Cis AB is a rare mutation in the ABO gene which complicates the basic inheritance pattern and blood-transfusion compatibility matching for ABO blood typing. There are different DNA mutations of either type A or Type B alleles that change several 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 reverse type B to type A and then, a new hybrid enzyme will 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). There are another 8 alleles reported in BGMUT, the most discovered reciently in China and Taiwan. In the cis-AB genotype, both antigens are expressed, like in a standard (trans) AB genotype. In a traditional AB phenotype, A and B antigenes are inherited separately from the father and mother while a cis-AB allele comes from one parent only. In a serum test, cis-AB tests almost the same as a traditional AB, but people with this rare type have problems with blood transfusions. Some of them need components like washed red blood cells or autotransfusion of serum and blood. Cis-AB type was studied first in Japan (Shishoku Island) and South Korea (Gwanju area) where this rare type is more common (Chinese coast provinces and Taiwan, too), although it is also seen in a few European families (from France, Spain, Belgium, Germany, and Poland). In the year 2004, the American Red Cross described a family in Nebraska with a father of type cis-AB rh negative, a mother type O and their baby of type cis-AB. Since then, several cases in America have been reported. Antigen expression is weaker than A1 or B.
Cis AB versus regular (trans) AB
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