MOLECULAR MECHANISM OF A MILD PHENOTYPE IN COAGULATION-FACTOR-XIII (FXIII) DEFICIENCY - A SPLICING MUTATION PERMITTING PARTIAL CORRECT SPLICING OF FXIII A-SUBUNIT MESSENGER-RNA

Congenital factor XIII (FXIII) deficiency is potentially a severe blee ding disorder, but in some cases, the symptoms may be fairly mild. In this study, we have characterized the molecular mechanism of a mild ph enotype of FXIII A-subunit deficiency in a Finnish family with two aff ected sisters, one of whom has even had two successful pregnancies wit hout regular substitution therapy. In the screening tests for FXIII de ficiency, no A-subunit could be detected, but by using more sensitive assays, a minute amount of functional A-subunit was seen, H-3-putresci ne incorporation assay showed distinct FXIII activity at the level of 0.35% of controls, and also the fibrin cross-linking pattern in the pa tients clotted plasma showed partial gamma-gamma dimerization. In West ern blot analysis, a faint band of full-length FXIII A-subunit was det ected in the patients' platelets. The patients have previously been id entified as heterozygotes for the Arg661 --> Stop mutation. Here we re port a T --> C transition at position +6 of intron C in their other al lele. The transition affected splicing of FXIII mRNA resulting in low steady state levels of several variant mRNA transcripts. One transcrip t contained sequences of intron C, whereas two transcripts resulted fr om skipping of one or two exons. Additionally, correctly spliced mRNA lacking the Arg661 --> Stop mutation of the maternal allele could be d etected. These results demonstrate that a mutation in splice donor sit e of intron C can result in several variant mRNA transcripts and even permit partial correct splicing of FXIII mRNA. Further, even the minut e amount of correctly processed mRNA is sufficient for producing prote in capable of gamma-gamma dimerization of fibrin. This is a rare examp le of an inherited functional human disorder in which a mutation affec ting splicing still permits some correct splicing to occur and this ha s a beneficial effect to the phenotype of the patients. (C) 1997 by Th e American Society of Hematology.