High frequency of mosaicism among megabase Nf1 deletions

Background: neurofibromatosis 1 is an autosomal dominant disorder characterized by tumors of the nerves systems and various cognitive deficits. The cause of neurofibromatosis is the defect of a tumor suppressor gene, NF1, 17q. A small group of the patients have facial anomalies associated with mental retardation and large number of tumors. Deletion of the entire NF1 gene has often been found in this patient group. However, most previous studies included only patients with the defined phenotypes and no screening has been carried out in a large number of unselected neurofibromatosis 1 patients.

Methods: 611 unselected neurofibromatosis 1 patients were initially screened for NF1 deletions by genotyping using microsatellite markers within or flanking the gene. FISH using probes covering the NF1 gene was carried out for 25 out of the 35 deletion candidates. 7 patients with identified deletions but normal facial configuration and normal intelligence were examined neuropsychologically and subjected to detailed FISH again using nuclei directly from peripheral leukocytes without culturing. From 4 patients, buccal cells were also obtained and examined.

Results: 35 case were identified as deletion candidates based on the apparent homozygosity of all 7 examined markers. From 25 of them, frozen or fresh blood specimen were available for FISH analysis which confirmed deletions in 23 cases. 7 patients exhibited normal facial configuration and intelligence. 6 of them turned out to be mosaic based on the finding of disomic FISH signals, and thus lack of the deletions in some leukocytes or buccal cells. We have thus found for the first time a high frequency of mosaicism in neurofibromatosis 1 patients with deletions (6 out of 23=30%). Our results also demonstrate that NF1 deletions are not exclusively associated with facial dismorphism and mental retardation and that a general screening of NF1 deletions should be applied for all NF1 patients regardless of their phenotypes.