|Blood Group Antigen Gene Mutation Database|
|Home | Systems | Resources | Administration | About||04-Dec-2016|
Uniqueness of blood group system genes
Among all human genes, those encoding the blood group antigens occupy a special place. This is because:
- By definition, their products reside on the erythrocyte surface and occur in polymorphic forms. Historically, their very discovery demonstrated the occurrence in populations of inherited gene products with polymorphic structures.
- Products of a small number of different allelic genes, residing on the same locus, are the most common polymorphic forms that occur with a high incidence, albeit in different ratios, in all world populations (common alleles); yet, for many blood group gene families, variants of those common alleles also occur, showing variable incidences (from less than 0.1% to as much as 99%), and often show a well-defined geographic and/or ethnic distribution (rare alleles). It is important to note that this definition is based on blood group phenotypes and therefore,in most cases, it focuses on DNA variation that occurs within, or affects the epitopic regions. Variation outside of those regions is documented when it occurs in cis to the epitopic regions or in cases where the presence of alleles was based on observations other than the blood group phenotypes So far all information indicates that each serological phenotype (common or rare) corresponds to a particular mutation(s) or DNA alteration affecting directly or indirectly the epitopic region, hence each serological phenotype specifies at least one particular allele.
- Nearly all blood group-specifying genes are expressed in erythroid tissues and their products are either membrane-associated protein antigens or enzymes, glycosyltrnsferases, which synthesize membrane-associated carbohydrates which also define the antigenic epitopes In a few cases the product is adsorbed to the erythocyte surface from plasma (Lewis, Chido-Rodgers antigens). Products of some genes are confined to erythroid tissues exclusively, whereas others show a more wide distribution and are present on surfaces of other cells or in soluble forms in the body fluids.
- In most cases, the expression of all erythroid polymorphic forms which carry a blood group antigen, common or rare, is consistent with normal physiologic function and is not associated with disease; additionally, in most cases, the absence of an erythroid gene product (null serological phenotype) does not cause disease; at this time, information is lacking as to whether the same is true for products, or lack of products, of non-erythroid alleles. Nevertheless, alleles of erythroid genes that express a common blood group phenotype may also cause disease (see SCL4A1).
- The expression of all erythroid polymorphic forms is independent of non-genetic factors such as sex, age, and environment.
- The resulting erythroid phenotypes can be assessed readily by serology, and documentation for their incidence and distribution exists for a large number of world populations. However, this information is fragmented as many populations were not tested, particularily for the presence of the rare alleles. Serological analysis is a powerful tool to identify variant individuals to examine and study their variant alleles. Currently, genotyping is becoming an important tool for identification of new alleles ( see ABO, Rh systems).
Return to Tutorials