13 December 2012
Fumiichiro Yamamoto and the ABO Histo-blood Groups and Cancer Laboratory discover the molecular genetic basis of the human Forssman glycolipid antigen negativity
In a paper published today in Scientific Reports the Yamamoto group resolves a classic question in Glycobiology. The heterophilic Forssman glycolipid antigen has a structural similarity to the histo-blood group A antigen and the GBGT1 gene that codes for the Forssman glycolipid synthetase (FS) is evolutionarily related to the ABO gene. Dr Yamamoto is well known for his ground-breaking work on defining the genetics behind the ABO blood groups, and this current work clarifies the functioning of the second in the family of four closely related genes, which specifies the expression of the Forssman antigen. Although humans are usually negative for Forssman antigen the group has shown that its production can be restored by the reversion of just two amino acid substitutions. Forssman antigen has been found in several types of tumor and recently a very rare blood group that is Forssman positive has been described.
The cells in the immune system discriminate friends and foes by recognizing antigens. The Forssman antigen was discovered in 1911 when Forssman carried out his classic experiments injecting rabbits with a suspension of kidney tissue from different species. In some cases the rabbits produced antibodies capable of destroying sheep red blood cells. Research revealed that many species were Forssman positive, such as mouse, dog, horse and chicken, whereas others are Forssman negative, including: human, the great apes, pig and frog.
The Forssman antigen is formed of a sugar chain, which is assembled from substrates in the cell by an enzyme of the alpha 1,3-Gal(NAc) transferase family called Forssman glycolipid synthetase. The GBGT1 gene codes for the enzyme and when it is present the antigen is produced.
The Yamamoto group carried out extensive in silico studies from known data on the genomes of many species, some of which were Forssman positive and some negative. From this they identified several potential mutations that could be responsible for the absence of the antigen in humans.
They followed this with DNA transfection experiments. This involves creating chimeras between the functional mouse and the nonfunctional human gene products and in vitro mutagenized amino acid substitution constructs and checking antigen production on the cell surface, which narrows down exactly which part of the human gene is inactivating the function. This showed that changing just 2 amino acids in the human code could restore the function of the FS glycosyltransferase.
The group also showed that loss of this function occurred recently in evolution and that one of the mutations is at the site that specifies the sugar specificity between the blood group A and B transferases coded by A and B alleles of closely related ABO gene.
In all, there are 4 genes encoding different alpha 1,3-Gal(NAc) transferases that produce a complicated map of positivity and negativity among various species. The Yamamoto group aims to complete this map for humans, thus providing a baseline for this set of antigens for healthy cells and therefore allowing the use of any changes for diagnosis and prognosis in some illnesses including cancers.