Study of the alpha-N-acetylgalactosaminyltransferase in sera and red cell membranes of human A subgroups

Detection and phenotype analysis of a novel Ael blood group allele

BACKGROUND AND OBJECTIVES

The presence of blood subtypes may lead to difficulties in blood group identification; however, third-generation sequencing (TGS) can help in accurately identifying difficult blood groups, and study the serological characteristics and molecular mechanism of Ael subtypes.

MATERIALS AND METHODS

ABO blood group was identified by the standard serological technique, weak blood group antigen was identified by adsorption-elution experiments, ABH substance in the saliva was determined and glycosyltransferase activity of A and B was detected. The ABO gene full-length sequence and promoter region were amplified by specific primers using single-molecule real-time sequencing, with the amplified products being sequenced directly and analysed in real time.

RESULTS

The patient was serologically identified as Ael subtype, and TGS analysis revealed new intron mutations in Ael patients (c.467C>T; c.29-10T>A).

CONCLUSION

The discovery of the new allele and the identification of ABO subtypes can be combined with serological characterization and molecular biological methods.

Molecular heterogeneity of the A3 subgroup

The molecular characterization of the subgroup A3 remains unclear. Four unrelated A3 blood donors were studied. Family studies were possible in three of them. The A3 subgroup was defined by immunohaematological evaluation with four different commercially available serums. Exons VI and VII of the ABO gene, responsible for 91% of the catalytic active part of the glycosyltransferase, were amplified and subjected to direct sequencing. The results in all samples showed heterozygosity for the G261 deletion. In the A3 allele, the following associations were found: C467T mutation and 1060C deletion in one A3 blood donor and in another G829A and 1060C. In one case, only the 1060C deletion was demonstrated in the A3 allele. One blood donor presented the T646A and the G829A mutations in homozygosity. It was concluded that the A3 blood group is very heterogeneous at the molecular level.

Molecular genetics of the ABO histo-blood group system

Molecular genetic study of the histo-blood group ABO system has elucidated the allelic basis of this genetic locus. Comparison of the nucleotide sequence has identified in the coding region differences which change amino acid sequences of the glycosyltransferases coded by these genes. Effects of the differences (mutations) on the specificity and activity of the glycosyltransferases have been examined.

Blood group antigens on human erythrocytes-distribution, structure and possible functions

Human erythrocyte blood group antigens can be broadly divided into carbohydrates and proteins. The carbohydrate-dependent antigens (e.g., ABH, Lewis, Ii, P1, P-related, T and Tn) are covalently attached to proteins and/or sphingolipids, which are also widely distributed in body fluids, normal tissues and tumors. Blood group gene-specific glycosyltransferase regulate the synthesis of these antigens. Protein-dependent blood group antigens (e.g., MNSs, Gerbich, Rh, Kell, Duffy and Cromer-related) are carried on proteins, glycoproteins and proteins with glycosylphosphatidylinositol anchor. The functions of these molecules on human erythrocytes remain unknown; some of them may be involved in maintaining the erythrocyte shape. This review describes the distribution, structures and probable biological functions of some of these antigens in normal and pathological conditions.

Molecular genetic analysis of the ABO blood group system: 3. A(X) and B(A) alleles

We have employed a PCR approach to determine the nucleotide sequences of the coding region in the last two coding exons of the histo-blood group ABO genes from one A(X) and one B(A) individual. Compared with A1 alleles, the (A(X)) allele has a single nucleotide substitution (T-->A at nucleotide 646) resulting in an amino acid substitution (phenylalanine-->isoleucine at amino acid 216). Compared with B alleles, the B(A) allele has two nucleotide substitutions (T-->C at nt. 657 and A-->G at nt. 703) resulting in an amino acid substitution (serine-->glycine at aa. 235). The amino acid substitution resulting from this B(A) allele is located at the second of the four amino acid substitutions which discriminate human A and B transferases, and the amino acid residue (glycine) is identical to that of A transferase suggesting the involvement of this amino acid or its surrounding area for the recognition and/or binding of the donor nucleotide sugars.

Molecular genetic analysis of the ABO blood group system: 1. Weak subgroups: A3 and B3 alleles

We have determined the nucleotide sequences of the coding region in the last two coding exons of ABO genes (which occupy 91% of the soluble form of A1 transferase) from 7 individuals with weak subgroup phenotypes. Four of the individuals had an A3 phenotype and 3 individuals had a B3 phenotype. We determined the nucleotide sequences based on PCR followed by subcloning and DNA sequencing of the amplified fragments. Two cases of the A3 allele and 1 case of the B3 allele were found to contain a single-base substitution which resulted in an amino acid substitution. However, no other cases of A3 and B3 alleles were found to contain differences in this region. This finding demonstrates for the first time heterogeneity among these weak subgroups at the nucleotide level.

Purification, properties and partial amino acid sequence of the blood-group-A-gene-associated alpha-3-N-acetylgalactosaminyltransferase from human gut mucosal tissue

An alpha-3-N-acetylgalactosaminyltransferase that transfers N-acetylgalactosamine from UDP-N-acetylgalactosamine to H-active structures to form A determinants was purified to homogeneity from human gut mucosal tissue of blood-group-A subjects. The mucosa was homogenized, then treated with Triton X-100, and the solubilized enzyme was purified by affinity chromatography on UDP-hexanolamine-agarose and octyl-Sepharose CL-4B. Enzyme activity was recovered in 44% yield with a specific activity of approx. 7 mumol/min per mg. The only effective acceptor substrates for the transferase were those containing a subterminal beta-galactosyl residue substituted at the O-2 position with L-fucose. The purified enzyme had a weak capacity to transfer D-galactose from UDP-D-galactose to similar acceptors to make blood-group-B determinants. H.p.l.c. and SDS/PAGE analysis indicated an Mr of 40,000 for the purified enzyme. For the first time a partial amino acid sequence Xaa-Ser-Leu-Pro-Arg-Met-Val-Tyr-Pro-Gln-Ile-Ser?-Val-Leu was obtained for the N-terminal region of the soluble alpha-3-N-acetylgalactosaminyltransferase.

Cellular localization of blood group antigens (including HLA markers) in human bladder urothelium

Expression of A, B, H, Lewis, I, i, HLA class I and class II antigens was studied on 81 urinary bladder samples. Striking variation in the expression of HLA class II and to a lesser extent of HLA class I determinants was observed. Expression of at least five distinct A (or B) determinants, the synthesis of which is controlled by H, A, B, secretor and Lewis genes, was demonstrated by using a panel of reagents directed against different A, B and H epitopes. These results suggest that evaluation of blood group changes during tumoral processes requires the precise determination of the specificity of reagents used and the knowledge of ABO, Lewis and secretor phenotypes for each patient. ABH and related antigens should now be regarded as tissue antigens with a complex genetic regulation and expression.

Genetics of ABO, H, Lewis, X and related antigens

The present knowledge on chemical, enzymatic, serologic and genetic aspects of ABH antigens is reviewed in an effort to produce a simple and coherent genetic model for the biosynthesis of these antigens and chemically related structures. The genetic control of type 1 (Le(a), Le(b), Le(c) and Le(d)), type 2 (X, Y, I, and H), type 3 and type 4 ABH and related antigens in different animal and human tissues is analyzed, taking into account the properties of the glycosyltransferases which are involved in their synthesis and considering possible competition for common acceptor and donor substrates. The phylogeny of ABH determinants shows that they appeared as tissular antigens much earlier than as red cell antigens. The ontogeny of ABH antigens suggests that they behave as differentiation antigens, and an effort is made to correlate their tissular distribution in the adult with the embryological origin of each tissue.

UDP-N-acetyl-D-galactosamine as a donor substrate for the glycosyltransferase encoded by the B gene at the human blood group ABO locus

The properties of the enzyme in the serum of blood group B individuals that catalyses the transfer of small amounts of N-acetyl-D-galactosamine to H-active precursor structures were compared with those of the blood group B gene-associated alpha-(1----3)-D-galactosyltransferase and with the blood group A gene-associated alpha-(1----3)-N-acetyl-D-galactosaminyltransferases in the serum of blood group A1 and A2 individuals. The biosynthetic products formed by the enzyme in B serum were identical with the A-active structures synthesised by the A1 and A2 gene-associated alpha-(1----3)-N-acetyl-D-galactosaminyltransferases but the enzyme differed from the A1 and A2 transferases in its apparent Km for UDP-N-acetyl-D-galactosamine, its heat susceptibility, its failure to bind to Sepharose 4B, and its adsorption to H-active sites on group O red cell ghosts under conditions which bind the B transferase but fail to adsorb the A1 and A2 transferases. The correlation between the levels of alpha-(1----3)-D-galactosyltransferase and alpha-(1----3)-N-acetyl-D-galactosaminyltransferase activities in all the group B serum samples tested, the maintenance of the same ratio of activities after successive cycles of binding to group O red cell ghosts, the retention of the ability to convert blood group O to A-active cells after treatment of the serum with Sepharose 4B, and the failure to detect any comparable activity in group O serum samples tested under the same conditions indicated that the enzyme in group B serum that utilises UDP-N-acetyl-D-galactosamine to make blood group A-active structures is the B gene-associated alpha-(1----3)-D-galactosyltransferase.

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.

Abnormal blood group galactosyltransferase in blood type A1B-subjects whose sera contain anti-B agglutinin

A blood type A1B female, whose plasma agglutinates B red blood cells from other subjects but not her own, was found. Her sister with blood type A1B (sister-1) exhibited the same peculiarity. The agglutination titer of red cells from these two subjects is lower than that of normal B. Her father is blood type B, and his plasma did not agglutinate B red cells from other subjects and his own. Her mother and another sister (sister-2) were usual blood type A1. Blood group galactosyltransferase (B-enzyme) activity of plasma from the propositus, sister-1, and their father was very low. Km for 2'-fucosyllactose of B-enzyme of these subjects was 1.3 mM, which was more than two times higher than that of the normal value. Km for UDP-Gal was similar, but the pH-activity profile differed for the two enzymes. Red cell membranes from her father contained about 70% ungalactosylated H-sites, whereas, virtually all H-sites are galactosylated in the usual B red cell membranes. The blood group ABH components are known to be heterogeneous. Because of the abnormal B-enzyme with low activity and low substrate affinity, some H components might not be galactosylated, particularly in A1B cases (i.e., the propositus and her sister-1), due to competition by A1 enzyme. The lack of certain B components is likely to be the cause of the existence of the anti-B agglutinin in their sera.

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.

Probable biosynthetic pathway for the synthesis of the B antigen for Bh variants

Red cells and serum from two Bh variants (B+H-cells) have been investigated for B and H blood group glycosyltransferases. The H enzyme could not be detected using either type 1 or type 2 chain acceptors. The B enzyme was present in normal amount when 2'-fucosyllactose was used as substrate, neither 6'-fucosyllactose nor 6'-fucosyllactosamine could act as acceptors for the B enzyme. Upon treatment of the Bh red cells by the B-degrading enzyme from Trichomonas foetus the B antigen was destroyed while H determinants were uncovered (B-H + cells). The cells thus treated could be further converted into A&H-red cells by the action of the A transferase from human blood group A serum. Previous treatment of the B-H + cells by the H-degrading enzyme from T. foetus, however, led to B-/-erythrocytes and prevented their conversion into A red blood cells by the A enzyme. The results clearly demonstrate that, as found in normal B individuals, the B antigen from Bh cells is built up from the H precursor and provide additional evidence that H is not a completely silent gene in Bh individuals.