Structural basis for red cell phenotypic changes in newly identified, naturally occurring subgroup mutants of the human blood group B glycosyltransferase.
BACKGROUND
Four amino-acid-changing polymorphisms differentiate the blood group A and B alleles. Multiple missense mutations are associated with weak expression of A and B antigens but the structural changes causing subgroups have not been studied.


METHODS
Individuals or families having serologically weak B antigen on their red cells were studied. Alleles were characterized by sequencing of exons 1 through 7 in the ABO gene. Single crystal X-ray diffraction, three-dimensional-structure molecular modeling, and enzyme kinetics showed the effects of the B allele mutations on the glycosyltransferases.


RESULTS
Seven unrelated individuals with weak B phenotypes possessed seven different B alleles, five of which are new and result in substitution of highly conserved amino acids: M189V, I192T, F216I, D262N, and A268T. One of these (F216I) was due to a hybrid allele resulting from recombination between B and O(1v) alleles. The two other alleles were recently described in other ethnic groups and result in V175M and L232P. The first crystal-structure determination (A268T) of a subgroup glycosyltransferase and molecular modeling (F216I, D262N, L232P) indicated conformational changes in the enzyme that could explain the diminished enzyme activity. The effect of three mutations could not be visualized since they occur in a disordered loop.


CONCLUSIONS
The genetic background for B(w) phenotypes is very heterogeneous but usually arises through seemingly random missense mutations throughout the last ABO exon. The targeted amino acid residues, however, are well conserved during evolution. Based on analysis of the resulting structural changes in the glycosyltransferase, the mutations are likely to disrupt molecular bonds of importance for enzymatic function.
Hosseini-Maaf
Bahram
B
Division of Hematology and Transfusion Medicine, Department of Laboratory Medicine, Lund University and Blood Center, Lund University Hospital, Lund, Sweden.
Letts
James A
JA
Persson
Mattias
M
Smart
Elizabeth
E
LePennec
Pierre-Yves
PY
Hustinx
Hein
H
Zhao
Zhihon
Z
Palcic
Monica M
MM
Evans
Stephen V
SV
Chester
M Alan
MA
Olsson
Martin L
ML
D000017:ABO Blood-Group System / Q000235:genetics*; D000483:Alleles; D004252:DNA Mutational Analysis; D004912:Erythrocytes / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D005260:Female; D005838:Genotype; D016695:Glycosyltransferases / Q000737:chemistry / Q000235:genetics*; D006801:Humans; D008297:Male; D008957:Models, Genetic; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D020125:Mutation, Missense; D010375:Pedigree; D010641:Phenotype; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship; D014961:X-Ray Diffraction
D000017:ABO Blood-Group System; D016695:Glycosyltransferases