[The Cis AB complex of the ABO system]

A novel cis AB allele derived from a B allele through a single point mutation

BACKGROUND

The very rare cis AB phenotype, first described in 1964, corresponds to a special ABO allele encoding a glycosyltransferase that is capable of synthesizing both A and B substances. Until now, gene sequences of only two cis AB alleles were partially characterized. One involved the A1*02 allele with a single nonsynonymous substitution at codon 268, whereas the second arose from a single nonsynonymous substitution at codon 266 in exon 7 of a B1*01 allele.

STUDY DESIGN AND METHODS

A cis AB phenotype was identified in a French family. The serologic characteristics of this phenotype were determined. The cis AB allele was characterized from exon 6 to exon 7 by cloning and sequencing.

RESULTS

The cis AB.tlse(*)01 allele is identical to B(1*)01 except for a single point mutation at nucleotide position 700, where a T replaces a C, implying a change of amino acid 234 (the B(1*)01 proline being replaced by a serine).

CONCLUSION

The cis AB.tlse(*)01 allele clearly differs from all previously reported ABO, including the two previous cis AB described.

A family exhibiting the cis-AB genotype

Studies on blood samples from a 30-year-old woman of Korean origin, her husband and children, showed the presence of the cis-AB genotype in the mother and one child. The cis-AB red cells showed abnormal reactions with both polyclonal and monoclonal anti-B reagents and an elevated level of H antigen. The serum contained weak anti-B that reacted with normal B antigen but not with that on cis-AB cells. Normal levels of A substance but reduced levels of B and H substances were present in the saliva. Conversely, the serum showed reduced A and normal B and H blood group transferase activity.

Inheritance of cis-AB in three generations (family Lam.)

In a case of disputed paternity genetic incompatibility was observed in the ABO blood group system between mother (O) and child (AB). From biostatistical evaluation of 21 genetic markers, including HLA phenotypes, a high value of probability for paternity, maternity and parentage was found between the child, the child's mother, the accused man and his mother. Substitution of the newborn was thereby excluded. The serostatistical evaluation of maternity and paternity could be supported by anthropological opinion. In serological investigations of the ABH system the A and B antigens of the 'A2B' cells were found to be abnormal in the child, the child's father, and the father's mother: the A was weaker than normal A1 but stronger than normal A2; the B was found to be abnormal which in addition was documented through occurrence of irregular anti-B antibodies in the child. In comparison to normal controls (A1, A2, B, A1B, A2B) diminished activity of alpha-D-N-acetylgalactosamine and alpha-D-galactosyltransferase were observed in the three 'A2B' propositi. These facts confirmed the existence of a cis-AB gene in the Lam. family which the child inherited from her father and the child's father from his mother. Assuming a population frequency of 1.1 X 10(-5) for the cis-AB gene, the probability for paternity was calculated from all genetic markers to be W = 99.9999985%.