Universal blood: eliminating blood-type mismatches
We are one step closer to generating universal blood from any blood type, according to a new study by a team of scientists led by Stephen Withers and Jayachandran Kizhakkedathu from University of British Columbia. The study appeared online this month in Journal of the American Chemical Society, ahead of print.
“This is a significant step towards developing [methods] for the complete removal of blood group antigens, allowing for blood transfusions, and organ and tissue transplants from donors that would otherwise be mismatched,” the researchers report.
In blood transfusion, it is adamant that the blood types of the donor and recipient are compatible. We all fall into one of the four blood types in the ABO blood group system: A, B, AB, or O. Blood type is determined by the presence or absence of certain sugars attached to the tips of parental sugar molecules at the surfaces of red blood cells (and also in other cell types, which is why organ and tissue transplants also require matching blood types), as shown in the diagram below. These molecules are called antigens. Antigens are targeted by antibodies and induce immune response. Type-O individuals lack these A- and B-antigens. Those who lack A- or B-antigen produce antibodies against the missing antigen. For example, if type A receives blood transfusion from type B, type-A individual’s antibodies will recognize the foreign B-antigen and could induce fatal immune response.
Type O blood, which lacks both A- and B-antigens, can be given to any blood type; type O is universal donor. If only we could convert any donated blood into type O… The idea sounds like science fiction, but the method has actually been proposed back in 1982. Scientists showed that an enzyme found in coffee beans can strip off B-antigen from type-B blood, converting it into type O. (Enzymes are proteins that catalyze biochemical reactions.) Various improvements have been made since then, such as discovering enzymes derived from bacteria that can cleave both A- and B-antigens.
Despite the advancement, the process is still largely “inefficient,” the researchers in the new study say. Also, the enzymes currently available are unable to completely strip off antigens, due to the complexity of how antigens are attached. The attachment of antigen sugar groups in the diagram above looks simple, but this is not so in reality. A- and B-antigens can be linked to the parent sugar group in several different ways. Types and locations of these linkages vary from one individual to another and depending on tissue and cell types. Finding an enzyme that is flexible enough to cleave every type of linkage has been a challenge.
In the new study, researchers demonstrate a method to engineer much improved versions of an enzyme naturally found in a bacterium. The approach they used is structure-guided directed evolution.
Firstly, the scientists looked at the structure of the naturally occurring enzyme and pinpointed areas that most likely affect the protein’s activity (that is, areas that interact with antigens). This is the structure-guided part. Next, the researchers made series of random mutations within the chosen regions and selected the resulting mutated enzymes with enhanced antigen-cleaving activities. These improved versions of the enzyme were subsequently put to multiple cycles of this regime: randomly mutating within specified regions and selecting resulting enzymes with desired properties for the next round. This is the directed evolution part.
After multiple rounds of engineering, the researchers were able to create new versions of the enzyme with up to 19-fold improvement. They then obtained the structures of the improved versions to apply further structure-guided directed evolution. Finally, the researchers came up with engineered enzymes with up to 170 times enhancement in the antigen removal.
The researchers say that the new variants are also superior than the original enzyme in that they are able to cleave some, although not all, types of antigen linkages that the original version could not.
“The resulting enzyme effects more complete removal of blood group antigens from cell surfaces, demonstrating the potential for engineering enzymes to generate…[universal] blood from donors of various types,” the researchers report. “This augurs well for future enzyme engineering efforts…[and this] work represents the first steps towards complete removal of all ABO blood group antigens with a single enzyme.”
The new study is the culmination of collaborations between two Canadian universities, University of British Columbia and University of Victoria, and Centre National de la Recherche Scientifique in France. The team leaders Withers and Kizhakkedathu are principle investigators belonging to Centre for High-Throughput Biology and Centre for Blood Research, respectively, at University of British Columbia.