Neurofibromatosis 2 (NF2) is a severe autosomal dominant disorder that predisposes to multiple tumours of the nervous system. About half of all patients are founders with clinically unaffected parents. The purpose of the present study was to examine the extent to which mosaicism is present in NF2 founders. A total of 233 NF2 founders with bilateral vestibular schwannomas (BVS) were screened by exon scanning. NF2 mutations were detected in the blood samples of 122 patients (52%). In 10 of the 122 cases, the ratio of mutant to normal alleles was obviously less than 1, suggesting mosaicism. Tumour specimens were available from 35 of the 111 subjects in whom no mutation could be detected in blood specimens. Mutational analysis by exon scanning detected typical NF2 mutations in 21 of the 35 tumours. In nine subjects, the alterations found in tumours could be confirmed to be the constitutional mutation based on finding of identical mutations in pathologically and/or anatomically distinct second tumours. In six other subjects with only a single tumour available, allelic loss of the NF2 gene was found in addition to the mutation in each tumour, suggesting that either the mutation or the deletion of the NF2 gene is probably the constitutional genetic alteration. Our results suggest that failure to find constitutional mutations in blood specimen from these 15 patients was not because of the limitation of the applied screening technique, but the lack of the mutations in their leucocytes, best explained by mosaicism. Extrapolating the rate (15/35 = 43%) of mosaicism in these 35 cases to the 111 NF2 founders with no constitutional NF2 mutations found in their blood, we inferred 48 mosaic subjects (111 x 0.429). Adding the 10 mosaic cases detected directly in blood specimens, we estimate the rate of mosaicism to be 24.8% (58/233) in our cohort of 233 NF2 founders with bilateral vestibular schwannomas.
