'True' genotype of chimeric twins revealed by blood-group gene products in plasma

Unravelling the biochemical basis of blood group ABO and Lewis antigenic specificity

The ABO blood-group polymorphism is still the most clinically important system in blood transfusion practice. The groups were discovered in 1900 and the genes at the ABO locus were cloned nearly a century later in 1990. To enable this goal to be reached intensive studies were carried out in the intervening years on the serology, genetics, inheritance and biochemistry of the antigens belonging to this system. This article describes biochemical genetic investigations on ABO and the related Lewis antigens starting from the time in the 1940s when serological and classical genetical studies had established the immunological basis and mode of inheritance of the antigens but practically nothing was known about their chemical structure. Essential steps were the definition of H as the product of a genetic system Hh independent of ABO, and the establishment of the precursor-product relationship of H to A and B antigens. Indirect methods gave first indications that the specificity of antigens resided in carbohydrate and revealed the immunodominant sugars in the antigenic structures. Subsequently chemical fragmentation procedures enabled the complete determinant structures to be established. Degradation experiments with glycosidases revealed how loss of one specificity by the removal of a single sugar unit exposed a new specificity and suggested that biosynthesis proceeded by a reversal of this process whereby the oligosaccharide structures were built up by the sequential addition of sugar units. Hence, the primary blood-group gene products were predicted to be glycosyltransferase enzymes that added the last sugar to complete the determinant structures. Identification of these enzymes gave new genetic markers and eventually purification of the blood-group A-gene encoded N-acetylgalactosaminyltransferase gave a probe for cloning the ABO locus. Blood-group ABO genotyping by DNA methods has now become a practical possibility.

Zulu XX/XX dispermic chimaera from Natal with two populations of red blood cells and patchy skin pigmentation

The chimaera is female and has two children. Her blood contains 99% group O, type AcP:BA, Pep-A:8-2 and 1% group A2B, type AcP:RA, Pep-A:1 red cells. H-, A- and B-transferase activities were demonstrated in her serum. The level of the H enzyme activity is low but is at the lower end of the normal range for group O persons. The levels of the A and B enzymes are also low but are higher than expected in a person with 1% A2B red cells in the blood. The levels of the A and B enzymes indicate that tissues other than the chimaera's haemopoietic tissue carry her genetically A2B cell line and are contributing the corresponding transferases to her plasma. Gross patchy skin pigmentation is present on the upper part of her body. The chimaera has evidently inherited two dissimilar germ nuclei from each parent.

Blood group chimeras. A review

More than 70 spontaneous chimeric propositi in man are known. This review attempts to summarize the information given by 32 twin chimeras, 32 dispermic chimeras and 11 chimeras of unestablished type.

Another example of haemopoietic (twin) chimaerism in a subject unaware of being a twin

A fourth human blood group chimaera studies in Birmingham is an example of haemopoietic (twin) chimaerism in which the subject was unaware of being a twin. Chimaerism was discovered during routine antenatal serological investigation in which it was shown that the proposita has two red cell populations, one of the rhesus genotype rr, and the other R1r. Further studies showed that she has two populations of lymphocytes, one with the female karyotype, 46XX, and the other with the male karyotype, 46XY. Skin fibroblasts were all 46XX.

Atypical transmission of ABO blood groups in a French family

A group AB mother (Mrs P.D.) gave birth to a group O female baby (C.D.). Extensive study of the blood group genetic markers in both the parents and the child, carried out on several occasions, showed nothing unusual outside the ABO system. Mrs P.D. then, gave birth to a second female baby who was also group O. Mrs P.D. had normal amounts of A, B, H and Lewis antigens in her saliva. The H, A and B agglutinability of her red cells was in the range of normal A2B group. This A2B blood group was characterized by very low A gene-specified glycosyltransferase activity in serum. Moreover this activity was undetectable in red blood cell membranes. These results are discussed in the light of various hypotheses in order to explain this unusual transmission of ABO blood group.

A human dispermic chimaera first suspected from analyses of the blood group gene-specified glycosyltransferases

The red cells of a normal male blood donor, K.S., were first grouped as B but he was found to lack anti-A in his serum. Closer investigation revealed that his red cells had very weak A activity, demonstrable only by absorption and elution of anti-A. He is a non-secretor of ABH and a secretor of Lea. Blood group A-, B and H-gene specified glycosyltransferases were detected in his serum. In contrast to the finding of a B antigen of normal strength on his red cells, the B transferase in his serum was only about 30% of the normal level and, despite the very weak A activity of K.S's red cells, the A transferase level was about 50% of that found in the serum of group A individuals with normal strength of A antigen. Moreover, the A transferase on the basis of its pH optimum, Km values for donor and acceptor substrates, activation by divalent cations, isoelectric focusing profile and capacity to convert O to A-active cells, was characterized as the product of an A1 gene. A family study showed that K.S's wife is group A2 and that they have two sons, one group A2 and the other group B. The group B son is assumed to have inherited a B gene from the propositus but the level of B transferase in the son's serum is three times as high as that in his father's serum. The wife of the propositus and his group A2 son have normal A2 transferases in keeping with their A2 red cell status. The A2 son therefore appears to have inherited an A2 gene from his mother but neither the A1 nor the B gene shown to be carried by his father. The distribution of transferase activities in K.S's red cells differs from that in his serum. A level of B transferase within the normal range was found in his red cell membranes but a very low level of A transferase was detected. The discrepancies between the serum transferases and ABO red cell group, together with the pattern of inheritance within the family, led to a suspicion of chimaerism. This was confirmed by the finding of fibroblasts with the female 46XX karyotype in cultures of the propositus' skin. These results suggest that K.S. is a dispermic chimaera with two different cell lines of the genotypes BO and A1O or A1A1. The group A2 son is assumed to have inherited an O gene from his father. It seems probable that K.S.'s bone marrow and reproductive organs are comprised predominantly of the XY cell line which carried the blood group BO genotype whereas his skin and other tissues which contribute the A1 transferase to his plasma, are partly made up of the XX cell line which carries the blood group A1O or A1A1 genotype.

Another example of haemopoietic chimaerism in dizygotic twins

Another example of haemopoietic chimaerism in dizygotic twins is described. Each twin had two distinct blood cell populations. The red cell populations differed in three blood group systems and in other genetic characters, and the white cell population in HLA types and in XX/XY karyotypes. The biological significance of these findings is discussed with special reference to the relative proportions of the two red and white cell populations, the ABH blood group gene-specified glycosyltransferase levels, and the HLA types.

Human blood-group A- and H-specified glycosyltransferase levels in the sera of newborn infants and their mothers

The level of blood-group A1-specified alpha,3'-N-acetyl-D-galactosaminyl-transferase in the serum of recently-delivered women was found to be appreciably lower than the level of this enzyme in the serum of non-pregnant adults and of newborn infants; a similar but less striking decrease was observed in the levels of the A2-specified alpha,3'-N-acetyl-D-galactosaminyltransferase and the H-specified alpha,2'-L-fucosyltransferase. Although the red cells of newborn infants are known to have relatively few A and H antigen sites, the serum of neonates was found to have a level of A1- and A2-dependent N-acetylgalactosaminyltransferases and H-dependent fucosyltransferase as high as, if not higher than, the serum of non-pregnant adults. This finding is compatible with the fact that the haemopoietic tissue contributes only about 20% of the serum transferase level.

A further example of human blood group chimaerism

Blood group chimaerism was detected in a healthy fertile woman, not known to be a twin. Her peripheral lymphocytes had a male karyotype (46/XY); fibroblasts cultured from her skin had a female karyotype (46/XX). The mechanism of chimaerism could not be established.

Further observations on the Birmingham chimaera

The appropriate ABH-gene specified glycosyltransferases in the plasma of the Birmingham chimaera were estimated. These observatiions and the demonstration of A1Leb blood group specific glycosphingolipid in the plasma indicate that the minority population of red blood cells probably represents the true blood groups of the patient.

Genetics of anti-A and anti-B levels

Hemolysins and agglutinins anti-A and anti-B were measured in 401 blood group O individuals belonging to 74 families. Levels of hemolysins and saline agglutinins reached a peak during childhood and decreased thereafter. Incomplete agglutinins showed a relative increase after childhood. Hemolysins and agglutinins were higher in blacks than in whites. Whites had higher anti-A than anti-B levels and the levels were higher in females than in males. In blacks the anti-B levels were almost as high as were the levels of anti-A and little sex difference was found. Levels of hemolytic anti-A and saline anti-A were closely correlated as were hemolytic anti-B and saline anti-B but low correlations were found between anti-A and anti-B levels. Anti-A and anti-B hemolysin and agglutinin levels were positively correlated with serum IgM levels but anti-A levels were not. Isoantibody levels were not correlated with IgA, IgD, and IgE levels. Correlation and regression analysis among relatives disclosed that from 20 to 30 per cent of the total phenotypic variation in isoantibody levels was accounted for by genetic variation. Age of the individual also contributed markedly to the variation in hemolysins and saline agglutinins, but about one-half of the total variation remained unaccounted for.

A and B and A1Leb substances in glycosphingolipid fractions of human serum

A and B and A1Leb substances were adsorbed onto red cells exposed to glycosphingolipid fractions prepared from the serum of group A and B and A1,Le(a minus b plus) donors. Group O cells exposed to fractions prepared from the serum of group A or B donors were agglutinated by an IgM cross-reacting antibody present in some group O sera. Cells exposed to fractions from A1,Le(a minus b plus) serum were agglutinated by anti-A1Leb. The amount of A substance in the fractions was related to the A subtype (A1 or A2) and to the Lewis and secretor phenotype of the donor. The uptake of blood-group substances from the lipid fractions was inhibited by the addition of whole serum to the fractions.