Somatic mosaicism of a greater than 1.7-Mb deletion of genomic DNA involving the entire NF1 gene as verified by FISH: further evidence for a contiguous gene syndrome in 17q11.2.

We report on a third case with neurofibromatosis type 1 (NF1) due to mosaicism for a gross deletion in 17q11.2 covering the entire NF1 gene. The deletion was suspected in Giemsa banded chromosomes and was confirmed by fluorescence in situ hybridization using the cosmids CO919 from the 5' region, GO2121 from the central, H10410 from the 3' region of the NF1 gene, and the 1.7-Mb YAC 947G11 spanning the entire 350-kb genomic DNA of the NF1 gene. The deletion was present in 33% of peripheral blood lymphocytes and 58% of fibroblasts. The clinical manifestations in this 6-year-old male patient were especially severe and extended beyond the typical features of NF1. The patient also displayed facial anomalies, severe and early-onset psychomotor retardation, seizures, spasticity, and microcephaly. These features differ from other large-deletion NF1 patients, even nonmosaic cases. The complex phenotype could be explained by the involvement of coding sequences flanking the NF1 gene, thus supporting the existence of a contiguous gene syndrome in 17q11.2.