Allelic affinities between the F13A common gene products inferred by the analysis of an (AAAG)n STR polymorphism within the 5' untranslated region.

Factor XIII a subunit (F13A) is the last enzyme in the blood coagulation cascade. It is characterized by extensive genetic polymorphism defined by 4 common alleles, F13A*1A, 1B, 2A and 2B and a few rare variants, some responsible for severe coagulation deficiencies. In order to infer the evolutionary affinities between the common F13A alleles we have applied PCR techniques to study, in a Northern Portuguese sample, a short tandem repeat polymorphism located within the 5' untranslated region of the F13A gene. The analysis of the molecular heterogeneity within the F13A gene products revealed that the four biochemical variants shared very similar, truncated, distributions of STR alleles and showed no signs of predominant haplotypic associations. These findings seem to support both the inferences that intragenic recombination played an important role in the generation of molecular diversity within each of the four main F13A alleles and that all the four F13A alleles must be rather old. Molecular heterogeneity levels allowed the identification of 1B as the oldest F13A allelic state, and 2A as the most recently generated allele, but were not different enough to accurately track the divergence of alleles 1A and 2B. However, additional analysis of linkage disequilibrium patterns indicates that 1B-->2B-->1A-->2A is the most likely evolutionary order of appearance of F13A main protein alleles, confirming and extending a previous hypothetical model inferred from their molecular features.