Activating mutations of the tyrosine kinase receptor FGFR3 are associated with benign skin tumors in mice and humansLogié A, Dunois-Lardé C, Rosty C, Levrel O, Blanche M, Ribeiro A, Gasc JM, Jorcano J, Werner S, Sastre-Garau X, Thiery JP, Radvanyi F.
Hum Mol Genet 2005;14:1153-60.
Specific germline activating point mutations in the gene encoding the tyrosine kinase receptor FGFR3 (fibroblast growth factor receptor 3) result in autosomal dominant human skeletal dysplasias. The identification in multiple myeloma and in two epithelial cancers-bladder and cervical carcinomas-of somatic FGFR3 mutations identical to the germinal activating mutations found in skeletal dysplasias, together with functional studies, have suggested an oncogenic role for this receptor. Although acanthosis nigricans, a benign skin tumor, has been found in some syndromes associated with germinal activating mutations of FGFR3, the role of activated FGFR3 in the epidermis has never been investigated. Here, we targeted an activated receptor mutant (S249C FGFR3) to the basal cells of the epidermis of transgenic mice. Mice expressing the transgene developed benign epidermal tumors with no sign of malignancy. These skin lesions had features in common with acanthosis nigricans and other benign human skin tumors, including seborrheic keratosis, one of the most common benign epidermal tumors in humans. We therefore screened a series of 62 cases of seborrheic keratosis for FGFR3 mutations. A large proportion of these tumors (39%) harbored somatic activating FGFR3 mutations, identical to those associated with skeletal dysplasia syndromes and bladder and cervical neoplasms. Our findings directly implicate FGFR3 activation as a major cause of benign epidermal tumors in humans.

Mosaicism of activating FGFR3 mutations in human skin causes epidermal nevi
Hafner C, van Oers JM, Vogt T, Landthaler M, Stoehr R, Blaszyk H, Hofstaedter F, Zwarthoff EC, Hartmann A.
J Clin Invest 2006; 116:2201-2207.
Epidermal nevi are common congenital skin lesions with an incidence of 1 in 1,000 people; however, their genetic basis remains elusive. Germline mutations of the FGF receptor 3 (FGFR3) cause autosomal dominant skeletal disorders such as achondroplasia and thanatophoric dysplasia, which can be associated with acanthosis nigricans of the skin. Acanthosis nigricans and common epidermal nevi of the nonorganoid, nonepidermolytic type share some clinical and histological features. We used a SNaPshot multiplex assay to screen 39 epidermal nevi of this type of 33 patients for 11 activating FGFR3 point mutations. In addition, exon 19 of FGFR3 was directly sequenced. We identified activating FGFR3 mutations, almost exclusively at codon 248 (R248C), in 11 of 33 (33%) patients with nonorganoid, nonepidermolytic epidermal nevi. In 4 of these cases, samples from adjacent histologically normal skin could be analyzed, and FGFR3 mutations were found to be absent. Our results suggest that a large proportion of epidermal nevi are caused by a mosaicism of activating FGFR3 mutations in the human epidermis, secondary to a postzygotic mutation in early embryonic development. The R248C mutation appears to be a hot spot for FGFR3 mutations in epidermal nevi.

Fibroblast growth factor receptor 3 mutations in epidermal nevi and associated low grade bladder tumors
Hernández S, Toll A, Baselga E, Ribé A, Azua-Romeo J, Pujol RM, Real FX.
J Invest Dermatol 2007;127:1664-6.
Epidermal nevi (EN) are benign lesions presenting at birth or in childhood. Based on the occurrence of fibroblast growth factor receptor 3 (FGFR3) mutations in seborrheic keratosis and urothelial carcinomas (UC), and the identification of two young patients with EN and UC, we hypothesized that mutations might occur in EN. The R248C mutation was found in 6/23 (26.1%) EN but it was absent from unaffected skin. In two patients with EN and UC, both lesions were FGFR3 wild type. Our findings indicate that: (1) FGFR3 mutations occur in mosaicism and can cause EN and (2) other genes are involved in EN.

Spectrum of FGFR3 mutations in multiple intraindividual seborrheic keratoses
Hafner C, Hartmann A, Real FX, Hofstaedter F, Landthaler M, Vogt T.
J Invest Dermatol 2007;127:1883-5.
Somatic FGFR3 mutations have recently been identified in seborrheic keratoses (SK). Affected individuals often show a large number of SK, but their spectrum of FGFR3 mutations has not been investigated yet. We analyzed 78 SK of four patients using a SNaPshot multiplex assay. FGFR3 mutations were detected in 46 of 78 SK (59%). The mutation rates of the patients ranged from 26 to 89%. Each patient showed at least four different mutated loci. FGFR3 mutations appear to be common genetic alterations in multiple SK with a varying interindividual mutation frequency but without specific intraindividual hot spots.