MUTATIONS CAUSING COAGULATION-FACTOR-XIII SUBUNIT-A DEFICIENCY - CHARACTERIZATION OF THE MUTANT PROTEINS AFTER EXPRESSION IN YEAST

We identified the mutations causing factor XIII A subunit deficiency i n two families, Two distinct mutations were identified in the S family : the nonsense mutation Tyr 441 --> stop in exon 11, inherited through the paternal line, and the missense mutation Asn 60 --> Lys in exon 3 , inherited through the maternal line. Two members of the J family wer e heterozygous for the previously described type 3 A subunit. The subs titution giving rise to the type 3 variant was found to be Gly 501 --> Arg in exon 12. The Asn 60 --> Lys and Gly 501 --> Arg mutations were constructed in cDNA clones and expressed in yeast (Saccharomyces cere visiae AH22). Although mRNA could be detected, protein containing the Asn 60 --> Lys substitution could not be detected, suggesting extreme instability or susceptibility to proteolysis, A subunits containing th e Gly 501 --> Arg substitution were expressed and found to be enzymati cally active in fresh yeast lysates. This variant has thermal instabil ity and lost activity during storage or purification. Gel filtration s tudies suggested that the type 3 variant assembled as a dimer, as do n ormal A subunits. The data suggest that the Gly 501 --> Arg (Type 3 va riant) would cause severe factor XIII deficiency if inherited in the h omozygous form or as a compound heterozygote with another deleterious mutation. (C) 1995 by The American Society of Hematology.