MOLECULAR MECHANISMS OF FVII DEFICIENCY - EXPRESSION OF MUTATIONS CLUSTERED IN THE IVS7 DONOR SPLICE-SITE OF FACTOR-VII GENE

In three Italian patients, two point mutations and a short deletion we re found in the intron 7 of factor VII gene, clustered in the donor sp lice site and located in the first of several repeats. The mutation 97 26+5G-->A, the most frequent cause of symptomatic factor VII deficienc y in Italy, as well as the deletion (9729de14) gave rise in expression studies to abnormally spliced transcripts, which were exclusively pro duced from the cryptic site in the second repeat. The insertion in the mature mRNA of the first intronic repeat caused (9726+5G-->A) a readi ng frameshift, abolishing most of the factor VII catalytic domain, or produced (9729de14), an altered factor with 11 additional residues, th e activity of which was not detectable in the cell medium after mutage nesis and expression studies. Studies of factor VII ectopic mRNA from leukocytes and expression studies indicated that the deleted gene prod uced 30% of normally spliced transcript. Differently, the 9726+5G-->A mutation permitted a very low level (0.2% to 1%) of correct splicing t o occur, which could be of great importance to prevent the onset, in t he homozygous patients, of most of the life-threatening bleeding sympt oms. The 9726+7A-->G mutation was found to be a rare and functionally silent polymorphism. These findings, which provide further evidence of the interplay of sequence and position in the 5' splice site selectio n, throw light on the heterogeneous molecular bases and clinical pheno types of FVII deficiency. (C) 1998 by The American Society of Hematolo gy.