Hemophilia B carrier determination based on family-specific mutation detection by DNA single-strand conformation...

Single-strand conformation (SSC) analysis can distinguish normal from variant DNA fragments containing single point mutations by conformation-induced electrophoretic mobility shifts in non-denaturing polyacrylamide gels. We studied 25 hemophilia B kindreds by using SSC analysis after polymerase chain reaction (PCR) amplification of the eight factor IX exons and their intron boundaries. Variant SSC fragments were unambiguously identified in 24 kindreds, and direct DNA sequencing of variant PCR fragments identified 20 different hemophilia B mutations. This technique was used for rapid and accurate carrier determination in female family members without the need for additional sequencing studies, because carriers have both normal and hemophilia family-specific SSC fragments. Of 25 obligate carriers from 15 kindreds, 24 were confirmed to carry variant fragments. The exception, a patient's daughter homozygous for the normal allele, was demonstrated by subsequent PCR genotyping to be the result of non-paternity. In the additional 32 at-risk females from 16 kindreds studied, 19 were identified as carriers and 13 as non-carriers. Eleven of the unique mutations affected restriction enzyme digestion sites, and carriers could then be identified by appropriate restriction enzyme digestion of amplified DNA. Our study, with hemophilia B as a model system, demonstrates the accuracy and efficiency of SSC analysis in screening and tracking unknown mutations in monogenic inherited disorders with known gene sequences.