Factor X deficiency is a rare haemorrhagic condition, normally inherited as an autosomal recessive trait, in which a variable clinical presentation correlates poorly with laboratory phenotype. The factor X (F10) genes of 14 unrelated individuals with factor X deficiency (12 familial and two sporadic cases) were sequenced yielding a total of 13 novel mutations. Family studies were performed in order to distinguish the contributions of individual mutant F10 alleles to the clinical and laboratory phenotypes. Missense mutations were studied by means of molecular modelling, whereas single basepair substitutions in splice sites and the 5' flanking region were examined by in vitro splicing assay and luciferase reporter gene assay respectively. The deletion allele of a novel hexanucleotide insertion/deletion polymorphism in the F10 gene promoter region was shown by reporter gene assay, to reduce promoter activity by ~ 20%. One family manifesting an autosomal dominant pattern of inheritance possessed three clinically affected members who were heterozygous for a splice-site mutation that was predicted to lead to the production of a truncated protein product. A model which accounts for the dominant negative effect of this lesion is presented. Variation in the antigen level of heterozygous relatives of probands was found to be significantly higher between families than within families, consistent with the view that the nature of the F10 lesion(s) segregating in a given family is a prime determinant of the laboratory phenotype. By contrast, no such relationship could be discerned between laboratory phenotype and polymorphism genotype.
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