Autosomal dominantly inherited tumor-prone syndromes are a substantial health problem and are amenable to epidemiologic studies by combining cancer surveillance registries with a genetic register (GR)-based approach. Knowledge of the frequency of the conditions provides a basis for appropriate health-resources allocations. GRs for five tumor-prone syndromes were established in the Manchester region of North West England in 1989 and 1990. Mapping birth dates of affected individuals from families onto regional birth rates has allowed an estimate of birth incidence, disease prevalence, and de novo mutation rates. Disease prevalence in order of frequency were for neurofibromatosis type 1 (NF1): 1 in 4,560; familial adenomatous polyposis (FAP): 1 in 18,976; nevoid basal cell carcinoma [Gorlin syndrome (GS)]: 1 in 30,827; neurofibromatosis type 2 (NF2) 1 in 56,161; and von Hippel Lindau (VHL) 1 in 91,111. Best estimates for birth incidence were: 1 in 2,699; 1 in 8,619; 1 in 14,963, 1 in 33,000; and 1 in 42,987, respectively. The proportions due to de novo mutation were: 42% (NF1); 16% (FAP); 26% (GS); 56% (NF2); and 21% (VHL). Estimates for NF1, NF2, FAP, and VHL are in line with previous estimates, and we provide the first estimates of birth incidence and de novo mutation rate for GS.

Neurofibromatosis type 1 (NF1) is characterized by cafe-au-lait spots, skinfold freckling, and cutaneous neurofibromas. No obvious relationships between small mutations (<20 bp) of the NF1 gene and a specific phenotype have previously been demonstrated, which suggests that interaction with either unlinked modifying genes and/or the normal NF1 allele may be involved in the development of the particular clinical features associated with NF1. We identified 21 unrelated probands with NF1 (14 familial and 7 sporadic cases) who were all found to have the same c.2970-2972 delAAT (p.990delM) mutation but no cutaneous neurofibromas or clinically obvious plexiform neurofibromas. Molecular analysis identified the same 3-bp inframe deletion (c.2970-2972 delAAT) in exon 17 of the NF1 gene in all affected subjects. The Delta AAT mutation is predicted to result in the loss of one of two adjacent methionines (codon 991 or 992) ( Delta Met991), in conjunction with silent ACA-->ACG change of codon 990. These two methionine residues are located in a highly conserved region of neurofibromin and are expected, therefore, to have a functional role in the protein. Our data represent results from the first study to correlate a specific small mutation of the NF1 gene to the expression of a particular clinical phenotype. The biological mechanism that relates this specific mutation to the suppression of cutaneous neurofibroma development is unknown.