Multimodal mechanisms of pathogenic variants in the signal peptide of FIX leading to hemophilia B.

Signal peptide (SP) is essential for protein secretion, and pathogenic variants in the SP of FIX have been identified in hemophilia B (HB). However, the underlying mechanism for the genotype-phenotype correlation of these variants has not been well studied. Here we systematically examined the effects of 13 pathogenic point variants in the SP of FIX using different approaches. Our results showed that these point variants lead to HB by missense variants and/or aberrant pre-mRNA splicing. The missense variants in h-region mainly affected the co-translational translocation function of the SP, and those in c-region caused FIX deficiency mainly by disturbing the co-translational translocation and/or cleavage of the SP. Almost absolute aberrant pre-mRNA splicing was only observed in variants of c.82T>G, but a slight change of splicing patterns was found in variants of c.53G>T, c.77C>A, c.82T>C, and c.83G>A, indicating that these variants might have different degree to affect pre-mRNA splicing. Although two 6-nt deletion aberrant pre-mRNA splicing products caused FIX deficiency by disturbing the SP cleavage, but they could produce some functional mature FIX and vitamin K could increase the secretion of functional FIX. Taken together, our data indicated that pathogenic variants in the SP of FIX caused HB through diverse molecular mechanisms or even a mixture of several mechanisms, and vitamin K availability could be partially attributed to varying bleeding tendencies in patients carrying the same variant in the SP.