XG: the forgotten blood group system

Area of Science:

• Genetics
• Immunology
• Molecular Biology

Background:

• The XG blood group system includes Xg(a) and CD99 antigens with a unique phenotypic relationship.
• The XG gene is located on the X chromosome, with parts in the pseudoautosomal region.
• The MIC2 gene, encoding CD99, is present in the pseudoautosomal regions of both X and Y chromosomes.

Purpose of the Study:

• To summarize the genetics, inheritance, and clinical significance of the XG blood group system.
• To highlight the relationship between Xg(a) and CD99 antigens.
• To discuss the implications of XG and CD99 in genetic disorders and cancer.

Main Methods:

• Literature review of studies on the XG blood group system.
• Analysis of genetic location and inheritance patterns of XG and MIC2 genes.
• Review of clinical data regarding anti-Xg(a) and anti-CD99 antibodies.

Main Results:

• Xg(a) exhibits X-linked inheritance, while CD99 is encoded by the MIC2 gene in the pseudoautosomal region.
• Anti-Xg(a) antibodies are rare and generally clinically insignificant, except for one case of severe hemolytic anemia.
• Anti-CD99 antibodies are extremely rare, and their clinical significance is undetermined.
• The XG gene is linked to several X-borne inherited disorders.
• Elevated CD99 levels are associated with certain cancers.

Conclusions:

• The XG blood group system provides valuable insights into X-linked inheritance and chromosome mapping.
• While generally benign, rare alloanti-Xg(a) can cause severe hemolytic anemia.
• CD99's role as an adhesion molecule and its association with cancer warrant further investigation.