol the relationship as the sporadic between cases gastrointestinal [79]. stromalRecently, tumors several (GISTs) reports and NF1. have NF1-associated-GISTs depicted the relationship tend to occurbetween at a youngergastrointestinal age in distal locations of the gastrointestinal tract. They generally have lower mitotic indexes and stromal tumors (GISTs) and NF1. NF1-associated-GISTs tend to occur at a younger age in do not show typical KIT/platelet-derived growth factor receptor alpha mutations [80]. distal locations of the gastrointestinal tract. They generally have lower mitotic indexes and 15.do Conclusionsnot show typical KIT/platelet-derived growth factor receptor alpha mutations [80].

15. ConclusionsNF1 is a genetic disorder which needs a collaborative and interdisciplinary approach to understand and manage the condition. The future direction of therapies seems to directNF1 cell type-specificis a genetic disorder growth which control needs pathways a collaborative with mutation-directed and interdisciplinary therapeutics approach in a personalizedto understand manner. and manage the condition. The future direction of therapies seems to direct cell type-specific growth control pathways with mutation-directed therapeutics in a per- sonalized manner.

Author Contributions: Conceptualization, V.P. and B.O.; methodology, T.R.; software, T.R.; valida- tion, G.A., C.S. and B.O.; formal analysis, V.P.; investigation, G.A.; resources, B.O.; data curation, T.R.; writing—original draft preparation, B.O.; writing—review and editing, V.P.; visualization,

Cancers 2021, 13, 463 14 of 17

Author Contributions: Conceptualization, V.P. and B.O.; methodology, T.R.; software, T.R.; valida- tion, G.A., C.S. and B.O.; formal analysis, V.P.; investigation, G.A.; resources, B.O.; data curation, T.R.; writing—original draft preparation, B.O.; writing—review and editing, V.P.; visualization, G.A.; supervision, V.P.; project administration, C.S.; funding acquisition, V.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.

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