Summary
Germline mutations in patients with hemophilia B generally have arisen within the
past 150 years. Evidence suggests that these germline mutations generally result from
endogenous processes. However, a unique pattern would be expected if a population
were exposed to a physiologically important germline mutagen since mutagens generally
produce characteristic patterns, or “fingerprints”, of mutation. To determine the
pattern of mutation in Mexican Hispanics, the regions of likely functional significance
in the factor IX gene were screened by di-deoxy fingerprinting (ddF) in 31 families
with hemophilia B. Mutations were found in 30 of these families. Haplotype analysis
was performed on individuals with identical mutations to help distinguish independent,
recurrent mutations from founder effects. Analysis of these 30 mutations, along with
7 mutations reported previously in Mexican Hispanic families, reveals a pattern of
independent mutation that is similar to the pattern of mutation observed in 127 U.
S. Caucasian families (p = 0.89). These results may reflect either an underlying pattern
of germline mutation due to endogenous processes or the presence of an ubiquitous
mutagen. Further analyses of the recurrent mutations revealed that two mutations,
T296M and R248Q, accounted for 19% of the mutations found in the Mexicans. Haplotype
data suggest that the multiple occurrences of T296M and R248Q are associated with
founder effects and that screening for these mutations may allow rapid mutation detection
and carrier diagnosis in a significant minority of Mexican families with hemophilia
B. These two mutations also are associated with founder effects in the U. S. Caucasian
population. However, the haplotypes are different in these two populations, indicating
independent origins. The occurrence of identical founder mutations in distinct populations
provides evidence for the previous hypothesis that the number of different mutations
giving rise to mild or borderline mild/moderate hemophilia B is small compared to
deleterious mutations causing more severe disease.