Blood Group Antigen Gene Mutation Database

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Xg (XG) Blood Group System

Introduction

The unique aspect of this blood group system is the location of its locus on the X chromosome at the pseudoautosomal boundary. The gene has been mapped to the distal part of the short arm of the chromosome. It consists of ten exons, the three 5' exons residing in the pseudoautosomal region and the remaining exons within the X-specific end. The latter region is not affected by chromosome inactivation. An apparently duplicated gene, CD99 (MIC2), resides in close proximity within the pseudoautosomal region. Its product, a glycoprotein, CD99 (also known as MIC2,12E7 antigen) bears 48% sequence homology to the XG product. A homologue of XG and MIC genes, XGPY pseudogene, is present on the Y chromosome, at position Yq11.21 The gene is transcribed.

The product of the XG locus, known as Xg^a glycoprotein, is a 180 amino acid, type II membrane protein, that contains 16 potential sites for O-linked glycosylation. Xg^a glycoprotein and CD99 are the two antigens that define this system. Only two phenotypes are known, Xg(a+) and Xg(a-); CD99 may be expressed in the latter individuals. It has been proposed that the Xg polymorphism is defined by a difference in the level of expression of the Xg protein at the surface of the erythrocyte rather than by an alteration in the amino acid sequence (Ellisset al.). The XG regulator gene, named XGR located between XG and MIC2 gene, appears to control, at the transcriptional level, the expression of XG and CD99 (Goodfellow et al., Fouchet et al.).

Function of proteins

CD99 is a T cell adhesion molecule. Xg function is not known.

Tissue distribution

CD99 can be detected on essentially all human cells and is highly expressed on all blood cells. Transcripts of Xg^a have been detected in thymus, bone marrow and fetal liver and several non-erythroid tissues.

Disease association

XG is linked to genes responsible for ichthyosis, occular albinism and retinoschisis; an elevated level of CD99 found in Ewing sarcoma, some neurectodermal tumors, lymphoma and acute lymphoblastic leukemia.

About the alleles

So far, no documentation is available on DNA variation in the XG, MIC2, or the regulator XGY genes. The Xg phenotypic polymorphism may be due to the level of expression of the XG product and/or regulation by XGR (Fouchet et al.). Phenotype: Males | Females - Xg(a+): 65.6 | 88.7; Xg(a-): 34.4 | 11.3

Other database IDs and links

Literature

New PubMed entries with the terms Xg (XG) and blood from the last 30 days.
NCBI Book Sections with the terms Xg (XG) and blood.

Contributors

Marion Reid and Christine Lomas-Francis, Immunohematology, Laboratory, New York Blood Center, 310 East 67th St., New York, N.Y., 10021

Contributors for specific alleles are listed with the alleles.

Links