Abstract
Background Factor IX (FIX) plays a critical role in blood coagulation. Complete deletion of
F9 results in severe hemophilia B, whereas the clinical implications of complete F9 duplication and triplication remain understudied.
Objective To investigate the rearrangement mechanisms underlying complete F9 deletion (cases 1 and 2), duplication (cases 3 and 4), and triplication (case 5),
and to explore their association with FIX expression levels and clinical impacts.
Methods Plasma FIX levels were detected using antigen and activity assays. CNVplex technology,
optical genome mapping, and long-distance polymerase chain reaction were employed
to characterize the breakpoints of the chromosomal rearrangements.
Results Cases 1 and 2 exhibited FIX activities below 1%. Case 3 displayed FIX activities
within the reference range. However, cases 4 and 5 showed a significant increase in
FIX activities. Alu-mediated nonallelic homologous recombination was identified as
the cause of F9 deletion in case 1; FoSTeS/MMBIR (Fork Stalling and Template Switching/microhomology-mediated
break-induced replication) contributed to both F9 deletion and tandem duplication observed in cases 2 and 3; BIR/MMBIR (break-induced
replication/microhomology-mediated break-induced replication) mediated by the same
pair of low-copy repeats results in similar duplication–triplication/inversion–duplication
(DUP–TRP/INV–DUP) rearrangements in cases 4 and 5, leading to complete F9 duplication and triplication, respectively.
Conclusion Large deletions involving the F9 gene exhibit no apparent pattern, and the extra-hematologic clinical phenotypes require
careful analysis of other genes within the deletion. The impact of complete F9 duplication and triplication on FIX expression might depend on the integrity of the
F9 upstream sequence and the specific rearrangement mechanisms. Notably, DUP–TRP/INV–DUP
rearrangements significantly elevate FIX activity and are closely associated with
thrombotic phenotypes.
Keywords
factor IX - genomic structural variation - hemophilia B - low-copy repeats - thrombosis