Cis AB
type of rare mutation in the ABO gene
Cis AB is a type of rare mutation in the ABO gene. It happens when the transferase allele contains a mix of amino acids from either A or B alleles, producing a bifunctional enzyme that can produce both types of antigens, usually with one weaker than the other. This results in a serum test result much like the standard, separate (trans) AB phenotype, although the weaker antigen can occasionally fail to be detected. It complicates the basic inheritance pattern (as the allele comes from one parent only) and blood-transfusion compatibility matching for ABO blood typing.
Different DNA mutations of either type A or Type B alleles change amino acids in enzyme transferase A or B, homologous enzymes differing in only four of 354 amino acids (R176G, G235S, L266M, and G268A). A single change in ABO gene DNA could switch type B to type A and then, a new hybrid enzyme can produce both weak B and A2 (in serum test, A2B, and A2B3). The most common mutation is an A105 allele variation in exon 7 nucleotide position G803C changing glycine (type A) to alanine (type B). Another 8 alleles are reported in BGMUT, most recently in China and Taiwan. Some Cis-AB carriers need components like washed red blood cells or autotransfusion of serum and blood.
Cis-AB type was studied first in humans in Japan (Shikoku Island) and South Korea (Gwangju area) where this rare type is more common (Chinese coast provinces and Taiwan also), although it is seen in a few European families (from France, Spain, Belgium, Germany, and Poland).
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