Linkage disequilibrium in the neurofibromatosis 1 (NF1) region: implications for gene mapping

To test the usefulness of linkage disequilibrium for gene mapping, we compared physical distances and linkage disequilibrium among eight RFLPs in the neurofibromatosis 1 (NF1) region. Seven of the polymorphisms span most of the NF1 gene, while the remaining polymorphism lies approximately 70 kb 3' to a stop codon in exon 49. By using Centre d'Etude du Polymorphisme Humain (CEPH) kindreds, 91-110 unrelated parents were genotyped. A high degree of disequilibrium is maintained among the seven intragenic polymorphisms (r > .82, P < 10(-7)), even though they are separated by as much as 340 kb. The 3' polymorphism is only 68 kb distal to the next polymorphism, but disequilibrium between the 3' polymorphism and all others is comparatively low (magnitude of 4 < .33, P values .27-.001). This result was replicated in three sets of unrelated kindreds: the Utah CEPH families, the non-Utah CEPH families, and an independent set of NF1 families. Trigenic, quadrigenic, three-locus, and four-locus disequilibrium measures were also estimated. There was little evidence of higher-order linkage disequilibrium. As expected for a disease with multiple mutations, no disequilibrium was observed between the disease gene and any of the RFLPs. The observed pattern of high disequilibrium within the gene and a loss of disequilibrium 3' to the stop codon could have implications for gene mapping studies. These are discussed, and guidelines for linkage disequilibrium studies are suggested.