MOLECULAR CHARACTERIZATION OF THE BREAKPOINTS OF A 12-KB DELETION IN THE NF1 GENE IN A FAMILY SHOWING GERM-LINE MOSAICISM

Neurofibromatosis type 1 (NF1) is caused by deletions, insertions, tra nslocations, and point mutations in the NF1 gene, which spans 350 kb o n the long arm of human chromosome 17. Although several point mutation s have been described, large molecular abnormalities have rarely been characterized in detail. We describe here the molecular breakpoints of a 12-kb deletion of the NF1 gene, which is responsible for the NF1 ph enotype in a kindred with two children affected because of germline mo saicism in the unaffected father, who has the mutation in 10% of his s permatozoa. The mutation spans introns 31-39, removing 12,021 nt and i nserting 30 bp, of which 19 bp are a direct repetition of a sequence l ocated in intron 31, just 4 bp before the 5' breakpoint. The 5' and 3' breakpoints contain the sequence TATTTTA, which could be involved in the generation of the deletion. The most plausible explanation for the mechanism involved in the generation of this 12-kb deletion is homolo gous/nonhomologous recombination. Since sperm of the father does not c ontain the corresponding insertion of the 12-kb deleted sequence, this deletion could have occurred within the NF1 chromosome through loop f ormation. RNA from lymphocytes of one of the NF1 patients showed simil ar levels of the mutated and normal transcripts, suggesting that the N F1-mRNA from mutations causing frame shifts of the reading frame or st op codons in this gene is not degraded during its processing. The muta tion was not detected in fresh lymphocytes from the unaffected father by PCR analysis, supporting the case for true germ-line mosaicism.