HOMONUCLEOTIDE TRACTS, SHORT REPEATS AND CPG CPNPG MOTIFS ARE FREQUENT SITES FOR HETEROGENEOUS MUTATIONS IN THE NEUROFIBROMATOSIS TYPE-1 (NF1) TUMOR-SUPPRESSOR GENE/

Neurofibromatosis type 1 (NF1) is among the most common human genetic disorders, having a constellation of cutaneous and skeletal manifestat ions, intellectual impairment, and an increased risk for a variety of malignancies. The NF1 gene has a high spontaneous mutation rate and is also associated with a variety of sporadic cancers in the general pop ulation. While a number of laboratories are involved in a coordinated effort to identify NF1 mutations, an important gap in our knowledge is an understanding of the mechanisms responsible for NF1 mutagenesis. I n this present paper we describe our analysis of the sequence environm ent in the NF1 gene at those sites where small deletions, insertions a nd nucleotide substitution mutations have been reported. Our objective was to determine whether specific nucleotide sequences commonly occur at these mutation sites within the NF1 gene. We assessed how frequent ly independent NF1 mutations occur at the site of short direct repeats , single nucleotide repeats (homonucleotides) and at CpG and CpNpG mot ifs. We have established that homonucleotide and short direct repeats are commonly involved in the majority of small deletions and insertion s analysed. Substitution mutations are frequently associated with homo nucleotide repeats and methylatable CpG dinucleotides and CpNpG trinuc leotides. We suggest that NF1 mutations are acquired and retained by c ells through an intricate balancing of repair and replication mechanis ms. Such mutations may provide a proliferative advantage for that cell and its progeny.