ABSENCE OF SOMATIC MOSAICISM IN 17 FAMILIES WITH HEMOPHILIA-B - AN ANALYSIS WITH A SENSITIVITY 10-FOLD TO 1000-FOLD GREATER-THAN THAT OF SEQUENCING GELS

Most estimates of germ-line mosaicism have been derived from families in which there has been transmission of a mutated allele to two or mor e children by an unaffected individual. Previously, analyses for somat ic mosaicism detected five such individuals by PCR-based sequencing an d haplotype analysis at a sensitivity of approximately 1 mutant per 10 wild-type alleles. To determine whether mutations that occur later in embryogenesis also give rise to somatic mosaicism, we analyzed leukoc yte DNA from 17 individuals in whom a mutation in the factor IX gene w as known to have originated. Methods capable of detecting 1 mutant all ele in 100-10 000 were utilized, and no further examples of somatic mo saicism were detected. If confirmed by future studies, the paucity of somatic mosaicism with mutant:wild-type allele frequencies ranging fro m 1:10 to 1:1000 (relative to the 11% of somatic mosaicism detected wi th mutant:wild-type allele frequencies of 1:1 to 1:10) may reflect a h igher mutation rate and/or germ-line lineage allocation very early in embryogenesis.