The “Malmo“ Epitope of Factor Ix: Phenotypic Expression of the “Viking“ Gene

Abstract

The epitope of a mouse monoclonal AB (9.9) which detects a Factor IX (F.IX) polymorphism in the plasma of normal persons (PNAS 82:3839, 1985) has been related to not more than 6 AA residues of F.IX by recombinant DNA technology. The same 6 residues define Smith’s polymorphic epitope (Am. J. Human Genet. 37:688, 1985 and in press). This region of F.IX contains the alanine:threonine dimorphism at residue 148 first suggested by McGraw et al. (PNAS 82: 2847, 1985) and established by Winship and Brownlee with synthetic DNA oligomers (Lancet in press). Using synthetic DNA probes, we have found that the DNA difference between positive and negative reactors to 9.9 is whether base pair 20422, the first pair in the codon for residue 148, is A:T or G:C. We can conclude that 9.9 reacts with F.IX containing threonine but not alanine at position 148.The F.IX immunologic polymorphism-whose epitope we are referring to as “Malmo”-is, not surprisingly, in strong linkage disequilibrium with two F.IX DNA polymorphisms, TaqI and Xmnl. The highest frequency of the rarer Malmo allele in 6 disparate ethnic groups was in Swedes (32%); a lower frequency (14%) was seen in White Americans whose ancestors came overwhelmingly from the Celtic regions of the British Isles; it was at very low frequency or absent in Black Americans, East Indians, Chinese and Malays. A maximum frequency in Swedes and absence in Africans and Orientals suggest that the transition from A:T to G:C occurred in Scandinavia and spread from there. The history of Europe and America plus the geographical distribution of the rare allele lead us to suggest that this locus might be designated: “the Viking gene”.